Effect of Temperatures on Polyphenols during Extraction

Background: Polyphenols are a set of bioactive compounds commonly found in plants. These compounds are of great interest, as they have shown high antioxidant power and are correlated to many health benefits. Hence, traditional methods of extraction such as solvent extraction, Soxhlet extraction and novel extraction technologies such as ultrasound-assisted extraction and subcritical water extraction (SWE) have been investigated for the extraction of polyphenols. Scope and Approach: Generally, for traditional extractions, the total phenolic content (TPC) is highest at an extraction temperature of 60-80 degrees C. For this reason, polyphenols are regularly regarded as heat-labile compounds. However, in many studies that investigated the optimal temperature for subcritical water extraction (SWE), temperatures as high as 100-200 degrees C have been reported. These SWE extractions showed extremely high yields and antioxidant capacities at these temperatures. This paper aimed to examine the relevant literature to identify and understand the mechanisms behind this discrepancy. Results: Thermal degradation is the most common explanation for the degradation of polyphenols. This may be the case for specific or sub-groups of phenolic acids. The different extraction temperatures may have also impacted the types of polyphenols extracted. At high extraction temperatures, the formation of new compounds known as Maillard reaction products may also influence the extracted polyphenols. The selection of source material for extraction, i.e., the plant matrix, and the effect of extraction conditions, i.e., oxidation and light exposure, are also discussed. The overestimation of total phenolic content by the Folin-Ciocalteu assay is also discussed. There is also a lack of consensus in TPC's correlation to antioxidant activity.

Automated summary

Polyphenols, bioactive compounds commonly found in plants, have high antioxidant power and are correlated to many health benefits. Traditional and novel extraction methods have been investigated for the extraction of polyphenols. The optimal extraction temperature for subcritical water extraction (SWE) can reach as high as 100-200 degrees C, leading to high yields and antioxidant capacities. The main factors affecting the extraction of polyphenols include thermal degradation, formation of Maillard reaction products, selection of plant matrix, and extraction conditions. The correlation between total phenolic content (TPC) and antioxidant activity lacks consensus.