In-vitro determination of nitrogen digestibility and lysine availability in meat and bone meals and comparison with in-vivo ileal digestibility estimates

The true ileal digestibility of nitrogen was determined in 20 meat and bone meal samples using 120 growing rats. Correction for endogenous ileal nitrogen excretion was based on feeding a further 15 rats a protein-free diet. The in-vivo digestibility estimates were compared with in-vitro values determined using either a multi-enzyme digestibility assay (method A), the pronase assay (method B) or a multi-enzyme digestion, pH-drop assay (method C). The in-vivo true digestibilities of lysine for 12 of the meat and bone meal samples were compared with fluoro-dinitrobenzene (FDNB) available lysine values. The mean in-vivo digestibility values ranged from 57.2 to 79.1% and the mean coefficient of variation between rats within meals was 7.6%. For methods A and B the correlations between in-vitro and in-vivo digestibility were low and not significant (P greater than 0.05). There was a significant positive correlation between pH at 10 min from the start of in-vitro digestion and in-vivo digestibility (r = +0.75; P less than 0.001) as was the case for pH at 20 min (r = +0.52; P less than 0.05). The positive relationship between pH at 10 or 20 min and in-vivo digestibility was contrary to expectation and may have been caused by the potentially high buffering capacity of meat and bone meal. The in-vitro methods A, B and C each showed a high degree of precision. The mean in-vivo true ileal digestibility of lysine in the meat and bone meals ranged from 64.7 to 86.9%. There was no significant correlation between in-vivo lysine digestibility and FDNB available lysine. There appeared to be an interaction between FDNB lysine availability and true ileal lysine digestibility, such that the meals with lower in-vivo digestibility had availability values higher than the corresponding in-vivo digestibility values, while the opposite was found for the more readily digested meals. The latter finding has important implications for the use of ileal digestibility coefficients as accurate predictors of available lysine levels in protein sources subjected to heating during processing.