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 dis persibility 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 autocla ving times at 121 C and 105 kPa and were included in 23% CP dextrose-SF die ts or 20% CP com-SF-corn gluten meal diets. Autoclaving times in Chick Assa ys 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 incre ased SF heating time (0 to 18 min in Chick Assay II 0 to 10 min in Chick As say 2, and 0 to 9 min in Chick Assay 3)1 with no additional improvement for longer autoclaving times. Urease index values (pH increase) were high init ially and at the shorter autoclaving times (1.65 to 2.4), and then decrease d suddenly to 0.3 or below as autoclaving time increased in two of the thre e chick assays. The KOH protein solubility values generally decreased as au toclaving time increased, but the responses were often inconsistent. Protei n dispersibility index displayed the most consistent responses to heating t ime: it decreased from above 70% to generally below 30% as autoclaving time increased from 0 to 30 or 36 min (mean r(2) from linear regression of PDI on autoclaving time was 0.92 for the three chick assays). The latter respon ses were particularly evident for the heating times, which yielded the grea test changes in chick growth performance. These results suggest that PDI is a more consistent and sensitive indicator of minimum adequate heat process ing of soybean meal than urease index or protein solubility in KOH.