A comparative study on physicochemical properties of Chinese-type soy sauces prepared using pure koji and mixed kojis

Proximate indices, water soluble peptides distribution, free amino acids (FAAs) and aroma active compounds in soy sauces prepared using Aspergillus oryzae koji (SSAO) and mixed kojis (SSAON, A. oryzae koji: Aspergillus niger koji = 3: 1, w/w) were determined. Results showed that SSAON resulted in 6.14, 5.97, 10.40, 71.59 and 9.62% of increases in contents of formaldehyde nitrogen, total FAAs, glutamic acid, peptides (<= 1 kDa) and reducing sugar when compared with those in SSAO, whereas there were no significant differences in contents of aroma active compounds except for 2-methoxy-4-vinylphenol between these two kinds of soy sauces. The higher activities of acid protease, glucoamylase present in the mixed kojis, were the main reason for the increases of formaldehyde nitrogen, total FAAs, glutamic acid, peptides (<= 1 kDa) and reducing sugar in SSAON. Moreover, results from sensory evaluation clarified that the taste of soy sauce prepared using mixture of A. oryzae koji and A. niger koji was obviously improved. Based on the results from the present study, soy sauce preparation using mixed kojis is an effective approach to improve its taste.