### Effect of High Temperatures on Phenolic Compounds

Phenolic compounds are a group of secondary metabolites found in plants that have been shown to have various health benefits, including antioxidant and anti-inflammatory properties. However, these compounds are susceptible to degradation under certain conditions, including high temperatures.

Several studies have investigated the effect of high temperatures on phenolic compounds. For example, a study on the effect of extraction temperature on total phenols and antioxidant activity of Gynura procumbens leaf found that the total phenolic content decreased with increasing extraction temperature [1]. Another study on the effects of drying temperature and long-term storage conditions on black rice phenolic compounds found that the total flavonoid content decreased above $40$ °C and the total phenolic content decreased above $80$ °C [1].

However, not all studies have found a negative effect of high temperatures on phenolic compounds. For instance, a study on the effects of heat treatment on the phenolic compounds and antioxidant capacity of quince fruit and its tisane's sensory properties found that although the heating process had a negative effect on phenolic content, it showed an increase in antioxidant activity [2]. Similarly, a study on the temperature-dependent studies on the total phenolics, flavonoids, antioxidant activities, and sugar content in six onion varieties found that the total phenol content increased with increasing temperature [6].

Overall, it appears that phenolic compounds are sensitive to high temperatures, and their content can decrease with increasing temperature. However, the effect of high temperatures on phenolic compounds can vary depending on the specific compound and the plant source.

It is worth noting that the degradation of phenolic compounds can be minimized by using appropriate extraction methods and storage conditions. For example, pressurized liquid extraction (PLE) studies have shown that high phenolic content can be obtained at high temperatures [4]. Additionally, most phenolic substances are susceptible to heat-mediated degradation when temperatures higher than 40 °C are used during extraction or storage, thus an upper limit temperature must be observed to preserve its useful components [3].

In conclusion, while phenolic compounds are sensitive to high temperatures, the effect of high temperatures on these compounds can vary depending on the specific compound and the plant source. It is important to use appropriate extraction methods and storage conditions to minimize the degradation of these compounds. Those working with phenolic compounds in research and industry must factor in these sensitivities to maintain their properties and benefits during processing, storage, or cooking.