Dietary fibre content and antioxidant activity of Manto Negro red grape (Vitis vinifera): pomace and stem

The general composition of two by-products of the vinification process of the Manto Negro red grape (Vitis vinifera) variety, namely pomace and stem, were determined. Both by-products present high contents of total dietary fibre (TDF), comprising three fourths of the total dry matter. The pomace had high protein (12.2%) and oil (13.5%) values and the stem large amounts of extractable polyphenols (11.6%). Due to the high fibre content, the soluble dietary fibre (SDF), insoluble dietary fibre (IDF), uronic acids (UA) and Klason lignin (KL) were analysed in both samples. Notable were the high percentage of soluble fibre (15%) in relation to the total dietary fibre for the pomace, as well as the high content of Klason lignin (KL) in both by-products, especially in the stem (31.6%). This fraction (KL) has important amounts of condensed tannins (CT) and resistant protein (RP).The free radical-scavenging capacity of the former by-products was determined using the DPPH method, resulting in EC50 values of 0.46 mg dm/mg DPPH (stem) and 1.41 mg dm/mg DPPH (pomace). Thus, both by-products of the vinification process of this variety of red grape (Manto Negro), particularly the stem, present excellent antioxidant properties as free radical-scavengers, clearly superior to those of other products described in the literature.