Evidence of enzymatic browning due to laccase-like enzyme during mash fermentation in Thai soybean paste

Enzymatic browning reaction in food systems is due to oxidation of phenolic compounds by oxido-reductase enzymes, e.g. polyphenol oxidase or ortho-diphenol oxidase (EC 1.10.3.1), tyrosinase (EC 1.14.18.1), laccase or para-diphenol oxidase (EC 1.10.3.2), as well as peroxidase (EC 1.11.1.7). Since tyrosinase and laccase are prevalent among fungi, these enzymes might be responsible for enzymatic browning occurring in soy sauce and soybean paste. Diphenolic compounds, i.e. resorcinol, catechol, hydroquinone, guaiacol, and tyrosine, induced browning in soybean paste moromi. This browning was related to enzymatic browning, since heating at 100 °C for 10 min destroyed the activity due to these substrates. Moreover, enzymatic browning inhibitors, i.e. ascorbic acid, KI, NaCl, or Na2SO3, suppressed this browning. Since resorcinol, which is an inhibitor of ortho-diphenol oxidase, also gave high browning activity, this indicated that the enzyme involved might be a laccase-like enzyme (para-diphenol oxidase) with a wide range of substrates. Moreover, such enzymatic activity was also detected in the culture of the mould used in the starter culture, i.e. Aspergillus oryzae MUTK. This activity also showed similar substrate specificity, as seen in the moromi. Supplementation of the soybean extract enhanced the enzyme activity in the cultured broth.