Effect of processing on the composition of bioactive compounds in soybean meal biscuits

Introduction
Soybean meal (SBM) is a byproduct of soybean oil industry that is mostly used as animal feed. SBM has high contents of proteins, fibers, minerals and phenolic compounds. Among the phenolic compounds, isoflavones are the most abundant class. They are naturally present as β-glycosides and malonyl glycosides. After various types of processing, acetyl glycosides and aglycones may be formed. Fermentation drastically modifies the isoflavones profile in a way that aglycones predominate over conjugated forms. In addition to fermentation, the application of food grade enzymes may also increase the aglycone content in soy foods. This change in isoflavones profile may be beneficial to human health, as aglycones are considered to be more bioavailable and of greater bioactive potential in comparison to conjugated forms.
Objective
To determine the isoflavones profile and antioxidant activity in biscuits prepared with SBM, fermented soybean meal (FSBM) and hydrolyzed soybean meal (HSBM).
Methods
Fermentation of soybean meal was carried out using Saccharomyces cerevisiae (0,5:75 SBM), incubated for 48h at 40°C according Silva et al., 2018. The SBM hydrolysis was carried out using a food grade cellulase (Celumax®) (0.1% w/w), incubated for 30 minutes at room temperature. SBM, FSBM and HSBM biscuits were produced by replacement of 95% of wheat flour from a standard formulation (AACC, 2000).
Biscuits were extracted in triplicate according to Silva et al., 2015. Isoflavones profile was assessed by LC-DAD-MS and antioxidant activity was determined by FRAP (Ferric Reducing Antioxidant Power) and TEAC (Trolox Equivalent Antioxidant Capacity) assays.
Results were compared by one-way ANOVA followed by Tukey’s test using GraphPad Prism software. Differences were considered significant when p<0.05.
Results and discussion
The isoflavones profile was modified after the hydrolysis and fermentation of SBM. Aglycones content was 6.5 times higher in FSBM biscuits (73.6 mg/100 g) and 5 times higher in HSBM biscuits (58.0 mg/100 g) in comparison to SBM biscuits (11.3 mg/100 g).
Antioxidant activity of FSBM biscuits was 68% and 50% higher when assessed by FRAP and TEAC assays, respectively, in comparison to the other biscuits. On the other hand, HSBM biscuits showed similar antioxidant activity to SBM biscuits (about 19 mmol Fe+2/g by FRAP assay and 15 µmol Trolox/g by TEAC assay), although these biscuits showed different isoflavones profiles. These results could be explained by the formation of compounds with antioxidant activity, besides isoflavones, during the fermentation process.
Conclusions
Although both fermentation with S. cerevisiae and hydrolysis with cellulase modified isoflavones profile in SBM, increasing aglycones, only the former process led to an increase in antioxidant activity.
References
Silva et al., 2018. Soybean meal and fermented soybean meal as functional ingredients for the production of low-carb, high-protein, high-fiber and high isoflavones biscuits
Silva, F. O. & Perrone, D., 2015. Characterization and stability of bioactive compounds from soybean meal
Financial support
CAPES, CNPq, FAPERJ