# Effect of Temperatures on Polyphenols during Extraction
## 2 Thermal Degradation
### Extraction Conditions
\begin{table}
\begin{tabular}{l l l l l} \hline \hline \multicolumn{1}{c}{**Source**} & \multicolumn{1}{c}{**Extraction**} & \multicolumn{1}{c}{**Temperatures Tested**} & \multicolumn{1}{c}{**Effect on Polyphenols**} & \multicolumn{1}{c}{**Reference**} \\ \hline Red grape pomace peels & TSE &
\begin{tabular}{l} Dried at 60, 100 and 140 \({}^{\circ}\)C; freeze-dried \\ samples served as controls \\ \end{tabular} & TPC \(\downarrow\) at 100 \({}^{\circ}\)C & [35] \\ \hline Grape seed flour (GSF) & TSE & Heated at 120, 150, 180, 210 or 240 \({}^{\circ}\)C & TPC \(\downarrow\) above 180 \({}^{\circ}\)C & [34] \\ \hline Black rice & TSE & Dried at 20, 40, 60, 80 and 100 \({}^{\circ}\)C & TPC \(\downarrow\) above 40 \({}^{\circ}\)C & [30] \\ \hline Spinach & PLE & Extractions between 50–190 \({}^{\circ}\)C & Flavonoids \(\downarrow\) at 130 \({}^{\circ}\)C & [41] \\  & & No decrease in TPC & & \\ \hline Black currants & TSE & Extractions between 20–60 \({}^{\circ}\)C & T \(\uparrow\), TPC \(\uparrow\) & \\  & & Anthocyanins \(\downarrow\) above 45 \({}^{\circ}\)C. & [16] \\ \hline Hemp, flax, canola & & & T \(\uparrow\), TPC \(\uparrow\) & \\ seed cakes & TSE & Extractions at 40, 50, 60, 70 \({}^{\circ}\)C & TPC \(\downarrow\) above 60 \({}^{\circ}\)C in flax and canola & [57] \\  & & & TFC \(\downarrow\) above 70 \({}^{\circ}\)C in hempseed cake & \\ \hline Peach & TSE & Extractions between 25–70 \({}^{\circ}\)C & TPC \(\downarrow\) above 60 \({}^{\circ}\)C & [55] \\  & & & TPC remains same between 25–70 \({}^{\circ}\)C & \\ \hline Mango peels and seed & TSE & Extractions at 25, 50, and 75 \({}^{\circ}\)C & TPC \(\downarrow\) at 50 and 75 \({}^{\circ}\)C & [58] \\ \hline Red grape skin & PLE & Extractions between 20 to 140 \({}^{\circ}\)C & Anthocyanins \(\downarrow\) above 100 \({}^{\circ}\)C & [62] \\  & & TPC \(\downarrow\) above 120 \({}^{\circ}\)C & & \\ \hline Elderberry, strawberry & & Heated at 95 \({}^{\circ}\)C & Anthocyanins \(\downarrow\) & [64] \\ and black carrot & & & & \\ \hline Red cabbage & TSE & Blanched at 94–96 \({}^{\circ}\)C & TPC \(\uparrow\) at 94–96 \({}^{\circ}\)C & [63] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Thermal Degradation. Ninety per cent of all phenolic compounds were recovered at all temperatures except for catechin and epicatechin, which saw a drop in recovery rate above 150 \({}^{\circ}\)C. The results of the boiling methanol extraction showed that the recovery rates were much lower than the PLE extraction. In fact, catechin saw a reduction of 40% in its concentration. The highest degradation observed in superheated methanol treatment was for catechins, but it was only 14%. The authors suggested that this degradation happened at moderate temperatures (around 65 \({}^{\circ}\)C) during boiling methanol extraction due to light exposure and oxidation by the oxygen present in the air. As no such degradation was observed in dark and oxygen-free environments during superheated methanol extraction, it was concluded that the lower recovery rates in methanol extraction were not due to thermal degradation. As catechins are known to be the most oxidisable compound among the ones tested, the 40% loss of catechins in methanol extraction compared with only 10% in PLE provides more evidence that the degradation is due to the presence of oxygen.