Trypsin inhibitors
Soybean contains trypsin inhibitors, which block the activity of the digestive enzyme trypsin. Average antitrypsic activity of whole soybeans was 39 TIU/mg, measured in France in 2003. However, it is very variable depending on the origin of the bean (from 3.4 to 71 TIU/mg) (Evrard et al., 2003). Heating destroys trypsin inhibitors, and soybeans fed to pigs or poultry usually undergo a process involving heating (see Processes in the "Description" tab). In heated soybeans, trypsin inhibitor activity is reduced to around 4mg/g (Clarke et al., 2007), compared to that of raw soybean, where trypsin inhibitor activity ranges from 20 to 35 mg/g. Other criteria used to characterize soybean seeds quality include urease activity (fall of pH) and protein solubility (see below).
Urease
Raw soybeans contain urease, an enzyme that releases ammonia from urea, and, therefore, should not be fed to ruminants in a diet already containing urea, as rumen microflora cannot handle the amount of NH3 produced (Newkirk, 2010; Fuller, 2004; Evrard et al., 2003). The recommended value for urease activity is below 0.3 pH unit (Clarke et al., 2007).
Protein solubility
The recommended value for protein solubility ranges from 70 to 85%. In over-cooked soybean seeds solubility is low and amino acid digestibility is reduced compared with under-cooked seeds, where solubility is high but protease inhibitors are not destroyed. The relationship between the concentration of protease inhibitors and amino acid digestibility has been described (Clarke et al., 2007). Up to 30% heated soybeans have been included in poultry diets with no evidence of depressed performance (Newkirk, 2010).
Lectins
Soybeans contain variable amounts of lectins. Mature seeds of Chinese varieties were reported to contain between 2.81 and 6.52 mg/g lectins (Chunmei Gu et al., 2010). The lectin content rises as the seed matures. Lectins are undetectable during the first 26 days after flowering but reach a maximum level by day 28 (Cornell University, 2012). Lectins bind to intestinal mucosa and prevent the absorption of amino acids, vitamin B12 and polysaccharides (Evrard et al., 2003). They are thought to be responsible for about 25% of the negative effects of feeding raw soybeans to poultry and pigs (Newkirk, 2010). Lectins can be destroyed by heat.
Oligosaccharides
Soybean seeds contain variable amounts (4-6%) of oligosaccharides (mainly raffinose and stachyose) that are not digested in the small intestine of monogastrics but are fermented by bacteria (Hayakawa et al., 1990). In pigs, levels as low as 2% oligosaccharides in the diet reduce energy digestibility, resulting in reduced growth (Grieshop et al., 2003). Oligosaccharides also reduce fecal consistency in weanling pigs (Liying et al., 2003).
Other antinutritional factors
Soybean seeds contain phytoestrogens, which reduce the reproductive efficiency of some animals, as well as goitrogens (iodine antagonists) (Fuller, 2004).
Rancidity
Soybean seeds contain phytoestrogens, which reduce the reproductive efficiency of some animals, as well as goitrogens (iodine antagonists) (Fuller, 2004).