Effect of Temperatures on Polyphenols during Extraction

Abstract

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 °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 °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–Ciocâlteu assay is also discussed. There is also a lack of consensus in TPC’s correlation to antioxidant activity.

Evaluation of Microwave-Assisted Extraction as a Potential Green Technology for the Isolation of Bioactive Compounds from Saffron (Crocus sativus L.) Floral By-Products

“…This information was in accordance with previous studies of natural compounds that have investigated the effect of drying temperatures in the antioxidant activity of grape pomace peels extracts. The authors have found that the antioxidant activity at 120 °C was lower (1.7 times) than that obtained at 20 °C [ 27 , 28 ].…”

“…This antioxidant power would be due to their bioactive content, such as polyphenols, since several research have reported that spices and herbs, with high phenolic content, are excellent sources of natural antioxidant compounds [ 30 ]. Nevertheless, the relationship between the structure of polyphenols and their antioxidant power is not yet well known [ 28 ]. Therefore, saffron floral by-products could be a source of natural antioxidants resulting from the saffron spice handling and processing and can be used to develop nutraceutical or functional food products with positive effects in human health such as the protection against oxidative stress [ 31 ].…”

Quantitative Structure-Property Relationship (QSPR) of Plant Phenolic Compounds in Rapeseed Oil and Comparison of Antioxidant Measurement Methods

“…Since the temperature in the P-DSC was much higher than in the Rancimat method, the values obtained were lower. It is known that temperature in particular can have a decisive impact on the stability of phenolic substances and can result in a decrease in antioxidative effect [83]. In some cases, a decrease in antioxidant activity was found even at temperatures of 40 and 60 °C, while in other studies, a decrease was found only at higher temperatures of 100 and 120 °C [83][84][85][86][87][88].…”

“…It is known that temperature in particular can have a decisive impact on the stability of phenolic substances and can result in a decrease in antioxidative effect [83]. In some cases, a decrease in antioxidant activity was found even at temperatures of 40 and 60 °C, while in other studies, a decrease was found only at higher temperatures of 100 and 120 °C [83][84][85][86][87][88]. Thereby, the type of phenolic compounds determines the influence of the temperature.…”

“…Thereby, the type of phenolic compounds determines the influence of the temperature. Flavonoids are considered to be particularly temperature-sensitive, and in particular, the subgroups of flavones, flavanols, and flavanones are known to be damaged even at lower temperatures [83,85,[89][90][91][92][93][94][95]. Accordingly, the numerical values of the two measurement methods are not comparable, and therefore, only the sequences of individual substances will be discussed in the following.…”