Biologically active components in by‐products of food processing

Abstract Food by‐products happen at various stages of production and processing at home and on commercial scales. In the recent years, because of the fast‐growing food companies and production, food processing by‐products have gained a lot of interest and attracted many technical and health professionals as well as policy makers internally and internationally. Also, concerns are increasing about food by‐products due to their ecological and environmental impact on the planet. This is particularly of concern when large companies emit. Large quantities of food by‐products are thrown into environment in which they can be exploited technically, medicinally, and pharmaceutically. This is due to their chemical component and biologically active compounds of the by‐products. Therefore, this systematic review focuses on the food by‐product biological compounds present in different parts of the food products, particularly in some common foods such as fruits, vegetables, cereals, dairy products, meat, eggs, nuts, coffee, and tea. Moreover, the review also explains the kind of biologically active compounds and their quantity not just in edible foods, but also in part and types of the by‐product which then can be reused and recycled into different processes in order to extract and get benefit from.


| INTRODUC TI ON
There are rapidly growing scientific data and literature focusing on the role of food processing secondary and by-products in relation to human well-being. Coincidently, there is an increase in the consumers' information regarding noncommunicable diet-associated diseases (Chernukhaa & Fedulova, 2015). Thus, the demand for nonchemical, natural, safe, and health-improving food components is also growing (Schieber, Stintzing, & Carle, 2001). Large quantities of agricultural secondary or by-products generated after food processing have become the main issue concerning food industry worldwide, since they could lead to environmental including pollution (soil, water, air).
Furthermore, the disposal of agricultural by-products could cost huge amount to treat under certain governmental regulations (Gowe, 2015).
Additionally, it is no longer pragmatic to discard by-products therefore; utilization of by-products has become an alternative method to overcome this issue (Lafarga & Hayes, 2014;Zhao, Chen, & Du, 2012).
Particularly, when food processing by-products have considerable quantity of valuable and favorable raw bioactive functional compounds, therefore they can be useful for both technological and pharmaceutical purposes (Azyyati & Yen, 2014;Schieber et al., 2001).
For recovery of biologically active compound in food by-products, different conventional methods including solvent-based extraction have been used. However, due to more demand for ecofriendly, cheap, and high-efficiency methods, novel techniques are superior to conventional methods (Gençdağ, Görgüç, & Yılmaz, 2020) and novel methods have been studied and examined. The methods used include membrane-based technologies including microfiltration,

| Fruits
Fruits are one of the food materials that undergo industrial processing which could lead to various waste materials in different forms and shapes during pre-and postharvesting process in growing, preparation, and processing (Joshi & Devrajan, 2007). As a result of that diversity, the by-product left is extremely dissimilar because of the difference in the fruits various industrial processes and producing different products. Some fruits result in 25%-30% by-product or nonedible waste products (Ajila et al., 2009). However, less waste

Amount of waste (1,000 ton) By-products (%)
Production, processing, and preserving of meat and meat products 150 2.5 Production and preserving of fish and fish products 8 3.5 Production and preserving of fruits and vegetables 279 4.5 Manufacture of vegetable and animal oils and fats 73 1.5 Dairy products and ice cream industry 404 2 Production of grain and starch products 245 1.5 Manufacture of other food products 239 2 According to the fruit waste part; peel, seed, and stones produced after fruit processing, they could be importantly utilized as sources of many bioactive components for agroindustry to role in the aspect and transform these by-products into valuable products (Tuchila, Jianu, Rujescu, & Butur, 2008). In a study about bioactive compounds, it was found that the mango peel and seed contained the highest quantity of bioactive compound. compounds than the end products (Ayala-Zavala et al., 2010).
Therefore, recently the modern technology has focused on the utilization and exploitation of these by-products in the production of new useful product with high technical and pharmaceutical properties as food additives and supplementation, since they possess many benefits which include antiviral, antibacterial, cardioprotective, and antimutagenic properties (Djilas, Canadanovic-Brunet, & Cetkovic, 2009). Despite having this usefulness, there is no comprehensive utilization and exploitation due to lack of understating the pharmaceutical and economic benefits, so there is an important opportunity to ascorbic acid/ g: 36.6, 41.5, 9.6, and 6.8 μmol of gallic acid equivalents/g and 2.4, 1.6, 1.3, and 0.90 mg of gallic acid equivalents/g, respectively (Soong & Barlow, 2004). Another research noticed that the peel and seed of "Uba" mango had a total phenolic content of 0.0572 and 0.08254 mg/g on dry matter basis. These values are 4.6 and 7.3 times more than those in the pulp (Ribeiro, Barbosa, Queiroz, Knödler, & Schieber, 2008). It has been found that the phenolic compound of muscadine grapes in the seed was the highest followed by contained hydrolyzable tannins and flavonoids, including anthocyanin 3,5-diglucosides, quercetin, myricetin, and kaempferol glycosides ( Sandhu & Gu, 2010). It has been found that the most common phenol compounds in grape seeds were seemed to be flavan-3-ols; most of them are gallocatechin gallate and catechin. The skins were mostly flavonols, that is, quercetin and myricetin. Determination of anthocyanins in the berry skin by ultra-high-performance liquid chromatography discovered twenty derivatives of malvidin, delphinidin, petunidin, cyanidin, and peonidin (Pantelić et al., 2016). Fruits by-products are also important for antimicrobial activity against some pathogenic, spoilage bacteria, and yeasts. The most abundant studied antimicrobials are essential oils. Essential oils are natural and volatile having strong odor produced by plants (Bakkali, Averbeck, Averbeck, & Idaomar, 2008). One of them is terpenes. They composed of a combination of 5-carbon and controlled rancidity during storage. Antibacterial activity of grapes extracts was studied.
They showed positive activity against some pathogenic bacteria.
Fruit by-products are also a good source of antioxidant that can be used as antibrowning agent that could happen to free cut fruits and reduced the quality of the products. It has been reported that low percentage of ascorbic acid reduced the browning reaction in fresh-cut peaches and apple slices and fresh-cut pineapple. Also, extracts from Palo Fierro rich in antioxidants reduced the browning of apple juice.
Fruit by-products are a good source of natural colorant and pigments due to high stability, purity, availability, and low cost particularly when the synthetic colorants are publically rejected due to health concerns. One of the common colorants that is extracted from different fruit by-products is grape pomace (Stintzing & Carle, 2004) blueberry (Bobinaitė et al., 2016) and some other guajiru, jambolao, jussara, and acai (Sousa De Brito et al., 2007).
Fruit by-products are significantly rich in bioactive dietary fibers which are crucial in prevention in many diet-associated diseases (Zhu, Du, Zheng, & Li, 2015). Dietary fiber and fiber-rich by-products of food processing present high technical and pharmaceutical properties. Thus, they have been supplemented to many food products

| Vegetables
Vegetable by-products composed of different parts: peels, seeds, stones, and leaves. They could be source of different materials such as antioxidants such as vitamins C and E, phenolic compounds

TA B L E 7 Elements in vegetable by-products
by-products from chicory made of only of kaempferol derivatives (Llorach, Tomás-Barberán, & Ferreres, 2004). Cauliflower by-products also showed to contain phenolic compounds. It has been stated that cauliflower by-products contain flavonoids and hydroxycinnamic acids (caffeic acid and sinapic acid). Flavonols such as kaempferol and quercetin with sinapic acid and glucose seemed to be the major phenolics available (Llorach, Espín, Tomás-Barberán, & Ferreres, 2003).
Dietary fiber contributes to the major part of the discarded by-products of vegetables. It has been studied that the total dietary fiber content in vegetable by-products ranges from 40% to 82% including all the types of soluble and insoluble dietary fibers (Table 4; Goñi & Hervert-Hernández, 2011). They impart a substantial quantity of biologically active compounds including polyphones and carotenoids associated with the fiber in the human digestive system.  (Asquer, Pistis, & Scano, 2013), particularly if they are used as a feed to livestock which could be a balanced diet in terms of micro-, macro-, and trace elements (Table 5).
Vegetable by-products contain various chemical and bioactive compounds which can be used in different ways. Ash content was high in summer squash vines (23.3%), whereas the lowest percentage (4.8%) was found in potato (Table 6). Moreover, lowest organic matter was found in summer squash vines (77.8%) and the highest was found (94.8%) in baby corn husk. Snow pea protein showed the highest percentage of 23.2%. In addition, cellulose in pea vines and hemicellulose in baby corn husk were found with highest per-

| Coffee
A large quantity of by-products are accumulated during the process of green bean coffee production which is estimated to be around 50% (Mussatto, Carneiro, Silva, Roberto, & Teixeira, 2010). There are different by-products that are generated after green coffee bean production according to the method used. The by-product of dry technique is primarily husk which includes the dried skin, pulp, and parchment by 0.18 ton per ton (Esquivel & Jiménez, 2012;Murthy & Madhava Naidu, 2012). The by-product of wet technique is mainly coffee pulp and the coffee silver skin, and the last by-product is spent coffee ground after brewing process.
The chemical composition of coffee by-products of different studies from 2000 to 2009 is presented in Table 8. The total carbohydrate is between 35 and 72.3, and total fiber is ranged from 24 to 43. Protein content is ranged from 5 to 11, but the lowest content is mineral which is up to 10%. Table 9 shows the procyanidin and flavonol content of coffee by-product in Arabica and Robusta from Mexico, India, and China.
The lowest amount of flavonols, 5 µg/g, was unraveled in Robusta TA B L E 8 Bioactive compounds of coffee by-products Saccharomyces Cerevisiae in sticky coffee husk fermentation resulted in best yield condition with the optimal temperature at 30°C and 3 g yeast/L. The yielded bioethanol was 8.49 ± 0.29 g/100 g dry husk (13.57 ± 0.45 g ethanol/L).
For health benefits, coffee by-products can be useful due to its content of bioactive compounds. Various studies have confirmed the fact that coffee by-products could be useful for health purposes. In a study about the benefits of coffee by-products in relation to prebiotic, antimicrobial, and antioxidant characteristics, it has been concluded that coffee silver skin and coffee spent grounds can potentially be used as functional ingredients. Furthermore, both of them can be useful as a source of prebiotic compounds, but melanoidins should be removed. Coffee silver skin and coffee spent grounds could be utilized as natural preservatives if used in large amount. Moreover, coffee silver skin and coffee spent grounds can be an important component to improve human health because of its antioxidant activity (Jiménez-Zamora, 2015). In another study, the antimicrobial activity of biologically active in coffee waste was studied, and it was found that coffee by-products showed an inhibitory activity against S. aureus and Escherichia coli.

| Tea
Tea is one of the most consumed drinks worldwide and possesses strong phenolic content (Vladić et al., 2016). One of the waste parts is stalk and stem which are rich in dietary fiber removed during tea processing. However, tea leave waste is the main by-products of tea leaf industry. Tea by-products are normally disposed of as compost, dumped into lands, and/or burned. But these are not a reasonable solution since they cause both environmental and economic problems (Hossain, Ko, & Yang, 2012 Alongside with the phenolic compound, it has also been reported that there are a number of amino acids in the tea leaf. Glutamic acid is the highest with 9.8 g/100 g protein, and cysteine and methionine are the lowest with 1.4 g/100 g protein (Table 11).
Regarding micronutrients, it has been found that tea by-products contain a number of elements as mentioned in

| Dairy products
Dairy industry sector is the main and essential fraction of global food industry with having a magnificent quantity of watery waste.
The most predominant waste that gained industrial attention is whey since it contains valuable bioactive nutrients.
Whey is one of the main by-products of the cheese manufacture process which is about the watery by-product left after the process.
These by-products contain many bioactive proteins such as β-lactoglobulin, α-lactalbumin, bovine serum albumin, and immunoglobulins (Galali & Hanee, 2019). It can be seen that the first two is particularly present in high concentration (Table 13). These perform important health functions. Therefore, this makes the whey valuable nutritionally (Asghar, Anjum, & Allen, 2011).
One of the main compounds of the milk by-products and whey is bioactive peptides. They have been labeled as protein fraction which imparts positive influence on body well-being through improving
Another by-product component in the cereal by-product is dietary fibers in different quantities. Corn bran seems to be richest by-product by 87.86 g followed by wheat bran 44.46 g and sesame coat by 42 g, whereas the lowest fiber content is in oat bran by 23.8 g (Table 16; Elleuch et al., 2011). These dietary fibers have been used in many products including breads (Al-Dmoor & Galali, 2014;Galali, 2014).

| Nuts
Nut by-products could include skin or testa, hard shell, green leafy cover, hull, and leaf. These are important and valuable sources of bioactive compounds that have multifunctional traits and antioxidant activity, and antimutagenic, anticarcinogenic, and antiproliferative TA B L E 1 5 Total phytosterol contents (as mg free sterols/g lipids) in total lipid extracts of cereal by-products Identified as cycloartenol.
c Identified as 24-methylenecycloartanol. d Unidentified compounds that had the same GC retention time as cycloartenol but had different MS spectra. e Unidentified compounds that had the same GC retention time as 24-methylenecycloartanol but had different MS spectra.
TA B L E 1 6 Dietary fiber content of some cereal by-products (% on dry matter basis) properties (Shahidi & Ambigaipalan, 2015). The bioactive compounds of these components involve in the protection of the body directly or indirectly through detrimental free radicals and diminish the risks of the diet-associated diseases. Thus, inclusion of these compounds in the daily meal is highly recommended by health expertise which can protect the body from harmful compounds (Alasalvar & Bolling, 2015

TA B L E 19
Amino acid composition of egg by-products (wastewater) (g amino acid/100 g total protein)

| Egg
Egg by-products mainly include eggshells and membranes (King'ori, 2017), wastewater from egg processing industry (Xu, Sheldon, Larick, & Carawan, 2002), and outcomes from breaking facilities Eggshell has been reported possessing many important nutrients such a calcium and trace amounts of other micro-elements, that is, magnesium, boron, copper, iron, manganese, molybdenum, sulfur, silicon, and zinc (King'ori, 2017). A medium-sized complete eggshell could give 750-800 mg of calcium. The calcium with magnesium and vitamin D increases mineral bone density (Schaafsma et al., 2018). Eggshells are also used as a cheap source of calcium in fertilizing plants (Amu et al., 2005). Shell membrane is an important by-product source for collagen which can be used medicinally and industrially (Ogawa, Portier, Moody, & Bell, 2004). Also, eggshell and membrane contain many biologically active peptides (Table 18; Nakano, Ikawa, & Ozimek, 2003).
Wastewater from egg industry is another by-product of egg that contains many biologically active peptides. It can be seen that there are a number of peptides in the wastewater which some of them are destroyed with acid hydrolysis (Table 19; Xu et al., 2002).
Protein peptides are also other bioactive compounds that are produced from egg processing by-products. In a study about the eggyolk protein by-product as a source of biologically active compounds, the following peptides sequences are produced: RASDPLLSV, RNDDLNYIQ, LAPSLPGKPKPD, and AGTTCLFTPLALPYDYSH.

| Meat
Meat like any other food products has a number of waste parts, but it is dissimilar depending on traditions, culture, and religion.
Blood is another by-product of meat which is rich in protein such as fibrinogen, globulins, and albumins and hemoglobin (Bah, Bekhit, Carne, & Mcconnell, 2013). Blood and other by-products are sources of bioactive peptides with different biological activities (Table 20; Lafarga & Hayes, 2014).
Animal horn is another waste product with many nutrients that can act and possess biologically active compound. In a study about elemental analysis of animal horn, there were a number of important elements including P, K, Ca, Mn, Fe, and Zn. The study also analyzed elemental composition of bone. It was found that there are same elements as horn plus Cr, Cd, Sn, and Ag (Buddhachat, Klinhom, & Siengdee, 2016).

| CON CLUS IONS
To summarize, food processing by-products accumulate in tons and cause huge environmental and economic problems in different stage and food industry sectors including fruits, vegetables, cereals, meat, dairy products, eggs coffee, and tea. If this would be studied and exploited carefully, they can be recycled and reused in different areas such as food industry, pharmaceuticals, and other biotechnical areas. These food by-products are still containing many important biologically active compounds including fatty acids, amino acids, vitamins, minerals, dietary fibers, and antioxidants which can be useful economically and pharmaceutically (e.g., antimicrobials) in different aforementioned sectors instead of throwing into the ground. It is worth mentioning that different novel techniques have been examined in order to increase the potent of recovery of bioactive compounds from food by-products.
They are superior to conventional methods. This attributes the fact that they are eco-friendlier, less hazardous, and less expensive. Therefore, it is important to choose a method that suits the intended bioactive compound to be extracted from different parts of the by-product.

ACK N OWLED G M ENT
The authors are grateful to Cihan University-Erbil for partial support of this work.

CO N FLI C T O F I NTE R E S T
The authors declare that they do not have any conflict of interest.

E TH I C A L A PPROVA L
This study does not involve any human or animal testing.