Ⅰ Background
The key in designing feed for piglets is to ensure optimal intestinal health, reduce prevalence of diarrhea, and promote growth. This can be achieved through two principal measures, safeguarding the intestinal cell structure to prevent damage and regulating the intestinal flora balance to prevent disorder. One of the factors that undermine the integrity of the intestinal cell structure is the presence of toxic and harmful substances in the feed.
The primary feed protein source, such as soybean protein, contains various ANF with high content. Exceeding a certain proportion can cause destruction of intestinal cell structure and flora balance leading to prevalence of diarrhea, and reduction in animal’s digestibility and absorption that suppressing growth. Eliminating these ANF from soy protein, therefore, is one of the crucial strategies to diminish intestinal damage and guarantee intestinal health.
Ⅱ Content
1.Species, physicochemical properties and anti-nutritional mechanisms in soy protein
Soy protein raw materials are commonly utilized as an ingredient in animal feed at 10-20%. Nevertheless, soybeans possess diverse ANF that restrict the proportion of employing. Acquiring knowledge of the species, content, physicochemical properties, as well as the mechanisms of ANF in soybeans, we can take appropriate measures to decrease or eliminate them.
2. Effective ways to eliminate main ANF
Based on the physicochemical properties of the main ANF in soybeans, it is possible to apply physical (high temperature, high pressure), chemical (alcohol, acid) and biological (fermentation, enzymatic hydrolysis) techniques to eliminate or break down these factors to different extents.
2.1. Antigenic protein
Antigenic proteins consist mainly of glycinin and β-conglycinin, which have unique spatial conformations. So to eliminate allergenic subunits in the quaternary and tertiary structures, can we eliminating anti-nutritional impact.
High pressure or high temperature can alter the quaternary and tertiary spatial structure of the antigen protein, while enzymatic hydrolysis or fermentation can break down the quaternary and tertiary structure of the protein, and even degrade the secondary and primary structure.
Extruded soybeans and SPC are treated with high temperature and high pressure resulting in antigen protein clearance between 50-70%. Fermented soybean meal, on the other hand, exhibits an average antigen protein clearance of 50% due to the variation in bacterial strains and fermentation conditions. In combined liquid enzymatic hydrolysis, enzyme selection and the control of conditions are based on the protein’s structural characteristics, achieving an antigen protein clearance over 90%.
2.2. Trypsin inhibitor
There are approximately 7-10 types of trypsin inhibitors, but only two have extensive and detailed investigation: the Kunitz-type trypsin inhibitor (KTI) and the Bowman-Birk-type trypsin inhibitor (BBI). All of them possess active centres capable of binding to proteases, inhibiting activity, disrupting active centres, and reducing anti-nutritional effects.
High temperatures can alter the conformation of the trypsin inhibitor’s active center structure. Additionally, enzymatic hydrolysis or fermentation can modify the spatial structure and degrade the primary and secondary structures, ultimately damaging the active center.
The clearance rate for high temperatures ranges from 75-95%, whereas the fermentation clearance rate varies between 50-90% due to differences in bacterial species and conditions. On the other hand, the combined liquid proteolysis clearance rate is approximately 90%.
2.3. Oligosaccharides
Soybean oligosaccharides are formed by linking 2-10 monosaccharide molecules via glycosidic bonds. They are soluble sugars in soybeans, encompassing various forms such as sucrose (disaccharide), raffinose (trisaccharide), and stachyose (tetrasaccharide).
α-Galactoside, an oligosaccharide composed of one or two galactoses linked by α-1,6 glycosidic linkages (e.g. Raffinose is a non-reducing 3-saccharide formed from galactose, glucose and fructose. Stachyose is a non-reducing 4-saccharide formed from galactose, galactose, glucose and fructose, and verbascose, etc.). As animals lack enzymes to decompose α-galactosides in the small intestine, it can only be utilized by microorganisms in the large intestine. Studies have demonstrated that the optimal method for removing oligosaccharides involves the enzymatic hydrolysis process. By employing α-galactosidase to hydrolyze stachyose and raffinose, up to 70-80% of these compounds can be removed. (Shang, 2018; Pan, 2011)
Ⅲ Summary
The key components of ANF within soy protein are antigenic proteins, trypsin inhibitors, and oligosaccharides. To increase the application potential of soy protein in feed, it is necessary to lessen their content and thus decrease their anti-nutritional influence through processing.
In practical applications, high temperature, high pressure, fermentation, and enzymatic hydrolysis may remove antigenic proteins and trypsin inhibitors to varying extents. Oligosaccharides can be specifically removed through enzymatic hydrolysis.
Enzymatic hydrolysis has the capability to eliminate over 90% of ANF in soybean, thus decreasing the injury to animal intestines. For this reason, the utilization of enzymatically processed soy protein is one of the most favourable options for creating antibiotic-free feeds and ensuring the intestinal wellness of animals.