Soybean trypsin inhibitor and urease activities and their correlations as affected by heating method, duration, sample matrix and prior soaking

Authors: Liu, Keshun; RUIZ, NELSON - Nelson Ruiz Nutrition, Llc

Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/18/2021
Publication Date: 7/6/2021
Publication URL: https://handle.nal.usda.gov/10113/7482152
Citation: Liu, K., Ruiz, N. 2021. Soybean trypsin inhibitor and urease activities and their correlations as affected by heating method, duration, sample matrix and prior soaking. Journal of the American Oil Chemists' Society. https://doi.org/10.1002/aocs.12514.
DOI: https://doi.org/10.1002/aocs.12514

Interpretive Summary: Soybeans are a major protein and oil source for human and animals. Yet, soybeans contain certain antinutrients, such as trypsin inhibitors, which can only be inactivated by heating for optimal nutrition. In the feed industry, although urease present in soybeans is not considered an antinutrient, its activity (UA) has historically been measured to indicate if heating has been sufficient to inactivate antinutrient factors, due to its easy measurement and observed correlation between UA and trypsin inhibitory activity (TIA) upon heating. Yet, over the years, controversy has emerged regarding reliability of UA as a heating index and surrogate parameter for TIA. To shed light on this issue, USDA researchers at Aberdeen, Idaho, carried out a study by subjecting raw soybean materials with four different matrices to three different heating methods for various times, measuring UA and TIA in treated samples with latest analytical methods, and determining their correlations. By combining all these treatments into a single study, the most comprehensive spectra can be obtained, regarding destruction rates of trypsin inhibitors and urease with heating time and their relationships in terms of correlation coefficients. This study confirms some previous findings, offered a few new findings, and clarified conflicting results among some reports. More importantly, it offers some new guidelines to the feed industry. For some soy products (such as defatted and toasted soybean meal) UA can be used as a reliable heating index, but for other soy products (such as low fat and full-fat soybean products heated by different methods), it is not reliable and thus TIA measurement is required.

Technical Abstract: Urease activity (UA) in soybeans has historically been measured to indicate heating inadequacy. Yet, over the years, controversy has emerged regarding the reliability of UA as a heating index and surrogate for trypsin inhibitor activity (TIA). In Experiments 1–4, raw soybean materials with different matrices (whole beans, flakes, full-fat and defatted flours) were selectively subjected to steaming, boiling, or dry oven toasting for various durations. For steaming or boiling soybeans, with or without prior soaking was another factor. Reduction rates of TIA and UA with heating time were compared, their correlation coefficients were determined and statistically treated. Experiment 5 entailed collecting 30 commercial soybean meals and measuring TIA and UA without further treatments. By combining the five experiments into a single study, the most comprehensive spectra regarding relative decreasing rates of TIA and UA with heating time and their correlations were obtained. Results show that the reduction rate of UA could be slower than, close to, or faster than that of TIA, depending on combinations of four factors (sample matrix, with or without prior soaking, heating method and interval). UA reached zero within shorter heating durations, while TIA maintained residual values at the longest durations. Consequently, positive correlations between TIA and UA varied from insignificant to very strong. UA was a reliable index for heating inadequacy and surrogate for TIA in soybean products heated by several single combinations of the four factors, but for those heated by other single or mixed combinations, it was unreliable, and TIA should be measured directly.