Analysis of raw protein structure and its influence on protein digestibility
The price of raw materials continues to rise, and feed companies generally use the methods of reducing protein to reduce the amount of protein raw materials, or using miscellaneous meal to replace soybean meal to control costs. The key to low-protein diet design is to improve protein digestibility, which is related to the digestibility of raw protein and the digestibility of the animal itself. When selecting protein raw materials, we need to understand the structural properties of raw protein, so as to effectively select protein raw materials with high digestibility.
In this issue, the editor will focus on the series of articles on "Ingredient Analysis" to decipher the structural characteristics of different raw materials and their impact on digestibility, so as to help you design low-protein diets, reduce formula costs, and ensure feed quality.
1.Classification and characteristics of proteins
Proteins can be divided into simple proteins (including albumin, globulin, histone, gliadin, glutenin, hard protein) and binding proteins (including nucleoprotein, glycoprotein, lipoprotein, metalloprotein, phosphoprotein) according to their chemical composition .
The physicochemical properties of simple proteins are as follows:
A. Albumin (albumin): small molecular weight, soluble in water, neutral salts, dilute acids and alkalis, and can be precipitated by saturated ammonium sulfate. Albumin is widely distributed in nature, such as albumin in wheat, serum albumin in blood and ovalbumin in eggs.
B. Globulin: Generally insoluble in water, soluble in dilute salts, dilute acids and dilute alkalis, and can be precipitated by half-saturated ammonium sulfate. Legumin in soybeans, myosin in muscle, hemoglobin in blood, and immunoglobulin in blood fall into this category.
C. Prolamin: insoluble in water and dilute salt solution, soluble in 70-80% ethanol. Mostly found in the seeds of grasses, such as zein and wheat gliadin.
D. Gluten: insoluble in water and dilute salt solution, soluble in dilute acid and dilute alkali. Such as rice glutenin in rice and glutenin in wheat.
E. Hard protein: insoluble in water, dilute salts, dilute acids and dilute alkalis. Hard protein mainly exists in skin, hair, nails, such as keratin, collagen, elastin, silk protein, etc.
2. Characteristics of protein in common raw materials in feed
(1) Protein raw materials
The first type of raw material that provides protein in feed is protein raw material, including vegetable protein, animal protein, and microbial protein. Vegetable protein such as soybean (soybean meal, etc.), rapeseed meal, cotton meal, etc., animal protein such as meat meal (fish meal, chicken meal, pork meal, etc.), plasma meal, milk powder, egg meal, etc., microbial protein such as yeast protein.
Plant proteins are mainly globulins, with compact tertiary and quaternary structures, high content of β-bend and randomcoils in secondary structures, low protein solubility and low digestibility. Animal protein is mainly myofibrillar protein and sarcoplasmic protein, with low tertiary and quaternary structure density, high protein solubility and high digestibility.
Table 1 Protein structural characteristics of different protein raw materials
[Source] Wang Ruihong, 2016; Zhao Dongdong, 2009; Guo Yuanyuan, 2011
(2) Energy raw materials
The second type of raw material that provides protein in feed is energy raw material. Due to the large amount of energy raw materials, the impact of the protein provided by these raw materials on the digestibility of the whole feed cannot be ignored. Feed commonly used energy raw materials such as corn, wheat (flour), broken rice, barley, sorghum and so on. The protein types in energy raw materials mainly include gliadin, glutenin, globulin, and albumin. The proportion of protein types contained in different raw materials is different, which determines the difference in protein digestibility.
Corn and sorghum have high prolamin content and low protein digestibility; broken rice, wheat and barley have high gluten and albumin content and high protein digestibility.
Table 2 Protein structural characteristics of different energy raw materials
[Material source] Jia Xiangxiang, 2011; Zhang Fei, 2012; Li Ming, 2006
3. The effect of protein structure on protein digestibility
Pulverization fineness of protein raw materials: The finer the pulverization particle size, the higher the contact area with animal digestive enzymes, and the higher the digestibility.
Protein tertiary and quaternary structure: disulfide bonds, hydrogen bonds, hydrophobicity, and electrostatic interactions affect protein digestibility.
Protein hydrophilicity is the external manifestation of the physical properties of protein tertiary and quaternary structures. In actual production, the hydrophilicity and water solubility of raw materials are usually used to indirectly judge the digestibility of raw materials.
The hydrophilicity of a protein mainly depends on the amount of charge on the surface of the protein molecule. If there are more charged amino acids on the surface of the molecule, the hydrophilicity will be high; if the content of hydrophobic amino acid residues is large, the hydrophilicity will be poor. Protein hydrophilicity is positively correlated with in vitro digestibility (Wang Weiguo, 2002).
Protein secondary structure: α-helix, β-strand, random coil, β-strand content affects protein digestibility.
Protein primary structure: The number of amino acids affects protein digestibility.
Table 3 Relationship between protein structure and in vitro digestibility
[Material source] Zhou Genlai, 2017; Wang Zhongjiang, 2017
[Summarize]
The protein structure in the raw material directly affects the digestibility of the protein. Animal protein, the commonly used protein raw material for feed, is better than vegetable protein, and rice and flour as energy raw materials are better than corn and sorghum.
In actual production, crushing, high temperature, high pressure, fermentation and enzymatic hydrolysis can change the structure of protein to varying degrees, thereby improving the digestibility of protein. Among them, deep enzymatic hydrolysis can reduce the primary structure of protein and significantly improve the digestibility of protein.
(For in vitro digestibility data of raw materials from different sources and different processes, please refer to "Analysis of Raw Materials (1) Quality Analysis of Feed Protein Raw Materials for Piglets")
The fourth phase of "Analysis of raw materials (3)" will continue to decipher the structural analysis of antigenic proteins that affect the digestive ability of animals and effective measures to remove antigens, so stay tuned!
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