Changes in molecular structure of protein and carbohydrate in soybean products with different processing methods and their effects on nutrient degradation characteristics of the products

In this study, four types of soybean products with different processing methods (soybean meal 1 and soybean meal 2, extruded soybean meal, fermented soybean meal and extruded soybean) were used to examine the effect of fermentation and extrusion on molecular structures of protein and carbohydrate. Extrusion and fermentation significantly decreased (P < 0.05) the values of related protein spectral intensities (height and area of amide and secondary structure) and the biggest reduction was found in extruded soybean compared to soybean meal 1 and soybean meal 2. Compared with extruded soybean meal, the area ratio of amide I to amide II and the height ratio of α-helix to β sheet in extruded soybean were significantly reduced (P < 0.05), and there was no difference in these spectral values between extruded and fermented soybean. Extrusion and fermentation significantly decreased (P < 0.05) the values of carbohydrate spectral intensities, including structural carbohydrate (STCHO) and cellulosic compounds (CELC) and total carbohydrate (CHO), compared to soybean meal 2. The ratio of α-helix to β-sheet was positively related to the DM of soybean degradability in the rumen (P < 0.05, r = 0.590), so was A-CELC to A-STCHO (P < 0.05, r = 0.747). A positive relationship was found between CP degradability in the rumen and the area ratios of amide I and amide II, CELC to CHO, and STCHO to CHO. Spectral intensity of CHO area was negatively associated with neutral detergent fibre (NDF) and acid detergent fibre (ADF) degradability in the rumen. The study indicated that extrusion and fermentation could alter the molecular structure of protein and carbohydrate and the degradation characteristics of soybean products in the rumen.