Tannins
Grape seeds contain important amounts of phenolic compounds (5-8% polyphenols), notably flavonoids (Chedea et al., 2016; Ye ZhiJing et al., 2016). The most abundant flavonoids in grape seeds are gallic acid, catechin, epigallocatechin gallate, epigallocatechin, epicatechin gallate, epicatechin, and proanthocyanidins (polymers of flavonoids, also called condensed tannins) (Gessner et al., 2015). These compounds form complexes with proteins in the feed and with digestive enzymes leading to disruption of the digestion process and to loss of nutrients. They also interfere with mineral absorption, causing damage to the mucosal lining of the gastrointestinal tract (Bekhit et al., 2016). The high tannin content of grape seeds and grape seed oil meal is, along with fibre, one of the main contributors to their low digestibility and to their generally low nutritive value. The catechins of grape seeds are largely responsible for their bitterness (Letaief, 2016).
Flavonoids are recognized as antioxidants with positive effects on the prevention of oxidative damage in tissues by the reduction of lipid oxidation and/or by blocking the production of free radicals (Letaief, 2016). For that reason, grape seeds and grape seed oil meal are being investigated for their potential beneficial effects, both on animal health and on the quality of animal products. Grape seed extracts are produced and marketed for a broad spectrum of pharmacological activities.