# Effect of Temperatures on Polyphenols during Extraction
## 4 Effects of Extraction Temperature on the Profiles of Polyphenols Extracted  
Many studies have found that the extraction temperature significantly impacts the type of polyphenols extracted since various polyphenols degrade at different temperatures [36; 39; 41; 47; 55; 58; 62; 73; 74; 75].  
[PERSON] et al. [47] performed pressurised liquid extraction (PLE) on dried thyme and studied the effect of temperatures between 50 and 200 \({}^{\circ}\)C and the profile of polyphenols being extracted at each temperature using HPLC. The results showed that temperature had a significant effect on the polyphenol subclasses (Figure 1, Table 2). Hydroxyphenylpronanoic acid (HPPA) concentration increased by almost three times when the extraction temperature was 200 \({}^{\circ}\)C, while 100 \({}^{\circ}\)C was optimal for hydroxycinnamic acids, flavones, flavonoids and total polyphenols. Additionally, higher extraction temperatures showed less diversity in the types of polyphenols extracted.  
Figure 1: Impact of temperature on polyphenols and subclasses. Extraction temperatures: (\(\Diamond\)) 50 \({}^{\circ}\)C; (\(\blacksquare\)) 100 \({}^{\circ}\)C; (\(\blacktriangle\)) 150 \({}^{\circ}\)C; (\(\times\)) 200 \({}^{\circ}\)C. Reprinted with permission from ([PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON]; [PERSON] Effects of temperature and time on polyphenonic content and antioxidant activity in the pressurized hot water extraction of deodorized thyme (_Thymus vulgaris_). J. Agric. Food Chem. 2012, 60, 10920-10929, doi:10.1021/jf3027759.). Copyright (2012) American Chemical Society.  
[PERSON] et al. [37] performed a sequential extraction at 100, 150 and 200 \({}^{\circ}\)C and analysed the extracts using HPLC; they found that polar phenolic compounds were extracted at low temperatures while less polar phenolics were extracted at higher temperatures. The polarity of water was reduced at higher temperatures, allowing it to solvate the nonpolar compounds and extract them. As a result, depending on the extraction temperature, phenolic compounds can be extracted with high selectivity. [PERSON] et al. [45] studied the extraction of polyphenols from rosemary, oregano and marjoram and found that TPC increased as the temperature increased from 66 \({}^{\circ}\)C to 200 \({}^{\circ}\)C. However, at temperatures above 150 \({}^{\circ}\)C, rosmarinic acid and carnosic acid decreases (Table 2). As the AOA remained very high, the authors suggested that rosmarinic and carnosic acid broke down to compounds with higher antioxidant power.  
[PERSON] et al. [36] performed a PLE for polyphenols from grape peels and seeds. For grape seed extracts, compounds were detected at 150 \({}^{\circ}\)C that were not present at 50 \({}^{\circ}\)C and 100 \({}^{\circ}\)C. [PERSON] et al. [73] extracted polyphenols from parsley, buckthorn, mint, caraway and birch and also performed acid hydrolysis (HCL in 50% aqueous methanol heated to 90 \({}^{\circ}\)C for 2 h) and compared the results to unhydrolysed samples. With the assumption that hydrolysis of the phenols occurs at high temperatures, we can compare the results of this study to other papers performing extractions at high temperatures. The results show that depending on the plant, many polyphenols appear or disappear after hydrolysis. Table 2 below summarises studies that showed a shifting profile of phenolics depending on extraction conditions.