Valorization of tropical fruit‐processing wastes and byproducts for biofuel production

Global energy demand is expanding due to an increasing population and the use of energy‐intensive technologies. Fossil‐based energy and transportation sectors have been blamed for greenhouse gas emissions and price fluctuations. Biofuel production has been suggested as a possible solution to this problem, despite the cost of production and difficulties with the availability of feedstocks, and the food versus energy debate, which is still a challenging issue. For many decades, researchers have been investigating how to valorize low‐cost and non‐edible biomass resources, including fruit‐processing waste and its byproducts, to produce various biofuels using innovative, cost‐effective, and green technologies to overcome these challenges. However, exhaustive studies and comprehensive and structured knowledge related to the biofuel production potential of fruit waste and byproducts are lacking because most of the research that has been done so far was specific to a single fruit, location, and method. As a result, this review paper emphasizes the evaluation of the physicochemical compositions of the wastes and byproducts derived from tropical fruit production and processing to determine their suitability in the production of different biofuels via various valorization techniques. The results of the review showed that fruit waste is a potential alternative raw material for producing different biofuels, like biodiesel, biogas, biohydrogen, and fuel pellets.


Introduction
T he production of tropical fruits is increasing significantly and accounts for 75% of global fruits. 1 Data obtained from the FAOSTAT 2 database has confirmed that there is an increasing trend in fruit production.As Fig. 1 shows, from 2010 to 2020, bananas, watermelon, apples, oranges, grapes, and mangoes were the top six fruits produced globally.The marketing (import and export) of tropical fruits has also increased radically, as shown in Fig. 2, 3,4 due to the increasing demand for consumption and the net economic return gained from production. 5 As mentioned by Teka et al. 6 changes in feeding habits, increasing numbers of fruit-processing plants, an increasing world population, a rapid rate of urbanization, industrialization, and growing consumer familiarity with tropical fruits, understanding the health benefits and the high nutritional value of tropical fruits, are the driving factors for increasing production, demand, and trade.Despite an increase in the overall trends of global fruit production, there have been variations in the amount of production from year to year and between different regions.According to Altendorf, 1 99% of total tropical fruit production takes place in developing countries, of which 80% is imported by developed countries.
With increasing fruit processing, culling of poor-grade fruit, and losses happening in different parts of the production system (like harvesting, transportation, and processing), a huge amount of fruit waste and byproducts are generated.Previous research has reported different amounts of waste and byproducts generated and discarded in the natural environment from fruit production and processing systems.According to a recent study by Isabela et al., 7 Yamaguchi et al., 8 Alzate Acevedo, 5 and Esparza et al. 9 fruit waste and byproducts account for 30-50% of total fruits weight whereas Ramirez-Pulido et al. 10 , Villacis-Chiriboga et al. 11 and Suri et al., 12 reported 60-70%.Today, the management of waste is a major threat all around the world. 13Unless this waste is properly managed, it will become a major menace all over the world by polluting environmental resources (air, soil, and water) 14 and increasing the economic costs associated with its transportation and handling in landfills. 15,16Beyond the environmental and economic impacts, 17 poor waste management also poses significant health impacts, such as outbreaks of different kinds of water-borne diseases (diarrhea, cholera, etc.), respiratory diseases (asthma, branchiate infection, etc.), and lung diseases. 18,19arallel to increasing waste generation, global energy demand also increased with an increasing rate of industrialization, urbanization, and continuous population growth. 20According to the International Renewable Energy Agency 21 and the International Renewable Energy Agency (IRENA), 22 approximately 1 billion people worldwide are currently without energy, and demand is expected to increase by more than 50% by 2050.High dependence on conventional energy sources (fossil fuel) for many decades resulted in environmental problems such as greenhouse gases and toxic pollutants emissions, which affect economic and social wellbeing significantly.To avert these interconnected challenges (increasing waste generation and energy demand) Figure 1.Global trends in fruit production over 10 years in millions of metric tons based on FAOSTAT data. 2 1809 different waste-management techniques and policies like the 2030 Agenda for Sustainable Development, which is related to waste reduction and prevention, reduction, recycling, and reusing of waste, have been developed, adopted and practiced by many countries. 23Shifting to renewable energy production from low-cost biomass resources as alternatives was also a common concern at a global level.2][33][34] A study conducted by Idayanti et al. 35 on ruminants reported that using pineapple fruit waste and byproducts as alternative ingredients in animal feed reduced the cost of conventional total feed by 30% while improving the quality of feed.5][36][37] Alnaimy et al. 38 stated that the high nutritional value of citrus by-products makes them play a key role in livestock fattening without altering animal health.Luzardo et al., 20 Scerra et al., 39 and De Evan et al. 40 also confirmed that feeding by mixing up to 35% of fruit waste/ byproducts with byproducts of cereal crops does not have a significant effect on the health and meat quality of ruminants, and hence suggested, as a promising method for improving feed quality, reducing the environmental impact and cost of feed without compromising animal health and meat quality.Beyond using them as animal feed, much research was also conducted to investigate whether to use fruit waste and byproducts for biofertilizer production, enzyme production, oil extraction, medicinal use, and human food supplements.The findings of Ghinea and Leahu 41 and Musa et al. 42 showed that organic waste from fruit processing has great potential to enhance the nutrient content of the soil after decomposition by microorganisms.The valorization of organic fruit processing wastes and byproducts into biofertilizers reduces the price of inorganic fertilizer, increases crop productivity, and lowers costs related to waste management practices. 43,44ruit wastes are also a good alternative material for enzyme production, acting as catalysts, 45 and help to save energy and chemicals in comparison with the conventional process of producing various products in large-scale processing industries.This is a win-win approach that enables the companies to comply with stringent legislation that strongly encourages processing industries to implement a circular economy (i.e., reduce and manage their waste). 20,39o satisfy the increasing demand for oil and simultaneously address challenges associated with poor waste management and debate over edible crop use for oil extraction, many researchers have also been searching for cheap raw materials.In this regard, waste, and byproducts from fruits like watermelon (Citrullus lanatus), Annona squamosa L. ('Gista'), 46 and avocado (Persea americana), 11 among | Biofuels, Bioprod.Bioref.17:1807-1842 (2023); DOI: 10.1002/bbb.2531other fruits, have been tried with promising results.Alzate Acevedo et al., 5 Rifna et al., 47 and Villacis-Chiriboga et al. 48tated that the essential compounds found in fruit waste and byproducts play a significant role in improving food quality and helping to address problems related to malnutrition in most developing and poor countries.In addition, fruit waste and byproducts generated by industrial and other processing units (large and small scale) can be viewed as opportunities for transitioning from fossil fuels to greener and cleaner energy sources. 49For satisfying the increasing demand for renewable energy sources (which was 3% in 2020 and continued increasing by 48% in the coming 20 years) 50 , and complying with a stringent regulation aiming at reducing emissions and organic waste; using fruit wastes as feedstocks for energy generation is very crucial.This is also an excellent opportunity for the full implementation and achievement of possible pathways outlined in the Sustainable Development Scenario for transforming the energy sector into net-zero emission pledges by mid-century (2050). 21Considering these issues, innovative research related to the management of fruit processing waste and byproducts has been developed with the intention of reducing waste and exploiting them to produce different biofuels.Even though different research has been conducted related to this issue in different areas of the world using different fruit waste (tropical and temperate fruits), technologies, and treatment techniques, they have not been summarized and unable to present organized information.Hence, the overall purpose of this review is to provide a comprehensive overview of trends of major tropical fruit production, processing, and waste generation, physicochemical compositions of fruit processing waste/ byproduct and their valorization into selected biofuels.

Tropical fruit-processing waste/ byproducts and composition
Global trade and processing of tropical fruits are increasing significantly. 5,8,51The consumption of fresh fruits for their refreshing taste and nutritional value has been common for a long time but the use of processed fruit products has become a fashion and is expanding due to an increase in fruit processing and preserving technologies.Industrial processing transforms fruits into commercially attractive byproducts (such as juices, jams, jellies, pickles, and natural flavors) and also generates significant amounts of different wastes such as peel, seed, pomace, pulp, stem, and others not directly used in human food but discarded as waste. 8,52,53Although reported differently by different researchers, this accounts for nearly half of the fruit by weight.For instance, the findings of Han et al., 13 found that papaya fruit processing generates a large amount of papaya waste in which seed waste constitutes up to 30% of the papaya by volume.][56] At the global level, a huge amount of waste is generated from fruit processing 57 and the amount of generation varies from country to country. 58A study conducted by Wongkaew et al. 59 in Thailand reported that, of 200 000 tons of food waste generated, 24% is mango peel waste.Another study conducted in India reported that the fruit processing industry generates about 5.6 million tons of waste annually, 60 at the producer and trader level due to poor handling, storage, transportation systems, and management.Data obtained from the FAOSTAT platform 2 also confirmed that a significant amount of waste is produced in the entire supply chain, which could pose a substantial risk to the ecosystem and human beings, and hence effective management strategies are demanded.

Major tropical fruits and the physicochemical compositions of their waste and byproducts
The amount and composition of fruit waste and byproducts generated differ between developed and developing countries.According to Esparza et al., 9 fruit waste accounts for 60-70% of total household waste.Its composition varies according to the culture, standard of living, and economic development of a country. 61The fruit-processing industry also generates huge amounts of waste and byproducts that are potential sources of different bioactive compounds (dietary fibers, amylopectin, protein, phenolics, carotenoids), vitamins, oils, and minerals (micronutrients), which serve as antimicrobial, antioxidant, anticancer, and anti-cardiovascular disease substances for humans. 11,12,26,27,62Basri et al., 63 Lee et al., 16 Miller et al., 64 and Mohsen et al. 62 also confirmed that these byproducts contain free fatty acids (saturated and unsaturated) and minerals like sodium (Na), magnesium (Mg), phosphorus (P), potassium(K), and calcium (Ca), which serve as ingredients in food supplements, animal feed, and for the production of different biofuels such as biodiesel and ethanol.The use and extraction of biochemical and bioactive compounds from tropical fruit waste and byproducts requires knowledge of their chemical composition and hence details of major tropical fruit waste, and their physicochemical composition is presented in the following sub-sections.

Apple waste and byproducts
Apple (Malus pumila) is a deciduous fruit that is cultivated widely throughout the world in temperate climates. 65A STATISTA 66 report showed that China is the leading applegrowing country in the world, followed by the USA, India, and Turkey.It is rich in vitamins, calcium, phosphorus, potassium, and organic acids.Sharma 67 stated that about 70% of apple fruits are consumed fresh while the remaining is processed into different value-added products like juice, apple cider, jams, jelly, puree, and other dried products.Industrial processing and consumption of apple fruits has experienced a powerful surge due to increasing demand for apples and apple products and hence generates enormous quantities of waste (solid and liquid) and byproducts.The solid residues consisting of a mixture of skin, pulp, and seeds are collectively called 'apple pomace' . 68Apple waste and byproducts, which make up 25-35%, are a potential source of bioactive molecules (such as protein, biopolymers, dietary fiber, and natural antioxidants), insoluble carbohydrates (like cellulose, hemicellulose, pectin, and lignin), simple sugars (glucose, fructose, and sucrose), minerals, proteins, vitamins, and organic acids. 26Furthermore, it is a good source of phenolic and terpenoid compounds that have antioxidant, antibacterial, and antitumor properties, providing important human health benefits. 69,70he amount and types of compounds present in apple wastes and byproducts vary depending on the varieties, 71 color (which varies between green, yellow, and red), and horticultural practices. 72In this line, chlorophylls, carotenoids, and anthocyanins are the compounds present in the color of these apples. 71In general, apple fruit waste encompasses different low-cost phytochemical and bioactive compounds that have nutraceutical, pharmaceutical, and other health benefits if properly valorized using appropriate technologies.Despite efforts to recover these essential compounds in various ways (e.g., as animal feed), about 80% of apple residues are considered waste rather than a resource and thus end up in landfills, resulting in greenhouse gas emissions and environmental pollution. 68,73Thus, apple residues can be viewed as potential feedstocks for the production of various biofuels, which aid efforts to reduce emissions from the energy sector.

Citrus fruit waste and byproducts
Citrus fruits are evergreen shrubs or small trees in the Rutaceae family and the genus Citrus and are widely cultivated in all climate zones (tropical, subtropical, and temperate). 74They include the most well-known fruits, such as sweet orange (Citrus sinensis), lemon (Citrus limon), mandarin (Citrus reticulata), bergamot orange (Citrus bergamia), grapefruit (Citrus paradisi), pomelo (Citrus maxima), lime (Citrus aurantiifolia), and citron (Citrus medica). 75Citrus fruits (CFs) are among the most widely cultivated, traded, processed, and consumed tropical fruits all over the world 76 due to their high nutritional value, numerous health benefits, and higher economic returns. 74In comparison with other fruits, the edible part of citrus fruits is small relative to the inedible parts.Although only 33% of the harvested citrus fruits undergo processing, 77 they generate a huge amount (~50-60%) of waste/byproducts (such as peels, seeds, pulps, and stones) due to increasing production and consumption worldwide. 25,75Fig. 3 below shows the waste produced during the processing of diverse types of citrus fruits.
In previous studies by Villacis-Chiriboga et al., 11 Pascoalino et al., 26 and Kaur et al. 77 reported that citrus byproducts are characterized by abundant quantities of soluble sugar content (glucose, fructose, and sucrose), lipids (linolenic, oleic, palmitic, stearic acids), essential oils (limonene), organic acids (citric, malic, oxalic acids), proteins, insoluble polysaccharides (pectin, cellulose, starch, dietary fiber, and hemicellulose), vitamins (vitamin A, B, and C), minerals (Ca, P, K), and various phytochemical (flavonoids, hesperidin, carotenoids) compounds, 11,26,77 as shown in Fig. 4 and Table 1.Furthermore, pectin and hemicelluloses are rich in galacturonic acid, arabinose, galactose, and small amounts of xylose, rhamnose, and glucose, which would be recovered using extraction technologies. 10,48ngo waste and byproducts Mango (Mangifera indica L.) is an evergreen branched tree with fleshy fruit containing stones, belonging to the Panes and Anacardiaceous genera and families, respectively.It originated in Southeast Asia and has over 1000 varieties.Nowadays, it is grown widely throughout the world's tropical and subtropical regions.83,84 The production and demand of mango fruit increased by 51% compared to other major tropical fruits (such as pineapple, avocado, papaya, and banana) in 2017.The Food and Agriculture Organization (FAO) 85 also reported that it is the most widely produced tropical fruit worldwide, followed by pineapples, papayas, and avocados.The global export of mango accounted for 29% of the global major tropical fruit trade and has shown an increasing trend (5.1% in 2020 in comparison with 120 000 tons in 2019).85 The volumes of waste released from mango processing units are expanding with increasing demand for processed mango products, mango fruit production, and the processing industry.86 Fresh mango fruit consumption generates an enormous amount of waste (about 57 200 metric tons per year) and byproducts consisting of seeds (stones), kernels, and peels.87 Figure 5 shows the anatomy of mango fruit and different waste/byproducts produced during its processing.The industrial processing of mango fruits also generates 15-25 million tons of solid waste (peels, stones, and kernels), which is approximately 25-40% of the whole waste, 27,86 and 30-50% of the raw fruits.88 The amount varies depending   25 1813 on fruit varieties, mango peel, and seeds account for about 7-24% and 20-60% of the fruit weight, respectively, while the kernel accounts for 45-75% of the seed weight.88 As presented in Table 2, mango waste and byproducts are potential sources of fat (6-15.2%),protein (6.36-10.02%),carbohydrates (32.34-76.81%),fiber (0.26-4.69%), macro and micronutrients (such as calcium, potassium, magnesium, phosphorus), and vitamins (A, E, K, and C), which can be used as raw materials in other industrial activities.27,30,59,94 Likewise, they are also sources of fatty acids and different chemical compounds like total reducing sugars, pentose sugar, carbohydrates, and lipids, which play an important role in waste-to-energy conversion. 27,30Mango seed contains 8.15% to 13.16% of oils 27 that serve as potential sources for energy fuel production.However, the potential yield and concentration of these compounds vary among fruit wastes and byproducts.
Wongkaew et al. 59 found that dietary fibers and other nutritional compounds of mango fruit waste are affected by the degree of ripeness, genetic differences, senescence, climate, and soil conditions.Maldonado-Celis et al. 33 also reported that ripe mango peel comprises about 15% of total sugars, among which sucrose is the principal sugar in ripe mango fruit, while fructose is the major monosaccharide  during the preclimateric phase.Bekele et al. 95 found that local mango varieties possess large seeds and small quantities of flesh/pulp relative to apple mango and other varieties.A similar source stated that the variation in the percentage of pulp is associated with varietal differences and the growing conditions of the fruits.The large amount of chemicals, minerals, vitamins, and other compounds in mango processing waste and byproducts suggests that can be utilized to make large-scale commercial products.In this regard, characterization, and knowledge of the physicochemical composition of the mango waste and byproducts is imperative before going ahead with valorization.Currently, many attempts have been made to recover and reuse these essential compounds.For instance, mango peel, which is a potential source of dietary fiber and contains 5-11% pectin, has been investigated by Wongkaew et al. 59 to recover food-grade mango peel pectin, which has commercial value in the food additives industry.Jahid et al. 96 also assessed the bioethanol production potential of different fruit peels, including mango peels, and concluded that they contain a high amount of fermentable sugar and suggested them as potential candidates for ethanol production.Such reuse of waste and byproducts creates an additional source of income for the mango-processing industry, in addition to following waste reduction regulations and reducing environmental problems like pollution and greenhouse gas emissions.

Papaya waste and byproducts
Papaya (Carica papaya) fruit is characterized by its high nutritional value, antioxidants, and low calories.The fruit is usually cylindrical with an average weight of 0.5-2.0kg and contains large quantities of seeds that comprise about 15% of the wet weight of the fruit. 97With an estimated 30-35% cull rate, a large amount of off-grade papaya waste is produced. 12urthermore, during its processing into various products (like jelly, jam, candy, pickles, and juice), a huge amount of waste and byproducts is generated and usually discarded. 97s stated by Pathak, 98 papaya fruit processing generates papaya peel and papaya seeds that constitute about 12% and 8.5% of the fruit weight, respectively, as shown in Fig. 6. 1815 These waste and byproducts are excellent sources serving as potential feedstocks for the production of different biofuel and bioactive compounds (carotenoids and antioxidants), vitamins (A, E, C, and B complex), carbohydrates, protein (27.3-28.3%),crude fibers (19.1-22.6%),lipids (28.2-30.7%)and minerals (like calcium and phosphorus) that are used as dietary additives, nutraceutical supplements, and pharmaceutical products. 28,61,97,99he fatty acid composition analysis of papaya seed oil showed that it had 72% monounsaturated fatty acids, predominantly 71% oleic acid, representing a promising new source of special plant oil for different applications 97 such as cosmetic use (skin and hair care) beyond biodiesel production.The nutritional value, chemical composition, and physicochemical properties of the papaya fruit waste and byproducts majorly depend on the ripeness/maturity level, cultivar, and processing conditions (e.g., drying method, dry temperature, particle size). 75,98,100The study by Dos Santos 99 in Brazil on two papaya varieties confirmed that there is a significant difference in the physicochemical composition of papaya byproducts (peel and seed), as shown in Table 3. Masresha and Mulate 101 also stated that growing seasons, environmental conditions (soil and geographical locations), and management practices are likely sources of variation in the nutritional, physical, and chemical compositions of papaya fruit.

Watermelon waste and byproducts
Watermelon (Citrullus lanatus) is an herbaceous, creeping tropical fruit in the Cucurbitaceae family that is mainly propagated by seeds. 102In 2022, global watermelon production was approximately 117.2 million tons and China is the world's leading watermelon producer (79.2 million tons), followed by Turkey (3.9 million tons). 103Traditionally, for many years, Citrullus lanatus was considered a sweet and non-functional fruit.However, nowadays, several studies have affirmed that it is a popular fruit rich in protein, vitamins (like vitamin A, vitamin B, especially vitamin B 1 and B 6 , and vitamin C), minerals (magnesium, potassium, phosphorous, sodium, zinc, manganese, and copper), folate, and some of the most important antioxidants like lycopene and different bioactive compounds (carotenoids). 104onsequently, its consumption and processing increased at a rate of 20-30% per year, and became the most popular fresh fruit among many people worldwide. 105Citrullus lanatus fruit processing industry generates a large amount (25-60%) of byproducts such as rinds, seeds, and peels (indicated in Fig. 7 below) and discards them, although they have great economic potential.These wastes/byproducts are underutilized despite having essential biomolecules that have multiple applications in biofuel production, pharmaceutical, and cosmetic industries.Table 4 shows some of the physicochemical composition of watermelon waste, particularly seeds, rinds, and peels.Vinha et al. 102 and Elsayed et al. 109 also reported that watermelon seeds are potential sources of proteins (27.4%), lipids (47.9%), carbohydrates (9.9%), vitamins B, minerals, crude fiber (48.26%), and oil.Watermelon seed oil showed a high fatty acid profile with 77.4% unsaturated fatty acid and 63.2% Table 3.Average values (in g 100 g −1 ) of the proximate composition of peel and seed flours from papaya waste/byproducts. 87

Parameters
Papaya peels Papaya seeds ). 109,110The rids of Citrullus lanatus are mainly composed of celluloses, hemicelluloses, pectin, lignin, and sugars that have a substantial contribution to biofuel production. 111Many researchers have evaluated different extraction methods and solvents to extract bioactive chemicals and compounds from watermelon byproducts and concluded that the variation was attributed to species in different countries, soil types, climatic conditions, and agronomic conditions. 112

Pineapple waste and byproducts
Due to the high nutritional value, attractive flavor, and refreshing sugar-acid balance, the demand, production, and consumption of pineapple (Ananas comosus (L.) Merrill) fruit in fresh and/or processed form has been increasing worldwide. 113This resulted in a massive amount of waste and byproducts (such as crowns, peels, leaves, cores, and stems) generation, accounting for 40-80% of the fruit parts. 114A recent study by Aili Hamzah et al., 115 and Roda and Lambri 113 attested that pineapple processing, transportation, and storage generate about 80% of waste and byproducts, which are simply discarded despite having potential benefits if properly valorized.Banerjee et al. 116 stated that the chemical composition of these wastes and byproducts varies based on the fruit varieties, growing conditions, and degree of ripeness.Pineapples also have higher concentrations of glucose and fructose, which are found during the fruit development stage, while the accumulation of sucrose is detected during the ripening stage. 115Table 5 shows the chemical composition of pineapple waste and byproducts.The waste and byproducts of pineapple fruit have a high moisture content, sugar, lignocellulose (cellulose, hemicellulose, and lignin), and protein compounds, 115 which are often discarded and contribute to environmental pollution and increase the risk from infectious disease-causing agents (like bacteria, viruses, and other microorganisms).

Physicochemical and fatty acid composition of different fruit wastes
Different fruit wastes and byproducts have different physicochemical composition and properties.This variation is attributed to species differences in different countries, and agronomic conditions like soil and climate conditions, as stated by Biswas et al. 112 Table 6 below shows the physicochemical composition of apple, mango, papaya, and pineapple fruit wastes.This fruit waste has less than 0.42 water activity, which is not conducive to the growth and reproduction of microorganisms (as it is below 0.6) and hence is stored for further processing without deterioration.As mentioned by Sancho et al., 56 the waste and byproducts of papaya, apple, pineapple, and mango fruits are acidic because their pH is less than or equal to 4.6, which does not allow the growth of microorganisms.However, these wastes are rich in sugars (like glucose and fructose), which determine their fermentation potential.Hence, they are suitable for fermentation to produce different biofuels, such as bioethanol, biomethane, and other fuels.Similarly, the availability of lipids in fruit waste also makes them a potential candidate for oil extraction, which could be used in biofuel  production and used for human consumption if they meet an international featured standard.Knowing the fatty acid profile of oils is very important because it determines their stability and oxidative properties.As shown in Table 7, the fatty acid composition of fruit residues showed a high concentration of palmitic and oleic acids, which are common in seeds and peels of fruits.Polyunsaturated fatty acids, like linoleic and linolenic acids, contribute to the prevention of atherosclerosis, cancer, heart disease, and diabetes.It is known that oils with a huge amount of unsaturation, particularly 18:2 fatty acids, are susceptible to oxidation and may produce products that contribute to arteriosclerosis and carcinogenesis.

Fruit waste and byproduct valorization into biofuels
Fruit-processing waste and byproducts (peels, seeds, pulp, and other parts of the fruits), which had been discarded for many decades without understanding their benefits, are now becoming a potential feedstock and are converted into diverse types of biofuels using various biochemical and thermochemical techniques.These biomass resources encompass different chemical compositions/compounds (which were discussed above), which can also be converted into other value-added products that have human health benefits.Studies by Dias et al., 100 Lee et al., 16 Suri et al., 12 and Nieto et al. 119 have reported that fruit wastes are rich in phytochemicals, antioxidants, dietary fibers, sugars (soluble and insoluble), vitamins, and minerals that are used as ingredients and natural colorants in the food processing industry and biofuel (liquid and solid) production.
Biofuels are fuels derived from biomass resources.They have a low carbon footprint that does not compete with the food sector and is recognized as a key element of an awaited sustainable energy system. 9Waste and byproducts from fruit processing units have recently emerged as promising feedstock for various biofuel productions [120][121][122] because they are inexpensive, biodegradable, less toxic, and abundantly available renewable resources.Beyond contributing to ensuring renewable and emission-free energy sources, utilizing processed fruit waste and byproducts also enables us to comply with stringent laws enforcing waste and emission reduction, reducing the cost of waste management and environmental pollution.Hence, to utilize these lowcost bio-resources and reduce the social, economic, and environmental impacts associated with increasing waste, environmentally sound and innovative approaches are needed. 123,124In this regard, different waste valorization techniques have been employed for many years, starting from direct use as animal feed and agronomic input to advanced value-added products, as presented in Figure 8.Among the different valorizations illustrated in Fig. 8, for this review, emphasis is given only to fruit waste conversion to different biofuels (such as biodiesel, bioethanol, biogas, biochar, and biohydrogen) and extraction of oils.Nowadays, the production of biofuels from low-carbon biomass resources such as fruit processing waste and byproducts has been attracting the attention over fossil fuels due to the spiraling demand for and cost of fossil-based fuels, the abundance of resources (organic wastes), their cost-effectiveness, and their renewability. 125l extraction from fruit waste/byproducts The increasing price of raw materials for oil extraction, coupled with increasing demand for oil for different purposes has led to a search for alternatives.Currently, there is an increasing tendency to use cheap raw materials for the extraction of oils that can be used for biodiesel production.
Beyond reducing the price of oil and diversifying the sources of income, the utilization of oil extracted from organic waste/residues enables the reduction of environmental impacts and generates alternative raw materials for biodiesel production. 97Besides feedstock quality, the oil extraction methods employed determine the quality of the oil extracted, which can also influence biodiesel production and quality.In this context, different methods of oil extraction and factors determining the quality and yield of oil produced from different fruit wastes and byproducts for biodiesel, biogas, bioethanol, and biohydrogen production are presented in the subsequent sections.

Oil-extraction methods
Oil extraction is the process of isolating oil from oilbearing seeds, animal byproducts/waste, and plants (fruits  Feedstock preparation: Before directly proceeding with extraction, selection, and pretreatment of the feedstocks are the primary activities.Preparation of feedstocks involves separating the different parts of fruits (like seeds, peel, and kernel), washing to remove dust and other contaminants, followed by drying to an appropriate moisture content. 127rying would be done in an oven or the sun for 3-5 days, or until the dry weight became constant.Dried seeds, kernels, or selected parts of the fruit would be ground to the appropriate size using a pestle and mortar or using any grinding or milling machines, then sieved, stored at room temperature, until ready for extraction.

Mechanical oil extraction
Mechanical oil extraction is the most conventional method, in which either a manual ram press or an engine-driven screw press is used for oil extraction. 128Mechanical pressing technology has been used by the indigenous community for extracting oil.It dates from the 1940s. 129It has been reported that a manual ram press gives an extraction efficiency of 60-65%, whereas an engine-driven screw press has an efficiency of 68-80%. 127,128There are two major types of mechanical presses: the expeller, which is also called 'a single screw press' and the extruder, which is used for oil extraction, and different additional functions. 129Expeller pressing is applied with a limited scope, like on-farm seed grinding.
Due to its low cost, no need for any solvents, ease of operation, and regulation of extraction temperature (under 50 °C), it is preferred by communities in developing countries for oil extraction. 129However, poor oil quality and recovery is one disadvantage of this method. 129The other shortcoming is that mechanical extractors are not suitable for all seeds (feedstocks). 127To improve oil yield and quality, mechanical extraction systems need further pretreatment and degumming.Optimizing the number of presses, using different screwing speeds, and extrusion of seeds (which depends on screw configuration) are among the mechanisms to enhance the yield and quality of oils from the intended feedstocks.Cheng and Rosentrater 130 also reported that a two-stage mechanical process that combines extrusion and expelling helps to improve oil yield and recovery over single-step expelling.Two-screw extrusion systems are used on a laboratory scale but they are preferable to single-screw systems because they allow thermomechanical treatment of seeds, and avoid further pretreatment steps (dehulling, cooking, and flaking), and produce high oil yields.
In response to the increasing demand for better oil quality and yield, oil extraction process optimization from fruit processing wastes has been investigated by Soly Peter et al., 131 Velic et al., 132 and Saleem et al. 133 using different solvents and extraction methods.Table 8 shows the merits and demerits of using mechanical and emerging oil extraction methods from different biomass resources, including fruit processing wastes and byproducts.

Steam distillation
Steam distillation is an extraction method that applies to temperature-sensitive plants containing aromatic compounds.
As can be seen in Fig. 9, the plant containing oil (feedstocks) is exposed to steam, and the steam passes through to produce a product containing vapor and crude oil.As mentioned by Ayoub et al., 138 the merit of this oil-extraction technique is the low thermal deterioration of compounds within the feedstocks because its operating temperature does not exceed 100 °C, whereas its demerit is a higher capital investment and the requirement for even distribution of feedstock for the better extraction of oil.

Chemical extraction methods
Solvent extraction.Solvent extraction techniques use organic solvents to remove oil from biomass residues and/ or oilseeds.It is one of the traditional extraction techniques that is widely used in commercial methods for separating oil and fats from solid or liquid oleaginous material or biomass residues.There are different types of solvent extractions.
Liquid-liquid and solid-liquid extractions are the most common. 139Figure 10 shows the solid matrix (feedstock) being placed in an extractor using a thimble, where it receives solvent continuously through condensation of its vapor through the distillation arm and is finally taken to the distillation chamber through a siphon tube.
In the solvent extraction technique, hexane, acetone, chloroform, benzene, and cyclohexane are among the organic solvents used to rupture the plant cell walls and disrupt the interactive forces between lipids and the tissue matrix to extract the oil. 140Satriana et al. 140 also indicated that an appropriate solvent must be selected by considering solubility, the low boiling point for easy recovery, toxicity, reusability, availability, and economic cost.Hence, the selection of appropriate types of solvent is the key factor that determines oil yield.Satriana et al. 140 indicated that the quantity and quality of the oil extracted using the solvent extraction method are influenced by the types and concentrations of solvent, the types of extraction techniques employed, the temperature, and the particle size of the feedstocks, as also mentioned by Odetoye et al. 141 Pereira et al. 142 investigated the effect of using different solvent types (compressed propane at 30-60 °C and pressures of 2-8 MPa, n-hexane, and ethanol) and extraction methods (subcritical fluid extraction, Soxhlet, and ultrasound-assisted extraction) on oil yield from yellow passion fruit seeds, and the results revealed that Soxhlet extraction using n-hexane followed by compressed propane at 30 °C and 8 MPa provided a higher oil yield of 26.12% and 24.68%, respectively.Keneni et al. 143 studied the effect of different extraction solvents on jatropha seed, indicating that solvents have different extraction capacities and efficiencies, which in turn influence the oil's quantity and quality.The authors also stated that, among the solvents under investigation (diethyl ether, ethanol, heptane, and hexane), the highest percentage (41.24%) of oil was extracted using hexane.Satriana et al. 140 also reported that the oil yield obtained from avocado seeds using hexane as a solvent is significantly higher than that obtained by other solvents.This makes n-hexane the most common and best organic solvent used in the oil industry due to its high efficiency in oil recovery, low cost, recyclability, non-polar nature, low heat of vaporization, and low boiling point (63-67 °C). 87gure 9. Steam distillation set up for oil extraction. 138

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The study conducted by Rengasamy et al. 144 on oil extraction from Jack fruit seed used different extraction methods (Soxhlet, microwave, and mechanical shaker) and solvents (methanol, cyclohexane, petroleum ether, and ethyl acetate).According to the authors, the maximum oil yield was obtained using the microwave, followed by the Soxhlet extraction method.The extraction was also conducted at different ranges of temperature (45-70 °C) and time (60 − 200 min for Soxhlet and mechanical extraction, but 0.5-3.5 min for microwave).The authors found that an increase in temperature and time increases the oil yield for different extraction methods and solvents.Besides the solvent types, as reported by Sarungallo et al., 145 the oil yield is influenced by temperature and heating time, and the highest oil yield from red fruit was recorded with increasing heating time before extraction.Hasibuan and Gultom 146 stated that oil extraction time is a factor that influences the extraction process, and a higher oil yield from the citrus lime peel is obtained at a longer extraction time, as presented in Table 9.
Until recently, conventional methods of oil extraction such as the use of solvents, steam distillation, and/or mechanical pressing have been used.Among these, solvent extraction methods are highly dependent on the diffusion of solvents into the plant cell wall and are technically challenging to handle. 149Furthermore, this extraction technique has some drawbacks, such as requiring more time for extraction in comparison with emerging extraction techniques; not being suitable for high-volume oil recovery, consuming large amounts of non-environmentally friendly material, requiring more additional purification, and thus contributing to the rising cost of waste disposal. 140,150,151ccelerated solvent extraction.Accelerated solvent extraction (ASE) was introduced in 1995 by the Dionex Corporation and is an automated rapid extraction technique that utilizes some liquid solvents at a mid-range temperature and pressures ranging from 60 to 200 °C and 35-200 bar, respectively. 152Extraction is conducted under pressure and the solvent is still below its critical condition to maintain the solvent in its liquid state at a high temperature.In this technique, as the temperature is elevated, the viscosity of the solvent is reduced, which thereby increases its ability to wet the matrix, solubilize, and increase the diffusion rate of the target analyte into the solvent, making extraction faster. 150xerting pressure on the solvent during extraction realizes working at an elevated temperature even with solvents having relatively low boiling points.Hence, the combination of elevated temperature and pressure allows extraction to occur very rapidly (12-20 min per sample) and completely. 150,152percritical fluid extraction.Oil extraction by supercritical fluid extraction (SFE) is conducted by using solvents above the specified temperature and pressure (i.e., above the critical point -the point above which gas and liquid do not exist as separate phases).The solvent is passed through a packed bed containing the solid sample, and then the

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pressure and/or temperature are reduced or increased. 138his extraction technique has many advantages relative to solvent extraction, including high reaction rates, the absence of toxic solvents, the ability to adjust solvent power for accurate extraction of target components, and the production of pure crude oils that are not contaminated with solvents. 153thanol, acetone, methanol, water, and carbon dioxide are common supercritical solvents that are predominantly used in these techniques due to their non-toxicity, the fact that they are non-explosive, and their ability to solubilize lipophilic substances and easily remove them from the final products. 137,153Furthermore, the low viscosity, high diffusivity, dielectric constant, and density of these fluids make them easily penetrate the sample and increase the extraction activity.For non-polar compounds, supercritical carbon dioxide is the most efficient solvent, whereas ethanol, methanol, acetyl chlorides, and an azeotropic mixture of hexane and ethanol extract polar compounds effectively. 154esides solvent types, extraction temperature, time, sampleto-solvent ratio, and pressure are process parameters that determine the quality and quantity of oil produced under supercritical fluid extraction techniques. 155Despite the several advantages, SFE also has limitations mainly associated with the high cost of the production process, the need for specialized equipment, the need to dry raw materials to reduce moisture content below 20%, and pretreatment. 153crowave-assisted extraction.In conventional methods, the mass transfer of analyte from the sample to the solvent by diffusion and osmotic processes limits the rate of extraction.To improve the extraction kinetics and efficiency, intensification of the extraction process is needed.In this regard, unconventional intensification techniques such as microwave-assisted extraction (MAE) have been developed to improve the yield, extraction efficiency, and quality of solvent-based processes.This operates under the principle of electromagnetic irradiation (with a frequency from 300 MHz to 300 GHz) of the polar solvent and the sample, which results in the superheating of the solid material, rupturing it. 137Hazarudin and Razali 156 stated that the MAE techniques involve three steps that take place one after the other.The first step is the separation of solutes from the active sample site under increased temperature and pressure.The second step is the diffusion of the solvent into the sample matrix.Finally, the solutes are released into the solvent.Microwave-assisted extraction is a green and novel technology that uses water or alcohol as solvents at controlled pressure and elevated temperature.Considering its higher extraction yield, lower energy and solvent consumption, low waste generation, and shortening of extraction time, its application for fruit seed oil extraction has been widely investigated. 139,156,157e efficiency of MAE depends on the nature and volume of the solvent, irradiation time, extraction time, microwave power, sample particle size, moisture content, temperature, and pressure.For instance, Cavdar et al. 158  the oil industry.As indicated in Fig. 11, UAE works on the principles of cavitation and oscillation phenomena (either using an ultrasound probe or bath that operates at 20 and 40 kHz frequencies).In this regard, the vibration generated by ultrasonic waves creates voids that help to transfer energy to solid particles in the extraction process, enabling the cavitation bubbles to grow and collapse at a higher amplitude, forcing the cell wall to rupture and accelerate the diffusion of oil into the medium. 161onsidering the advantages of UAE over conventional extraction, many studies have been done related to its application in oil extraction from seeds and other byproducts of fruit processing industries. 162Table 10 shows the optimum UAE conditions for the extraction of oil from different fruit seeds, and the yield ranges from 17.64% to 32.27%.
The variation in the percentage of oil yield is due to variations in extraction time, temperature, solvent-to-sample ratio, ultrasound power, and types of solvents used during extraction.As Table 10 shows, the percentage of oil yield from seeds of different fruits is different under different operating conditions.Zhang et al. 164 tried to optimize process parameters for oil extraction from papaya seeds using continuous UAE and compared the oil yield relative to other conventional extractions.The study concluded that about 32.27% of papaya seed oil was obtained using UAE within a shorter extraction time and minimum solvent consumption whereas 25.27% obtained using Soxhlet extraction techniques.This yield is also relatively higher than the yields in the studies by Samaram 163 and Senrayan, 161 which used UAE techniques for oil extraction with papaya seeds.This might be due to variations in solvent-to-sample ratio, temperature, extraction time, and power difference applied in the ultrasound system.Zhang et al., 164 and Thirugnanansambandham et al. 167 also stated that the removal of oil from seeds of different fruits increases with an increase in process parameters like time, temperature, ultrasound power, and solvent-to-seed ratio.

Enzyme-assisted oil extraction method
Enzyme-assisted extraction is a green oil extraction technique that uses a compound that selectively and efficiently acts on substrates under investigation and enhances oil extraction and other chemical processes without significantly influencing the environment. 47,168In this method, enzymes of different kinds (e.g., natural enzymes, garbage enzymes) that are produced from different biological substances would be used. 169By rupturing and disrupting the pores in pre-extraction steps, enzymes facilitate the digestion of intertwined cells lignocellulosic of biomass and other polymers (that hinder extraction) and ease the diffusion of the oil into the extraction medium. 170Environmental safety, 1825 the absence of volatile compound formation, low solvent consumption, and the ability to extract oil and protein simultaneously without losing oil quality are potential benefits of using enzyme-assisted oil extraction (EAOX).Syed et al. 136 also stated that using EAOX helps to reduce undesirable side products, reduce waste treatment costs, and acid development and oxidation of the oils during further processing and storage.The efficiency of EAOX is a function of enzyme type, composition, and concentration, in addition to feedstock types and reaction conditions. 151For instance, unless the concentration is properly optimized, excess enzymes above the saturation point of the substrate active site lead to the formation of odors, bitterness, and caramelization of the sugar, which hinders the extraction process and the quality of the oil to be extracted. 171Similarly, the moisture content must be optimized, as the formation of a thick suspension inhibits enzyme activity and reduces extraction yield.Cellulase, pectinase, phospholipase, and proteases are some of the common enzymes used in oil and other bioactive extractions.Numerous studies have reported that oil extraction using different enzymes helps to shorten the time needed for extraction and improve oil quality.The substrate particle size is a factor influencing the quality and yield of oil.Previous work by Kalia and Rashmi 172 and Vikram et al. 173 reported that reducing substrate particle size to the optimum level increases the intensity of oil extraction from biomass feedstocks.Temperature and pH also determine extraction efficiency.Enzymes perform their activity at a temperature of about 30-45 °C, and if it exceeds this limit, denaturation of the protein and inhibition of the reaction process takes place.Hence, optimization of all process parameters enables better yield and quality.Despite having potential benefits, largescale (industrial) applications of enzymes are difficult because of the elevated cost of enzymes, 138 and their application is limited by environmental conditions. 137

Biodiesel production from oils extracted using fruit waste and byproducts
Biodiesel is a non-toxic, environmentally friendly, biodegradable, and renewable biofuel produced from plant and animal fats through the transesterification of triglyceride oil with alcohol in the presence of a catalyst. 1746][177] Biodiesel is among such fuels that are used in different transportation sectors with different engines, either in their pure form and/or mixed/blended with other diesel fuels in different proportions up to 30%. 178nlike conventional fuels, biodiesel has been produced from a range of locally available and renewable resources, such as plant oils (edible and non-edible), wastes (organic waste from urban, household, and industrial sources), animal fats, and oleaginous microorganisms (algae). 179Worldwide, for many decades, a huge amount (95%) of biodiesel has been produced from edible crops that have human nutritional roles. 180However, using these crops has led to complex socio-economic and ecological impacts like deforestation, destruction of soil, pollution, and food versus energy debates.Another major obstacle in biodiesel production is associated with feedstock costs, which account for about 60-70%. 179o reverse the situation and reduce the cost associated with biodiesel production, different alternative feedstocks which are non-food crops (inedible) like Jatropha curcas, 181 jojoba (Simmondsia chinensis), 182 cottonseed (Gossypium hirsutum), 183 moringa (Moringa oleifera), 184 and others have been exhaustively tested by different researchers.
More recently, the utilization of waste and by-products from fruit processing is under investigation, as indicated in Table 11 below.For instance, biodiesel production from citrus seed oil is gaining attention due to its fatty acid composition and low cost in comparison with other vegetable oils. 25Almasi et al. 190 reported that bitter orange (Citrus aurantium) seed oil can be used as a novel feedstock for biodiesel production, and a maximum yield of 97% was obtained via a base-catalyzed transesterification reaction at 60 °C, 1 wt.% catalyst concentration, and an 8:1 methanol to oil ratio.In Australia, a study conducted by Azad 188 and Rashid et al. 189 related to biodiesel production using oil extracted from Citrus reticulata via an alkali-catalyzed transesterification process reported a 42.29% biodiesel yield.Citrus sinensis seed oil extracted using Soxhlet and n-hexane solvent at 66 °C was converted into biodiesel using alkali-catalytic transesterification (at a temperature of 60 °C for 60 min), and about 76.93% biodiesel was obtained, and the authors concluded that Citrus sinensis is a good feedstock for biodiesel. 185Jeyakumar et al. 192 optimized alkaline transesterification of jackfruit seed oil and obtained a biodiesel yield of 92.5% at a temperature of 60 °C, with an 8:1 alcohol to oil molar ratio for 90 min, and with a catalyst loading rate of 0.6 wt.%.As indicated in Table 11, Rengasamy et al. 144 also reported about 92% biodiesel yield using oil extracted from jackfruit seed at a temperature of 65 °C, a methanol to oil ratio (1:9), a stirring speed of 400 rpm for 120 min, and 1% w/t of NaOH as a catalyst.The authors concluded that jackfruit seeds are a potential feedstock for biodiesel production.
As can be seen in Table 11, the final yields reported are below the EN14214 standard value, therefore none of these biofuels can be used directly in a combustion engine.Improvement in these fuels can be achieved easily by upgrading them via decantation, filtration, and distillation, among other processing alternatives.However, which one should be used depends on each specific case.

Methods of biodiesel production
Different methods of converting oils from different sources (vegetables, animal fats, etc.) into suitable and viable biodiesel fuels have been investigated/researched. 197  has been the dominant technique for biodiesel production over many decades.Aktas et al. 198 and Gebremariam and Marchetti 199 mentioned that dilution/blending, thermal cracking, and microemulsion as conventional means of producing biodiesel by simply mixing with other fuels (gasoline and diesel) and have been practiced for many years.Noting the shortcomings of conventional techniques, many advanced (nonconventional) technologies, such as microwave-assisted, ultrasound-assisted, supercritical transesterification, and so forth have been investigated.The details of these techniques are presented as follows: Blending, also called dilution, is the process of mixing oils (vegetable, fruit waste, and animal fat oil) with diesel fuel in certain proportions.So far, biodiesel mixing ratios of B20, B30, B40, B50, and B80 with diesel fuels are widely used.Blending aims to reduce the problems associated with the direct use of oils in diesel engines, such as high viscosity, acid composition, carbon deposits, lubricating oil thickening, and gum formation during storage and combustion. 200icroemulsion is a technique to reduce the high viscosity of vegetable oils and animal fats using viscosity-lowering alcohol (like methanol and ethanol as additives), higher alcohol and alkyl nitrates as surfactants, and cetane improvers, respectively. 199This method helps attain or reach the maximum viscosity requirement/level of oil used in a diesel engine.Beyond viscosity reduction, it also increases the cetane number and spray properties of biodiesel.The drawback of using this technique is that the micro-emulsified oils have a lower heating value in comparison with petroleum diesel due to the presence of alcohol 198 and also cause problems like injector needle sticking, gum formation, carbon deposit, and incomplete combustion. 199,200ransesterification is the chemical reaction of triglycerides and alcohol to form a monoalkyl ester of fatty acids (biodiesel) and glycerol in the presence of a catalyst. 201igure 12 shows the transesterification reaction in which one mole of triglycerides reacts with three moles of alcohol and is converted into three moles of fatty acid alkyl ester and one mole of glycerol (byproduct).In this process, the triglyceride is converted step by step into diglyceride, monoglyceride, and glycerol.
Transesterification is among the most common and widely used techniques for biodiesel production due to its relative simplicity of operation. 198The process is also called alcoholysis (e.g., methanolysis if methanol is used, whereas ethanolysis if ethanol is used). 202Several types of alcohol (methanol, ethanol, butanol, propanol, amyl alcohol, etc.) are used in transesterification reactions, among which methanol and ethanol are frequently used.This is associated with their renewability (ethanol) and low cost (methanol). 184,198To obtain the desired quality biodiesel, the incoming oil must be filtered to remove any solid residues, and the level of free fatty acids and water content of the oil must be monitored before directly proceeding with the conversion reaction.
In transesterification reactions, diverse types of catalysts and alcohol are used.A catalyst is classified as acidic (homogenous or heterogeneous), alkaline (homogeneous or heterogeneous), or enzymatic.Accordingly, transesterification can be referred to as an enzyme-catalyzed transesterification, an acid-catalyzed transesterification, or a base-catalyzed transesterification process.If the catalyst used in the reaction exists in a liquid form (base catalysts like KOH, NaOH, CH 3 ONa, and H 2 SO 4 ; and an acid catalyst such as HCl, H 3 PO 4 , or BF 3 ), the process is called a homogenous reaction, whereas it is heterogeneous if it is in solid-state (e.g., CaO and MgO). 184The selection of catalyst types depends on the scale of production, the free fatty acid content of the oil, and the nature of the feedstocks.Cellulase, pectinase, and hemicellulase (a common name of several enzymes with hemicellulolytic activity -for example, xylanases, glucuronidases, galactosidases, mannanases, arabinofuranosidases, and acetyl or feruloyl esterases) are some of the most common enzymes used in oil and other bioactive extraction. 203Novozymes 435 and Eversa Transform are among the commercial enzymes that are commonly used for oil extraction. 204For instance, at the industrial level, homogenous catalysts are widely used for mass production/ high yield, fast reaction rate, and minimal reaction conditions, which are requirements of industrial processes. 176,182Base catalysts are commonly used if the free fatty acid content of the oil is lower than 0.5 wt%, otherwise, it results in the formation of soap and water, which make biodiesel recovery very difficult, reduce biodiesel yield, and increase production costs. 127On the other hand, if the free fatty acid content of the oil is higher than 0.5 wt%, it is advisable to adopt a two-stage transesterification (first esterification with an acid catalyst followed by transesterification) to enhance yield and quality biodiesel and reduce reaction time and cost. 179he other type of transesterification process that takes place without the application of a catalyst to produce biodiesel is called non-catalytic transesterification.This mechanism gives a high yield with a high speed and enables problems (catalyst consumption, traces of catalyst mixture in products, low glycerin purity, generation of wastewater, etc.) associated with chemical and enzymatic catalytic transesterification to be overcome. 153,179A supercritical transesterification reaction is an example of such a process.This type of transesterification uses any type of raw material (feedstocks), especially those that are difficult to treat using conventional methods, such as animal fats and oils with high free fatty acids.Temperature, pressure, and alcoholto-oil molar ratios are the common operating parameters or factors influencing biodiesel yield under supercritical conditions.The yield increases with an increase in oil to a solvent molar ratio (from 1:1 to 1:41) and temperature. 184As mentioned by Pikula et al., 153 methanol, ethanol, propanol, butanol methyl acetate, dimethyl carbonate, and dimethyl tert-butyl methyl ether are the most commonly used solvent in supercritical transesterification reactions.The addition of co-solvent and increasing temperatures and pressures of the reaction processes increases the solubility of triglycerides, which in turn increases the production of biodiesel.A study by Okoro et al. 205 related to the comparison of biodiesel production under catalyst-free supercritical, and subcritical conditions noted that biodiesel yield and glycerin production varied with pressure and temperature.The authors reported that biodiesel production using the supercritical transesterification method was successful with feedstocks of high free fatty acid content at a temperature of 350 °C using methanol.Supercritical transesterification also enabled the production of biodiesel from feedstocks containing high moisture content (e.g., microalgae) through simultaneous extraction and transesterification without the oil extraction step. 206Recently, supercritical transesterification has become a promising technique for large-scale biodiesel production from several feedstocks, including low-grade biomasses. 207,208owever, due to its excessive cost, low energy efficiency, high temperature, pressure, and oil-to-solvent requirement, the expansion and application of this biodiesel production technique are restricted.Hence, further studies on how to reduce these limitations are deemed necessary for its future application in large-scale industrial biodiesel production.

Quality of biodiesel produced from fruit waste and byproducts
The American Society for Testing and Materials (ASTM D6751) and EN14214 standard specifications were used by different researchers to check the quality of biodiesel produced from oils of different feedstocks, including fruit waste, and byproducts.The most common fuel property parameters used by ASTM and EN14214 are specific gravity, kinematic viscosity, flash point, cloud point, pour point, calorific value, carbon residue, and acid value.
As Table 12 shows, the majority of the biodiesel produced from fruit waste and byproducts meets the requirements of the basic ASTMD6751 and EN14214 standards under consideration.For kinematic viscosity, which is an important parameter that determines the fuel properties of biodiesel, and influences its lubricity, fluidity, and atomization, all of the biodiesel from the fruit fulfills the standard.Biodiesel with low viscosity results in the wear of engine parts, whereas high viscosity causes poor combustion, which in turn increases emissions.Biodiesel produced from fruit waste and byproducts complies with this parameter.For instance, Rengasamy et al. 144 reported 5.7 mm 2 /s for biodiesel produced from jackfruit seed oil that agree with ASTMD445 standard (i.e, within a limit of 1.9-6.0).However, Jeyakumar et al. 192 reported 2.1 mm 2 /s which is a unique value for the same type of fruit.As reported by Almasi et al. 190 and Ezekoye et al., 185 the reported kinematic viscosity of biodiesel from bitter orange and Citrus sinensis seed oil are within the ASTM standard (as indicated in Table 12) and conventional diesel (2-4.5 mm 2 /s at 40 °C).As reported by Rengasamy et al. 144 it is slightly higher than conventional diesel, whereas it is within the range reported by Jeyakumar. 192 Specific gravity is also another physicochemical property of biodiesel.The study conducted by Almasi et al. 190 on the 1829 physicochemical properties of biodiesel produced from bitter orange seed oil showed good compliance with ASTM standards.As Table 12 shows, the density of bitter orange biodiesel, which is an important parameter that determines the atomization of fuels in an airless combustion system, complies with ASTM and EN standards and is also less than that of biodiesel from rapeseed (0.8825 g/cm 3 ) and moringa (0.883 g/cm 3 ). 190The kinematic viscosity, which causes incomplete combustion, carbon deposition, and smoke exhaust in the engine and affects the overall performance and emission of the engine whenever beyond the limit, is also in agreement with ASTM for biodiesel produced from better orange oil. 190According to the author's 190 findings, bitter orange oil is a potential new feedstock to produce biodiesel that can be used in a diesel engine because the values of other physicochemical properties are in good agreement with the ASTM requirement, as seen in Table 12.The other study conducted by Rashid et al. 189 related to biodiesel production from Mandarin (Citrus reticulata) seed oil showed that the physicochemical properties of the fuel comply with ASTM and EN standards and hence, like other citrus seed oil, it has good potential to augment the supply of biodiesel.The physicochemical properties of biodiesel produced from Citrus sinensis also showed agreement with international standards for biodiesel, except for flash points, which are higher than the limit. 185This can be considered an advantage because it indicates the non-flammability of the biodiesel relative to petro-diesel.The authors also concluded that Citrus sinensis seed oil is potentially a good feedstock and used alkaline catalyzed transesterification to provide a high yield and excellent quality biodiesel.As for other fruits (which are indicated in Table 12), biodiesel of mango seed meets quality standards, except for cloud point and acid value.However, in another study by Olatude et al., 193 the cloud point of biodiesel from mango seed oil was in compliance with standards.These variations are possibly associated with several factors, such as high saturated fat content, presence of contaminants (such as residual glycerides, moisture, and other substances), incomplete transesterification (which results in unreacted triglycerides remaining in the final product).The quality of biodiesel mainly depends on the type and nature of the solvent used, types of extraction methods, level of pretreatment and purification applied before and during the conversion of oil to the biodiesels.The findings of Kittiphoom and Sutasinee 213 affirmed that the acid value and other quality parameters of biodiesel from mango seed oil vary based on the types of solvent used.The study of Awolu et al. 209 on biodiesel production from mango seed oil, extracted using supercritical CO 2 along with different solvents, serve as evidence that higher yields were obtained when using acetone and ethanol as a solvent.However, the requlting product's quality was compromised due to inclusion of extra impurities that were extracted along with the oil.Using n-hexane and petroleum ether; and by optimizing the extraction conditions (temperature, and pressure), the author obtained good-quality biodiesel that met the standards.Hence, to address challenges associated with quality standards of the biodiesel, appropriate extraction techniques, solvent types, sufficient pretreatment and purification, optimization of extraction methods and proper blending with Petro diesel are very important.

Bioethanol production of fruit waste/ byproducts
Bioethanol (ethyl alcohol) is an eco-friendly liquid biofuel that is produced primarily by fermentation of different lignocellulosic biomasses that are rich in carbohydrates, sugars, starch, and, more recently, from microalgae. 214Long-term criticisms of bioethanol production from first-generation feedstocks (particularly food crops) stimulated shifting to non-edible biomass, and this makes biofuel an attractive option to replace fossil fuels due to its renewability, costeffectiveness, and contribution to alleviating the energy crisis and environmental pollution. 215The study by Khandaker et al. 216 and many other research findings reported that any fruit wastes and byproducts that are rich in fermentable sugar and carbohydrates are potential feedstocks or substrates for bioethanol production.In this regard, several bioethanol production techniques such as simultaneous saccharification and fermentation, pre-hydrolysis simultaneous saccharification and fermentation, and simultaneous and co-fermentation have been developed and investigated. 115,116Table 13 shows different fruit waste/byproducts that have been investigated for their bioethanol production potential, and most of the findings concluded that they are potential candidates even though their yield differs between different fruits.For instance, in the study conducted by Khandaker et al. 216 in Malaysia, the bioethanol production potential of fruits (pineapple and orange) and other two vegetables (potato and tomato) using Sacchromyces cerevisiae was reported and the pineapple waste/byproducts generated the highest (5.37%) yield followed by tomato wastes (5.07%).Fish et al. 221 stated that high cellulose, hemicellulose, and other lignocellulosic materials in watermelon waste and byproducts produce bioethanol fuel through the fermentation process using yeast (Ethanol Red).The findings of Akin-Osanaiye et al. 217 reported that papaya fruit waste and byproducts were also potential substrates to produce ethanol (content of 2.82-6.60%)using active baking yeast (Saccharomyces cerevisiae) and its production increases at the optimum time (72 h), yeast concentration, pH (4.5), and temperature (30 °C).A study conducted in Spain by Guerrero 219 stated that banana wastes are an interesting feedstock for secondgeneration bioethanol production (yield ranging between 77% and 81%) and are suitable for scaling up to a commercial scale.Conversely, Khaliq et al. 220 conducted a bioethanol production test on waste and byproducts (pseudo stems) of three different species of banana in Indonesia and reported a very small average yield (1.28%) after 9 days (216 h) of fermentation using Saccharomyces cerevisiae.This might be associated with the nutritional composition and variety of the feedstocks, temperature, pH, and concentration of yeast and feedstocks.Jahid et al. 96 stated that the extraction method employed had an impact on yield while comparing bioethanol production potential of four fruit wastes (peels of banana, pineapple, papaya, and mango) using enzymatic hydrolysis and fermentation methods.Accordingly, Jahid et al. concluded that banana peel and pineapple peel are potential candidates for bioethanol production under enzymatic hydrolysis rather than fermentation with Saccharomyces cerevisiae.Furthermore, Jahid et al. 96 stated that the degree of ripeness, genetic differences, climate, and soil conditions also have an impact on the concentration of fermentable sugar, which in turn influences the overall bioethanol yield of fruit waste.

Biogas production from fruit waste/ byproducts
Biogas production is among technologies that enable the production of renewable and versatile energy sources through a process called anaerobic digestion. 222 1831 digestion (AD) is a biochemical process in which complex organic matter decomposes in the absence of oxygen (under managed conditions) and is suitable for temperature by different types of microorganisms. 121,223As shown in Fig. 13, the AD process consists of a series of interlinked stages of production.The first stage is the hydrolysis process, and it is the breakdown and transformation of complex organic components of substrates (such as carbohydrates, lipids, and proteins) into smaller and soluble organic compounds such as amino acids, saccharides, and fatty acids by extracellular enzymes generated by the hydrolytic bacteria.A previous study mentioned that hydrolysis has an optimum temperature and pH between 30 and 50 °C, and 5-7, respectively.In the second stage (acidogenesis), volatile fatty acids and other byproducts such as alcohol, ammonia, carbon dioxide, and hydrogen are produced.Due to the shorter regeneration times of acidogenic bacteria, this stage proceeds at a faster rate than all other stages of AD.Acetogenesis (the third stage) is the process by which these volatile fatty acids and other byproducts are converted into acetate and hydrogen, which are finally transformed into methane by acetoclastic methanogenesis and hydrogen methanogenesis. 224,225At this stage, methanogenic microorganisms have a slower regeneration time and require a higher pH than in other stages of AD. 222 Even though a relatively small amount of biogas is produced during hydrolysis, it reaches its maximum peak during the methanogenesis stage.For many decades, biogas production has been limited to using animal manure and agricultural residues (stalks, straws, leaves, roots, husks, seed shells, and farm waste) as potential feedstocks. 120As shown in Table 14, different fruit wastes are also potential candidates for biogas production.Considering the multiple functions and demands of biogas, feedstocks from different sources, including waste from fruit and vegetable production and processing, have been investigated, and have attracted interest due to their availability and high organic matter and moisture content. 226Beyond composition, the high moisture content and high availability accounting for 33%  of fruit and vegetable waste and byproducts make them a potential candidate for biogas production.The findings of Chakravarty and Sachin 227 revealed that utilizing fruit (such as peel, pomace, seeds, and kernels of orange, watermelon, papaya, banana, and pineapple) in an anaerobic digestion process together with co-substrates like animal manure helps to generate an alternative biofuel.However, the amount of biogas yield is influenced by the types of reactors, types, amounts, and composition of biomass, NH 3 concentration, the water content of the feedstock, and other operating conditions like temperature and pH. 223Feedstocks mainly consisting of carbohydrates (like glucose, cellulose, and hemicellulose) produce low amounts of methane, whereas those that contain high-fat content contribute to high methane production. 223There is a direct relationship between biogas yield and the temperature of the process, whereas it is an inverse relation with hydraulic retention time.As shown in Table 15, at lower temperatures, very few microorganisms, particularly those that are psychrophilic participate in the AD process, and the conversion of biomass to biogas takes a longer time.Similarly, at higher temperatures (43-55 °C), very few thermophilic microorganisms engage in the AD process and generate biogas within a shorter time.
The studies conducted by Carissa 228 and Chakravarty 229 mentioned that co-digestion of fruit processing waste with cow manure provides more biogas yield than individual digestion of fruit waste.For instance, according to Carissa, 228 except for apple waste, which requires greater supplements due to its high C/N ratio and high carbohydrates that make it easy to degrade, other fruit wastes do not undergo digestion without a minimum of 20% manure supplement.Another study by Narayani 230 on fruit peels wastes (from apples, pineapple, grapes, custard apples, and sweet lime) reported a higher biogas yield of 405 mL using 75% fruit waste and 25% cow dung, followed by 381 mL from 50% fruit and an equal ratio of co-substrate (25% each of cow dung, and rice bran).Other studies recommend the application of different pretreatment techniques (physical, chemical, and biological) to enhance the biogas yield from different feedstocks.Morales-Polo et al. 226 confirmed this by applying freezing, ultra-freezing, and lyophilization as a pretreatment of fruit waste and byproducts, and generating 91.28 NmL per 100 g and 32.25% biogas and methane, respectively.According to them, the application of these pretreatments resulted in more than a 50% increase in biogas and methane yield by increasing the solubility of the substrate, porosity, and accessibility of microorganisms to the substrate.Biogas valorization as a biofuel provides numerous possibilities, including direct heating, cooking fuel, or electric power generation in gas (micro) turbines and engines as well as in co-generation installations. 227

Biohydrogen
Biohydrogen is a clean and renewable fuel that is produced from biomass resources by biological fermentation (dark fermentation).Considering its higher energy content (approximately 122 kJ/g) and wider applications in many sectors like transportation, power generation, building heating, and new decarbonized industrial processes, currently, its demand has increased by 4-5% worldwide and will continue in the coming 20 years 231 as seen in Fig. 14.
Understanding the resultant impacts of using fossil fuels as feedstock for hydrogen fuel production, currently, attention is diverted to using organic waste materials due to their renewability and ease of access, and means of ensuring the transition to net-zero emissions by 2050. 233n this context, the increasing volume of waste generated from fruit and other agro-processing units is considered a good opportunity for biohydrogen production.For instance, a study by Erwin et al. 233 reported 70.7% hydrogen production from banana waste using the in-liquid plasma method.Beyond reducing the cost of hydrogen production, utilizing organic waste/biomasses from different sources (agriculture, industry, etc.) would help to achieve the target set by IRENA to reduce the global average temperature to 1.5 °C21,234 as per the Paris agreement.

Biohydrogen production methods
Currently, hydrogen fuel is produced using various technologies like reforming (steam, partial oxidation, and auto-thermal), pyrolysis, electrolysis, thermolysis, photoelectrolysis, and biophotolysis. 235Figure 15 shows the biological/organism process of hydrogen fuel production from fossil fuels.Patel et al. 237 and Moussa et al. 238 stated that gasification, microbial electrolysis, bio photolysis, and fermentation (dark and photo fermentation) are the most commonly used biohydrogen production techniques.The efficiency and use of feedstocks vary among different production technologies.In this regard, fermentation, microbial electrolysis cells (MECs), and gasification are

Conclusions and future perspectives
Globally, waste generated from the production, consumption, and processing of fruits is increasing and poses a significant impact.Likewise, the demand for green and renewable energy is also increasing.To secure the energy demand and alleviate pollution problems associated with poor waste management and emissions from fossil-based fuels, utilizing low-cost biomass from fruit processing for biofuel production is a viable option.This review focuses on utilizing tropical fruit wastes and byproducts to extract oils used for biodiesel production and other different biofuels (biogas, bioethanol, and biohydrogen), including an evaluation of different innovative technologies for production and the chemical composition of fruit wastes.Different conventional and emerging methods of biofuel production from diverse fruit wastes and byproducts were also reviewed in this paper.The results of the review showed that fruit waste and byproducts are potential candidates for producing different biofuels using green technologies.Before choosing any biofuel production technique, a proper evaluation of its advantages and  disadvantages (as indicated in Table 16) is required, including the technology's cost, time of extraction, and yield.As a future direction, planning the production of different biofuels (biodiesel, biopellets, biogas, including biohydrogen) from similar feedstock and integrating different production processes are crucial to attain maximum benefit (social, economic, and environmental) from fruit wastes and byproducts.An exhaustive investigation related to technologies and the economic feasibility of biofuel production from other fruit wastes is therefore required.

Figure 2 .
Figure 2. Global aggregate volume of major tropical fruit export.3

Figure 3 .
Figure 3. Major citrus fruits and their byproducts/wastes generated after processing them into various products.25

Figure 4 .
Figure 4. Anatomy of CF and different bioactive compounds available in waste/byproducts.25

Figure 13 .
Figure13.Stage of biogas production by anaerobic digestion.223 use organic biomass/wastes as common feedstocks, whereas photo fermentation and bio photolysis are dependent on solar energy.

Figure 14 .
Figure 14.Global hydrogen fuel demand forecast by sector from 2019 to 2070 in a sustainable growth scenario.232

Figure 15 .
Figure15.Overview of biological biohydrogen production processes.236 1812 © 2023 The Authors.Biofuels, Bioproducts and Biorefining published by Society of Industrial Chemistry and John Wiley & Sons Ltd.

Table 2 .
The physical and chemical properties of mango waste (peel, kernel, and seed) reported by different studies.

Table 9 .
Effect of extraction methods and solvent types on oil yield from fruit wastes and byproducts.

Table 10 .
The operating and optimized conditions of ultrasound-assisted oil extraction from selected fruit seeds.

Table 11 .
Transesterification Summary of potential fruit waste and/or byproducts used as feedstocks for biodiesel production by transesterification method.

Table 13 .
Anaerobic Potential fruit waste and byproducts used for bioethanol production.

Table 15 .
223gas production thermal stage and corresponding retention time.223

Table 16 .
136mary of the advantages and disadvantages of different oil extraction methods.136Itrequires high capital investment, and a lowvalue product is obtained.• Continuous wetting is needed to supply the cold temperature that feeds into the system Solvent extraction • A small amount of solvent can extract enormous quantities of oil.• Agitation during solvent extraction promotes the removal of the targeted compound.• It is cheaper and easier to manage • This extraction technology does not support a sustainable environment due to the use of toxic and flammable solvents Enzymatic extraction • It is a sustainable oil extraction process that does not hurt the environment • It requires excessive cost, incubation time, and de-emulsification during down-stream operations Supercritical fluids extraction • It produces a higher amount of oil due to enhanced solubility with solvent.• The use of CO 2 as a solvent makes it a cheaper process due to the easier availability and nonflammability of CO 2 .The use of microwave heating for oil extraction nullifies the release of CO 2 .• Only a fraction of energy is required as in comparison with conventional heating • This technique is not applicable when the solvent or desired compound is non-polar or volatile