1. Introduction

The agro-industry produces a huge amount of waste, such as peels and seeds from fruits (juice industry), coffee husks, coffee pulp, spent coffee grounds, cocoa husks, cocoa bean shells, acerola bagasse, soybean expeller, rice straw, wheat straw, and sugar bagasse. Most of these wastes contain value-added substances such as phenol-type compounds, which are important for their antioxidant activity. Phenolic compounds possess an aromatic ring, bearing one or more hydroxyl substituents, and whether they have low or high molecular weights (from one to several aromatic rings), all of them are generally referred to as polyphenols. The chemical complexity of polyphenols and the ease of extraction from vegetal tissues divide them into two main groups. The first group is comprised of low-molecular-weight phenols (LMWP) such as flavonoids, hydroxycinnamic acids, stilbenes, and benzoic acids, which are found in free form or as glycosides (Figure 1) [1–3]. They are easily extracted by aqueous-organic solvents which is why they are named extractable polyphenols (EP). The second group of compounds are low- or highmolecular-weight polyphenols that include (i) lignans which are phenolic acids or flavonoids associated with the cell wall, such as highly condensed phenylpropanoids [4] and (ii) tannins of high-molecular-weight polyphenols, which can be polymers

Figure 1. Extractable (EP) and non-extractable polyphenols (NEP) from ARs.

of phenolic acids and sugars (hydrolysable tannins) or polymers of polyhydroxyflavan-3-ol (condensed tannins or proanthocyanidins) [5]. Due to structural complexity, low solubility, and matrix or vegetal tissue availability, these polyphenols are not easy to extract and therefore are considered nonextractable polyphenols (NEP).

EP and NEP from agro-industrial residues (ARs) represent sources of value-added compounds with potential uses as ingredients in functional foods [6] or dietary supplements due to their health benefits, including antioxidant activity [7–10].

Some problems associated with the recovery of antioxidant phenols or polyphenols are the low availability from matrix (NEP), chemical complexity (NEP), low extraction yields (NEP and EP), and the reduction of antioxidant activity during the extraction process (NEP and EP). All of them present challenges to be overcome for the best use of ARs and economic feasibility. In this review, chemical complexity, extraction methods, and antioxidant activity described in the most recent bibliography are presented. Research in the use of agro-industrial waste involves applying nonconventional extraction methods and establishing conditions that prevent the degradation of polyphenols and, consequently, the loss of antioxidant activity.

Phenolic compounds with antioxidant potential in ARs described to date can be grouped into five classes according to the number of carbon atoms in the basic skeleton: C6C1, C6C3, C6C1C6, C6C2C6, and C6C3C6 [11]. There are benzoic acids 1–6 (C6C1), hydroxycinnamic acids 7–14 (C6C3), benzophenones 15–16 and xanthones 17–18 (C6C1C6), stilbenes 19–26 (C6C2C6), flavan-3-ols 27–30, anthocyanidins 31–35, flavonols 36–40, flavanones 41–45, flavones 46–49, isoflavones 50–55 and dihydrochalcone 56 (C6C3C6) Figure 2. Table 1 shows low-molecular-weight phenolic compounds and their occurrence in ARs,

Figure 2.

67

Structures of low-molecular-weight polyphenols obtained from Ars [14, 31, 50–53].

Antioxidant Compounds from Agro-Industrial Residue DOI: http://dx.doi.org/10.5772/intechopen.85184

Antioxidant Compounds from Agro-Industrial Residue DOI: http://dx.doi.org/10.5772/intechopen.85184

Figure 2. Structures of low-molecular-weight polyphenols obtained from Ars [14, 31, 50–53].

of phenolic acids and sugars (hydrolysable tannins) or polymers of

Extractable (EP) and non-extractable polyphenols (NEP) from ARs.

extractable polyphenols (NEP).

activity [7–10].

Figure 1.

Antioxidants

antioxidant activity.

66

polyhydroxyflavan-3-ol (condensed tannins or proanthocyanidins) [5]. Due to structural complexity, low solubility, and matrix or vegetal tissue availability, these polyphenols are not easy to extract and therefore are considered non-

EP and NEP from agro-industrial residues (ARs) represent sources of value-added compounds with potential uses as ingredients in functional foods [6]

Some problems associated with the recovery of antioxidant phenols or polyphenols are the low availability from matrix (NEP), chemical complexity (NEP), low extraction yields (NEP and EP), and the reduction of antioxidant activity during the extraction process (NEP and EP). All of them present challenges to be overcome for the best use of ARs and economic feasibility. In this review, chemical complexity, extraction methods, and antioxidant activity described in the most recent bibliography are presented. Research in the use of agro-industrial waste involves applying nonconventional extraction methods and establishing conditions that

Phenolic compounds with antioxidant potential in ARs described to date can be grouped into five classes according to the number of carbon atoms in the basic skeleton: C6C1, C6C3, C6C1C6, C6C2C6, and C6C3C6 [11]. There are

benzoic acids 1–6 (C6C1), hydroxycinnamic acids 7–14 (C6C3), benzophenones 15–16 and xanthones 17–18 (C6C1C6), stilbenes 19–26 (C6C2C6), flavan-3-ols 27–30, anthocyanidins 31–35, flavonols 36–40, flavanones 41–45, flavones 46–49, isoflavones 50–55 and dihydrochalcone 56 (C6C3C6) Figure 2. Table 1 shows low-molecular-weight phenolic compounds and their occurrence in ARs,

or dietary supplements due to their health benefits, including antioxidant

prevent the degradation of polyphenols and, consequently, the loss of


mainly pomace, peels, seeds, and fibers from fruits such as acerola (Malpighia) [12], coffee [13], mandarin oranges (Citrus) [14], berries [15], yuzu (Citrus) [16], mangoes [17], apples [3], pears [3], oranges [18], jocote (Spondias purpurea L.)

[19], cocoa husks [20], and grapes [21].

Polyphenols described in agro-industrial residues.

Daidzin 50, glycitin 51, daidzein 52, glycitein 53,

Polyphenol Example of ARs

Antioxidant Compounds from Agro-Industrial Residue DOI: http://dx.doi.org/10.5772/intechopen.85184

Flavonol

Flavanone

Flavone

Isoflavones

genistein 54

Table 1.

69

Dihydrochalcone

Cyanidin 32 Saskatoon berry pomace [15], grape skin [21],

Peonidin 34 Grape skin [21], grape pomace [24], blueberry

Quercetin 36 and glycosides Saskatoon berry pomace [15], Vidal grape pomace

Kaempferol 37 and glycosides Vidal grape pomace [27], grape skin [21], mango

Isorhamnetin 38, glycosides Apple fiber [3], grape skin [21]

Rhamnetin 40 glycosides Mango peels [27], jocote peels [19]

Naringenin 41 Orange peels [18], lemon pomace [34]

Hesperetin 45 Orange peels [18], lemon pomace [34]

Diosmetin 46 Orange peels [18], sugarcane bagasse [25, 26]

Tricin 49 Milled rice straw extract [36], sugarcane bagasse [25, 26]

Genistin 55 Soybean okara [37], cherry pomace [38], sugarcane

Phloretin 56 and glycosides Apple fiber [3], yuzu peels (Citrus junos) [16]

Naringin 42 Orange peels [18], lemon pomace [34], yuzu peels

Hesperidin 43 Orange peels [18], lemon pomace [34], yuzu peels

Myricetin 39 glycosides Grape skin [21]

Narirutin 44 Orange peels [18]

Tangeritin 47 Orange peels [18] Luteolin 48 Cocoa bean shells [35]

Petunidin 35 Grape pomace [24], blueberry waste [33]

Malvidin 33 Grape skin [21], blueberry waste [33]

blueberry waste [33]

[27], pear fiber [3], grape skin [21], mango peels [31], jocote peels [19], grape pomace [24], lemon

peels [31], jocote [19], lemon pomace [35],

waste [33]

pomace [34]

pomegranate seeds [29]

(Citrus junos) [16]

(Citrus junos) [16]

Soybean okara [37]

bagasse [26]

#### Antioxidant Compounds from Agro-Industrial Residue DOI: http://dx.doi.org/10.5772/intechopen.85184

Polyphenol Example of ARs

p-Hydroxybenzoic acid 2 Mandarin peels [14] Vanillic acid 3 Mandarin peels [14] Gallic acid 4 Mango peels and seeds [17]

Gallic acid derivates theogallin 5 and ellagic

Hydroxycinnamic acids and derivates

Caffeoylquinic 14, coumaroylquinic, and

Hopeaphenol 22 and isohopeaphenol, ampelopsin 23, miyabenol C 24, trans-ε-viniferin 25, r-2-viniferin 26, and

Maclurin 15, riflophenone 16 Mango peels [17]

Mangiferin 17, isomangiferin 18 Mango peels [17, 31]

Trans-piceid 20, cis-piceid and piceatannol 21 Grape skin [21], grape cane [32]

(+)-Catechin 27 Vidal grape pomace [27], cocoa husk [20],

()-Epicatechin 28 Vidal grape pomace [27], cocoa husk [20], acerola

Procyanidins B1 29, procyanidins B2 30 Grape skins [21], grape pomace [24], Vidal grape

Delphinidin 31 Saskatoon berry pomace [15], blueberry waste [34],

bagasse [12]

grape pomace [24]

feruloylquinic acids

Benzophenones

Xanthone

Stilbenes

ω-viniferin

Flavan-3-ols

Anthocyanidin

68

Protocatechuic acid 1 Mandarin peels [14], grape bagasse [22], spent

Ferulic acid 7 Coffee pulp [13], grape seed oil press residues [28],

Caffeic acid 8 Grape seed oil press residues [28], grape pomace [24] p-Coumaric acid 9 Grape seed oil press residues [28], mandarin peels

Caftaric acid 11 Grape seed oil press residues [28], grape pomace [24] Fertaric acid 12 Vidal grape pomace [27], seed oil press residues

Coutaric acid 13 Seed oil press residues [28], grape pomace [24]

jocote [19]

Trans-resveratrol 19 Grape pomace [24], grape skin [21], grape cane [32]

Grape cane [32]

pomegranate seeds [29]

pomace [27], pomegranate seeds [29]

Sinapic acid 10 Mandarin peels [14], grape pomace [24]

ground coffee grounds [23], grape pomace [24],

Orange peel [18], acerola bagasse [12], Vidal grape pomace [27], jocote [19], grape pomace [24]

Vidal grape pomace [27], pomegranate seeds [29]

[14], acerola bagasse [12], Vidal grape pomace [27]

Coffee pulp [13], apple fiber [3], Saskatoon berry pomace [15], mandarin peels [14], acerola bagasse [12], green coffee seed residue [30], pear fiber [3],

sugarcane bagasse [25, 26]

[28], grape pomace [24]

Benzoic acids

Antioxidants

acid 6


#### Table 1.

Polyphenols described in agro-industrial residues.

mainly pomace, peels, seeds, and fibers from fruits such as acerola (Malpighia) [12], coffee [13], mandarin oranges (Citrus) [14], berries [15], yuzu (Citrus) [16], mangoes [17], apples [3], pears [3], oranges [18], jocote (Spondias purpurea L.) [19], cocoa husks [20], and grapes [21].
