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 fluorodinitrobe nzene (FDNB)available lysine (FDNB-lysine) as predictors of apparent ileal nitrogen digestibility in blood meals. Twenty blood meals representing a wi de 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 h eating and then air-dried at 50 degrees C for 5 h. A control sample of bloo d was freeze-dried at -30 degrees C, The control and heated blood meals wer e also subjected to ileal nitrogen digestibility and pronase assays. The me an apparent ileal nitrogen digestibility values for the 20 meals were highl y 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 s ignificant correlations (p < 0.01) between in vivo ileal nitrogen digestibi lity and pronase solubility (R-2 = 0.55), FDNB-lysine (R-2 = 0.62) and the reported time in drier (R-2 = 0.56). Heating coagulated blood at Fo 1.0 (eq uivalent to 121.1 degrees C for 1 min) significantly (p < 0.01) reduced the mean ileal nitrogen digestibility from 91 to 73% but further heating had n o effect on digestibility. The pronase solubility values did not follow a c onsistent pattern, with the control sample having the lowest solubility. Th e in vitro pronase assay and in particular FDNB-lysine are useful for predi cting the in vivo ileal digestibility of nitrogen in blood meals. A combina tion of heating duration and temperature during processing appears to be im portant in determining protein quality. (C) 1999 Elsevier Science B.V. All rights reserved.