Feed nutrients can only be absorbed after being fully digested by the animal, and digestion of protein is the key.
Reason 1. When protein is fully digested and absorbed, some of them provides nutrition for intestinal development, the others enters the body for growth and deposition.
Reason 2. If the protein cannot be fully digested and absorbed in the front section of small intestine, it will enter the rear section and be fermented by microorganisms to produce inorganic ammonia, organic amines, etc., resulting in an imbalance of the flora. In actual production, insufficient protein digestion and absorption is an important cause of diarrhea, slow growth, and low feed conversion rate. These problems will be more prominent in the context of AGP-free.
Therefore, improving the digestibility of protein in raw materials is one of the effective measures to help intestinal health. Usually, there are two ways to improve the digestibility of protein:
One is to increase the digestibility of the protein in feed, the other is to improve the animal's capacity to digest protein.
Today we’re going to talk on the topic of digestibility of feed protein raw materials.
The protein structure of raw materials is the main factor affecting digestibility.
① The disulfide bond, hydrogen bond, hydrophobicity and electrostatic interaction in the tertiary and quaternary structure of protein affect its digestibility.
② Protein hydrophilicity is the external embodiment of physical properties of protein tertiary and quaternary structure, which indirectly reflects the digestibility of protein raw materials. The hydrophilicity of protein mainly depends on the change on the surface of protein. When there are many charged amino acid on the surface of protein, the hydrophilicity is high; when the content of hydrophobic amino acid residues is in large amount, the hydrophilicity is poor. The water solubility of protein is positively correlated with in vitro digestibility (Wang Weiguo, 2002). In practical production, the digestibility of raw materials is usually judge by hydrophilicity and water solubility of raw materials.
③ The content of α- helix and β- turn, random coil and β- sheet affect the digestibility of protein.
④ The amount of amino acid in the primary structure of protein affects the digestibility of protein.
Table 1. Relationship between protein structure and digestibility
Measure to improve the digestibility of raw protein
In actual production, physical (crushing, heating, high pressure), chemical (acid-alkali hydrolysis), biological (enzymatic hydrolysis) and other processes are generally used to change the quaternary, tertiary, secondary and primary structure of protein raw materials to achieve the purpose of improving digestibility (Guo Weibo, 2019). The following introduces the effects of crushing, high temperature, high pressure, and enzymatic hydrolysis process on the effect of improving protein digestibility.
Crushing
Conventional crushing can increase the contact area between protein from raw material and digestive enzymes of animals. Therefore, the digestibility of protein can be improved by decreasing the particle size of raw materials. In vitro pepsin-trypsin digestion method is used for determination of digestibility in vitro. It was found that the in vitro protein digestibility increased with the decrease of particle size of raw materials. In general, when fineness increased from 60 mesh to 80 mesh, and the digestibility increased by 3%.
Table 2. In vitro digestibility of protein from different raw materials with different crushing particle size (%)
Heating
Heating can change the secondary structure of the protein. As described above, the α-helices content in the secondary structure of protein is positively correlated with digestibility in vitro. However, β-sheet content was negatively correlated with digestibility in vitro.
Studies show that α-helices first increase and then decrease with the increasing of temperature, while β- sheet first decrease and then increase. The data showed that when soy protein was heated at 75-80℃, the α-helices increased, β-sheet decreased, and in vitro digestibility increased gradually. When heated at 85-100℃, the α-helix decreased, the β-sheet increased, and in vitro digestibility increased gradually (Wang Zhong-jiang, 2017). This is due to the gradual denaturation of soybean protein with increasing temperature from heating. Protein molecular subunits are dissociated, and the ordered structure stretches and unfolds, increasing the exposure of enzymatic sites. At 85℃, protein hydrolysis degree was higher. Too high temperature causes excessive denaturation of protein and produces thermal aggregates with high molecular weight and low surface charge density, which is not conducive to protein digestion and enzymatic hydrolysis, and the degree of protein hydrolysis is reduced.
Excessive heating will also increase the cross-linking of disulfide bonds in the tertiary structure of proteins and increase the hydrophobic interaction between proteins, reducing the in vitro digestibility of proteins. Rocha (2002) also found that 85-135℃ heat treatment of soybean protein, as the temperature rises, the disulfide bond increases, the protein cross-links and aggregates, and the protein digestibility decreases. And heating at high temperature will cause Melard reaction between sugar and protein, the product will become darker and smelly, and protein digestibility will decrease.
In practical production, soybean meal and extruded soybeans are all heated, suggesting that the optimal heating temperature should be 80-85℃. But overall, the digestibility improving is limited, and improper heating may destroy heat-sensitive anti-nutritional factors, but reduces the digestibility.
Table 3 The effect of temperature on the secondary structure content of soybean protein (%)
High pressure
By compressing the volume of protein molecules, changing the secondary bonds of the tertiary and quaternary structure of the protein, broken the hydrogen bonds between the protein molecules, weaken the total hydrophobic force and decrease the ionic bonds, causing protein depolymerization, molecular structure extension, etc. to improving protein digestibility. Ultra-high pressure treatment does not affect the primary structure of the protein. When it is less than 150MPa, the quaternary structure of the protein is changed; when it is above 150MPa, the tertiary structure of the protein is changed; when it is above 200MPa, the secondary structure of the protein is changed due to the destruction of the hydrophobic effect and ionic interaction. In actual production, both extruded soybeans and SPC are treated with high pressure, suggesting that the optimum pressure should be 150-200MPa. But overall, the digestibility increases by 5-8%.
Table 4. Effect of high pressure on protein structure and digestibility
Acid and alkali treatment
Strong acid and alkali treatment can change the tertiary structure of protein (breaking hydrogen bond and expanding structure), but it will have an extremely adverse effect on the primary structure. For example, alkali hydrolysis reduces the content and proportion of Lysine and Arginine; acid hydrolysis destroys tryptophan. In practical production, the nutritional value of protein is seriously damaged by the treatment of strong acid and alkali.
Enzyme treatment
A. Microbial fermentation
Use extracellular enzymes, such as proteases produced by microbial metabolism to degrade the protein of raw materials. Take fermented soybean meal as an example, the protease produced by fermentation first destroys the spatial structure of the protein, and further decomposes part of the long-chain peptides in the primary structure into small peptides to improve the digestibility. The types of microorganisms used for fermentation, fermentation conditions, etc. directly affect the increasing level of protein digestibility. In practical production, the TCA soluble protein of some fermented soybean meal can reach 8-10%, which increased the digestibility by 5%. Improving the stability of each batch is the goal.
Table 5 TCA soluble protein content of different batches of fermented soybean meal (FSBM)
B. Enzymatic hydrolysis
Enzymatic hydrolysis can open the three-level and four-level spatial structure of the protein, expose the enzyme cleavage site, hydrolyze the peptide bond of the protein, and degrade the large molecule protein into small molecule peptides, thereby improving the digestibility of the protein. Enzymatic hydrolysis has the characteristics of high catalytic efficiency, strong specificity, and adjustable enzyme activity through environmental condition control, so as to achieve the stability of the process and products. In actual production, more and more protein materials such as soybean meal, wheat protein, yeast, fish meal, rice rolls, etc. are used for enzymatic hydrolysis to improve the digestibility of protein and release functional peptide components. In practical production, the TCA-soluble protein content of products with different levels of enzymatic hydrolysis of different raw materials is 15-40%, and the protein digestibility can be increased by 10-20%.
Table 6. Acid soluble protein content of different Enzymolysis protein materials
1. Fine crushing, heating, extruding, fermentation or hydrolysis process of a single raw material or compound feed can change the protein structure and improve the digestibility of the protein to different level.
2. From the above analysis, it can be seen that the liquid enzymolysis process is one of the effective measures to improve the digestibility, stability and efficiency of protein, and at the same time release the functional components from raw materials.
3. Hydrolyzed protein is the best choice to ensure the full digestion and absorption of protein, help on intestinal health under the background of AGP-free!
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