Relationship Between Molecular Structure Characteristics Of Feed Proteins And Protein In Vitro Digestibility And Solubility

Nutritional value of protein in feedstuff not only depends on its amino acid content and composition proportion, but also on its molecular structure characteristics. Therefore, the molecular structure characteristics of protein in feedstuff can be used as important information and criteria for evaluating the nutritional value of feed. This study, the molecular structure characteristics of the typical protein feedstuff(soybean meal, fish meal, distiller’s dried grains with solubles, corn gluten meal, feather meal and soybean products) were determined by Fourier transform infrared molecular spectroscopy(FTIR). Then, the solubility and in vitro digestibility of feed proteins were measured by(0.2%)KOH solution kjeldahl and pepsin-pancreatin two-step enzymatic methods, respectively. Statistic method is used to analyze the correlation between protein molecular structure characteristics and protein in vitro digestibility as well as solubility. This paper will provide more scientific evidence for further assessment of nutritional value of feed proteins.Experiment one: Relationship between Molecular Structure Characteristics of Feed Proteins and In Vitro Protein Digestibility and SolubilityTo address this question, we analyzed soybean meal, fish meal, corn DDGS(distiller’s dried grains with solubles), corn gluten meal, and feather meal. The protein molecular structures(amide I, amide II, α-helix, β-sheet and α-helix/β-sheet)were then quantified using Fourier transform infrared molecular spectroscopy(FTIR). The protein solubility and in vitro digestibility were analyzed by(0.2%)KOH solution kjeldahl and pepsin-pancreatin two-step enzymatic methods,respectively.The results showed that: 1. Height and area of amide I had a significantly positive correlation with protein solubility and digestibility in vitro(P<0.001); height and area of amide II had a significantly positive correlation with protein solubility(P=0.003) and in vitro digestibility(P<0.05); the relatively quantitative amounts ofα-helices, random coils, and α-helix to β-sheet ratio in the protein secondary structures were positively correlated with in vitro digestibility and solubility(P<0.004). On the other hand, the percentage of β-sheet structures was negatively correlated with in vitro digestibility and solubility(P < 0.002). 2. The results of cluster analysis(CLA) showed that: the molecular structure characteristics of amide I and amide II in the infrared spectral range 1 720 to 1 479 cm-1 can be used as an important basis to distinguish feed proteins. 3. Regression analysis based on the protein molecular structure characteristics to predict in vitro digestibility and solubility provides a mathematical model for evaluation of protein nutrition.Experiment two: Effect of Different Heat-treatments on Protein Molecular Structure Characteristics, Protein Solubility and in Vitro Digestibility in Full-fat SoybeanIn this study, full-fat soybean as a feed protein was treated with moist process(autoclaved(0.1MPa) at 120 °C for 7.5 min) and dry process(heated at 120 °C for15 min), and extruded fat-full soybean without heat treatment. The protein molecular structures(amide I, amide II, α-helix, β-sheet and α-helix/β-sheet) were then quantified using Fourier transform infrared molecular spectroscopy(FTIR). The protein solubility and in vitro digestibility were analyzed by(0.2%)KOH solution kjeldahl and pepsin-pancreatin two-step enzymatic methods, respectively.The results showed that: 1. Different heat treatments significantly affected protein molecular structure characteristics, solubility and in vitro digestibility of three kinds of soybean samples. The most significant effect was observed on the extruded soybean. 2. Height and area of amid I and amide II had a significantly positive correlation with protein solubility and digestibility in vitro(P<0.05); The contents of α-helix and ratio of α-helix to β-sheet had the most significantly positive correlation with the protein solubility and digestibility in vitro(P<0.001). Whereas the contents of β-sheet had a least significantly negative correlation with the protein solubility and digestibility in vitro(P=0.002). 3. A significantly positive correlation was found between protein solubility and digestibility in vitro(P<0.001).In conclusion, novel data were obtained concerning molecular structure characteristics of feed protein in this study. We found that all measured molecular structure characteristics of feed proteins were correlated with in vitro digestibility and solubility. The α-helix to β-sheet ratio can be used to predict the nutritional value of feed proteins.