Determination of soluble dietary fibre content of Okara treated with high hydrostatic pressure and enzymes: a comparative evaluation of two methods (AOAC and HPLC-ELSD).

ABSTRACT: Okara is a promising by-product from soybean and a rich source of dietary fibre, chiefly insoluble (IDF). To increase its solubility and functionality, a treatment with high hydrostatic pressure (HHP) assisted by food grade enzymes -Ultraflo® L or Viscozyme® L- has been performed. To monitor the effectiveness, an analysis of dietary fibre, mainly the soluble fraction (SDF), was accomplished by the AOAC enzymatic-gravimetric method with dialysis followed by both, spectrophotometric methods and High Performance Liquid Chromatography with Evaporative Light Scattering Detector (HPLC-ELSD) analysis of the soluble fraction. A significant increase in SDF (≈1.5-times) to the expense of a decrease in IDF was shown and chromatograms revealed two peaks of 95 and 22 kDa. Thus, treated Okara possessed a more balanced and convenient ratio of soluble to insoluble dietary fibre, which could have health benefits as prebiotic. Inbuilt interferences of the official AOAC's method for dietary fibre were confirmed and the direct HPLC-ELSD approach was about twice more sensitive than spectrophotometric methods. Consequently, the direct HPLC-ELSD analysis of the supernatant is proposed as a cheaper, faster and reliable method. Combined HHP-treatments plus specific enzymes represent a promising alternative for the valorisation and preservation of agrofood by-products.