Effect of drying temperature on the stability of polyphenols and antioxidant activity of red grape pomace peels

The effect of drying temperature (60, 100, and 140 degrees C) on the polyphenols' content and antioxidant activity of red grape pomace peels was studied. Freeze-dried samples were used as reference. Differences on the CIE-LAB color, total extractable polyphenols, condensed tannins, UV-vis spectra, and antioxidant activity were evaluated. When drying temperature was 100 and 140 degrees C, a significant reduction in both total extractable polyphenols (18.6 and 32.6%) and condensed tannins (11.1 and 16.6%) was observed, as well as a decrease of 28 and 50% in the antioxidant activity of the samples, respectively. Hue angle and total color difference in the sample dried at 140 degrees C were significantly higher than in the freeze-dried reference material. A red color loss at 140 degrees C was also confirmed by lower absorbance values in the spectra at 525 nm. Drying at 60 degrees C did not significantly affect the sample characteristics evaluated.