Characterization of surface Aspergillus community involved in traditional fermentation and ripening of katsuobushi

Abstract

A soup stock made from katsuobushi is an important element of, and the basic seasoning responsible for the taste of, traditional Japanese cuisine. Fermented and ripened katsuobushi, called karebushi, is manufactured via a repeated molding process on the katsuobushi surface. Our aim was to characterize the surface Aspergillus community and their enzymes involved in the fermentation and ripening. Five dominant Aspergillus species isolated from the karebushi surface were identified—A. amstelodami, A. chevalieri, A. pseudoglaucus, A. ruber, and A. sydowii. Analyses were performed on final molding stage-samples from different manufacturers, and 1st to 4th molding stage-samples from the same manufacturer. The composition ratios of the five Aspergillus spp. varied according to the manufacturer of the karebushi. A. amstelodami and A. chevalieri tended to be detected as dominant species when the water content of the karebushi fillet was >15% and the fat content was >3.5%, respectively. In samples from a given manufacturer, the dominant species in the final molding stage tended to be A. chevalieri and A. pseudoglaucus. Mixed molds were cultured by solid-state fermentation using katsuobushi powder medium at two different water activity (aw) levels. Crude extracts from each culture showed lipase, aminopeptidase, carboxypeptidase, and protease activities. Notably, the crude extracts cultivated at 0.85 aw showed higher protease activity toward hemoglobin and lipase activity toward p-nitrophenyl palmitate than those at 0.95 aw. These hydrolytic enzymes are probably involved in decolorization of katsuobushi and lipid degradation during the long fermentative and ripening period. In addition, mixed cultures could transform 2,6-dimethoxyphenol into 1,2,3-trimethoxybenzene, previously reported as an attractive and mild flavor component. Our results may help promote the use of desirable Aspergillus spp. as starter cultures for manufacturers to stabilize and improve the quality of fermented and ripened karebushi.