Protein quality in blood meal: II. The effect of processing on in vivo nitrogen digestibility in rats, protein solubility and FDNB-available lysine

The aim was to investigate the pronase solubility assay and fluorodinitrobenzene (FDNB)-available lysine (FDNB-lysine) as predictors of apparent ileal nitrogen digestibility in blood meals. Twenty blood meals representing a wide range of commercial processing methods were subjected to a rat apparent ileal nitrogen digestibility assay, and pronase solubility and FDNB-lysine contents were also determined. In the digestibility assay, each blood meal was included in a semi-synthetic corn starch-based diet as the sole source of protein. Chromic oxide was included as an indigestible marker. On day 14 of the assay, ileal digesta were collected after slaughter of the rats, 3 h from the start of a single meal. In a second study, coagulated blood was subjected to different durations (Fo 1.0, Fo 3.0, Fo 12.0 and Fo 30.0) of heating and then air-dried at 50°C for 5 h. A control sample of blood was freeze-dried at −30°C. The control and heated blood meals were also subjected to ileal nitrogen digestibility and pronase assays. The mean apparent ileal nitrogen digestibility values for the 20 meals were highly variable, ranging from 17 for a batch-dried meal to 95% for a spray-dried meal. Pronase protein solubility ranged from 11.7 to 63.0% and FDNB-lysine content from 60.3 to 100.2 mg g−1 dry matter. There were statistically significant correlations (p < 0.01) between in vivo ileal nitrogen digestibility and pronase solubility (R2 = 0.55), FDNB-lysine (R2 = 0.62) and the reported time in drier (R2 = 0.56). Heating coagulated blood at Fo 1.0 (equivalent to 121.1°C for 1 min) significantly (p < 0.01) reduced the mean ileal nitrogen digestibility from 91 to 73% but further heating had no effect on digestibility. The pronase solubility values did not follow a consistent pattern, with the control sample having the lowest solubility. The in vitro pronase assay and in particular FDNB-lysine are useful for predicting the in vivo ileal digestibility of nitrogen in blood meals. A combination of heating duration and temperature during processing appears to be important in determining protein quality.