Protein dispersibility index as an indicator of adequately processed soybean meal

Three chick assays (8 to 17 or 21 d) were conducted to evaluate protein dispersibility index (PDI) as an indicator of minimum adequate heat processing of soybean meal compared with the urease index and protein solubility in 0.2% KOH. Solvent-extracted soyflakes (SF) were subjected to various autoclaving times at 121 C and 105 kPa and were included in 23% CP dextrose-SF diets or 20% CP corn-SF-corn gluten meal diets. Autoclaving times in Chick Assays 1, 2, and 3 were 0 to 36 min, 0 to 30 min, and 0 to 12 min, respectively. Body weight gains and gain-to-feed ratios increased (P < 0.05) with increased SF heating time (0 to 18 min in Chick Assay 1, 0 to 10 min in Chick Assay 2, and 0 to 9 min in Chick Assay 3), with no additional improvement for longer autoclaving times. Urease index values (pH increase) were high initially and at the shorter autoclaving times (1.65 to 2.4), and then decreased suddenly to 0.3 or below as autoclaving time increased in two of the three chick assays. The KOH protein solubility values generally decreased as autoclaving time increased, but the responses were often inconsistent. Protein dispersibility index displayed the most consistent responses to heating time: it decreased from above 70% to generally below 30% as autoclaving time increased from 0 to 30 or 36 min (mean r2 from linear regression of PDI on autoclaving time was 0.92 for the three chick assays). The latter responses were particularly evident for the heating times, which yielded the greatest changes in chick growth performance. These results suggest that PDI is a more consistent and sensitive indicator of minimum adequate heat processing of soybean meal than urease index or protein solubility in KOH.