The influence of physiological pH conditions and myrosinase activity on glucosinolate hydrolysis and isothiocyanate formation during the in vitro digestion of mustard by-products

This study examines the hydrolysis of three glucosinolates-sinigrin, glucotropaeolin, and sinalbin-from the green tissues of two mustard species (Brassica carinata and Sinapis alba) into their corresponding isothiocyanates (ITCs) (allyl-, benzyl-, and 4-OH-benzyl-isothiocyanate, respectively) throughout the successive stages of human digestion (oral, gastric, and intestinal). The results show that the three isothiocyanates tend to form during the oral phase (5.80, 9.64, and 20.31 μmol/g, respectively), undergoing progressive degradation in the subsequent phases of the digestive process (gastric and intestinal) with values in the intestinal phase of 3.72 and 5.11 μmol/g for allyl- and benzyl- ITCs, respectively. Their instability was particularly significant for 4-OH-benzyl-ITC, which was no longer detectable at the end of the intestinal phase. Interestingly, under the stomach's acidic pH conditions, the enzyme myrosinase retained up to 50% of its activity, enabling glucosinolate hydrolysis and, thus, the formation of isothiocyanates during the gastric phase. In addition, glucosinolates were mainly found in the bioaccessible fraction, whereas isothiocyanates tended to concentrate in the non-bioaccessible fraction due to the molecules' lipophilic characteristics.