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In this work, the critical role of temperature (the values assayed were 22, 37 and 60 °C), the chemical characteristics of the medium (ethanol, methanol and water) and the reaction time on the antiradical capacity of phenolic systems was studied. An initial increase and a following decrease in antiradical activity were observed for catechin and resveratrol solutions in all solvents assayed. The maximum antioxidant activity was higher and occurred in a shorter time as the storage temperature was increased or the solvent polarity was decreased. The maximum values of the latter variable in the catechin and resveratrol cases (reaching an antiradical activity value higher than 50% in comparison with the initial one) were detected when oxidation was carried out in ethanol at 60 °C after 6 and 24 h of storage, respectively. Such variations were due to different reaction pathways. In fact, oxidative polymerization and oxidative formation of hydroxyl groups were found to be responsible for the enhancements of antiradical activity in catechin and resveratrol, respectively. A similar trend with variations of temperature in the different media was also observed in the grape extract case. The evolution of antiradical activity was followed by high-performance liquid chromatography analysis, which pointed to the phenol polymerization of the monophenols detected (catechin, gallic acid, epicatechin and quercetin) as mainly responsible for such variations.

Grape byproducts were subjected to an extraction process under various different experimental conditions (namely, solvent type, temperature, solvent-to-solid ratio, time contact, and raw material) in order to study the effect of these conditions on the yield of phenolic compounds and the corresponding antiradical activity of extracts. Although the order of decreasing capacity to extract soluble materials was ethanolmethanolwater, methanol was the most selective for extracting phenolic compounds. Temperature and solvent-to-solid ratio were found to have a critical role in extraction efficiency; values of 50 degrees C (between 25 and 50 degrees C) and 1:1 (between 1:1 and 5:1) maximized the antiradical activity of phenolic extracts. In addition, extracts from grape samples previously subjected to distillation reached higher antiradical values in comparison to those coming directly from pressing; in both cases, seed extracts showed better results than those of stem when ethanol or water was employed, whereas the opposite occurred in the case of methanol. These differences were attributed to the different phenolic compositions of the considered fractions.

Lignocellulosic residues, such as pine sawdust and almond hulls, were solvent-extracted under different experimental conditions to optimize the yield of polyphenolic antioxidant compounds, which were quantified. The antioxidant power of extracts was evaluated by ability to scavenge the 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical. Both materials were found to be important sources of phenolic antioxidants, although the efficiency of the extraction varied with the experimental conditions. Among the three solvents used (ethanol, methanol and water), ethanol was the most favourable for total extractables, although methanol was more selective for extracting polyphenolics. For these latter, pine sawdust offered the best results, with a 3–10 times higher (0.1122 g/100 g in dry basis) total phenolics content than almond hulls but, despite this, phenols from hulls showed a higher antioxidant capacity (58 vs 34% of inhibition).

Extracts from Murta leaves are used by Chilean natives for their benefits on health and cosmetic properties, which are mainly due to the presence of polyphenolic compounds. Extraction of such compounds is strongly influenced by several variables, the effects of which are studied in this work; the antioxidant power of the resulting extracts was measured by two different methods [2,2-diphenyl-1-picrylhydrazyl (DPPH) and thiobarbituric acid reactive substances (TBARS)]. On the whole, maximum values of polyphenolic yields and antiradical power (DPPH method) were attained at 50 degrees C (from 25 to 50 degrees C) and a solvent-to-solid ratio (v/w) of 15:1 (15:1-25:1). The solvents assayed were ethanol, methanol, and water. The highest polyphenolic yield values (2.6% expressed as gallic acid) were reached with methanol, whereas maximum EC50 was attained by the ethanol extract (0.121 mol gallic acid/mol DPPH). Contact time was shown to have only a slight influence in alcoholic extraction, while in water a remarkable effect of increasing contact times (30-90 min) was observed. Just water was the solvent that offered the best result when the antioxidant power was measured by the TBARS method. High-performance liquid chromatography-mass spectrometry analysis revealed the presence of polyphenols, basically flavonols and flavanols, sometimes glycosilated; myricetin and quercetin glycosides were detected in all extracts, whereas epicatechin was present in alcoholic extracts and gallic acid was only present in water.

The efficiency of liquid–solid extraction of phenols from natural products was enhanced by effect of coupling certain arrangements with the conventional extraction process (control). In fact, whilst 2.20 g phenols/100 g sample were obtained from grape byproducts using the control system, values of 2.69 and 3.21 g of phenol/100 g sample were attained when process was carried out by passing bubbling nitrogen through the packed bed or by using pulsed flow solvent, respectively. Likewise, values of antiradical activity (measured by DPPH) were at least twofold higher than those of the control. HPLC analysis confirmed a delay in oxidative polymerization of phenols when bubbling nitrogen was used. The latter avoids the formation of high polymerization degree species and allows one to obtain extracts with higher antiradical capacity. Catechin, epicatechin, gallic acid and quercetin were the main monomeric phenols identified in the grape extracts obtained.