Effect of Grape Variety and Storage Conditions on Polyphenols Stability and Antioxidant Capacities of Grape Seed Extracts.

The utilization of grape seeds, a by-product of winemaking, and the production of valuable bioactive products from them is a very promising concept in the field of industrial waste management. The aim of this study was to investigate the effect of grape variety and storage conditions on the polyphenol stability and antioxidant capacities of grape seed extracts over a long period of time (10 months). The separated seeds from the pomace of the red grape varieties Cabernet Sauvignon and Cabernet Franc were dried at 23 °C for 10 days and at 40 °C and 60 °C for 24 h. The polyphenol extraction from all types of dried seeds was carried out under the same conditions: 70% aqueous ethanol, magnetic stirring at 600 rpm, for 3 h. Seeds dried at 40 °C yielded extracts with the highest total phenolic content. The extracts obtained from them were prepared in three physical forms: liquid concentrated form, lyophilized form and dried form at 40 °C. All the obtained extract forms were stored at 4 °C and 23 °C for 10 months. The effects of grape variety, extract physical forms and extract storage conditions on the total phenolic content, total flavonoids, procyanidins and antioxidant capacity of the extracts were studied. All parameters were determined at 0, 3, 6 and 10 months. It was found that the two types of dried extracts showed the highest stability of polyphenols at 4 °C for 10 months. The loss of total phenolic content of lyophilized Cabernet Franc GSE under these conditions was only 8.17%. All the physical forms of Cabernet Franc GSE exhibited lower losses of antioxidant activities compared to those of the Cabernet Sauvignon GSE. It was found that the reduction of antioxidant activity followed first-order kinetics. The individual compounds of the liquid and lyophilized Cabernet Franc and Cabernet Sauvignon seed extracts stored at 4 °C and 23 °C for 0 and 10 months were determined by HPLC, and their percentage loss was calculated. After 10 months of storage, the greatest degradation losses were recorded for gallic acid, procyanidin C1 and epicatechin, followed by procyanidin B3. Among the analyzed polyphenolic compounds, procyanidin B1 demonstrated the highest stability, with a loss of only 0.6%, followed by catechin and procyanidin B2, with losses of 4.02% and 2.35%, respectively. The effect of differences in the chemical composition of the two grape varieties on the stability of polyphenols and antioxidant activity of the extracts was also evaluated.