Evaluating Nutritional Quality of Single Stage- and Two Stage-fermented Soybean Meal

This study investigated the nutritional quality of soybean meal (SBM) fermented by Aspergillus ($FSBM_A$) and/or followed by Lactobacillus fermentation ($FSBM_{A+L}$). Both fermented products significantly improved protein utilization of SBM with higher trichloroacetic acid (TCA) soluble true protein content, in vitro protein digestibility and available lysine content, especially in $FSBM_{A+L}$. Moreover, $FSBM_{A+L}$ produced a huge amount of lactic acid resulting in lower pH as compared to the unfermented SBM or soybean protein concentrate (SPC) (p<0.05). $FSBM_A$ and $FSBM_{A+L}$ raised 4.14% and 9.04% of essential amino acids and 5.38% and 9.37% of non-essential amino acids content, respectively. The ${\alpha}$-galactoside linkage oligosaccharides such as raffinose and stachyose content in $FSBM_A$ and $FSBM_{A+L}$ decreased significantly. The results of soluble protein fractions and distribution showed that the ratio of small protein fractions (<16 kDa) were 42.6% and 63.5% for $FSBM_A$ and $FSBM_{A+L}$, respectively, as compared to 7.2% for SBM, where the ratio of large size fractions (>55 kDa, mainly ${\beta}$-conglycinin) decreased to 9.4%, 5.4% and increased to 38.8%, respectively. There were no significant differences in ileal protein digestibility regardless of treatment groups. SPC inclusion in the diet showed a better protein digestibility than the SBM diet. In summary, soybean meal fermented by Aspergillus, especially through the consequent Lactobacillus fermentation, could increase the nutritional value as compared with unfermented SBM and is compatible with SPC.