Tannins
Grape berries and their by-products, particularly the seeds and the skins, contain important though variable amounts of phenolic compounds, particularly flavonoids, which include anthocyanins, flavonols, and flavan-3-ols (catechin, epicatechin) (Chedea et al., 2016). Condensed tannins (proanthocyanidins) are polymers of flavan-3-ols of different molecular weights (Ye ZhiJing et al., 2016). Grape pomace can contain as much as 20% of DM as condensed tannins (Molina-Alcaide et al., 2008). 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). Vinification methods influence the tannin content of grape pomace: particularly, higher fermentation temperatures and extended maceration times increase the release of phenolic compounds in the wine, and reduce their amount in the by-products (Ye ZhiJing et al., 2016). The high tannin content of grape pomaces is, together with fibre, one of the main reasons for their low digestibility and low nutritive value.
Flavonoids are recognized as antioxidants, with positive effects on the prevention of oxidative damage in tissues by the reduction of lipidic oxidation and/or blocking the production of free radicals (Letaief, 2016). For that reason, the inclusion of small amounts of grape pomace in the diets of ruminants, poultry, and pigs, is being investigated for its potential beneficial effects, both on animal health and on the quality of animal products. The effect of grape pomace tannins on the reduction of methane emissions by ruminants is also being studied (Moate et al., 2014).
Copper toxicity
There have been reports from Brazil of lethal intoxication of sheep fed grape pomace derived from grapevine treated with copper-based fungicides (Reis et al., 2015).
Pesticide residues
The grapevine is treated with a variety of pesticides. In Australia, an assessment of animal products derived from livestock fed grape pomace treated with registered pesticides found that the risk of residues was low with the majority of chemicals (MacLachlan, 2010).
Spoilage
Fresh grape pomace normally spoils readily. Depending on the conditions and on the nature of the product, it can remain edible for up to a week (Février et al., 2009), but it can also become inedible in less than 24 hours (Hentges et al., 1982).