Counteracting roles of lipidic aldehydes and isoflavone antioxidants on soy protein oxidation revealed by chemometric survey of solvent and mechanically extracted soybean meals

Soybean meal (SBM) is a premier source of protein for feeding production animals. Protein oxidation negatively affects the nutritional value of SBM, but its causative associations with soybean oil extraction methods and non-protein components in SBM were not well examined. In the current study, 40 solvent extracted SBMs (SSBM) and 8 mechanically extracted SBMs (MSBM) collected from different commercial producers and geographic locations were profiled by chemometric analysis.
The results showed that the extraction methods led to diverse differences between SSBM and MSBM. In gross composition, SSBM had greater crude protein, ash, moisture, and water activity while MSBM had greater crude fat and crude fiber. In protein oxidation, MSBM had greater protein carbonyl contents than SSBM (9.2 vs 5.2 mmol/kg protein, p<0.0001). In lipid oxidation, MSBM also had greater p-anisidine values than SSBM (1.25 vs 0.48, p<0.001), and more lipidic aldehydes, including 2-hexenal, 2-heptenal, 2,4-heptadienal, octanal, 2-octenal, 2-decenal, and nonanal. In antioxidant status, SSBM had greater Trolox equivalent antioxidant capacity (TEAC) than MSBM (25.0 vs 23.6 µmol/g, p<0.0001), which was in consistent with the total phenolics content (1.17 vs 0.86 mg caffeic acid equivalent/g, p<0.0001) and the total content of isoflavones, including genistin, genistein, daidzin, and daidzein (2147.0 vs 1481.8 µg/g, p<0.0001). Interestingly, MSBM had greater contents of tocopherols, including α- and g-tocopherol, than MSBM due to higher residual oil content.
Pearson correlation analysis showed that protein carbonyl content was positively correlated with the p-anisidine value and total aldehydes, but inversely correlated with the TEAC value. Among antioxidants, total phenolics and isoflavones contents had inverse correlations with protein carbonyl content, total aldehydes, and p-anisidine value, respectively.
Overall, extraction methods significantly affected gross composition, oxidation, and antioxidant status of SBMs. Lipidic aldehydes and phenolic antioxidants play counteracting roles in the oxidation of soy protein.