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Towards a better understanding of anti-nutritional factors, reactive lysine, KOH protein solubility, and rooster amino acid digestibility in commercial soybean meals

Hack, A., B. Parsons, P. Utterback, J. Emmert and C. Parsons

The two primary objectives were 1) to determine trypsin inhibitor (TI) and chymotrypsin inhibitor (CI) levels using two different methods and 2) to determine near infrared spectroscopy reactive Lys (NIR RL) (Evonik), KOH protein solubility (KOHPS), and in vivo rooster amino acid (AA) digestibility in commercial samples of solvent extracted dehulled soybean meal (SBM) and extrusion expeller mechanically processed SBM. A total of 99 samples of SBM were obtained; 86 solvent and 13 mechanical. After sample evaluation using NIR TI and SOY CHEK (LSB Products, Manhattan, KS), 54 SBM (44 solvent and 10 mechanical) were selected for further analyses including trypsin units of inhibition (TUI/g) and chymotrypsin units of inhibition (CUI/g) using the new AOAC method (Liu et al., 2021), and trypsin inhibition units (TIU/g) and chymotrypsin inhibition units (CIU/g) using a new, fast, and cost-effective procedure (Kim and Krishnan, 2023). In addition, 36 of those SBM were evaluated for in vivo AA digestibility using 4 to 6 individually caged cecectomized roosters per sample. Roosters were fasted for 26 h prior to crop intubation with 25 g of SBM and excreta were quantitatively collected for 48 h. Data were analyzed using regression analyses to determine linear and quadratic relationships among variables. The probability level for statistical analyses was P < 0.05. No differences in NIR RL were observed between solvent and mechanical SBM. Values for TUI/g ranged from 1.1 to 17.2 among SBM, and mechanical SBM had a higher mean TUI/g of 13 compared with a mean of 7 in solvent SBM. The NIR TI was positively correlated (P < 0.05) with TUI (R2 = 0.56). Mean TUI values were higher than TIU values primarily because many SBM with low TUI levels had 0 TIU/g. Rooster Lys, Met, and Cys digestibility ranged from 70 to 96, 79 to 91, and 69 to 84% among SBM, respectively. For the regression analyses, NIR RL exhibited a quadratic relationship with rooster Lys and Arg digestibility (R2 = 0.40 to 0.42). Analyzed KOHPS was positively correlated with NIR RL and rooster Lys and Arg digestibility (R2=0.47 to 0.73). Maximum rooster AA digestibility was predicted to occur at a lower TUI level for solvent SBM than for mechanical SBM. Results for CUI, TIU, and CIU were similar to those for TUI and there was generally no advantage of measuring CUI or CIU in addition to TUI.

The results of this study indicate that 1) NIR RL can detect heat-damaged SBM, 2) analyzed TI varies among analytical methods, 3) mechanical SBM contains higher TI and CI than solvent SBM, 4) NIR RL, NIR TI and wet lab TI and CI are correlated with in vivo rooster AA digestibility, and 5) KOHPS is correlated with NIR RL and rooster Lys and Arg digestibility.