Tannins
Tomato pomace and skins contain tannins, which may limit their use for monogastrics (Kavitha et al., 2005).
Tomatine
Unripe tomatoes and the green parts of ripe tomatoes contain tomatine, a solanine-like alkaloid (saponin) that may be toxic to insects, dogs, and to a lesser extent to herbivores, causing diarrhoea, vomiting and intestinal irritation. However, tomatine disappears as the tomato ripens and is not a problem in tomato pomace (Milner et al., 2011; Straney, 1998). Tomatine may have medicinal properties such as antibiotic, anticancer, anti-cholesterol, anti-inflammatory, antinociceptive and antipyretic effects (Milner et al., 2011).
Pesticide residues
Tomato by-products may be contaminated with chemicals such as insecticides, weed killer, etc. (Campos et al., 2007). The skins are directly exposed to these chemicals and pomace is thus prone to contamination. Maximum residue limits for food and feed are fully described in the Codex Alimentarius (Codex Alimentarius, 2011).