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Standardization of the KOH protein solubility test for soybean meal in a single commercial laboratory and correlation to in vivo poultry amino acid digestibility

Ruiz, N., C. Parsons, B. Steinlicht, D. Taysom, X. Wu and F. Liu

The KOH protein solubility (KOHPS) test is used to evaluate whether a lot of commercial soybean meal (CSBM) has undergone the Maillard reaction during processing. The method was introduced to the animal feed industry in 1990 by Araba and Dale and is widely used worldwide. However, there are at least two limitations to this important measure of CSBM quality that are still poorly explained: First, large inter-lab variability is routinely observed. Second, nutritionists and formulators are unable to estimate the amino acid digestibility (AAD) for lysine (LYS) and arginine (ARG) in lots of CSBM affected by the Maillard reaction because a quantitative interpretation of the results in terms of AAD has not been developed. The objective of this study was to standardize the KOHPS test in a single commercial lab starting from the description by Araba and Dale (1990), utilizing CSBM samples for which the in vivo AAD was already known. Two CSBM samples, one with high solubility (HS) (KOHPS ~ 80-85%) and another with low solubility (LS) (KOHPS ~ 50-55%) were used as reference samples. After a preliminary protocol with the HS reference sample testing ≥ 80% solubility was developed, adjustments in the intensity of agitation were made with 4 CSBM samples of known in vivo LYS digestibility (88-96% digestible LYS) to match their KOHPS values in the 80-85% solubility range. The resulting standardized protocol yielded 83.08% KOHPS for the HS CSBM reference sample (mean value of 72 test results) with repeatability standard deviation, 1.45%; repeatability relative standard deviation, 1.74%; (Within-Lab) reproducibility standard deviation, 3.91%; and (Within-Lab) reproducibility relative standard deviation, 4.70%. It also yielded 52.77% KOHPS for the LS CSBM reference sample (mean value of 71 test results) with repeatability standard deviation, 1.20%; repeatability relative standard deviation, 2.27%; (Within-Lab) reproducibility standard deviation, 5.32%; and (Within-Lab) reproducibility relative standard deviation, 10.08%.

In conclusion, a standardized protocol is available for the determination of KOHPS of CSBM that will allow the building of regression equations for the estimation of AAD to be used in least-cost formulation.