Assessing Protein Quality in Heat-Treated Soybean Products

Dr. Maurice Eastridge, Professor, Department of Animal Sciences, The Ohio State University

Protein is a critical nutrient in the diets of food-producing animals. It provides amino acids to tissues for cellular structure and function (e.g., muscle is about 20% protein) and can be used as an energy source. Thus, it is vital for maintenance, growth, and production by animals. Crude protein (CP) generally consists of 12 to 20% of the diets for food-producing animals, depending on age and productive stage of the animal, with 16% being a typical concentration. For ruminants, the protein is supplied via concentrates and forages, but for non-ruminants, the protein is primarily supplied by grains. Soybean products are by far the most abundant source of supplemental protein in diets for food animals in the US. Of the soybeans produced in the US, about 97% are processed for animal feed, biofuels and vegetable oil; 2.6% is directly used for animal feed; and 0.1% used directly for human food. The primary ingredient for supplemental protein in farm animal diets is soybean meal (SBM). The oil is extracted either by solvent or mechanically, with about 99% of the SBM available being from solvent extraction because the solvent process results in a higher recovery of the oil than mechanical extraction. Solvent extracted SBM will have about 1.5% residual oil versus 5 to 7% with mechanically extracted SBM. Solvent extracted SBM typically contains 48% (CP, as-fed basis) if the hulls are left out during processing or 44% CP if the hulls are added back into the meal. Soybean hulls can be purchased as a separate commodity, but they only contain about 14% CP. Mechanically processed SBM is priced higher because of the residual oil for energy and the higher level of rumen undegradable protein (RUP) in comparison to solvent extracted SBM.

There is growing interest in feeding full-fat soybeans (SB) to food animals, especially to lactating dairy cattle for the energy and protein. Typical SB contain about 48% of the fatty acids (FA) as linoleic acid (C18:2) and 25% as oleic acid (C18:1). In the new variety of high oleic acid Plenish SB, C18:2 is at about 6% and C18:1 at 74% (Bales and Lock, 2024). The lower level of unsaturation results in lower risks to disrupting ruminal fermentation. Heat treating both types of SB has interest to reduce the antinutritional properties and increase the level RUP in the SB, whereby more of the protein is degraded in the small intestine instead of in the rumen. Therefore, the goal is to adequately heat the SB to reduce the antinutritional properties but not overheat the protein source whereby the total tract digestibility of the protein is reduced. The heat processing (roasting) may be done at a feed plant or on-farm. In either case, the temperature used in the roasting, steeping time, and cooling mechanisms post-roasting are critical for optimal results. Thus, various options for assessment of protein quality in SBM and full-fat SB are described in Table 1. Several commercial laboratories offer these analytical methods.

Conclusions

Soybeans are a source of high-quality protein and typically at an economical advantage for dairy farms compared to other feed ingredients. However, further heat processing can increase the value of the meal or beans as discussed above. The additional heating needs to be adequate for the desired outcome and overheating can be detrimental to protein digestibility and amino acid absorption, but underheating limits animal response while having increased the cost of the ingredient. The analyses described above provide insightful information, but oftentimes, the absolute number from the analysis for a given feed will be less valuable than using the analytical data to assess the difference in values among different processors or a change in a value caused by a processing change.

Sources

Bales, A.M., and A.L. Lock. 2024. Feeding high oleic acid soybeans to lactating dairy cows. Proceedings 32^nd Tri-State Dairy Nutrition Conference, Columbus, OH. https://www.tristatedairy.org/

Boucher, S.E., S. Calsamiglia, C.M. Parsons, H.H. Stein , M.D. Stern, P.S. Erickson, P.L. Utterback, and C.G. Schwab. 2009. Intestinal digestibility of amino acids in rumen undegradable protein estimated using a precision-fed cecectomized rooster bioassay: I. Soybean meal and SoyPlus. J. Dairy Sci. 92 :4489–4498.

Food and Agriculture Organization of the United Nations. 2025. Accessed via OurWorldinData.org/agricultural-production

Multi-step in vitro protein evaluation (MSPE). Cumberland Valley Analytical Services. Water, Waynesboro, PA, https://www.foragelab.com/Services/Forage-and-Feed/Invitro-Digestibility

NASEM. 2021. Nutrient requirements of dairy cattle. 8^th rev. ed. National Academies of Sciences, Engineering, and Medicine, National Academies Press, Washington DC.

Raver, K. and J. Goeser. Technical Insights: Evaluating soybean roasting efficacy. Rock River Laboratory, Inc. Watertown, WI. https://rockriverlab.com/file_open.php?id=557 

van Eys, J.E., and N. Ruiz. 2021.Quality manual and analysis for soybean products in the feed industry. 3^rd ed., US Soy and US Soybean Export Council. https://ussec.org/wp-content/uploads/2022/07/QualityManualAnalyses_NewBrandColors_0629_Final.pdf

Table 1. Analytical measures to assess protein quality in soybean meal (SBM) and soybeans (SB).