Recovery of Antioxidants from Agro-Industrial Side Streams

Recovery of Antioxidants from Agro-Industrial Side Streams: Comparison

Please note this is a comparison between Version 1 by Radu Claudiu Fierascu and Version 2 by Peter Tang.

Agro-industrial side streams covers a wide range of products available as raw materials. These can serve as a source of other added value products, under the nomenclature of biowaste. Food waste are generated from different sectors of food industry, such as vegetables, fruits, milk, meat, fish, and wine production. Recovered compounds can be re-utilized as food additives, functional foods, nutra-/pharmaceuticals, cosmeceuticals, beauty products, and bio-packaging.

food waste
extraction
re-utilization
active compounds
fruits
vegetables

1. Introduction

In a general definition, agro-industrial side streams represent the organic and inorganic material generated as residues from different sectors: agriculture, livestock, dairy production, food, and beverage industry, etc. Due to the impressive amounts of wastes generated, the community of researchers is striving to find cost-competitive solutions for recovery of biologically active compounds from these materials. Agro-industrial side streams covers a wide range of products available as raw materials. These can serve as a source of other added value products, under the nomenclature of biowaste. Food waste are generated from different sectors of food industry, such as vegetables, fruits, milk, meat, fish, and wine production [1]. At a European level, up to 37 million tons from the food and drink industry are estimated as biowaste [2]. Another source for biowaste production is agro-forestry. These wastes consist of stems of the crop plants like wheat, rice, sugarcane, etc. [3]. Some statistics presented by different authors show that there are large amounts of wastes generated from agro-forestry sector: 709 million tonnes of wheat straw, 673 million tonnes of rice straw, 102 million tonnes of bagasse [4], 0.1 million tonnes of coir [5], and 2.96 million tonnes of solid olive waste [6].

These large amounts of wastes can produce pollution and economic loss [7] and causes landfilling to be no longer sustainable. With an estimation of increasing human population of 9.3 billion people in 2050 [2], the actual concern of scientific community is to transform these materials into added-value products for other industries.

Improper management of wastes leads to irreversible environmental issues, like global warming [8], just to name the most known and currently discussed environmental effect. The impact of nitrous oxide and methane generated during composting of different waste is estimated to be 20 times higher than carbon dioxide release in the next years [9]. In developed countries, the wastes are transformed in energy or commodity chemicals, while in less developed countries there is a gap for proper waste management due to insufficient infrastructure, policy framework and funds [10].

Antioxidants are found among several groups of natural compounds definitely worth to be recovered from agro-industrial waste and by-products (Table 1).

Table 1.

Examples of different antioxidants from agro-industrial side-streams.

Compound Group	Source	Extracted Compounds	Ref.
Phenolic compounds	Apple seeds	Phloridzin, ellagic acid, caffeic acid, ferrulic acid, protocatechuic acid, gallic acid	^[11][12]
Phenolic compounds	Avocado seeds

48]. One of the advantages of PLE is represented by the selective recovery of target compounds. Pereira and coworkers obtained anthocyanin-rich fraction separately from other phenolic compounds with application of PLE ^[48][49]. Application of enzymatic pretreatment can also increase the yield of recovery, even few times (e.g., for pomegranate peels) ^[49][50]. PLE is also used as a pre-treatment for other techniques ^[50][51] or as a complementary technique ^[51][52]. The complementary use of PLE enables to improve the yield of recovery and to shorten the extraction time ^[52][53].

The use of ultrasounds can facilitate the recovery of antioxidants with high reproducibility and low solvent consumption ^[29][30]. Ultrasound-assisted extraction (UAE) was presented as a “green” and safe technology, promoting the release of extractable compounds (like polyphenols and pectins) from fruit peel wastes ^[53][54], vegetables ^[54][55], or wine industry wastes ^[55][56].

Microwave-assisted extraction (MAE) utilizes high frequency, non-ionizing electromagnetic waves which can facilitate the extraction due to a highly localized temperature and pressure. These changed conditions results in the reduction of extraction time and solvent consumption ^[56][57]; however, the used solvents have to be permanent dipoles. The use of solvent mixtures extends MAE applications ^[57][58][58,59] to different types of agro-food by-products, such as brans ^[59][60], peels ^[60][61], seeds ^[61][62], wine shoots ^[62][63], etc.

2.2. Recovery of Antioxidant Compounds from Edible Oil Industry Wastes

The edible oil manufacturing processes generate a substantial amount of side streams, which can be further utilized in order to recover different target compounds or as an energy source (some examples presented in Table 2). It is estimated that in manufacturing and processing the vegetable and animal oils, 3.9% represents the remains materials, to be named wastes ^[63][74].

Table 2.

Some examples of recovery of antioxidant compounds from edible oil industry wastes

Waste	Extraction Method	Optimized Extraction Conditions	Obtained Compounds	Antioxidant Assay	Ref.
Flaxseed hulls	PEF	Electrode area (cm²)—95 pulse length (µs)—10 Temperature (°C)—20 Electric field (kV/cm)—20	Tocopherols, polyphenols, phytosterols	-

Waste

Extraction Method

Optimized Extraction Conditions

Obtained Compounds

Antioxidant Assay

Ref.

Flaxseed hulls

PEF

Electrode area (cm²)—95

pulse length (µs)—10

Temperature (°C)—20

Electric field (kV/cm)—20

Tocopherols, polyphenols, phytosterols

; for other authors, this represents “the decrease in edible food mass throughout the part of the supply chain that specifically leads from raw material to food for human consumption” ^[76][100]. However we define those wastes, this industry presents a large amount of by-products. Fruit and vegetable production have been increased, reaching approximatively 0.9 billion tons of fruit and more than 1 billion tons of vegetables, in 2017 ^[77][101]. In the fruits sector, the most produced are citrus, watermelons, banana, apples, and grapes, while for vegetables are potatoes, tomatoes, onions, cucumbers and cabbages ^[78][102]. According to FAO statistics, a large amount of foods ends up in the garbage (approx. 1.3 billion tons) distributed on different sectors: from fruits and vegetable industry resulting 66% by weight of total food losses, roots and tubers account for 44 and 20% by weight ^[79][80][103,104].

Also, there is the category of “sub-standard products”, which represent the fruits and vegetables that have small dimensions and do not fulfill quality standards, regulated by EC Regulation No 1221/2008, and are used for some derivatives, like juices, vinegar, etc., generating in turn different by-products.

The inedible wastes resulting from processing of fruits and vegetables is represented by peels, pomaces, seed and depending on source, have different ratios: for bananas, pineapples and citrus (25–46%), apples (12%), cauliflower (43%), carrots (20%), and garlic (22%) ^[81][105]. Due to the large amounts of moisture, these residues are perishable ^[82][106] and implies various difficulties in storage ^[83][107], being necessary a pre-treatment for further utilization for recovery of target compounds ^[84][108]. Side streams from fruits and vegetable sector represent a rich source in bioactive compounds, especially antioxidants (some examples presented in Table 3). For these wastes, antioxidants, especially phenolic compounds composition, can be modified by fermentative processes which can take place in large amounts of waste. Some authors suggested that fermentation increases the phenolic content ^[85][86][109,110], while others observed a decrease for antioxidant concentration ^[87][111], or even a compound degradation ^[88][112]. So, a proper and rapid extraction treatment is necessary to recover antioxidants from wastes (pomace, peels, etc.).

Table 3.

Some examples of recovery of antioxidant compounds from fruits and vegetable wastes

Waste	Extraction Method	Optimized Extraction Conditions	Obtained Compounds	Antioxidant Assay	Ref.
Apple pomace	Cec	Methanol, ethanol and ethyl acetate	Phenolic compounds and triterpenic acids	DPPH, FRAP, ABTS	^[89][113]
Apple pomace	MAE	Solvent—70% acetone and 60% ethanol, Microwave power (W)—735, Solvent volume to sample ratio (mL/g)—5.65 Time (s)—149	Phenolic compounds	DPPH	^[90][114]
Mango peels	ScE	Pressure (MPa)—25.0 Temperature (°C)—60 Solvent—15% w/w ethanol	Carotene	-	^[91][115]
Orange peel	LSE	Solvent: cyclopentyl methyl ether, ethyl lactate, isopropyl alcohol, polyethylene glycol 300, isopropyl acetate, dimethyl carbonate, methyl ethyl ketone, 2-methyl-tetrahydrofuran and ethyl acetate Temperature (°C)—70 Time (min)—150 Solid -liquid ratio—1:10

^[99][129]. Beside the losses resulted from the production processes, round 88 million tons of food (meat, fish and other animal origin products) are wasted annually only in the EU, with associated costs estimated at 143 billion € ^[100][130]. Even though the losses in animal origin products industry are in general less than in agro-industry (20% comparative with crops production –30%), meat waste has the highest negative environmental impacts estimated by greenhouse emissions ^[101][131]. In this respect, the recovery of active compounds from wastes and their further utilization is a solution mitigating negative environmental impact (Table 4).

Table 4.

Some examples of recovery of antioxidant compounds from other different industries wastes

Waste	Extraction Method	Optimized Extraction Conditions	Obtained Compounds	Antioxidant Assay	Ref.
Squid muscle	SbWE	Temperature (°C)—250 for aminoacids; 160 for peptides	Amino acids Peptides		^[61][62]
	ABTS			^[	^102][139]	Palm pressed fibers Procyanidin B2, epicatechin, rans-5-O-caffeoyl-	PLE, Sx, Pc d-quinic acid, procyanidin B1, catechin	Temperature (°C)—35, 35, 78.4 Flow rate (g/min) 2.4 Pressure (Mpa)—0.1; 0.1; 4 ^[12][13]
	Carotenoids			-	^[64
Poultry wastes	SbWE	Temperature (K) 533 Reaction time (min)—28 H₂SO₄ concentration in reactant system 0.02%.	Amino acids	-	^][79]	^[103][140]	Phenolic compounds
	Palm pressed fiber Rapeseed cake	PLE Sinapine, sinapic acid and canolol		Solvents: CO₂ and compressed liquefied petroleum gas Temperature (°C)—60 Pressure (MPa)—25.0 ^[13][14]
Waste chicken breast muscle	OT	Sets of high voltage short pulses and by low voltage long pulses Energy (J/g)—38.4 ± 1.2	β-sitosterol, α-tocopherol, squalene		Proteins HPX/XOD	DPPH ABTS ^[65][80]	^[104][142]	Phenolic compounds
Palm pressed fiber	Citrus peel	UAE Total phenolic content	Ultrasound intensity (W.cm⁻²)—120 Pulse cycle 0.4Limonene
Fucus vesiculosus		Temperature (°C)—20	-		MAE	Pressure (psi)—120 Time (min)—1 1g alga/25mL water ^[14][15]
	β-sitosterol,		α-tocopherol, squalene,	Fucoidan DPPH ABTS	^[66][81]	Phenolic compounds
		-		^[	^92][116]
	^[	¹⁰⁵	^]	[	149]	Olive leaves	Cocoa shells
Saccharina japonica Aresch	Coconut shell	UAE, HC	ASE Total phenolic content		MAE Temperature (°C)—190 Hexane, hydro-alcoholic solution (70:30 EtOH/HLeaf moisture content (%)—5 ₂Aqueous ethanol concentration (%)—80 O) ternary mixture (30:49:21 Hex/EtOH/H₂O) cycle number 47.1, cycle time (s)—5 residence time (s)—5	Solvent: 55% ethanol total residence time (min)—3.93 ^[15][16]
	Irradiation power (W)—400 solid/solvent ratio 1:8;	Time (min)—25	Oleuropein, Luteolin-7-O-glucoside	Catechins epicatechins theobromine caffeine DPPH			Phlorotannins DPPH ^[67][85]	- ^[93][119]
		^[	¹⁰⁶	^]	[152]	Phenolic compounds
Olive tree pruning biomass Olive mill leaves	Tomato seeds Grape marc, Orange peel,	UAE Strawberry, Citrus pulp Camelina cake	Total phenolic content	UAE Power (W)—400 Frequency (kHz)—24. Liquid/solid ratio of extraction (v/w	Power (W)—90 hexane-acetone-ethanol 2-1-1 ^[16][17]
)—20 mL/g.			Phenolic compounds Flavonoids	Lycopene DPPH, ABTS, FRAP
Undaria pinnatifida and		-			^[68][86]	^[94][121]	Phenolic compounds
Olive pomace	Black currant Sea buckthorn	UAE, MAE, Se Delphinidin 3-O-rutinoside, delphinidin 3-O-glucoside, cyanidin 3-O-rutinoside, cyaniding-3-O-glucoside, ellagitannins, proanthocyanidins, p-coumaric acid, caffeic acid-hexosides, coumaroylquinic acid-hexosides, vanillic acid-hexoside, (+)-Catechin, (−)-epicatechin, Quercetin 3-O-rutinoside, 3-O-glucoside, and 3-O-(6′′-malonyl)-glucoside	Ethanol concentration (%)—90, Temperature (°C)—50, Time (min)—5 Liquid /solid ratio (mL/g)—30 Ultrasound intensity (W/cm²
Tomato seeds	MAE, OT	)—135.6 Ultrasound frequency (kHz)—60	Temperature (°C)—70 Time (min)—15 Solvent—70% ethanol	Rutin	-	^[95][123]
Onion waste

^][161,162,163,176,177]. Besides film wrappings, biodegradable containers being in direct contact with foods are also designed. Incorporation of herbal antioxidants is especially advantageous in fatty food applications. Such containers enriched with e.g., achiote or yerba mate decreases oxidation processes within food, hence prevent the changes in sensorial and nutritional product characteristics ^{[131][132][133]}[178,179,180].

Circulation of redundant food biomass within food industry is ensured likewise through utilization of secondary food products into functional foods. High fiber waste as pineapple peel and core, mango rind, cactus pear peel or broken rice is easily introduced into cereal bars or cakes ^[134][135][181,182]. Red fruits concentrates are basis of sports drinks rich in easily available energy from glucose, fructose, sucrose or maltodextrin/glucose polymers. Polyphenols from various berries, cocoa, ginger, vegetables and seeds are often found in fortified antioxidant functional beverages, which are also available as fermented products ^[136][183]. Since healthy eating and healthy lifestyle attracts a lot of attention in a modern society, functional foods are a well-established avenue for introduction of recovered bioactive compounds.

Healthy products are usually associated with natural or ecological as well. This brings a current eco-beauty movement, based on re-utilization of food wastes into daily cosmetics showing new applications of food derived antioxidants. Cosmetic companies participate is so-called “circular beauty” by introducing to the markets fast-growing number of products including repurposed food waste. The food and drink manufacturers serve directly as a source ingredient which are transformed into: coffee scrubs made from reclaimed coffee grounds; range of hair products and hair dyes rich in anthocyanins obtained from blackcurrant residues; lip balms and colors using processed fruit waste; hand soap and candles from cooking grease ^[112][113][159,160]. The added value products obtained from agro-industrial side streaming supply also the production of cosmeceuticals. These skin “quasi drugs” are aimed to exert anti-aging, whitening or sunscreen effects which are based mainly on the action of plant antioxidants. Market products rich in resveratrol, arbutin, organic acids, rutin, but also vegetable oils, create the demand for pure compounds obtained in the eco-friendly and sustainable way. Similar market demand is still factual in case of nutraceuticals and herbal drugs, which are frequently composed of blended plant extracts or purified compounds ^[121][168].

The proved beneficial effects exerted by natural antioxidants are observed mainly in prevention of cardiovascular disease and cancer and mitigation of diabetes complications. The inhibition of low-density lipoprotein oxidation underlies the successful delay of atherosclerosis development. Action of antioxidants within plasma and mitochondrial membranes results in beneficial effects on platelet aggregation. Additional scavenging of peroxynitrite generated by the reaction of nitric oxide and superoxide anion ensures the proper amount of nitric oxide for maintenance of flexible blood vessels for normal blood flow ^[137][138][184,185]. The same action of antioxidants in endothelium of veins is beneficial in management of consequences of diabetes. Since higher levels of reactive oxygen species generated in diabetes were proved by clinical evidence, the deleterious effects of this disease are usually associated to oxidation. High blood glucose levels additionally promote auto-oxidation of glucose to form free radicals. Supplementation with antioxidants helps to protect beta cells of the pancreas and make them function correctly ^[139][186]. What is more, lipophilic antioxidants, were shown to selectively regulate peroxisome proliferator-activated receptors, being a ligand-regulated transcription factor playing essential role in energy metabolism ^[140][187]. This action of antioxidants on molecular level results in improvement of body glucose utilization and insulin sensitivity ^{[138][141][142]}[185,188,189]. Antioxidants were also shown to have ability to modulate molecular mechanisms in cancer cells. Their cancer-preventive and -therapeutic effects result from suppression of inflammation, oxidative stress, proliferation and angiogenesis ^[143][190]. Positive effects in cancer treatment were frequently observed for fruit antioxidants in animal models and human clinical trials ^[144][191].

Some examples of antioxidants from the above-described industries and their applications are listed in Table 5.

Table 5.

Recovery of antioxidant compounds from agro food side streams and potential applications.

Waste	Active Compounds	Application	Ref.
Applications of antioxidant compounds recovered from edible oil industry wastes
Palm pressed fiber	β-Sitosterol, α-tocopherol, squalene	Cosmetic formulation with high sun protection factor	^[66][81]
Sunflower leaves	Diterpenoids, flavonoids	Natural herbicide	^[72][92]
Sunflower seed	Phenolic compounds	Antioxidant additive for sunflower oil	^[145][192]
Sargassum fusiforme
	MAE coupled with HSCCC	Solvent: ethanolic KOH solution (1.5 mol/L) Irradiation power (W)—500 Liquid/solid ratio 20:1 Time (min)—20 Revolution speed (RPM)—800	Fucosterol, 24-methylenecholesterol, phytol	-	^[107][153]
		^[	^17][18]
	Hydroxytyrosol, maslinic acid, oleanolic acid			-
Porphyridium cruentum	UAE		Solvent: 2mL of ethanol, 10mg ascorbic acid, 3mL of n-hexane, Time (min)—20	Tocopherol	- ^[69][87]	^[108][154]	Phenolic acids, flavonoids	Grape skin	Gallic acid, caffeic acid, epicatechin,
Olive leaves and tree bark	SCe SbWE(PT)	p-coumaric acid, rutin, catechin gallate		Temperature (°C)—145 Time (min)—15 intense pulsed light (V)—1200 ^[18][19]
Temperature (°C)—60,		Pressure (bar)—300	α-tocopherol, squalene	-	Time (s)—60
Nannochlorops sp.	PEF		Quercitin		^[70	- ^][88]		The electric field (kV/cm)—20 Consecutive pulses 1–400	Carotenoids ^[96][124]
	-			^[	^109][155]	Flavonols	Pistachio hulls
Olive waste	UAEH	Gallic acid, penta-O-galloyl-β-d-glucose, anacardic acid	Phenolic compounds Cellulase, pectinase ^[19][20]
Frequency (kHz)—40		Power (W)—200	Phenolic compounds	DPPH, ABTS, FRAP	^[71]	Pomegra-nate waste [	UAE 89	Temperature (°C)—51.5; Amplitude level—58.8% Solvent—sunflower oil	Food industry ]	Carotenoids	^[11][12] -	Flavonoids, carotenoids	Passion fruit peel
	^[	⁹⁷	^]	[	127]	Sunflower leaves	PLE, ESE β-carotene, provitamin A, quercetin, lycopene	CO₂ and mixture of solvents (ethanol in water from 0 to 100%)
Berries		Phenolic compounds	^[20][21]
Pressure (bar)—400		Temperature (°C)—55	Diterpenoids, flavonoids	-	^[72][92]	Carotene	Carrot pomace
Rapeseed press-cake	HVED	α- and β-carotene	High voltage pulsed power (kV)—40 Intensity (kA)—10 ^[21][22]
Needle diameter (mm)—10			Protein, polyphenols and isothiocyanates	TEAC	^[73][95]	Lycopene	Tomato peel	Lycopene	^[22][23]
Non phenolic compounds	Lettuce	Ascorbic acid	^[23][24]
Non phenolic compounds	Sugarcane molasses	Pullulan	^[24][25]
Non phenolic compounds	Rice bran oil	Tocopherol	^[25][26]

2. Recovery of Antioxidants from Agro-Industrial Side Streams

2.1. Recent Advances in Recovery of Antioxidants from Agro-Industrial Side Streams

The use of agro-industrial side streams as a source of bioactive compounds cannot be performed without “green” advanced extraction techniques. The general flow chart of an extraction process is based on three general steps: pre-treatment, extraction, and purification (Figure 1), the yield of recovery of interest compounds being influenced by different parameters, as pre-treatment, solvent, temperature, agitation rate etc. The pre-treatment process is the first parameter considered. Pre-treatment based on heat is not applicable in antioxidants recovery, because it affects the process in a negative manner, with a simultaneous reduction of the phenolic concentrations and antioxidant capacity of the extracts ^[26][27]. Pre-treatment processes such as foam mat, electro-osmotic de-watering and micro-filtration are proposed by different authors, in order to keep biological activities unaltered and to remove microbes from vegetal material ^[27][28]. Alcohol precipitation is the most used technique for the separation of small particles (such as polyphenolic compounds or minerals) from macromolecules ^[28][29]. Ultra-filtration was applied for removing pectin and potassium from olive mill wastewater ^[29][30], while enzymatic treatment for extracting flavors and colors from plant materials ^[30][31].

Figure 1.

Flow chart of extraction process.

The use of the solvent depends on the solubility and volatility of the target compounds. Phenols are easily solubilized in polar protic mediums ^[31][32], carotenoids are more liposoluble in polar aprotic or non-polar solvents ^[32][33]. The recovery of pectin and hemicelluloses requires complex solvent treatment with ethanol followed by alkali ^[28][29]. Several environmentally friendly solvents, alternatives to the use of classical organic solvents emerged in the last decades, such as ionic liquids, alcohols or terpenes, surfactant solutions or natural deep eutectic solvents. The extraction techniques applicable for the recovery of antioxidants from plant matrices are solid-liquid methods. Classical extraction method (such as Soxhlet extraction, percolation, maceration, hydro distillation, and steam distillation) are based on solid-liquid extraction with various solvents. They have significant drawbacks, especially in terms of long extraction time, relatively large quantities of organic solvents used and low yields of recovered target compounds ^[33][34]. Advanced methods are considered pressurized liquid extraction (PLE) (supercritical and subcritical fluid extraction—SFE, enhanced solvent extraction (ESE)—involves the use of mixtures water or organic solvents with CO₂ as solvents, accelerated solvent extraction—ASE), microwave-assisted methods (MAE) and ultrasound-assisted methods (UAM), alongside with non-conventional ohmic technologies (technology relies on ohmic heating by passing electrical current through materials, instead of conductive heat transfer) such as pulsed electric field (PEF) and high voltage electric discharge (HVED), and they can be successfully used for the recovery of antioxidants from agro-industrial by-products. These modern and advantageous techniques require less solvent and energy consumption, and provide enhanced yields of recovery of active compounds ^{[34][35][36][37]}[35,36,37,38]. Another innovative technique which can be used in the case of antioxidant recovery is solid-liquid dynamic extraction (RSLDE) by using Naviglio extractor which finds application in various sectors, such as the pharmaceutical, cosmetic, herbal, food and beverage sectors ^[38][39]. For this procedure the advantages are working at room temperature, the possibility to recover compounds sensible to the temperature, the principle of the method being based on generating of a gradient pressure between the inner and the outlet of solid matrix ^[39][40].

All of these modern techniques have a lot of advantages: increased selectivity in recovering different classes of compounds, small amounts of solvents used, decreased extraction time and less amounts of remaining wastes. However, the major drawbacks appearing during industrial scaling up is the cost of the equipment and its maintenance, the use of adjacent installations needed to provide solvents and other necessary conditions for a proper process at large scale. On the other hand, very good optimization of the parameters is easy to obtain in reproducible conditions, which is often different from laboratory scale.

In case of pressurized liquid extraction (PLE), parameters such as pressure, time, use of co-solvent, etc. are crucial in order to optimize the method and do not affect compounds extracted ^[40][41]. PLE is suitable for recovery of non-polar, polar and semi-polar compounds from different matrices of by-products, such as cereals ^[41][42], wine making wastes ^[42][43], fish industry ^[43][44], olives ^[44][45] or fruits ^[45][46]. However, in this type of extraction, undesirable compounds can be generated (e.g., hydroxymethylfurfural) ^[46][47]. The phenomenon can be avoided through the addition of increased amounts of solvents, which increases the temperature of extraction ^[47][


Pumpkin seeds


UAE

MAE

	Frequency (GHz)—2.45	Ethanol concentration (%)—60 Time (min)—20 UAE-EtOH—60% UAE-hex/EtOH/ H ₂O—30:49:21%	Phenolic compounds	DPPH	^[74][97]

¹ Where: ABTS—2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid; ASE—accelerated solvent extraction; DPPH (assay)—2,2-diphenyl-1-picrylhydrazyl; ESE—enhanced solvent extraction; EtOH—ethanol; FRAP (assay)—ferric reducing ability of plasma; hex—hexane; HVED—high voltage electric discharge; MAE—microwave-assisted extraction; Pc—percolation; PEF—pulsed electric fields; PLE—pressurized liquid extraction; SCe—supercritical extraction; Se—solvent extraction; Sx—Soxhlet extraction; TEAC (assay)—Trolox equivalent antioxidant capacity; UAE—ultrasound-assisted extraction; UAEH—ultrasound-assisted enzyme hydrolysis; HPX/XOD—Hypoxanthine/xanthine oxidase system (superoxide radical scavenging activity).

2.3. Recovery of Antioxidant Compounds from Fruits and Vegetable Wastes

In this sector, researchers have different opinions regarding the terms and definitions. This concept is being very controversial: for some authors, the term “fruit and vegetable waste” represent the inedible parts of fruits or vegetables that are obtained during collection, handling, transportation and processing ^[75][99]

¹ Where: ABTS—2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid; Cec—classical extraction with centrifugation; DPPH (assay)—2,2-diphenyl-1-picrylhydrazyl; FRAP (assay)—ferric reducing ability of plasma; HC—hydrodynamic cavitation; LSE—liquid solid extraction; MAE—microwave -ssisted extraction; OT—ohmic technologies; SbWE(PT)—Subcritical water extraction with physical pretreatment; ScE—Supercritical extraction; UAE—ultrasound-assisted extraction.

2.4. Recovery of Antioxidants Compounds from other Different Industries

Other industries from agro-food sector, which can produce large amounts of side streams are animal origin derived products as meat (with side streams bones, tendons, skin, parts of the gastrointestinal tract and other internal organs, and blood), and fish. They have a large annual production of 263 million tones and 128 million of tones, respectively ^[98][128]. In the fish industry, more than 50% is considered to be waste

¹ Where: ABTS—2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid; DPPH (assay)—2,2-diphenyl-1-picrylhydrazyl; HSCCC—high-speed counter current chromatography; PEF—pulsed electric fields; OT—ohmic technologies; MAE—microwave-assisted extraction; SbWE—subcritical water extraction; UAE—ultrasound-assisted extraction.

3. Potential Applications of Antioxidants Recovered from Food Waste and by-Products

Besides colorants, preservatives or texturizing agents produced in circular economy, antioxidants find its second application mainly as food additives ^[110][157], functional foods and nutra-/pharmaceuticals ^[111][158], but recently also as functional cosmetics (cosmeceuticals as anti-aging products, whitening products and sunscreens), beauty products (makeup products) ^[112][113][159,160] and biopackaging ^{[114][115][116]}[161,162,163]. Plant derived antioxidants are compatible with food products and were already confirmed to be helpful in shelf life extension. Extracts from prune, grape, bearberry, grape seeds, rosemary, clove and mango shell are effective in conservation of meat and meat products, significantly decreasing lipid oxidation comparing to control under refrigeration conditions with greater effect in raw meats than in cooked ones ^{[117][118][119][120][121][122][123]}[164,165,166,167,168,169,170]. Potato peel, sugar beet pulp and rosemary successfully control the oxidation in sunflower oil and soybean oil ^[124][125][171,172]. The bakery products enriched by plant extracts or powdered plant materials are appreciated for their taste. Nevertheless, the addition of lavender or Melissa waste, grape or moringa extracts results in the extension of enjoyable consumption of breads and cookies ^{[126][127][128]}[173,174,175].

The reuse of food waste and food by-products within the same industry facilitates the waste management and lowers financial outlays spent for eco-friendly production. The eco-aspect is furthermore covered by production of bio-packaging. The focus is placed on development of active packaging from natural biodegradable polymers supported by plant antioxidants. Soy protein isolate or fish gelatin may be transformed into films enriched with mango kernel extracts, licorice residue extract, pomegranate peel powder or pine bark extract to name just a few, resulting in functional release of antioxidants over time and delayed spoilage of covered products ^{[114][115][116][129][130}


Soy bean waste

	Proteins	Biopackaging	^[146][193]
Olive waste extract	Phenolic compounds	Food industry (increasing shelf life of meat)	^[147][194]
Olive mill wastes	Phenolic compounds	Food antioxidants	^[148][195]
Applications of antioxidant compounds from fruits wastes
Apple seeds
	Pharmaceutical formulations		^[144][191]
Mango peels	Carotene	Antioxidant additive for edible oil	^[91][115]
Banana peels	Caffeic acid	Cosmetic formulations	^[149][196]
Citrus peels	Phenolic compounds, essential oils and flavonoids	Pharmaceutical formulations	^[150][197]
Citrus wastes	Phenolics and flavonoids	Cosmetic formulations	^[151][198]
Citrus peels	Terpinene, cymene	Pharmaceutical formulations	^[152][199]
Cocoa	Total extract	Larvicidal nanoparticles	^[153][200]
Grape pomace	Phenolic compounds	Food industry	^[154][201]
Applications of antioxidant from vegetable wastes
Tomato wastes	Lycopene	Health related applications	^[155][202]
Beetroot pomace	Betalains	Medicinal and food applications	^[156][203]
Carrot pomace	Carotenoids	Pharmaceutical formulations	^[157][204]
Garlic waste	Ethanolic extract	Food additive to increase products shelf life	^[158][205]
Onion waste	Phenolic compounds	Food industry	^[159][206]
Cauliflower by-products	Isothiocyanates	Food industry	^[160][207]
Applications of antioxidant compounds from other industries
Meat industry wastes	Gelatin Heparin	Pharmaceutical formulations (antioxidant and antihypertensive)	[1]
Algal biomass	Sulfated polysaccharides	Pharmaceutical formulations	^[161][208]
Algal biomass	α-Carnitine	Nutraceutical products	^[162][209]
Squid waste	Astaxanthin	Pharmaceutical industry	^[163][210]
Shrimps shells	Astaxanthin	Food packaging material	^[164][211]
Shrimps shells	Carotenoprotein	Supplementary nutritive feed	^[165][212]