Predicting the standardized ileal protein digestibility of processed soybean meal and rapeseed meal in growing pigs using two in vitro methods

A study was conducted to compare protein digestibility of processed ingredients using 2 in vitro methods with known standardized ileal digestibility of CP (SIDCP) measured in growing pigs. The SIDCP in soybean meal (SBM), rapeseed meal (RSM), and both ingredients retoasted in the presence of lignosulfonate, resulting in processed SBM (pSBM) and processed RSM (pRSM), was determined in a trial with growing pigs surgically fitted with a steered ileocecal valve cannula. Toasting in the presence of lignosulfonate was performed to induce protein damage. Initial pH and degree of hydrolysis after 10 min (DH10) and 120 min (DH120) were determined using the pH-STAT method. Hydrolysis was performed using trypsin, chymotrypsin, and peptidase at pH 8. Size-exclusion profiles of the resulting peptides after hydrolysis were also determined. Crude protein digestibility was determined using a 2-step enzymatic method, with pepsin at pH 2 and pancreatin at pH 6.8. The SIDCP in SBM, pSBM, RSM, and pRSM were 83.9, 71.6, 74.9, and 64.6%, respectively. Initial pH of ingredient solutions measured at constant N concentration was 6.9, 5.9, 6.1, and 5.5, respectively, and was highly positively correlated to SIDCP (r = 0.99, P <0.01). The DH10 using the pH-STAT method was 10.8, 7.3, 8.7, and 7.0%, respectively, and was positively correlated to SIDCP (r = 0.95, P = 0.046). There was no correlation between DH120 and SIDCP. Similarly to the SIDCP, the size distribution of peptides in the 120-min hydrolysates were affected (P <0.001) by the type of ingredient and processing. Digestibility of CP with the 2-step enzymatic method was 89.6, 83.4, 78.9, and 68.8% for SBM, pSBM, RSM, and pRSM, respectively, and tended to be correlated to SIDCP (r = 0.91, P = 0.092). In conclusion, both in vitro methods gave correlations similar to SIDCP, indicating that both might be used as indication for the SIDCP of thermally processed SBM and RSM in growing pigs.