Thermal stability of major classes of polyphenols in skins, seeds and stems of grape pomace

Grape pomace (GP) is a by-product of wine industry, in which remains significant amount of polyphenols. The freshly pressed pomace is perishable and has to be dried for use at later time. Depending on the drying methods, significant loss in polyphenols may occur during drying. This chapter describes the impacts of dehydration methods such as room temperature drying and vacuum drying on the retention of polyphenols in the pomace of Cabernet Sauvignon and Muscadine Nobel grapes using freeze dried sample as control. Seeds, skins and stems were separated manually after drying and the effects of drying methods on the stability of GP polyphenols were evaluated by the retentions of total polyphenol (TP), total anthocyanins (TA) and total flavonoids (TF). Results show that seeds and stems from both grape varieties were rich in TP and TF but low in TA, while skins were rich in anthocynin. Compared to freeze drying, both room temperature and vacuum oven drying methods resulted in significant loss of TP, TA and TF in different parts of Muscadine pomace. Among different groups of polyphenols, anthocyanin was affected the most by drying methods. Flavonoids in Cabernet pomace exhibited higher heat stability than that in Muscadine pomace indicating the differences in flavonoid composition between the two grape varieties. Therefore, the polyphenol composition and stability of GP are variety dependent. The selection of drying method needs to consider what type of polyphenol to be preserved, variety of GP and the cost of drying method.