Effect of temperature on the free radical scavenging capacity of extracts from red and white grape pomace peels

Free radical scavenging capacity of the extracts from red and white grape pomace peels (RGPP, WGPP) and as a reference for two commonly used antioxidants (BHA (synthetic) and DL-alpha-tocopherol (natural) heated at processing temperatures (80, 100, and 120 degrees C) was evaluated using the 2,2-diphenyl-1-picrylhidrazyl (DPPH.) reagent. Kinetic behavior of the sample extracts did not change when they were heated, following a general multiplicative model in which remaining DPPH. was more affected by temperature than the time needed to reach the steady state. The reduction of this property in the sample extracts heated at 120 degrees C as compared to those at 20 degrees C followed an increasing order: BHA(15.3%) < WGPP (22.9%) < RGPP (28.3%) DL-alpha-tocopherol (69.8%). Color losses were also measured by a tristimulus colorimeter HunterLab and two parameters were used to express the sample extract color: hue angle (tan b*/a*)(-1) and a*/b* ratio. The higher red color losses in RGPP extracts (12.9 times in hue angle) as compared to the yellow color losses in the WGPP extracts (1.5 times in a*/b* ratio) during heating may explain their differences in the free radical scavenging capacity.