Opportunities and challenges in single‐cell protein production using lignocellulosic material

The increase in world population has led to an increase in global food demand, which is estimated to grow by 35–56% from 2010 to 2050. As a result, the risk of hunger is also expected to rise. Protein consumption between 2000 and 2018 rose by 40% worldwide and is expected to increase at 9.3% cumulative annual growth rate in the near future. Hence, there is a huge demand and potential market available for human and animal protein. Therefore, newer and more protein sources will be required to meet the longer food value chains. Hence, a microbial protein produced using waste biomass could offer socio‐economic benefits and consumer acceptance owing to its vegetarian nature. Moreover, waste biomass‐based proteins are environmentally benign and very good ingredients in food and feed. Dry microbial protein, described as single‐cell protein, grown on agricultural residue, has been considered as a potent source of protein for both humans and animals. Thus, single‐cell protein will open up new opportunities to meet the growing protein demands for humans and animals. Microbially produced beer, cheese and yogurt are all examples of microbial protein sources for humans and animals. The feedstock, like agriculture residue, is relatively cheap and could be a potential source for large‐scale protein production in the future.


Introduction
T he finite availability of land, climate change, rising population and developing economies are generating pressure on the food material supply chain.The global population is estimated to increase by about 9.7 billion by 2050. 1 It is estimated that the global food demand will increase by 35% in 2010 and by 56% by 2050, 2 while the risk of hunger is expected to increase at the same pace. 3rotein consumption rose 40% globally between 2000 and 2018 4 and is expected to increase at about 9.3% cumulative annual growth rate in the coming years.Hence, a large market is available as the demand for human and animal protein will grow in the future.Existing animal-based proteins are hindered by their negative environmental and health impacts. 5Therefore, new and more protein sources are needed to develop larger value chains and pay attention to issues such as production costs, food safety, scalability, sustainability and consumer acceptance.Hence, a protein produced using agricultural residue could offer socioeconomic benefits and a high level of consumer acceptance owing to its vegetarian nature.Moreover, agro-based proteins are environmentally friendly and good for human health.Single-cell protein grown on agricultural waste has been considered a source of food/feed for humans and animals. 57][8][9] All of these feedstocks are relatively cheaper and could be a potential source of protein production in the future.The concept of microorganism-based protein has been explored for centuries. 10,11][14] Single-cell protein is dried and dead microbial cells derived from a pure and mixed culture of yeast, algae, fungi or bacteria and is used as a substitute for protein. 15,16All of these cultures are grown on various carbon residues and can be an alternative to protein.Single-cell protein comprises fats, carbohydrates, minerals, vitamins and nucleic acids as well as protein.Therefore, it holds significantly higher nutritional value than that conventional protein.The main source of conventional protein is milk, cheese, eggs, chicken, fish, meat, beef, peanuts, etc.8][19] However, the cost of milk or animal protein is very high, and poor economic nations are starved of protein.Therefore, SCP cultivated on the waste residue could help to add to the protein supply value chain.It is cheaper to produce than conventional protein as it is grown on cheaper carbon.
A large number of microbes have been used for the production of SCP.These can be divided into yeast, bacteria, fungi and algae.1][22][23][24][25][26] The yeasts include Candida, Saccharomyces, Rhodosporidium and Rhodotorula and contain about 40-60% protein in the dry mass. 22,23easts are suitable for SCP production due to their superior nutritional value, high lysine content and larger size, making them easier to cultivate and harvest.They contain high levels of tryptophan, can be grown at low pH and have been found to have a low nucleic acid content.The bacteria include Brevibacterium, Cellulomonas, Alcaligenes, Rhodopseudomonas and Lactobacillus, and contain about 50-65% protein in the cell-dried mass. 20,27,28The bacteria Clostridium butyricum is a very well-accepted microorganism for food purposes in the EU.Fungi are filamentous in nature, which facilitates the harvesting process; however, they have several demerits, such as lower growth rate, poor acceptability and lower protein content (31-43%), and some species, such as Penicillium Aspergillus and Fusarium, are dangerous to the health. 29,30Hence, these harmful fungi must be avoided for SCP production.The algae include Chlorella and Spirulina and contain about 41-62% protein. 25,31ingle-cell protein has been widely used as a food supplement for humans and as a feed for animals like pigs, poultry and farmed fish.In the food industry, it is widely used as a meat substitute, flavouring material, emulsifier, micronutrient carrier and in soup. 32,33Apart from this, SCP is being produced under different commercial names like Brovile®, AlgaVia®, Quorn®, Vitam-R®, Pruteen®, Marmite® and FermentIQ™, etc. 34,35 Japan, Finland, France, Russia and the UK have commercialized the production of SCP for decades. 36,37However, cheaper carbon sources and optimal process parameters are still being researched.Moreover, the search for new microorganisms are still being conducted by the scientific community.Purple phototrophic bacteria (PPB) are emerging as a potential technology for resource recovery  38 This biomass can be used as a SCP, with a high protein content.As a result, it has the potential to increase the impact and the economic feasibility, which justifies higher capital costs for scale-up studies in regard to PPB photobioreactors.However, the amount of information available for technoeconomic assessment of single-cell protein from PPB is still limited and can only be determined in dedicated larger-scale, outdoor systems.Larger-scale units are required to supply feed for bigger cohort trials.
Primarily carbohydrates are the main carbon source for SCP production.However, in recent time several waste residues, like food waste, agricultural waste and aquacultural waste, have been exploited. 39,40Among these, agricultural waste is an abundantly available and cheaper carbon source for SCP production.Therefore, in this review, we have compiled the literature from the last 10 years mainly on the carbon source called lignocellulosic material (LCM), considering its high availability and low cost.

Production process of SCP
Single-cell protein production is performed in a fermenter equipped with a material inlet and outlet ports, an agitator, pH control, temperature control and an oxygen inlet system.This is fully mechanized and does not require manual interference.It is equipped with all kinds of sensors like pH sensors, temperature sensors and oxygen level monitors.The process requires the growth of cells in the fermenter, an outlet mechanism and separation of the unused material.After fermentation, the cell biomass is harvested for downstream processing, such as cell disruption, washing and protein extraction followed by purification.][43] Fermentation needs to be conducted in axenic conditions so as to avoid any unwanted microbial growth.Unwanted growth can contaminate the SCP and could have the adverse impact on human health.Three types of fermentation are reported for SCP production.These are submerged, semisolid and solid-state fermentation methods. 44,45In submerged fermentation, the microorganism is grown in free-flowing media, wherein the carbon source is either soluble or suspended in the media.In solid-state fermentation, the microbes are grown on a solid substrate like wheat bran and agricultural residues like straws and stovers without free-flowing water.Solid-state fermentation is practiced extensively to produce various products such as SCP, feed, food, enzymes, flavoring materials, pigments, vitamins and biologically active secondary metabolites.In contrast, semi-solid fermentation is the process where liquid content is increased to improve the availability of various nutrients for microbial growth and mass transfer.Molasses, soluble sugars, etc., are the common carbon source for submerged fermentation.Although the handling of the process in submerged fermentation is much better, it requires huge capital costs and a high operating cost.A schematic diagram for fermentation is given in Fig. 1.The diagram includes all of the necessary steps required for fermentation. 46ermentation can be performed in batch mode, fed-batch mode and continuous mode.In a batch mode, the microbial culture is inoculated into the fixed volume of the media kept in the fermenter.At the end of the process, the broth is removed in batch fermentation, and the fermenter is again charged with the same material.In continuous fermentation, all of the nutrients, carbon sources and media are added in together continuously, 47 whereas fed-batch fermentation is a hybrid of the batch mode and continuous fermentation.In general, the final fermentation broth comprises 1-5% solids, and these are separated, washed, dried and used as they are or mixed with other materials.For drying the material, drum or spray drying methods are employed, considering the lower operating cost of converting the product into powder form.The final product is free from cells, high in nutritional value and light in color.

Process parameters affecting the SCP production
Fermentation is a very sensitive biochemical process and must be performed in fully automated fermenters in a highly controlled manner.The feedstock needs to be preprocessed or pre-treated and enzymatically saccharified.Once fermentable sugars are produced the fermentation is performed.Yield and productivity are dependent on many process parameters like temperature, the pH of the media, inoculum size, inoculum age, dissolved oxygen, aeration rate and the carbon and nitrogen source used to cultivate the microbes.Apart from this, the microbe's properties also matter a lot for good yield and productivity.For example, microbes should have high nutrition value and low production time, low nucleic acid content, a wide range of pH tolerance and high digestibility.Moreover, the microbial strain should be non-pathogenic and non-toxic.It must fall in the category of GRAS (generally recognized as safe) and have the ability to be cultivated on a wide range of carbon sources, tolerance to high cell density, good downstream processing, stability during cultivation and very good organoleptic properties. 48any studies have been conducted to optimize the process parameters like temperature, pH and process time on various 313 microorganisms while producing SCP.It is reported that the optimal temperature for all microbes is from 28 to 32°C. 49owever, the pH of media varies from one species to other.For example, yeast is grown in the pH range of 2.5-8.5 with an optimal range of 4.0-5.0.Generally, fungi need lower pH than bacteria for their cultivation.For fungi, the optimal pH is 3.8-6.0,whereas they can grow in a wide range of pH 2.0-9.0. 46,50ignocellulosic material contains more than 60% of fermentable sugars; however, it is recalcitrant and highly crystalline in nature.Therefore, sugars present in plant cell walls are very difficult to extract, hence a pre-treatment process is required to prepare the LCM to make it amenable for microbial attachment.Owing to its resemblance to LCM, fruit waste has been considered LCM with some variations.This abundantly available biomass comprises carbohydrates like insoluble pectin, cellulose and hemicelluloses rich in galacturonic acid, arabinose and galactose. 51These properties resemble agriculture waste; therefore, fruit waste along with LCM has been considered a feedstock for protein production in this review.It is reported that these fruit wastes contain some antimicrobial compounds which can inhibit the growth of the microbes; therefore, a low-severity pre-treatment is required for processing. 52This process works as sterilization of the feedstock.It removes the antimicrobial compounds from the peels and can be utilized as a carbon source for protein production.

Substrate for the production of SCP
In general, carbohydrates are the primary feed for most microbes to be employed for protein production. 53herefore, SCP production is highly dependent on the feedstock type, which is reported to have a significant impact on the output of the SCP.Feedstocks used for protein production should be non-toxic, cheap in price, renewable in nature and have a high carbohydrate content.Out of these, LCM and fruit waste are considered waste from the agricultural sector.Other feedstocks compete with food materials.Therefore, LCM and fruit waste have been considered potent sources of carbohydrates and have been included in this review.Agriculture residue is called lignocellulosic material and comprises mainly cellulose (35-50%), hemicellulose (15-30%) and lignin (10-30%). 51,54Cellulose is a homopolymer of glucose, highly ordered and recalcitrant, which is present in the form of bundles in the plant cell wall.Hemicellulose and lignin are heteropolymers of pentose/hexose sugars and phenyl propanol, respectively, but are highly amorphous.Lignin and hemicelluloses are covalently bonded with each other and wrap the cellulose bundles, creating a very hard and recalcitrant biomass.Therefore, extracting fermentable sugars from the LCM poses significant challenges, so to extract these sugars from LCM, pre-treatment and saccharification steps are required.Figure 2a-e provides a pictorial representation of the LCM cell wall components.

Physico-chemical properties of fruit waste and agricultural residue
The physicochemical properties of any feedstocks play a vital role in producing SCP.The LCM contains about 50% fermentable sugars that are present in polymers called lignin, cellulose and hemicelluloseas, as presented in Fig. 2a,c,d, respectively. 55Lignin is a polymer of aromatic material and contains mainly three units called p-hydroxyphenyl, guaicyl and syringyl alcohols present in different ratios from one biomass to another, as presented in Fig. 2b.In comparison, cellulose and hemicellulose are polymers of hexoses and pentoses sugars, respectively. 56The ratio of these two polymers also differs from one biomass to another.Cellulose is present in the form of bundles.In contrast, lignin is

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attached to hemicellulose with a covalent bond and wraps up the cellulose bundles, making the material very recalcitrant and highly crystalline.
The physicochemical properties of fruit waste are like those of LCM to an extent.The fruit waste contains large amounts of soluble sugar in the form of starch.Meanwhile insoluble sugars are present in the form of cellulose, hemicellulose and pectin, as presented in Fig. 2c-e, respectively. 57,58ellulose is a very important structural material of the cell wall of plants and many forms of oomycetes and algae.Some species of microbes secrete it to form biofilms with an industrial application.Structurally, cellulose is a polysaccharide comprising a linear structure containing cellobiose units of several hundred to several thousand units linked by β-(1 → 4) linked d-glucose units.This is the most abundant organic polymer on the planet.Cotton fiber comprises 90% cellulose, wood contains about 50%, whereas dried hemp includes approximately 57%.Cellulose is used to produce pulp and paper.Some animals, particularly termites and ruminants, can digest cellulose with the help of symbiotic microorganisms in their guts.In human nutrition, cellulose is a non-digestible part of insoluble dietary fiber, acting as a hydrophilic bulking material for feces and aiding defecation.Hemicellulose, also called xylan, mainly comprises a backbone of β-(1 → 4)-linked residues and can be divided into hetero-xylans and homo-xylans. 59Homo-xylan is the polymer of xylose, whereas the hetero-xylans are polymers of pentose and hexose sugars.

Literature description (Table 2) of SCP production using various agri-wastes
The use of orange residues as a carbon source for producing SCP using Candida utilis in submerged fermentation has been reported. 60The carbon source was present at 10% while three different fermentation media were used at 150 rpm, 30°C and 96 h.The results show that the highest production of protein biomass was 13.42 g L −1 .Upon optimization of the medium and fermentation using Box-Bencken 33 design, the biomass production was 15.71 g L −1 with a crude protein of 6.2%; therefore, it was concluded that orange peel is an excellent carbon source for the production of SCP.Locally available watermelon (Citrullus lanatus), pineapple (Ananas comosus), sour orange (Citrus medica), papaya (Carica papaya), mango (Mangifera indica) and banana (Musa acuminata) peel wastes were tested to establish their suitability for the production of SCP using palmyra (Borassus flabellifer) with naturally mixed bacteria and yeast culture under submerged fermentation. 46The peel waste was taken at 10% (v/v) in the shaking incubator at 100 rpm and 48 h.The optimum process parameters for the fermentation of papaya waste were pH 5.0 and 25°C, for 24 h, producing 52.4% protein content, followed by pineapple, watermelon, banana, sore orange and mango peel, producing 49.7, 45.2, 30.4,29.5 and 24.6% protein respectively.
Investigation of SCP production using Candida lipolytica with an alkaline treatment of olive residue and its use as a carbon source has been reported. 61Optimization of the physical and chemical parameters was conducted using nutritional media to obtain a high rate of SCP production.The optimal state was 0.8 m sodium hydroxide at 100°C for 60 min.These process parameters demonstrated the most increased sugar production of up to 12.5%, the optimal growth of the yeast and the production of SCP in 4 days of incubation using 150 rpm at 30°C and a pH value of 5.5.This concludes the yeast growth to be 11.24 g L −1 of the SCP. 61It is reported 62 that an oleaginous yeast species called Yarrowia lipolytica grows on various substrates where optimization of the fermentation process to get a consistent and high yield of protein was performed.A fractional design of the experiment was applied, and the process temperature and pH were reported to impact protein production significantly.An SCP concentration of 8.3% was achieved at pH 5.0 using biofuels as a carbon source.This study demonstrated the benefit of cultivating Y. lipolytic A-101 on waste biofuels for producing SCP and amino acids.
It has been reported 63 that cashew bagasse and guava peel were used for solid-state fermentation to produce SCP.The fermentation was conducted at 30°C, with 70% equilibrium humidity, with 3% of the initial concentration of Saccharomyces cerevisiae yeast while employing 5% peels and bagasse.Multivariate analysis was used to analyze the results.The overall results indicated that adding protein-rich material is an alternative nutritional and economic method.Nurmalasari et al. 64 reported using S. cerevisiae to determine the effect of adding fructose and sucrose on the production of SCP.The results suggest that the addition of sugars to the media had a very significant impact on the pH, protein content and cell dry weight.With the addition of fructose, it had a pH of 3.81, a protein content of 69.08 mg L −1 and a dry weight of 0.4203 g, whereas with the addition of sucrose, the pH was 4.33, the protein content of 85.55 mg L −1 , and the dry weight of the cells was 0.3385 g.Therefore, adding fructose as a carbon source results in a greater cell dry weight than adding sucrose carbon.Mujdalipah et al. 65 reported using pineapple peels and rice water washings to produce SCP by employing S. cerevisiae.The main objective of this work was to find the effect of the ratio of pineapple and rice waste.

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Various ratios of these two carbon sources were applied.However, the best results were obtained at a 1:2 ratio in 56 h of fermentation with a protein concentration of about 60.0% in the biomass.
Umesh et al. 66 reported using S. cerevisiae using pineapple peels as a carbon source.In the best experiment, more than 9.8% of protein was obtained, while experimenting with a 25°C temperature and 5.0 pH in 120 h for the fermentation media.The use of pineapple, orange, banana, cucumber and watermelon waste for SCP production has been reported 67 employing Aspergillus niger as a microbe.The results suggested that using banana waste results in the highest growth of crude protein.The protein content was reported to be 2.29 g L −1 .Using ammonium nitrate as the nitrogen source yielded 3.20 g L −1 protein yield.Hence, the investigation revealed that A. niger could produce protein biomass from food waste, and a higher yield can be obtained by using ammonium nitrate as a nitrogen source.
Wu et al. 68 describe the production of SCP and xylitol using a yeast strain, Candida intermedia FL023, using lignocellulosic hydrolysates from corn cobs.The strain was reported to ingest the hexoses and pentoses sugars for SCP production.The crude protein content was 48.4% on a dry biomass basis.The pretreatment of corn cob was conducted using alkaline conditions followed by saccharification.Thus, the obtained hydrolysate was used as a carbon source in this process.Apart from getting the SCP, xylitol was also obtained with a productivity of 0.38 g L −1 h −1 .Mahan et al. 69 reported that Rhodococcus opacus PD630 and DSM 1069 have been employed to convert (i) lemon pulp, juice and peel, (ii) orange pulp, juice and peel, and (iii) corn stover effluent for the production of SCP.Both strains could utilize the agri-residue as a carbon source, resulting in the production of SCP.The study concluded that R. opacus has the potential to produce SCP from agri-waste.The strain PD630 worked better than the others, resulting in up to 56.9% protein compared with 47.0%.Kurcz et al. 70 described the use of glycerol and potato water as a carbon source and nitrogen source, respectively to produce SCP employing the yeast strain C. utilis ATCC 9950 at 200 rpm and 28°C for 72 h using 5% glycerol.It resulted in 12.2 g L −1 of protein at a concentration of 40.6% in the crude biomass.Zaki et al. 71 investigated using rice straw pulp and urea as a carbon and nitrogen source, respectively, to grow Trichoderma reesei as an SCP.The results suggested that crude protein content in mixed SCP biomass increases with the increase in fermentation time and reaches 25% when the C/N ratio used is 20:1.Somda et al. 72 described the use of mango waste to produce SCP using C. utilis.Analytical methods were applied to determine the biomass yields and amino acid sequence.The biomass yield was 6.5 g L −1 , and the protein concentration in the biomass was reported to be up to 56.4%.Jiru et al. 73 reported that Torula yeast (Cyberlindnera sp.) can produce SCP using banana hydrolysate as a carbon source.The highest amount of biomass, i.e. 8.82 g L −1 , was obtained in 48 h and the protein content in the biomass was 10.3%.
Mensah et al. 34 described using pineapple waste to grow microbial biomass for protein production.Saccharomyces cerevisiae was used to produce the microbial biomass.The impact of pineapple juice concentration was also observed.A maximum yield of SCP (3.01 kg m −3 ) was observed for the 60% (v/v) pineapple concentration, and a 100% (v/v) substrate concentration resulted in the least SCP of 2.5 × 10 −2 kg m −3 .Aruna et al. 74 reported the solid-state fermentation of yam peels using S. cerevisiae as a fermenting yeast.The microbial biomass grown was dried at 60°C.The protein content obtained in the biomass was 15.5% when fermentation was continued until 96 h and by supplementing the broth with ammonium sulfate.The results concluded that higher yields were obtained owing to ammonium sulfate as a nitrogen source.Yunus et al. 75 described the use of C. utilis and Rhizopus oligosporus to produce microbial biomass.The maximum biomass yields were reported as 10% (v/w) in 48 h, and the full protein content was said to be 41.0% in the microbial biomass.All the experiments were conducted at 30°C.Azam et al. 76 reported SCP production using two microbes, A. niger and S. cerevisiae utilizing orange peel as a carbon source.Aspergillus niger was used in solid-state fermentation, whereas S. cerevisiae was used in submerged fermentation.It was observed that A. niger could produce high protein contents of 29.8, 27.2 and 29.0% for Citrus aurantium, Citrus sinensis, and Citrus paradisi, respectively.The increase in protein content was attributed to the higher sugar content.Mondal et al. 77 investigated the production of SCP using fruit processing leftovers.Saccharomyces cerevisiae was used as a biomass source in a submerged fermentation.The results indicated that the cucumber peels produced higher protein content than the orange peel, with 53.4 and 30.5%, respectively.However, protein content was lower, i.e. 17.5%, when fruit hydrolysate was used.At the same time, adding glucose in a fruit hydrolysate results in a higher protein content, i.e. 60.3%.Therefore, SCP growth is dependent on the media composition and growth substrate.

Challenges and opportunities
Using LCM and fruit waste poses a series of challenges and opportunities in utilizing these feedstocks as a carbon source in the production of SCP.Lignocellulosic material requires size reduction and physical, chemical and biochemical conversion to produce fermentable sugars.After release from LCM, these sugars can be utilized by the microbes to produce SCP.The process block flow diagram for SCP production from LCM is presented in Fig. 3.These processes are performed at a specific pH and solid loading.The parameters differ from the SCP production process.Additionally, pre-treatment of the LCM generates fermenting microbe inhibitors like furfurals and acetic acid.Therefore, the pre-treated biomass must be conditioned to perform the fermentation to produce SCP.All the necessary nutrients must be added to the fermentation process as these LCMs do not contain nitrogen and other nutrients required for SCP production.Even after pre-treatment, the microbes do not consume polymers like cellulose, starch, hemicellulose and pectin.Therefore, to release all of the sugars present in the pre-treated LCM need to be enzymatically saccharified using cellulases.The cellulases are consortia of various enzymes like endo-glucanases, exo-glucanases, βglucanases and xylanases.All of these add to the cost of the SCP production using LCM.
Fruit waste is also part of the LCM, composed of the same biopolymers.However, these are relatively soft but contain some antimicrobial components; therefore, before producing SCP, microbial components need to be removed by solvent extraction of the fruit waste.The block flow diagram of the SCP production from fruit waste is presented in Fig. 4. As a result of solvent extraction, a significant amount of sugar is also lost.Recovering these sugars or valorizing them for some other application needs lots of energy, which adds to the cost.Apart from this, removing the antimicrobial component can also be performed using expellers.However, this process also results in the loss of sugars.Therefore, utilizing LCM needs deeper understanding of the process, the feedstock, and other cost components.Additionally, bioconversion to SCP needs specific temperature, pH and loading of the feedstocks.Therefore, developing new microbes that can work in a broader range of process parameters is still challenging.While using LCM, getting pure protein is impossible as some of the unreacted LCM will always accompany the single-cell protein as both of these components are insoluble in the aqueous solutions.Therefore, producing SCP for human consumption may not be possible as the SCP contains all the residual LCM.However, it could be an excellent protein source for animals and fisheries.
Although the production of SCP poses many challenges, it provides, at the same time many opportunities.LCM is an abundantly available low-cost feedstock for SCP production.Researchers have many opportunities to find more robust microbes that can work in a broader range of process parameters, and to find more effective methodologies to extract sugars from LCM. Cellulases are very expensive to produce or purchase; therefore, utilizing cellulasesproducing microbes along with the SCP-producing microbes could eliminate the enzyme cost.Since protein titer in the fermenting broth is low, developing new methodologies for high-titer protein while fermentation is an opportunity for the scientific community.The concept for the production of SCP using LCM will result in utilizing waste and consequently clean the environment.

Public health and safety
The public health and safety of the consumers are paramount.The microbes used in the production of microbial protein production are generally accepted as safe to use and handle.Contamination can take place with microbial agents (harmful for humans) at any stage of the food supply chain, from farm to food.Therefore, when producing microbial proteins, an excellent hygienic and manufacturing practices must be strictly followed along the entire food chain to prevent the growth of microbiological food contaminations, which can cause high incidences of morbidity and mortality among consumers.Lately, research has been conducted on implementing innovative technologies for enhancing the quality and safety of food without compromising its organoleptic and nutritional values.Furthermore, studies should be addressed to develop simple, less expensive and fast tests for monitoring and controlling microbial contamination of food protein, as well as to explore new food manufacturing processes.

Conclusions
Lignocellulosic material is reported to be an abundantly available and cheap source of carbon for producing the SCP using various microbes.However, extracting the sugars from the LCM poses significant challenges to utilize it.Therefore, pre-treatment or pre-processing is a prerequisite before the microbial fermentation produces SCP.When using agri-waste, all sugars are not released 319 by conducting the pre-treatment, and an additional process of saccharification is also required to get good yields of microbial proteins.Fruit waste also requires preprocessing to remove the fermenting inhibitors.Similarly, a precondition step is also necessary for agri-waste to remove fermenting inhibitors like furfurals and acetic acid.To utilize this waste efficiently for SCP production, a deeper understanding of the process, feedstock and other cost components is required.Bioconversion to SCP needs specific temperature, pH and loading of the feedstocks.Therefore, developing new microbes that can work in a broader range of process parameters is still challenging.When using LCM, getting pure protein is impossible as some of the unreacted LCM will always accompany the protein as both of these components are insoluble in aqueous solutions.Therefore, producing SCP for human consumption may not be possible as the SCP contains all of the residual LCM.Nevertheless, it could be an excellent protein source for animals and fisheries.

Figure 4 .
Figure 4. Processing of fruit waste to single-cell protein (SCP).

Table 1 .
Chemical composition (average on a dry basis) of some protein-rich food and SCP.

Table 2 .
Single-cell protein on agriculture waste in annual chronology.