Recovery and Utilization of Bioactives from Food-Waste

Abstract:

Food and Agriculture Organization estimated a trillion of US dollar loss in food waste annually in 2013. The land used equivalence of food waste was big enough to the size of Canada. The dumping of food waste in landfill leads to 8% of total harmful gases. With the growing population (which is going being double in 50 years) and in demand for processed food, the food waste becomes a critical challenge in and agriculture industry. Therefore, there is an urgent need to reduce waste and find a beneficial use for safe food presently thrown away. Food wastes are full of many bioactive compounds such as carbohydrates (starch, cellulose, and hemicelluloses), lignin, proteins, lipids, organic acids, smaller inorganic parts minerals, phenolic compounds antioxidants, and vitamins. These potential bioactive compounds can be used in the food, pharmaceutical, cosmetics, and textile industries. Therefore, in the present, an attempt has been made to summarize the various studies conducted to recover the bioactive compounds from food wastes and their utilization as nutraceuticals as well as pharma products. Recovery and utilization of these compounds from food wastes not only lead to of the goal of sustainability in food production and consumption but also lead to achieve of zero wastes.

Key Words: Food Wastes; Bioactives; Recovery; Utilization; Proteins; Polysaccharides; Phenolic Compounds

 

 

 

Contents:

 

1. Introduction

 

2. Recovery of bioactives from various plant-based food wastes

 

1.  Proteins
   1.         Food industries using soybean
   2.         Canola oil Industry
   3.         Peanut industries
   4.         Hazelnut oil Industries
   5.         Sunflower
   6.         Proteins from Palm oil industries
2.   Polysaccharides
   1.         Cereals
   2.         Vegetables
   3.         Fruits
3. Phenolic compounds

3. Utilization of Bioactive compound from food wastes
4. Conclusions
5. Acknowledgments
6. References

 

 

1. Introduction

According to the United Nation’s Save Food initiative, Food and Agriculture Organization (FAO), United Nations Environment Programme’ (UNEP), and stakeholders:ReFED ( Rethink Food Waste) food waste can be defined as “any removal of food from the food supply chain which is or was at some point fit for human consumption, or which has spoiled or expired, mainly caused by economic behavior, poor stock management or neglect”(http://www.thinkeatsave.org/index.php/be-informed/definition-of-food-loss-and-waste). European project FUSIONS’’ defined food waste as” any food, and inedible parts of food, removed from (lost to or diverted from) the food supply chain to be recovered or disposed (including composted, crops ploughed in/not harvested, anaerobic digestion, bio-energy production, co-generation, incineration, disposal to sewer, landfill or discarded to sea. (1)

According to FAO, this food waste is around 1.4 billion tons per year that’s roughly one-third of the global food supply, which can feed approx. 2 billion people each year. (http://www.fao.org/food-loss-and-food-waste/en/). As per USDA (2014) these loss or wastes are around 133 billion pounds/ per year which were 31% of the country’s annual available food supply, or 429 pounds per person, per year. In terms of money, this food loss was worth about $161.6 billion at retail prices in 2010 (https://www.usda.gov/foodlossandwaste). Food waste is expected to rise to about 126 Mt by 2020, (2) Figure 1(a-b).

Figure 1a: Distribution of food waste known around the world per capita per year (in kg/person/year). The map was prepared from the data retrieved from Food sustainability index 2017 on Nov 19-2018	Figure 1b: Top 10 countries with the highest food waste known around the world per capita per year (in kg/person/year) during the year 2017. The graph was prepared from the data retrieved from Food sustainability index 2017 on Nov 19-2018

Food losses and waste per year are roughly 30% for cereals, 40-50% for root crops, fruits and vegetables, 20% for oilseeds, meat, and dairy, plus 35% for fish globally (http://www.fao.org/save-food/resources/keyfindings/en/) (Figure 2). Food waste production is higher in developed countries followed by developing and undeveloped countries. In Europe and North America, food waste production ranges from 95-115 kg/year/per person, while in sub-Saharan Africa and South/Southeast Asia is only 6-11 kg/year. This high extent of food waste in medium- and high-income countries can be explained simply by throwing away the food, which is still suitable for human consumption and in low-income countries food is mostly lost during the production-to-processing stages.

Figure 2: Graph showing percentage loss of food in different food industries globally. The data are presented as rough estimates of food losses, which occurs every year. (Graph is generated from the data downloaded from http://www.fao.org/save-food/resources/keyfindings/en/).

The food is wasted throughout the food supply chain. This waste production occurs at various stages of food production, post-harvest handling, and storage, processing, distribution and consumption (3, 4). The highest amount of food loss occurs at the farm level (5). According to Rethink Food Waste through Economics and Data 9.2 billion kilograms of food lost at the farm level annually in the US (6). During food processing in food industries food losses and food waste occurs at all stages of processing such as damage during transport or non-appropriate transport systems, problems during storage, losses during processing or contamination, inappropriate packaging. An erratic way of handling or conservation and lack of cooling/cold storage are the main cause of food loss and waste at the market level (Parfitt et al., 2010). At consumer level food is wasted due to over- or non-appropriate purchasing, bad storage conditions, over-preparation, portioning and cooking, etc (7, 8).

Food waste is a complex problem rather than the socioeconomic problem. As there are numerous risks associated with waste for humans, animals, and the environment, billions of dollars are spent on the treatment of agricultural and food waste. Food waste mostly disposed of as a landfill and this is a big source of harmful methane gas production (9). When food waste is incinerated, it causes harmful air pollution and loss of chemical values of food waste. Interactions among food waste, water & energy resources, environmental quality, and social justice make it a more serious problem and require immediate attention from individual to global level (10) as well as appropriate management (11).

Though not impossible but it is hard to eliminate food waste. However, there are significant ways to reduce food waste and find a good and beneficial use for safe food that is presently thrown away.

Food wastes are rich in carbohydrate polymers (starch, cellulose, and hemicelluloses), lignin, proteins, lipids, organic acids, smaller inorganic parts minerals, phenolic compounds antioxidants and vitamins (12). These bioactive compounds possess antibacterial, antitumor, antiviral, antimutagenic, and cardioprotective activities (13–16). Thus, food wastes can be used to extract and isolate potential bioactive compounds that can be used in the food, pharmaceutical, cosmetics, and textile industries. This utilization of bioactive compounds from food wastes not only could reduce the risks and the costs for treatment of waste, but also could potentially add more value for agricultural and food production.

Recovery and utilization of these bioactive compounds is a very challenging process. The biggest challenge for the recovery of these compounds is to find the most appropriate and environment-friendly extraction technique able to achieve the maximum extraction yield without compromising the stability of the extracted products. The extraction of the high‐value components must be economically feasible to perform. This target can be accomplished by isolating the interested compounds through individual and additionally consolidated physical and biochemical methodologies with the end goal to give a scope of segments, all of which would add to accomplishing whole‐waste exploitation (17–19).

Therefore, in a present chapter, an attempt has been made to review the previous literature for various methods of recovery of these bioactive compounds and re-utilized them in the food, cosmetic and pharmaceutical industries. At present, we are giving emphasis on the plant-based food wastes.

2. Recovery of bioactives from various plant-based food waste

Food waste related to cereals comprise of rice bran, wheat bran and brewers spent grain. Root and tubers wastes include potato peel, sugar beet, and molasses. Wastes from oil crops and pulses include sunflower seeds, soybean seed, and olive pomace etc. (Figure 3). Fruits, vegetables, roots, and tubers have the highest wastage rates of any food (https://www.usda.gov/foodlossandwaste). Food wastes from these plant-based food industries comprised of proteins, polysaccharides, phenolic compounds, carotenoids, and other compounds.

Figure 3: A few examples of food waste classified based on their origin

3.1 Proteins

Proteins are large biomolecules consisting of one or more long chains of amino acid residues known to perform many functions within organisms. Proteins serve as macronutrients in our diet and known for their building blocks activity in the body. They are not a good source of energy because each gram of protein contains only 4 calories. The recent advancement in nutrition science leads to the identification of many nutritional benefits of plant proteins (20). In the edible oil industry, proteins are the main components of food waste, with a relatively high content of protein are present in the defatted meals obtained from oil sunflower, canola, rapeseed, palm, etc. These defatted by‐products generated from oil refineries (oil cake, stem, and grain husk) are not only good sources of proteins but are also available in large quantities and at a low cost. Due to the rapidly growing plant protein market, there is great competition to evaluate oil-seeds, fruits and vegetables, cereals and soy alternative legumes and their processing wastes as a source of commercial proteins. There are few studies had been reported for the bioactive and functional properties of protein extracted from meals of major oilseeds such as for canola (21).

The recovery of protein from different biomass matrices has been intensively studied to obtain nutritionally important products. First, based on their solubility, plant proteins have typically been extracted with alkali. This technique involves alkaline solubilization of proteins, removal of insoluble material by centrifugation, precipitation of the protein at the pH corresponding to the isoelectric point, and collection of precipitated proteins by centrifugation. The second technique is based on micellization involving protein extraction using a salt solution, centrifugation, and precipitation. Here, the protein is recovered from a salt extract by ultrafiltration, diafiltration membranes, or dilution in cold water, followed by centrifugation. The isoelectric precipitation technique leads to higher yields of extracted proteins than the micellization methods (22). Below are the examples of how different food waste is utilized for protein extraction?

3.1.1 Food industries using soybean:

Soybean is a legume that contains no cholesterol and is low in saturated fat. Soya seeds are a rich source of plant proteins where dry seeds almost contain 42% of proteins relative to other components. Soybean seed protein is also called soy protein and it is often used to replace animal proteins in an individual’s diet. Soybeans are the only vegetable food that contains all eight essential amino acids among nine essential amino acids required for human body. Besides this, they are also a good source of fiber, iron, calcium, zinc, and B vitamins (23–26). Food wastes from the soy oil and milk industries are rich in protein and are commercially available as protein supplements.

Soybean curd residue is the main waste from the soy-industries responsible for making soymilk, tofu, etc. In Japan, soybean curd residue (SCR) is called as okara and is the main waste of soybean products. 1.1 (kg) of soybean curd residue generated from each kg of soybeans processed into soymilk or tofu (27). Soybean curd residue is a relatively inexpensive source of protein that is known for its high nutritional and excellent functional properties. SCR contains 27% protein with good nutritional quality. This can be a potential source of low-cost vegetable protein for human consumption (28, 29). Protein extraction from SCR is mainly affected by the low water solubility of the protein in SCR. Therefore, solubility improvement is a key process reported in the literature. One specific example is the recovery of up to 53% of proteins at pH 9.0 and 80 °C for 30 min (30). This method was further modified where mild acid treatments were done in order to improve the emulsifying and foaming properties (31). 93.4% of the protein was recovered from the okara flour by using a three-step-sequential extraction which includes, the introduction of primary and secondary grinding steps (32). This method also showed how emulsification, water, and fat binding, and foaming properties of protein from SCR were comparable to the commercial soy isolate (32). Free amino acid and soy peptide were produced after fermentation of SCR [31]. Therefore, the development of protein resources from soybean residue has great potential.

Food waste from tofu (a kind of curd made from mashed soybean seeds) industry includes whey protein, which is rich in branched-chain amino acids. About 14 million pounds of soybean whey protein of high biological value is disposed of as a waste (33). Smith et al.,  (34) describes the recovery of proteins from whey by forming insoluble complexes with anionic materials. Whole whey protein was prepared by dialysis, and whey was fractionated by heating into heat‐coagulable and supernatant proteins (33).

The residue left after oil extraction from soybean is known as soybean meal. Defatted soybean meal, containing no hulls has an intermediate energy concentration. The main reason for the popularity of soybean meal is the unique composition of amino acids. It is particularly a good source of both lysine and tryptophan. The higher nutritional value of soybean meal is responsible for its excellence as a poultry, livestock, and companion animals feed. Dehulled and defatted soybeans residue generated in the soy industry is used to produce soy flour, concentrates, and isolates. These are commercial products used as a protein supplement with varying protein concentration soybean flour (~40% protein), soy protein concentrate (SPC~70% protein), and soy protein isolate (SPI, >90% protein). Protein extraction for production of soy protein concentrates and isolates were done by wet extraction methods and dry fractionation methods (35). Preparing protein isolates through alkali extraction and isoelectric precipitation is commonly used for soybean (36). Lee et al., (37) Extracted and purified the protein of soybean flakes and meals by lime treatment followed by ultrafiltration. Xing et al., (35) used a custom-built bench scale electrostatic separator for soy protein enrichment.

3.1.2 Canola oil Industry

Canola is the second largest oilseed produced in the world next to soybean, which produces protein-rich meal during oil extraction (36, 38). Canola meal protein is very rich in essential amino acids including sulfur-containing amino acids. Total protein content in the defatted canola meal is being around 32%. Cruciferin, napin and lipid transfer proteins are the main three proteins present in canola meal. Out of these, cruciferin and napin are the predominant storage proteins. Their Protein Digestibility Corrected Amino Acid Score (PDCAAS) was reported to be similar to the soy protein isolate i.e. 0.86 (36, 39, 40). PDCAAS is a method of evaluating the quality of a protein, based on digestibility and requirement of the amino acids in humans. Peptide mixtures and hydrolysates derived from canola proteins are beneficial for human health such as lowering of blood pressure by inhibiting angiotensin I-converting enzyme (41). Besides this, these peptides and hydrolysate fractions are rich in antioxidant, antidiabetic, anorexigenic, anticancer, antiviral, hypercholesterolemic and bile acid binding properties (40, 42). Amino acid profile of isolates indicated high nutritional quality for use in products for 10-12-year old kids. More than 99% of the protein was extracted from crude commercial hexane defatted canola meal when a 5% w/v suspension in 0.4% w/v NaOH was agitated for 60 min at room temperature in baffled flasks on an orbital shaker at 180-200 rpm. Protein recovery was 87.5% upon precipitation with acetic acid (43).

Tzeng et al., (44) designed a method for canola protein production by alkaline extraction followed by precipitation and membrane processing. The Edward Donald, (45) patented a protein recovery method  using salt for forming a canola protein isolate of high protein content in a gentle non-denaturing process in which fat is substantially removed.

Canola protein isolates can also be prepared through ultrafiltration/diafiltration of aqueous protein extracts. However, this wet processing of canola protein isolates involves high water usage and energy costs, which is not feasible at commercial level. However, use of advanced centrifuges with reduced energy requirements and higher G-forces not only reduces the cost but enhance the effective separation with higher purity, and quality of  protein. Another method is enzyme-assisted, chemical-free processes that can extract protein in presence of oil (such as from expeller-pressed meal). The fact that there are no solvents or chemicals used would allow for a clean label, but the process cost may still be a challenge. On commercial scale such as “Burcon Nutra Science” has been using aqueous extraction, combined with membrane filtration were able to produce three different canola protein isolates with excellent functionality and a neutral flavor by using a clean and gentle extraction process. In order to avoid heat damage during these extraction process, cold pressing and low-temperature desolventizing have been used (36).

3.1.3. Peanut Oil Industry

Wastes from peanut processing byproducts are a rich source of natural high-quality protein, the protein amount varies from 50–55% (46, 47). An average of 5.78 million metric tons of peanut meal was prdouced  during 2000 to 2010 all over the world (47).  Depending on the oil content peanut meals are divided into the fresh or dry meal. Protein digestibility corrected amino acid score (PDCAAS), of peanut proteins, is nutritionally equivalent to meat and eggs for human growth and health (47). Thus, it can serve as a cost-effective source of protein for poor countries.

Peanut proteins possess high emulsifying activity, emulsifying stability, foaming capacity, excellent water retention and high solubility (48). Different methods like isoelectric precipitation, alcohol precipitation, isoelectric precipitation with alcohol precipitation, hot water extraction and alkali solution with isoelectric precipitation are used for the separation of peanut proteins (49). However, these separation methods resulted in the production of wastewater that is responsible for the serious environmental pollution as these methods consumed large amount of acid and alkali. In order to improve the extraction efficiency of proteins, many new methods have been designed. These include the enzyme, superfine grinding, radiation, microwave and ultrasonic (US) processing (50). For the production of protein concentrate, protein recovery has been done by isoelectric precipitation; aqueous precipitation; alcohol precipitation; isoelectric precipitation and alcohol precipitation; hexane and aqueous alcohol precipitation and ultrafiltration (UF) (47–49, 51). Alkali solution and isoelectric precipitation; ultrafiltration (UF) is the recovery methods used for protein isolates productions (48, 51, 52).

3.1.4. Hazelnut oil Industries

Hazelnut oil cake is a residue from the hazelnut oil industry and it is also a source of functional proteins. The hazelnut cake contains 54.4% protein. While studying the bioactive, functional and edible film-forming properties of isolated hazelnut (Corylus avellana L.) isolated proteins from untreated (HPI), hot extracted (HPI-H), acetone washed (HPI-AW), and acetone washed and hot extracted (HPC-AW-H) Aydemir et al., (21) found that isolated meal protein is rich in antioxidants, iron chelation, antiproliferative activities on colon cancer cells and good oil absorption properties. Further, they showed that bioactive, solubility and gelation properties of hazelnut proteins could be improved by simple processes like acetone washing and/or heat treatment.

3.1.5. Sunflower oil Industries:

Protein contents in sunflower seeds range between 10% and 27.1% dry weight (DW) basis. 85% of these proteins mainly belongs to storage proteins (53). In sunflower two major classes of globular proteins are present, 11S globulin (or helianthinin) and 2S albumins (sunflower albumins (SFAs)) (54). The quality of sunflower meal depends on the plant characteristics (seed composition, hulls/kernel ratio, dehulling potential, growth and storage conditions) and on the processing (dehulling, mechanical and/or solvent extraction (55). The protein content in the defatted meal depends on the processing. It is 40% during mechanical extraction, 50% during solvent extraction and when prepared from dehulled seeds protein content varies from 53–66% (56, 57). Although a good source of protein but the presence of phenolic compounds, especially chlorogenic acid (CGA) and protein denaturation during processing for oil extraction hampered the recovery and utilization of the functional proteins and also protein solubility, emulsion, foam, and gelling properties.

3.1.6 Proteins from Palm oil industries:

Unlike other oil cake, the palm kernel cake (PKC) is not as popular as is the other oil cake explained in the previous section. Protein content in PKC is 16%-18% (58). The chemical composition of PKC is very similar to that of corn gluten or rice. There are only 85% of amino acids present, which is lower than the most oilseed meals (58–60). The essential amino acids like lysine, methionine, histidine and threonine contents are also less in PKC (61). The crude protein composition of PKC depends on the processing method employed and the type of palm kernel used (61).  Due to low protein contents and lack of a commercially feasible extraction method, the protein concentrates and isolates are not commercially produced, rather the extract is used for the production of wood adhesive formulations (60, 62).

Among the several methods used for protein extraction from PKC, the alkaline method was able to recover 11.91g/100 g (63). Arifin et al., (64) also confirmed the efficacy of saline treatment over alkaline treatment by using saline and alkali treatment method for extraction of palm kernel protein. They also applied the Central Composite Designs of Response (CCDR) surface methodology and identified the optimized conditions for better yield. For saline condition, protein recovery was 28.39-88.38% and optimized conditions were pH-9.0, NaCl concentration-0.02M and Solvent/meal 60:1 ratio. The optimized conditions in alkaline treatment were 0.03 M NaOH, at a temperature of 35°C with a liquid/solid ratio of 30:1. The recovery in alkaline treatment varies from 10.5-74.5%.

Chee et al., (60) used the response surface methodology to optimize the trypsin-assisted assay conditions for PKC protein extraction. Here the recovery of protein was 61.99% i.e. 0.74 g/100 g of the original protein content of PKC. Their recovery yield is five times higher than the alkaline method yield from trypsin-assisted extraction procedure was almost (10.21±0.24g/100 g). This method not only significantly improved the PKC protein recovery but also improved the solubility and emulsifying properties.

Ng and Mohd Khan, (65) successfully recovered 68.50±3.08% crude protein by using an alkaline solution at pH 11, at a ratio of 1:10 (g/ml).) They also used the enzymatic hydrolysis to produce palm kernel extract (PKE) protein hydrolysates or crude PKE peptide (PKEP). According to this study, pepsin was found to be the least efficient protease to hydrolyze the PKEP. In another study, they optimized the enzymatic hydrolysis of palm kernel cake protein (PKCP) with trypsin to obtain PKCP hydrolysates (PKCPH) by using response surface methodology (RSM) (66).

3.1.7. Proteins from the cereals waste

The abundance of protein content in cereal waste is next to fibers, which is a byproduct in the brewing process and present as Brewer’s spent grain (BSG). BSG is a major insoluble solid residue obtained after beer wort production in the brewing industry (67, 68). This represents the ~85% of the residue obtained after the mashing process (69, 70). Annual global production of BSG is estimated to be ~39 million tonnes (69). Brewer’s spent grain mainly contains fiber (30–50% w/w) and protein (19–30% w/w) (69). Besides proteins and fibers, BSG also contains a variety of minerals, phenolic compounds, and sugars (71, 72). The most abundant proteins in the BSG includes hordeins, glutelins, globulins, and albumins (73). Among proteins, the essential amino acids represent ~30% of the total content, with lysine being the most abundant (14.3%) (74).

There are many methods has been used for the recovery of the proteins from the BSG these include extraction with alkali, salt solutions, detergents and aqueous enzyme-assisted methods (45, 75–78). Ernster (1986) patented a process for extracting protein solids from spent brewer’s grains includes alkaline extractions of the grains followed by ultrafiltration of the product to yield a highly purified protein solid.

Celus et al., (73) prepared the BSG protein concentrate (BPC) by alkaline extraction of BSG (17% w/v) with 0.1 M NaOH at 60 °C. He further enzymatically hydrolyzed BPC in a pH-stat setup by commercially available proteases (alcalase, flavourzyme, and pepsin) to obtain hydrolysates with different degrees of hydrolysis (DH). During physical processes, such as pressing and sieving of wet BSG, resulted in two fractions, protein fraction (rich in protein and fat and low in fiber) and a fiber fraction (low in protein and rich in arabinoxylans (80, 81). Schwencke (81) described chemical extractions of BSG protein fraction in alkaline medium, namely the alkaline extraction of BSG at pH 11–12 and 104–121°C. The protein yield in ultrasonic-assisted extraction using response surface methodology was 96.4 ± 3.5 mg/g of BSG which was in agreement with the predicted value 104.2 mg/g BSG (82).

Wahlström et al. (78) described the preparation and characterization of new carboxylate salt- urea called as deep eutectic solvents (DESs) for BSG protein extraction. With 10 wt% water addition this is an excellent solvent for protein extraction from the brewer’s spent grain (BSG). The yield of extracted protein was 80%, due to the dissolving of insoluble protein during the mashing process.

3.2 Polysaccharides

Polysaccharides are carbohydrates consisting of long chains of monosaccharides linked by glycosidic linkages, which on hydrolysis give the constituent monosaccharides or oligosaccharides. Their structure varies from linear to branch. Polysaccharides are commonly heterogeneous, containing slight modifications of the repeating unit. About 90% of total natural polysaccharides produced on earth belongs to plants (83). Starch, cellulose, hemicelluloses, pectin, inulin etc. are some examples of polysaccharides and are called as a dietary fiber. Dietary fiber/ fiber is the non-digestible constituents making up the plant cell wall. These are resistant to enzymatic digestion (84, 85)

Based on their water solubility these fibers can be divided into the insoluble dietary fiber (cellulose, hemicelluloses, lignin etc.) and soluble dietary fibers (pectin, inulin, gums, and mucilage). Dietary fiber exerts a great impact on health-promoting food for mankind (86). Dietary fiber intake reduces the risk of coronary heart disease, stroke, hypertension, diabetes, obesity, gastrointestinal disorders (87). In addition, dietary fiber improves serum lipid concentrations, lowers blood pressure, improves blood glucose control in diabetes,  promotes regularity,  aids in weight loss, and appears to improve immune function (88–95).

Cereals, vegetables, fruits, and nuts are a natural source of dietary fiber (96). Wastes from the cereals, fruits, and vegetable processing are the most widely investigated substrates for the extraction of several types of dietary fibers (97). A number of methods have been used for dietary fiber extraction from these wastes which includes, dry processing, wet processing, chemical, gravimetric, enzymatic, physical, and microbial or a combination of many methods (98). Although these methods are being used for a long time, these are responsible for modifying the structure and functionality of the extracted fiber. Therefore, with advances in technology the latest methods for extractions like ultrasound, microwave, and high-pressure processing etc. are now in use. These improved methods not only reduce processing times and temperatures but also enhance the yield and quality (99, 100). Below are the few examples of polysaccharides extraction from various food wastes.

3.2.1. Cereals

Wheat, rice, corn, barley, sugarcane are the main agro-wastes and are potential sources of polysaccharides. The polysaccharides from these agro wastes are produced during cultivation, harvesting, and post-harvesting steps. These polysaccharides rich wastes are also called as ‘lignocellulosic residues’: i.e. the complex of cellulose, hemicellulose, and lignin (83). Isolation of hemicelluloses involves alkaline hydrolysis of ester linkages to liberate them from the lignocellulosic matrix followed by extraction into aqueous media(101).

 Wheat straw is extensively studied for hemicelluloses extractions. Earlier, hemicellulose was extracted from lignified wheat straw using aqueous alkali method but it often results in brown colored hemicellulose. Therefore, with the time alkali method was modified for better yield and quality of the hemicellulose. Lawther et al., (102) studied the effect of varying concentrations of KOH, H₃BO₃ under different temperature and time of extraction. They reported the varying nature of the alkali (calcium hydroxide, sodium hydroxide, lithium hydroxide, and liquid ammonia ) for the optimal extraction and isolation of hemicellulose and cellulose from wheat straw. At an optimum concentration of 24% KOH/2%H₃BO₃ and 20°C for 2 h the maximum yields for hemicellulose and cellulose of 34.23 and 35.96%, respectively was achieved.

Sun et al., (103) when treated the wheat straw with 2% H₂O₂ at 50°C and pH 11.5 for 4±30 h or with 2% H₂O₂±0.05% anthraquinone at 50°C and pH 11.5 for 4.5 h resulted in solubilization of 79-86% of the original lignin and 77-91% of the original hemicelluloses, respectively. Ultrasonically assisted extraction produced a higher yield of wheat straw hemicelluloses and lignin than those of the classical alkali procedure (104). When surface methodology was optimized with extrusion parameter it showed that extrusion cooking had a positive effect on total and soluble dietary fiber and negative on the insoluble dietary fiber (105). For extraction of wheat bran dietary fiber, alkali (2% NaOH) in a combination of proteinase resulting in relatively pure dietary fiber (100).

Total polysaccharides yield from rice waste depends on the kind of plant and raw materials (husk or straw) The yield varies from 8.2% to 26.1%(106) Glucans are the main water-soluble polysaccharides in rice, Polysaccharides after alkaline extraction of rice husks contain arabinose, xylose, glucose, and galactose. Hemicelluloses are one of the most abundant natural polysaccharides and comprise over 30% of the dry matter of rice straw. A comparative study of the extraction of rice straw hemicellulose by alkaline and hydrogen peroxide treatments resulted in the development of a fractionated treatment procedure for rice straw hemicellulose with maximum yield but minimal degradation and light color (103).

Bagasse is a dry pulpy residue left after extraction of juice from sugar cane. It has 32–34% cellulose, 19-24% hemicellulose, 25–32% lignin, 6–12% extractives and 2–6% ash (107). Earlier, alkaline extraction was the best and most efficient way to remove lignin from bagasse since a higher concentration of the solution. Chemical extraction of sugarcane bagasse showed that high alkaline concentration is responsible for the production of thin fibers with a lack of tenacity, of bending rigidity, and of bending hysteresis (108). Ultrasounds and mechanical action after the chemical extraction lead to a further improvement (108). The treatment with alkaline hydrogen peroxide (AHP) has also affected the physical and chemical properties of sugarcane bagasse (SB) where brightness, water-holding capacity (WHC) and oil binding capacity (OBC) have increased by 34, 96 and 55 percent (109). It also leads to removal of lignin by 53% thereby color of Solka Floc® 900, a commercial dietary fiber, was pure white (L=93.51) (109). The reduction of loaf volume and softness of bread was found At 5% of AHP-SB and Solka Floc® 900 substitutions (109).

Corn hulls are the inexpensive byproducts of the dry milling and wet milling processes and it contains hemicelluloses (30%-50%), cellulose (approximately 20%), phenolic acids (approximately 4%, mainly ferulic and ferulic acid), starch (9–23%), proteins (10–13%), lipids (2–3%) and ash (2%) (84, 110, 111). In defatted corn hull the maximal yield of dietary fiber A (DFA) which reached 33% was obtained by hot-compressed water (HCW) 150°C for 60 min (111). The yield of dietary fiber B (DFB) has also increased from 2.0% to 56.9% as the temperature increased from 110 to 180°C, while the yield of solid residue (SR) decreased from 88.7% to 27.7% (111).

3.2.1. Vegetables

Fruit and vegetable wastes including seeds, peels, leaves, roots, tubers, skin, pulp, seeds, stones, pomace etc. are the rich sources of dietary fibers, and other bioactive compounds (15, 112–114). Many fruits and vegetables generate at least up to 25% to 30% of unusable waste materials (115). The food waste varies from type of fruits and vegetable as well as the part used for eating. The waste amount in apples is 10.91% while in papaya 53% of the final product. In Mandarin, peeling is responsible for almost 16% of wastes and while in pineapple this waste amount goes to the even higher level of 48% of the final product (116, 117).

Onion peel, potato dietary fibers, by‐products (stems and florets) from cauliflower, carrot pomace, tomato pomace are mainly used for the production of dietary fibers. Dietary fibers study on onion showed that the dietary fiber content depends upon the variety and processing methods (118, 119). The main components of the dietary fibers are cellulose and pectic polysaccharides (120). The dietary fibers solubility in water depends on the presence of galactan side chain (118). Potato peel, pulp and potato solid wastes all are a rich source of dietary fibers (121, 122). Potato pulp is richer in rhamnogalacturonan I (123). In cauliflower, after comparing the stems and florets wastes for non-starch polysaccharides it was found that higher amounts of polysaccharides are present in stems than the florets (124).  Both stems and florets wastes are rich in insoluble fibers with pectic polysaccharides as the main component of non-starch polysaccharides. Carrot pomace and tomato pomace are also rich in dietary fibers. On the basis of dry matter, carrot pomace has 63.6% and tomato has 50% of dietary fibers. (125, 126). When edible snail’s enzymes were used on carrot pomace it leads to the production of soluble dietary fiber from carrot pomace (127). It was observed that 77.3 g water-soluble fibre/100 g of carrot pomace was produced after 96 h of enzymatic hydrolysis (127).

3.2.3. Fruits

Fruit waste has been utilized for pectin as a soluble dietary fiber. A number of methods have been tried to extract pectin from various sources so far (128–132). Industrially pectin is chemically obtained from apple pomace and citrus peels by using strong acids such as oxalic, hydrochloric,  nitric, and sulphuric acids  (133–137). Acid treatment is not environmentally friendly and thus extraction methods were further modified. These modifications include enzymatic extraction i.e. use of enzymes such polygalacturonase (hemi) cellulose, protease and microbial mixed enzymes (138, 139), ultrasonic, autoclave, microwave, and extrusion-assisted (138, 140–143) treatments to extract pectins from apple pomace and orange peel etc.

Apple pomace a waste material from apple juice processing contains significant amounts of dietary fiber. Total dietary fiber in vacuum‐dried pomace varies from  442 to 495 g/ kg and 480 g/ kg in freeze‐dried pomace (138, 144). Li et al., (145) compared the extraction of water-soluble dietary fiber (SDF) from apple pomace (AP) by cellulase, microwave and ultrasound-assisted methods with the conventional acid method. Microwave-assisted methods showed a drastic efficiency and the cellulase method provided the highest soluble dietary fiber yield.

The citrus industry generates a considerable number of by-products (or waste) with high amounts of valuable bioactive components. The annual world production of citrus fruits is over 100 million metric tonnes. Skin, pulp, seeds, and wastewater represent the byproducts obtained from the lemon processing. These wastes are rich in bioactive molecules. Peel from citrus industry is a very rich source of insoluble dietary fiber. The dietary fibers in peels were significantly higher than in peeled fruits. Soluble dietary fibers in different citrus fruits ranging between 34.2% and 46.6% of the total dietary fiber amount (146–148).

Chau and Huang (149) evaluated and compared the chemical composition and physicochemical properties of various fractions of dietary fibers (soluble and insoluble dietary fiber, alcohol-insoluble solid, and water-insoluble solid) prepared from Liucheng sweet orange peel by different methods. The studied peel was rich in insoluble fiber-rich fractions mainly composed of pectic polysaccharides and cellulose. Kratchanova et al., (150) showed that microwave pretreatment of fresh orange peels led an increase in the capillary-porous characteristics and the water absorption capacity of the plant material and heating inactivated the pectinesterase activity in the oranges. Thus, a considerable increase in the yield and quality of extractable pectin. Zykwinska et al., (151) used proteases and cellulases to isolate pectins and pectic oligosaccharides from the citrus peel at 50 °C for 4 h. For extraction of high methoxy pectins of high molar mass were extracted with three different enzyme mixtures. Wang et.al. (152) studied the pectin composition in peels of eight varieties of citrus. They used ethanol and water sequential extraction. A similar method was used by Mandalari et.al, (153) for pectin extraction from Citrus bergamia Risso fruit peel.

Apart from oranges grapes are the world’s largest fruit crop (154). Therefore, a large amount of solid remains of grapes fruit called ‘grape pomace is produced after juice extraction. It contains the skins, pulp, seeds, and stems of the fruit. Grape pomace is a good source for many bioactive compounds including dietary fibers, namely, hemicelluloses, cellulose, and small proportions of pectin (154).

Llobera and Cañellas (155) studied the composition of pomace and stem of the Manto Negro red grape (Vitis vinifera) variety, namely pomace and stem and showed high contents of total dietary fiber. Besides this, they also studied the soluble dietary fiber, insoluble dietary fiber, uronic acids and Klason lignin in both samples. In another study by Llobera and Cañellas (156) on white grape (Vitis vinifera) variety, Prensal Blanc was analyzed for the dietary fiber components together. A comparative analysis of red and white grapes varieties were done by González et. al.(157). They evaluated the grape pomaces and stems, from ten different grape (Vitis vinifera L.) varieties (six red and four white) as raw materials for the production of dietary fiber concentrates. They also analyzed the carbohydrate and functional properties of the dietary fiber.  They reported pectin as main components of the cell wall. In the stem, cellulose was higher than the other components. The red grape cultivar “Tempranillo” had the highest dietary fiber content in the pomace (36.90 g/100 g FW), stem (34.80 g/100 g FW), and fruit (5.10 g/100 g FW). Deng et. al. (158) analyzed the skins of two white wine and three red wine grape pomace from US Pacific Northwest for their dietary fiber (DF) by DF gravimetric–enzymatic method with sugar profiling by HPLC–ELSD and reported that red grape pomace has significantly higher DF than those of white grape pomace but reverse was true in case of but soluble sugar with low soluble sugar in red grape pomace.

Zheng et. al.(159) used enzymatic-gravimetric method for recovery of dietary fiber from grape juice pomace and evaluated their functional properties of red grape variety ‘Amur’ grape.

Mango (Mangifera indica L. Anacardiacea) is the most popular tropical fruit ranked 5th in total world production among the major fruit crops. 20% of the fruits are processed for products such as puree, nectar, leather, pickles, canned slices etc. (115). Peel and Kernel are most important byproduct with high amounts of dietary fibers (115, 160–164).

Another common and important tropical fruit crop is Banana. It is also one of the earliest crops cultivated in the history of human agriculture. Due to its cultivation and consumption in recent decades, it is now second largest fruit crop (165). Banana by-products have been used for wrappings foods and clothes (166). The fiber content in the peel is approximately 50% which include both the soluble fraction (pectins, gums) and the insoluble fraction (Cellulose, lignin, hemicelluloses, β-glucans) (167). Banana peel is a good source fiber of lignin (6-12%), pectin (10-21%), cellulose (7.6-9.6%), hemicelluloses (6.4-9.4%) and galactouroninc acid (168). Beside this, it also contains sugars including glucose, galactose, Arabinose, rhamnose, and xylose (168). Sequential H2O/ Chelating agent/ acid extraction was mainly used for fiber extraction (167).

In recent years sour cherry byproducts have received much more attention(169). The sour cherry has an astringent taste since their acid/ sugar ratios are higher in comparison to the sweet cherry (Prunus avium L.). Uronic acid is the main sugar component of the alcohol-insoluble solid following with cellulose, arabinose, and galactose in pomace. Pectin had a low methylation degree and are rich in simple sugars, arabinose, and galactose (170).

3. Phenolic compounds

Phenolic compounds are the plant secondary metabolites responsible for the sensory characteristic and are also natural antioxidants present in plants, foods, and beverages (171–173). Phenolic compounds are responsible for their chemopreventive properties such as antioxidant, anticarcinogenic, or antimutagenic and anti-inflammatory effects (174, 175). Phenolic compounds are present in all plant part such as bark, stalks, leaves, fruits, roots, flowers, pods, seeds, stems, latex, hull etc. (171). Structurally they contain one or more aromatic rings along with one or more hydroxyl groups in their basic structure (176). These can be classified into flavonoids (flavonols, flavanones, flavones, flavanonols, isoflavones, flavanols, and anthocyanidins), tannins, stilbenes, phenolic acids and lignans (177). For extraction of phenolic compound many novel extraction techniques have been developed including ultrasound-assisted extraction, supercritical fluid extraction, microwave-assisted extraction, and accelerated solvent extraction (178).

Cereal barn is rich in ferulic, vanillic, p-coumaric, caffeic, and chlorogenic acids (179). Wang et. al. (180) optimized the ultrasound-assisted extraction of phenolic compounds from wheat bran. In order to obtain the optimal conditions for phenol extraction, ultrasound-assisted extraction parameters such as the solvent, extraction temperature and extraction time were optimized using response surface methodology (RSM), by employing central composite rotatable design (CCRD). Ethanol concentration, 64%, extraction temperature, 60 °C, and extraction time, 25 min were the optimal conditions obtained by RSM for the extraction of phenolic compounds under ultra-sonication.

Rind, peel, and seeds of fruits and vegetables are the rich source of phenolic compounds (181). Potato peel is a major source of phenolic which is 50% of the total bioactive compounds(181). Luthria (182) evaluated the influence of different extraction parameters to optimize phenolic acids extraction by using a pressurized liquid extractor. Solvent composition, extraction time, particle size, flush volume, temperature, pressure, and solid-to-solvent ratio are the studied parameters. Samarin et al. (183) studied the extracts of five different solvents (water, ethanol, hexane, methanol, and acetone) and two solvent extraction methods (Solvent and ultrasound-assisted) for extraction of phenolic compounds.

Choi and others (2016) analyzed the bioactive compounds in whole potatoes, peels, and pulps of “Superior” variety of Korean potato. Phenolic extraction from sweet potato peels was analyzed by modeling and optimization by response surface modeling and artificial neural network. Thus, this model studied the effect of solvent to solid ratio, time and temperature on the extraction of phenolic compounds (184).

Wastes from citrus juice industries include peels and seeds. These wastes are known to have high natural antioxidant properties. Citrus seeds possessed greater antioxidant activity than peels. Citrus peel contains high amounts of flavanone glycosides (hesperidin, neohesperidin, narirutin, naringin), lower amounts of polymethoxylated flavones (sinensetin, tangeretin, nobiletin), and traces of flavonols, glycosylated flavones, and hydrocinnamic acid (185, 186). A phenolic compound extracted from peel by many methods such as conventional solvent extraction, supercritical CO2 extraction (SC-CO2), subcritical water extraction (SWE), pressurized fluid extraction (PFE), ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE) and combined approach (186). Phenolic composition of some citrus peels and seeds was described by Bocco et. al (187). They studied the antioxidant activity of several citrus peels and seed extracts obtained by methanol extraction for free phenolic compounds and by alkaline hydrolysis for bound phenolic compounds in a model system based on accelerated citronellal oxidation. They reported methanol extracts are richer in flavones and glycosylated flavanones, whereas hydrolyzed extracts contained mainly phenolic acids. A review on extraction methods of citrus peel phenolic compounds by M’hiri et.al. (186) concluded that conventional solvent extraction method can give reasonable yield, but it causes degradation of the thermolabile compounds of the extract. Others methods such as high-hydrostatic-pressure extraction, high temperatures (50–200 °C), and high pressures (10–15 MPa) although time savor but led to degradation of the phenolic compounds and represented especially by flavonoids, (characterized by their antioxidant activities). This review compiles the different extraction methods for phenolic compounds from citrus peel and shows that conventional solvent extraction method can give reasonable yield, but it causes degradation of the thermolabile compounds of the extract. They suggested ultrasound-assisted extraction for thermolabile components and microwave-assisted extraction to separate both polar and nonpolar phenolic compounds. It is a less matrix-dependent method and it needs less selective conditions by using a wide spectrum of organic solvents. This review shows that there is a lot to explore in terms of extraction process parameters and technologies, but few articles deal with a multifactor optimization extraction method of citrus peel phenols content. This review suggested a need for multifactor optimization of an extraction method for better extraction efficiency and preservation activities of phenolic compounds.

Tavares et al., (188) analyzed the aqueous waste obtained after hydrodistillation of lemongrass (CcHD) in Cymbopogon citratus (Cc) (Lemongrass) oil industry. The CcHD is rich in phenolic acids and flavonoids, namely luteolin and apigenin derivatives.

Banana peel contained large amounts of dopamine and L-dopa, catecholamines with a significant antioxidant activity (189). Evaluation of phenolic compound in peel and pulp or flesh of pomegranate and peaches reveled the higher phenolic compounds in peel than the pulp or flesh. (190, 191).

The grape seed from wine industry showed the highest amount of phenolic compounds such as gallic acid, catechin and epicatechin, and a wide variety of procyanidins whereas, skins revealed the highest levels of anthocyanins and p-coumaric acid hexoside (192, 193). Ultrasound-assisted extraction technique was used by Ghafoor et al. (194) for extraction of anthocyanins and phenolic compounds from grape peel Another extraction method i.e microwave-assisted extraction of polyphenolic antioxidants from grape seeds was also studied and response surface methodology (RSM) was used to evaluate the effect of microwave power, solvent concentration, extraction time, and their interactions by krishnaswamy (195).

Mango kernel is very rich in gallic acid, ellagic acid, gallates, gallotannins, condensed tannins (196). Mango peel contains anthocyanins, quercetin-glycosides, kaempferol-glycoside, xanthone-glycosides, cyanidin 3-O-galactoside anthocyanidin hexoside, γ-tocopherol, quercertin, mangiferin pentodise syringic, ellagic, gallic acid, condensed tannins etc. Mangiferin present in leaves of Mangifera indica has been extracted by microwave assisted extraction using water as a solvent by Kulkarni and Rathore (197).

Apple peel and pomace are the rich in the many phenolic compounds these include catechins, procyanidins, phloridzin, phloretin glycosides, caffeic acid, and chlorogenic acid; the peel possesses all of these compounds and has additional quercetin glycosides and cyanidin glycosides. The air-dried and freeze-dried apple peels had the highest total phenolic, flavonoid, and anthocyanin contents these includes contains neochlorogenic acid, 3-p-coumaroylquinic acid, chlorogenic acid, quercetin glucoside, and rutinoside, kaempferol-rutinoside, isorhamnetin-rutinoside, quercetin, kaempferol, isorhamnetin, anthocyanin (198)

4. Utilization of Bioactive compound from food wastes

A bioactive compound extracted from the food wastes had a lot of potential as a nutraceutical, functional foods, and food additive(17, 199). Bioactive compounds present in the food wastes are known have antioxidant and radical scavenging activities responsible for delaying and inhibiting the oxidation of DNA, proteins, and lipids thus lower the risk of developing on many diseases. like cancer, Alzheimer, cataracts, and Parkinson etc. Some of the examples of the utilization of bioactive compounds are as follows:

Oilcake a waste from the oil industry is highly popular feed utilized in dairy and poultry industries. The olive by-product, so-called “pâté,” a natural source of bioactive compounds characterized by the presence of hydroxytyrosol, β-hydroxyverbascoside, oleoside derivative, luteolin etc., is a potential ingredient for nutraceuticals preparations or feed industry (200).

Protein isolates and protein concentrates from the various oilcake meal major and minor oilseeds such as soybean, peanut, rapeseed, sunflower, almond, groundnut, and walnut are well utilized as a food supplement. Protein degrading enzymes from the food wastes can also be used in meat or brewing industries.

Pectin another bioactive compound extracted from the fruit pomaces is now been used as gelling agents in jams, fillings, sweets, etc Besides this fruit pomace also used to extract many food additives such as dietary fibers, lactic acid, pigments, vinegar, natural sweeteners and cellulose (201).

Mangiferin a bioactive  compound obtained from the peel and leaves of mango is now been used to treat the cancer cell as single administration of mangiferin or in combination other anticancerous drugs has shown the potential benefits in brain, lung, cervix, breast and prostate cancers, and leukemia besides its antioxidant and anti-inflammatory properties (202).

Lignan concentrates obtained from the flaxseed oil cake is also rich in anti-cancer, antioxidant, antibacterial, antiviral, and anti-inflammatory properties (203).

γ-Oryzano obtained from rice bran is commercially available and is used as a cardioprotective and reduce the menopausal symptoms.

β-glucans extracted from cereals especially barley flour used the production of pasta, noodles, breakfast cereals, and dairy products. Addition of flavonoids and saponins from black bean seed coat to whole wheat bread formulation was resulted in retention of added flavonoids and saponins as well as anthocyanins in bread after baking with many health benefits of prepared bread (204).

Lignans rich flaxseed meal is used in the preparation of bread, muffins and other bakery products. Ferulic, vanillic, and syringic acids and other phenolic compounds available in cereal bran provide resistance against free radical damage, cancer and cardiovascular diseases (205).

As compared to the number of food wastes produced and it is potential in food, health, and pharmaceutical industries food wastes are not fully utilized, due to non-availability of cost-effective efficient methods with less harmful waste production. The production cost of these compounds is much higher than the output. Subsequently, there is a need to utilize novel extraction innovations fit for lessening dissolvable utilization, consequently equipped for expanding the general eco-supportability of the sustenance life cycle and diminish the creation cost.

Therefore, there is a need to use novel extraction technologies with less solvent consumption and capable of increasing the overall eco-sustainability with low the production cost.

Conclusion:

There are several bioactive compounds present in unutilized food and these form landfills which leads to production of harmful gases. With the advance ment of science and discovery of various extraction method

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