Soybean products
Soybeans are the most widely used protein in the world and is the primary protein source in most swine diets. Soybean products used in swine diets include soybean meal, full-fat soybeans, fermented soybean meal, enzyme-treated soybean meal, soy protein concentrate, and soy protein isolate.
Soybeans contain anti-nutritional factors that reduce nutrient utilization, most notably trypsin inhibitors. The trypsin inhibitors have to be inactivated by heating or toasting soybeans prior to use in swine diets. Raw soybeans are not recommended for use as such in swine diets. Moreover, pigs have a transitory hypersensitivity reaction to soybean meal induced by allergenic proteins, namely glycinin and β-conglycinin, and indigestible carbohydrates of soybeans. Pigs experience a transitory period of poor nutrient absorption and low growth performance following the first exposure to a diet with high amounts of soybean meal (Li et al., 1990). To alleviate this problem, pigs should be gradually acclimated to diets with increasing amounts soybean meal after weaning. Furthermore, soybean meal can be further processed to remove the allergenic compounds and improve the utilization of soy proteins by weanling pigs (Jones et al., 2010).
Soybean meal
Soybean meal is the standard protein source in swine diets and is used as the reference ingredient for protein quality. The amino acid profile, balance, and digestibility in soybean meal is better than any other plant protein source used in swine diets. Soybean meal is produced from hulled or dehulled soybeans. Dehulled soybean meal is often referred as high-protein soybean meal and contains approximately 48% crude protein, whereas hulled soybean meal contains approximately 44% crude protein and is referred as low-protein soybean meal (NRC, 2012). Processing methods to extract oil from soybeans include expelling and solvent extraction. In the expelling method, oil is mechanically extracted from soybeans after an extrusion process is used to inactive trypsin inhibitors. In the solvent extraction method, oil is extracted using a solvent and then a toasting process is used to inactive trypsin inhibitors. The expelled soybean meal contains higher oil content than solvent-extracted soybean meal because mechanical extraction is less efficient in de-oiling soybeans.
Full-fat soybean
Full-fat soybean are produced by avoiding the oil extraction process from soybeans. Thus, full-fat soybeans are a good source of both protein and energy. Full-fat soybeans have an oil content of approximately 15% and protein content of 35-45% (NRC, 2012). The inclusion of full-fat soybeans is also a means of providing oil to the diet but it must be greater than soybean meal to maintain a similar protein and amino acid level in the diet.
Fermented or enzyme-treated soybean meal
Soybean meal can be further processed by microbial fermentation or enzymatic treatment to remove the allergenic proteins and indigestible carbohydrates that induce a hypersensitivity reaction in pigs (Stein et al., 2016). Fermented soybean meal and enzyme-treated soybean meal have greater concentration of amino acids than conventional soybean meal, but the digestibility is similar (Cervantes-Pahm and Stein, 2010). In general, specialty soy protein products provide an excellent opportunity to using plant protein sources in diets for weanling pigs.
Soy protein concentrate and isolate
The allergenic proteins and indigestible carbohydrates that are harmful to pigs are mostly removed during production of soy protein concentrate and soy protein isolate (Stein et al., 2016). Soy protein concentrate must contain at least 65% crude protein and is produced by removing the water-soluble sugars, minerals, and other minor constituents from de-oiled soybean meal. Soy protein isolate is the most concentrated soy protein source and must contain at least 90% crude protein. It is produced by protein isolation and removal of all sugars, fat, and minerals from de-oiled soybean meal. The digestibility of amino acids is high in soy protein concentrate and isolate, which makes excellent sources of protein to use in nursery diets (Cervantes-Pahm and Stein, 2010).