5. Conclusions

improves yields of polyphenols because it promotes release of NEP, but polyphenols can loss their antioxidant capacity. A method that significantly improved the TPC without reducing the AA of the extracts is the hydrodynamic cavitation (HC), which was used to assist polyphenols extraction from cocoa bean shells with hexane/ethanol/water mixtures [35]. The authors compared the efficiency of this method with UAE (ethanol/water); AA were similar (EC50 62 3.2 and

66.9 2.4 μg/mL), while TPC was 125 and 197.4 mg GAE/g for extracts obtained by

In general, SFE (with the optimal conditions) is an extraction method more convenient in order to avoid degradation reactions and therefore reduces the AA of extracts. Blueberry waste extracts were obtained by SFE (90% CO2, 5% H2O, 5% ethanol, 20 MPa) and PLE (40 °C, 20 MPa, 15 min). Both methods showed extracts with similar AA (1658 160 and 1746 71 μM TE/g), but SFE yields extracts with TPC 48% more higher (134 11 mg GAE/g) than those from PLE (90.2 2 mg GAE/g) (Table 2) [33]. However, extracts obtained by UAE (ethanol) and Soxhlet (ethyl acetate) from coffee residues showed better AA values (EC50 235.1 and 202.23 μg/mL) than those observed in the extracts obtained by SFE (200 bar/

Pretreatments as solid-state fermentation before polyphenolic extraction have shown effects on TPC and AA, for example, in spent coffee grounds, an increase in TPC and AA of 36%, and 15% were observed in fermented extracts by Bacillus clausii followed by ethanol/water extraction [73]. The same increase in AA was observed when soybean residues were subjected to solid-state fermentation using Saccharomyces cerevisiae (20.39–24.04 mM TE/g) [37]. However, the use of Penicillium roqueforti in this fermentation type with cocoa shells showed a weak increase of AA from 79.2 to 81.3% (2.6%) and the reduction of TPC from 2120 20 mg

Grape cane residues are rich in stilbene compounds, which can be extracted with mixtures of water/ethanol or acetone/water, and their antioxidant activities depend on stilbene type present in extracts, e.g., quantitative structure-antioxidant activity relationship studies showed structural facts as planar geometry of transisomers has direct relation with the AA because polyphenols increase their free radical-stabilizing properties [74]. This may explain why AA is lower in extracts with higher stilbene contents; water/ethanol extracts with 8.93 mg resveratrol eq./g showed an AA of 1259.6 μM TE/g (ORAC), while acetone/water extracts with stilbene total content of 2.62–3.30 mg/g showed from 1700 to 5300 μM TE/g. Therefore, stilbene extraction accelerated by temperature can have consequences on stereoisomer content and therefore on the AA. Extraction of grape skins at 50 °C assisted by MAE and UAE showed better stilbenes contents for MAE (1.5 mg/ 100 g), but authors did not describe the AA [21]. Heat resistance phenol compounds and with significant AA are those found in residues previously treated to release NEP such as 56, 76-79 [36, 75] (Figures 2 and 7). For example, apple pulp was subjected to reflux with water for 2 h, and the EtOAc extract showed an AA of IC50

323.15 K, CO2 + 8 or 4% ethanol) (EC50 630 and 516.2 μg/mL) [23].

GA/100 g to 926.6 61 mg/100 g (56%) [63].

Heat resistance phenol compounds identified in ARs [36, 75].

Figure 7.

78

UAE and HC, respectively.

Antioxidants

Most of the bibliography related to the study of waste is focused on the search for conditions for the greater extraction of polyphenols from ARs and evaluating the feasibility of using these residues as a source of antioxidants. To evaluate the extraction efficiency of the proposed methods, the content of total phenols (TCP), the quantification and/or identification of specific polyphenols and determination of AA have been described. Antioxidant activity of polyphenols varies mainly by the temperature, which could promote the compound degradation or only small structural changes, mainly with anthocyanidins and stilbenes. Extraction methods applied to ARs described in this review showed the use of nonconventional technologies such as SFE and LPE for EP extraction while chemical, enzymatic, or thermic hydrolysis has been used to transform NEP to EP to apply the EP extraction methods and recover antioxidants. Moreover, significant contributions to the knowledge of the chemistry of ARs are summarized and representative compounds are shown that cover most types of phenols that exist in the plant kingdom and that are present in such residues. The chemical structures of 79 low-molecular-weight compounds, mainly EP and some examples of tannin and lignin residues, are described. Therefore, the use of ARs to recover polyphenols is growing due to the knowledge of ARs chemistry and to the development of nonconventional extraction methods and more efficient dry methods.
