# Effect of Temperatures on Polyphenols during Extraction
## 2 Thermal Degradation  
[PERSON] and [PERSON] [54] used three mechanisms to explain the behaviour of polyphenols at high temperatures. First, the insoluble phenolic compounds may be released when the lignin bonds to phenolic acids are broken. It has been shown that the quantity of bound phenolic acids (measured after hydrolysis of plant tissue) is twice that of free phenolic compounds [54]. Secondly, lignin itself may be degraded at high temperatures, giving rise to more phenolic acids. This could explain the increase in phenolic yield with increasing temperature in PLE extractions. Lastly, at high temperatures, thermal degradation of the polyphenols may occur. Thermal degradation is the most common mechanism used to explain the fall in polyphenol yield during high-temperature extractions. However, thermal degradation alone does not explain the behaviour of polyphenols, as conventional extraction studies have shown that thermal degradation occurs at 80 \({}^{\circ}\)C compared with 150-200 \({}^{\circ}\)C in PLE. The other two mechanisms discussed above could be attributed to the conflicting results in the literature. The increase in TPC observed at high temperatures in PLE extractions could be due to the lignin-phenolic acid bonds breaking or due to the breakdown of lignin itself, giving rise to more phenolic acid. [PERSON] et al. [35] reported a significant drop in total phenolic content when the plant material was dried at temperatures above 100 \({}^{\circ}\)C, also showing a decrease in antioxidant activity along with a decrease in TPC and suggesting that thermal degradation of the polyphenols may have occurred. Another study was conducted by the same authors to investigate the effect of drying temperatures between 20 \({}^{\circ}\)C and 120 \({}^{\circ}\)C. They observed that antioxidant activity at 20 \({}^{\circ}\)C was 1.7 times higher than the AOA observed at 120 \({}^{\circ}\)C. They suggested that the oxidation of polyphenols at high temperatures may also lead to a loss in observed antioxidant activity [28]. [PERSON] et al. [34] studied the effect of heating grape seed flour at various temperatures (120-240 \({}^{\circ}\)C) on polyphenols extracted with ethanol. The study found that the TPC yield dropped when the temperature was raised beyond 180 \({}^{\circ}\)C, while total flavonoid content (TFC) dropped at a lower temperature of 120 \({}^{\circ}\)C, suggesting that flavonoids are more sensitive to temperatures. A significant increase in TPC was observed at 150 \({}^{\circ}\)C, which could be due to the liberation of phenolic compounds from the plant matrix and/or the breakdown of lignin. At an extremely high temperature of 180 \({}^{\circ}\)C, similar to [PERSON] et al. [35], the authors suggested that the main mechanism behind the reduction is thermal degradation. When yields of specific polyphenols are calculated by HPLC, it is observed that (1) catechin drops at above 150 \({}^{\circ}\)C, (2) gallocatechin increases massively with increasing time and temperature, (3) epicatechin is not highly affected at 120 \({}^{\circ}\)C but drops at above 150 \({}^{\circ}\)C and (4) gallic acid increases massively with time and temperature [34]. Other studies have also found that flavonoids are more heat sensitive, and the degradation of flavones and flavanols/flavanones are observed at lower temperatures [39; 40; 54; 55; 56; 57; 58; 59]. Anthocyanins are one of the most abundant flavonoid constituents found in fruits and vegetables [60; 61]. One study tested the thermal effects between 20-60 \({}^{\circ}\)C and observed that, initially, an increase in temperature causes an increase in anthocyanin extraction [16]; however, a sharp decrease in anthocyanin extraction was observed at 45 \({}^{\circ}\)C. The anthocyanin content in the TPC dropped from 70% to 54% with an increase in temperature beyond 45 \({}^{\circ}\)C. The initial increase in yield with temperature could be due to the increase in solubility of anthocyanins in ethanol. However, the TPC showed the opposite trend and increased with temperature.