Assessment of the effects of soy varieties and processing on protein quality, digestibility and intestinal bioactivity (DPP-IV inhibition and total GLP-1 release)

Soybean is widely used to produce food and feed livestock for its high nutritional value and technological properties. Traditionally, soy proteins are wet-extracted from meal using heat and basic pH to produce high yields extracts. However, when proteins were extracted under these conditions some aspects related to protein quality were still overlooked. In this study, the effect of processing of two varieties of soy, Energy and Namaste, on protein digestibility and bioactivity were evaluated. Soybean meal was characterised and subjected to two different protein extraction conditions to evaluate their impact on in vitro gastrointestinal digestion, estimated by characterising the protein solubilisation. Furthermore, the bioactivity of the digested mixtures was studied by evaluating the effects on the total GLP-1 secretion and on the inhibition of the DPP-IV enzyme. In all conditions, soy meals and extracts showed potential bioactivity on total GLP-1 secretion. Depending on the variety and extraction conditions, some changes were observed. Noteworthy, different results were observed when proteins were extracted under more rough basic conditions, presenting the highest protein extraction yield (60-70 % in rougher and 40 % in milder conditions), hydrolysis of the high-molecular-weight proteins and stronger potential inhibitory activity on DPP-IV enzyme (27 % and 17 % respectively from Energy and Namaste, at the lowest concentration tested). However, this condition produced an increasing % of D-amino acid formed and lower protein solubilisation after digestion. These findings highlighted the relevance of employing an integrated approach, combining the evaluation of chemical and biological properties when the nutritional value is a target.