Total phenolic content, antioxidant, tyrosinase and α-glucosidase inhibitory activities of water soluble extracts of noble starter culture Doenjang, a Korean fermented soybean sauce variety

In this study, samples of Korean Doenjang, a dietary soybean fermented food, were prepared using three fungal strains (Aspergillus oryzae J (A), Mucor racemosus 15 (M15), and Mucor racemosus 42 (M42) and bacterial strain Bacillus subtilis TKSP 24 (B) as starter cultures. The resulting Doenjang samples were evaluated for their total phenolic contents, antioxidant activities based on DPPH and nitrite radical scavenging and reducing power abilities, as well as alpha-glucosidase and tyrosinase inhibitory activities in vitro. The starter culture Doenjang samples showed significant antioxidant activities with DPPH and nitric oxide radical inhibitory rates of 59.92 and 92.83%, respectively. The starter culture Doenjang samples yielded phenolic contents in the range of 25.06-29.49 mg/g gallic acid equivalents (GAE). The starter culture Doenjang samples also strongly inhibited tyrosinase and a-glucosidase enzymatic activities by 58.93-62.25% and 58.23 +/- 1.28-67.14 +/- 1.14% with IC50 values of 14.14-25.93 and 27.40 -40.98 mg/ml, respectively. Moreover, starter culture Doenjang samples showed potent antioxidant efficacy in terms of reducing power ability. These findings confirm the therapeutic potential of starter culture Doenjang samples as a potential source of natural antioxidants for use in food and medicine industries along with their possible applications for treatment of skin depigmentation and diabetes mellitus type-2 related disorders. (C) 2015 Elsevier Ltd. All rights reserved.