3.1 Tannins

units. These are also known as proanthocyanidins because the butanol/HCl/heat treatment produces a red anthocyanidin [54]. Hydrolysable tannins are multiple esters of gallic acid with glucose and products of oxidative reactions, and they can be soluble (EP) or non-soluble (NEP) (Figure 3) [55]. Lignin is a phenylpropanoid (C6C3) polyphenol where the monomeric units are p-coumaryl 71, coniferyl, and sinapyl. Occurrence of monomeric units in lignin varies according to the taxonomic

Non-extractable polyphenols are common in almost all ARs and represent significant polyphenol percentages of total phenol content (TPC) in vegetal tissues. For example, NEP quantification in peels from apple, banana, kiwi, mandarin, mango, nectarine, orange, pear, and watermelon showed that, of the total phenols found, 7–82% correspond to NEP [58]. Currently research in appropriate methodologies for the extraction of NEP is a priority topic because of the economic advantages of the use of ARs. Some examples of research on the best conditions for the extraction of NEP from ARs are those for the use of cocoa by-products, which involve extractions assisted by ultrasound [20, 35], thermic treatment [59, 60], hydrodynamic cavitation [35], pressurized liquids [50], pulsed electric field [61], subcritical water hydrolysis [62], and solid fermentation [63]. There are also reports on detailed chemical studies of the NEP structure thanks to modern analytical instrumentation, such as liquid chromatography (LC) coupled to matrix-assisted

Structures of condensed tannins (proanthocyanidins), hydrolysable tannins (gallotannins), and lignin

origin of the ARs, e.g., gymnosperms or angiosperms [56].

Antioxidants

Figure 3.

72

(5-hydroxyguaiacyl residue) [55, 57].

Studies in pomegranate by-products led to the identification of several polyphenols. EP was extracted with acetone 70% (ultrasound-assisted 20 min, 30 °C), and NEP was previously subjected to basic hydrolysis of insoluble residues before extraction under the same conditions. Ellagic acid 6 and monogalloyl-hexoside 57 were the main compounds, in addition to ellagic acid derivates, valoneic acid bilactone I and II 58, punicalagin 59 and isomers, trigalloylglucopiranose I and II, and granatin, among 34 hydrolysable tannins described in Figure 4. Moreover, condensed tannins such as procyanidin dimers 30 and gallocatechin (or catechin) hexoside were described. Antioxidant activity for polyphenol (EP and NEP) fractions was 0.12–3.58 mmol TE/g dm (DPPH) and 5.34–208.73 μmol TE/g dm (ABTS radical-scavenging activity) [66]. Oligomeric proanthocyanidins such as dimer, trimer, tetramer, pentamer, and hexamer units (MW 600, 889, 1177, 1465, and 1753 amu) were identified in coffee pulp by MALDI-TOF-MS analysis. Extraction of these tannins was made with acid aqueous acetone [67]. Acetone/water also was used to extract anthocyanidins from litchi pericarp (Litchi chinensis), after its incubation with Aspergillus awamori. Total extractable tannin recovery was increased up to 59%, and ESI-TOF-MS analysis revealed A. awamori degraded B-type condensed tannin and showed low capacity to degrade the condensed tannin A-type.

Figure 4. Hydrolysable tannins identified in pomegranate by-products [55, 69].

Figure 5.

Condensed A- and B-type condensed tannins. A-type C4-C8 couple C2-O-C7 linkage and B-type C4-C8 linkage [68].

Differences between both compound types are C4-C8 couple C2-O-C7´ linkages for A-type tannin, while C4-C8 linkages are found in B-type tannin (Figure 5) [68].

#### 3.2 Lignin

Studies of chemical composition of NEP corresponding to lignin were made for that recovered from black liquor (BL) and milled rice straw extract (RSE). BL is produced from basic hydrolysis/heat of rice straw (straw alkali oxygen cooking). Lignin and phenolics from rice straw were obtained by Soxhlet extraction (ethanol/benzene 1:2, v/v, 8 h). Detailed studies by infrared and nuclear magnetic resonance spectroscopy showed a number of residues from rice straw lignin, including β-O-4'ethers 60, β-O-4'ethers with acylated γ-OH 61, phenylcoumarans 62, resinols 63, dibenzodioxocyns 64, α,β-diaryl ethers 65, tricin units 66, Cαoxidized guaiacyl unit 67, syringyl units 68, Cα-oxidized syringyl 69, phydroxyphenyl units 70, p-coumaroyl 71, guaiacyl units 72, feruloyl 73, cinnamyl alcohol end-groups 74, and cinnamyl aldehyde end-groups 75 (Figure 6). Antioxidant evaluation (DPPH• and ABTS• methods) showed lignin from BL had a better radical-scavenging ability than RSE, which was due to the release of p-hydroxyphenyl units by rice straw alkali oxygen cooking. This process caused the destruction of tricin [70]. Lignin and phenolics could also be extracted from milled rice straw by hydrothermal treatment (210 °C) to release cellulosic components, delignification (ethanol/water 60.5%, 130 °C) to fractionate the lignocellulosic biomass and separation (nanofiltration cutoff 280 Da) to isolate the polyphenols [71].

Extraction of NEP from residues that come from industrial processes has been done using methodologies applied to EP because they were released by the industrial process involved. For example, polyphenolics from sugarcane bagasse were extracted with 95% ethanol (maceration 7d) and p-coumaric acid; tricin 66, luteolin, tricin 7-O-β-glucopiranoside, diosmetin 46, 6-C-glucoside, and protocatehuic acid 1 (Figure 2) were isolated from ethanol extracts (ethanol 95%, 7d, maceration). TPC from extract was 3.0 mg GAE/100 mg dm [25]. Antioxidant activities (ORAC assay) of tricin 49, p-coumaric 9, and protocatechuic 1 acids were 9.76 1.01, 7.22 0.73, and 6.40 0.62 μmol TE/μmol, respectively. Phenols yields were from <0.2 to 2.12 mg/kg dm. Other compounds reported in this residue were genistein 52 (15.22 1.28 mg/g), genistein 53 (trace), and

quercetin 36 (9.98 0.40 mg/g DW) [26]. Polyphenol recovery was improved (117.1%) when residue was treated with glacial acetic acid and hydrogen peroxide at 60 °C for 7 h and later subjected to hydrolysis with xylanase (Clostridium thermocellum ATCC 27405). The antioxidant activity was increased 73%

Other residue that has undergone industrial processes enough to release polyphenols is the lignin from the ozone, soaking aqueous ammonia pretreatment of wheat. Py/GC/MS analysis showed the presence of 17 phenolic compounds derived from guaiacyl 72, syringyl 68, and p-hydroxyphenyl 70 units in ratios of 65.09, 23.36, and 11.5%, respectively, the main compounds being Phenol 2-methoxy (guaiacol) 76, phenol 2-methoxy-4-vinyl (4-vinylguaiacol) 77, and 2,6-dimethoxy

(695.8 105.3 μmol trolox eq./L) [72].

Residues identified in lignin from rice straw [70].

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

Figure 6.

75

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

Figure 6. Residues identified in lignin from rice straw [70].

quercetin 36 (9.98 0.40 mg/g DW) [26]. Polyphenol recovery was improved (117.1%) when residue was treated with glacial acetic acid and hydrogen peroxide at 60 °C for 7 h and later subjected to hydrolysis with xylanase (Clostridium thermocellum ATCC 27405). The antioxidant activity was increased 73% (695.8 105.3 μmol trolox eq./L) [72].

Other residue that has undergone industrial processes enough to release polyphenols is the lignin from the ozone, soaking aqueous ammonia pretreatment of wheat. Py/GC/MS analysis showed the presence of 17 phenolic compounds derived from guaiacyl 72, syringyl 68, and p-hydroxyphenyl 70 units in ratios of 65.09, 23.36, and 11.5%, respectively, the main compounds being Phenol 2-methoxy (guaiacol) 76, phenol 2-methoxy-4-vinyl (4-vinylguaiacol) 77, and 2,6-dimethoxy

Differences between both compound types are C4-C8 couple C2-O-C7´ linkages for A-type tannin, while C4-C8 linkages are found in B-type tannin (Figure 5) [68].

Condensed A- and B-type condensed tannins. A-type C4-C8 couple C2-O-C7 linkage and B-type C4-C8

Studies of chemical composition of NEP corresponding to lignin were made for that recovered from black liquor (BL) and milled rice straw extract (RSE). BL is produced from basic hydrolysis/heat of rice straw (straw alkali oxygen cooking). Lignin and phenolics from rice straw were obtained by Soxhlet extraction

(ethanol/benzene 1:2, v/v, 8 h). Detailed studies by infrared and nuclear magnetic resonance spectroscopy showed a number of residues from rice straw lignin, including β-O-4'ethers 60, β-O-4'ethers with acylated γ-OH 61, phenylcoumarans 62, resinols 63, dibenzodioxocyns 64, α,β-diaryl ethers 65, tricin units 66, Cαoxidized guaiacyl unit 67, syringyl units 68, Cα-oxidized syringyl 69, p-

hydroxyphenyl units 70, p-coumaroyl 71, guaiacyl units 72, feruloyl 73, cinnamyl

Antioxidant evaluation (DPPH• and ABTS• methods) showed lignin from BL had a better radical-scavenging ability than RSE, which was due to the release of p-hydroxyphenyl units by rice straw alkali oxygen cooking. This process caused the destruction of tricin [70]. Lignin and phenolics could also be extracted from milled rice straw by hydrothermal treatment (210 °C) to release cellulosic components, delignification (ethanol/water 60.5%, 130 °C) to fractionate the lignocellulosic

Extraction of NEP from residues that come from industrial processes has been done using methodologies applied to EP because they were released by the industrial process involved. For example, polyphenolics from sugarcane bagasse were extracted with 95% ethanol (maceration 7d) and p-coumaric acid; tricin 66, luteolin, tricin 7-O-β-glucopiranoside, diosmetin 46, 6-C-glucoside, and

protocatehuic acid 1 (Figure 2) were isolated from ethanol extracts (ethanol 95%, 7d, maceration). TPC from extract was 3.0 mg GAE/100 mg dm [25]. Antioxidant activities (ORAC assay) of tricin 49, p-coumaric 9, and protocatechuic 1 acids were 9.76 1.01, 7.22 0.73, and 6.40 0.62 μmol TE/μmol, respectively. Phenols yields were from <0.2 to 2.12 mg/kg dm. Other compounds reported in this residue were genistein 52 (15.22 1.28 mg/g), genistein 53 (trace), and

alcohol end-groups 74, and cinnamyl aldehyde end-groups 75 (Figure 6).

biomass and separation (nanofiltration cutoff 280 Da) to isolate the

3.2 Lignin

Figure 5.

linkage [68].

Antioxidants

polyphenols [71].

74

phenol (syringol) 78. The residue was identified as a potential source of antioxidants because it showed 86.9 0.34% of inhibition of DPPH radicals similar to that of commercial BHT 103.3 1% [36].

Residue/ref. Extraction method Antioxidant activity TPC/(dry matter)

(ABTS)

(DPPH)

(DPPH) 169 � 3 μM TE/g (ABTS)

(DPPH)

(ABTS)

(DPPH)

(FRAP)

(ABTS)

(DPPH) 199 � 20 μM TE/g (ABTS)

(ABTS)

(ORAC)

Water/EtOH 40.4 and 55.4%, 84 °C 238.6 μM TE/g (ABTS)

Acetone/water 6:4, room temperature 1700–5300 μM TE/g

82–153 μg/mL (DPPH)

141 � 1 μM TE/g (ABTS)

161 � 3 μM TE/g (ABTS)

160.13 � 13 μM TE/g

326.0 � 5.7 mg TE/L

17.894 μM TE/100 g

1658 � 160 μM TE/g

1746 � 71 μM TE/g (DPPH)

66 � 1 μM TEAC/g

1259.6 μM TE/g (ORAC)

24.04 mM TE/g (DPPH) 20.65 mM TE/g (ABTS)

Increase 15%

UAE, ultrasound-assisted extraction; EC50, effective concentration at 50%; PLE, pressurized liquid extraction; PEF, pulsed electric field; SFE, supercritical fluid extraction; TPC, total phenol content; MeOH, methanol; EtOH, ethanol;

ethylbenzothiazoline-6-sulfonic acid) diammonium salt; DPPH, 2,2-diphenyl-1-picrylhydrazyl; ORAC, oxygen radical absorbance capacity; MAE, microwave-assisted extraction; TE, trolox equivalent; FRAP, ferric-reducing

AcOEt, ethyl acetate; CAE, chlorogenic acid equivalent; GAE, gallic acid equivalent; ABTS, 2,2<sup>0</sup>

nd Stilbenes 1.5 mg/

nd Stilbenes 0.71 mg/

1791.9 � 126.3 mg FSE/L

7.61–17.40 mg GAE/L

36 � 1 mg CAE/g

133.3 � 0.6 mg CAE/g

57 � 3 mg CAE/g

182.6 � 28.2 mg CAE/g

302.5 � 7.1 mg GAE/L

1051 mg GAE/ 100 g Increased 36%

134 � 11 mg GAE/g Wet matter

90 � 2 mg GAE/g Wet matter

8.93 mg resveratrol eq./g

100 g

100 g

116 mg GAE/10 g


Stilbene total content 2.62– 3.30 mg/g

Water, 92 � 3 °C, 2 min EC50 18–27 μg/mL

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

UAE, EtOH EC50 235.1 μg/mL

SFE 200 bar/323.15 K, CO2 + 4% EtOH EC50 516.2 μg/mL

Soxhlet extraction, EtOAc EC50 202.23 μg/mL

Hydroalcoholic solvent (50%) at 40°C,

Solid state fermentation with Bacillus clausii (37 °C, 39 h), defatted (hexane) and EtOH/water (80:20) extraction (orbital shaker, 30 °C, 50 rpm, 3 h)

SFE 90% CO2, 5% H2O, 5% EtOH,

PLE 40 °C, 20 MPa, 15 min, 5 mL cell,

MAE, 600 W, 2450 MHz, 50 � 5 °C, water/EtOH/phosphoric acid 50:50:1

UAE, 130 W, 40 kHz, 50 � 5°C, water/ EtOH/phosphoric acid 50:50:1

Solid state fermentation with Saccharomyces cerevisiae r. f. bayanus 72 h. Water/MeOH (80%) extraction

antioxidant power; FSE, ferrous sulfate equivalent; nd, not described

Antioxidant activity and extraction methods in agro-industrial residues.

SFE 200 bar/323.15 K, CO2 + 8% EtOH EC50 630 μg/mL (DPPH)

Coffee pulp [13]

Coffee husk [23]

Spent coffee grounds [23]

Coffee silver skin [1]

Spent coffee grounds [73]

Blueberry waste [33]

Grape cane [47]

Grape cane [32]

Grape skins [21]

Soybean okara [37]

Table 2.

77

60 min

20 MPa

EtOH/water 1:1
