Antitryptic activity
Trypsin inhibitors are the main antinutritional factor in peas, although peas are one of the grain legumes with the least trypsin inhibitor content, usually lower than 2% of the protein content (raw soybeans contain 8 times this amount). There are large varietal differences in antitryptic factors. The trypsin inhibiting activity of 33 European spring pea varieties ranged from 1.69 to 7.56 trypsin inhibiting units (TIU), while the level in winter peas was 7.34-11.24 TIU (Leterme et al., 1998). Smooth peas contain more trypsin inhibitors than wrinkled peas (Leterme et al., 1989; Perrot, 1995).
Trypsin inhibitors bind with trypsin in the small intestine, preventing protein digestion. They also induce pancreatic enlargement and increase protein secretion, causing lower N retention, lower growth and lower feed efficiency in monogastric species, including pre-ruminant animals (Perrot, 1995; Rackis et al., 1986). In ruminants, trypsin inhibitors are degraded in the rumen and are not a concern (Fuller, 2004).
Tannins
Tannins are known to reduce protein digestibility in monogastrics because they bind with protein prior to their digestion. Tannin content is related to the seed color, grains with dark seed coats containing more tannins (Myer et al., 2001). Tannin content is also much lower in white flowered peas than in coloured flowered peas (Grosjean et al., 1986; Canbolat et al., 2007; Prolea, 2008).
Lectins
Lectins are proteins able to bind glycoproteins and carbohydrates. They act in the small intestine by interfering in the absorption of the end-products of digestion by binding and disrupting the epithelial cells (Dixon et al., 1992). They represent about 2.5% of pea protein (Perrot, 1995).
Improving the nutritive value of peas by decreasing trypsin inhibitors and tannins is the goal of many breeding programs (Gatel, 1995). Modern cultivars of "protein peas" are tannin-free and have low concentrations of trypsin inhibitors, which makes them particularly suitable for animal feeding, even in the unprocessed form for monogastrics (Mihailovic et al., 2005).