Production of Sufu, a Traditional Chinese Fermented Soybean Food, by Fermentation with Mucor flavus at Low Temperature

Abstract

This study evaluated biochemical changes, especially those related to protein degradation, occurring in sufu fermentation with Mucor flavus at low temperature. The effects of incubation temperature on M. flavus growth, biomass accumulation and protease production were described. Results indicated that M. flavus could grow well on the surface of soybean curd and produce considerable protease to degrade soybean proteins during sufu fermentation at lower temperature such as 15℃. Almost no subunits of protein could be observed after maturation for 8 weeks in M. flavus-type sufu. Post-fermentation was the main stage responsible for the hydrolysis of protein together with the increase in the content of amino-type nitrogen and free amino acid. Our results demonstrated that M. flavus had possible application as an alternative strain for sufu manufacturing at low temperature.Keywords: sufu, Mucor flavus, fermentation, protein degradation *To whom correspondence should be addressed. E-mail: yinljmail@yahoo.com.cn IntroductionIn Asian countries, soybean is consumed in many forms, including soymilk, soybean curd products, and fermented soybean food such as miso, soy sauce, tempeh, natto and sufu. Sufu is a traditional fermented soybean curd originating in China (Steinkraus, 1996). It is a soft cheese-like product with a spreadable creamy consistency, and it has been consumed widely as an appetizer for centuries in China (Zhang and Shi, 1993;Han, et al., 2001;Han, et al., 2003a). There is a similar product called tofuyo in Okinawa, Japan (Yasuda, 2000(Yasuda, , 2001. Several types of sufu can be distinguished, according to different local processes or colour and flavour in China. Mould-fermented sufu is the most popular type due to its attractive colour and strong flavour (Han, et al., 2001).Sufu is produced by means of solid-state fermentation of soybean curd after inoculation with pure culture moulds. In commercial practice, Actinomucor spp., Mucor spp. and Rhizopus spp. are used for sufu preparation. Among fungal genera, Actinomucor elegans and Actinomucor taiwanensis seemed to be the most frequently used for commercial sufu production in China. However, these two mould species generally grow well at temperatures ranging from 25℃ to 30℃, hence making it difficult to produce sufu when the indoor temperature of the factory exceeds 35℃ during hot summer or below 20℃ during cold winter. Recently, the screening of alternative strains for the commonly used starter cultures in sufu production is receiving much attention. (Han, et al., 2003b;Hu, 2006;Hu, et al., 2008;Hu and Zhao, 1998a;Hu and Zhao, 1998b).Fermentation temperature greatly affects the growth of fungus and the variations in the enzyme activities produced by microorganisms, which are thought to have a close relation to sufu quality. Protein degradation is one of the important biochemical events during sufu fermentation. It had been reported that most of the soybean proteins were degraded into peptides and amino acids in well-qualified sufu. With the exception of the ...

Influences of fermentation and ripening temperatures on the enzymatic activity and physicochemical and sensory properties of salted egg white sufu

“…During processing, proteases produced by starter organisms hydrolyze and degrade proteins into small molecules and liberate some free amino acids, thus contributing to the unique flavors and textures of sufu (Cheng et al, ; Chou & Hwan, ). Figure shows that the protease activity in the douchi koji reduced markedly during the initial 5 days of fermentation.…”

“…This was probably because of the leaching of enzymes from the koji (Li et al, ). The protease activity of koji during processing depends on the production conditions including incubation temperature and duration, relative humidity, and the addition of salt and ethanol (Cheng et al, ; Han, Ma, et al, ). In this study, the protease activity of the infusion and the SEWS prepared at 35°C and 45°C was higher than that of the 25°C sample.…”

“…Chou et al () reported that the growth rate and enzyme production of Actinomucor taiwanensis cultured in a sufu pehtze preparation were greatly affected by the incubation temperature and humidity. During sufu production, Cheng et al () observed that the enzyme produced by Mucor flavus and incubated at 15°C had higher protease activity than that incubated at 10°C. The same study determined that protease activity increased in conjunction with mycelial biomass, and that fermentation temperature substantially affected mold growth and protease production.…”

“…Protein is the major constituent, apart from water (Chou & Hwan, ; Li et al, ), and is degraded into small peptides and free amino acids during processing, eventually contributing to the unique flavors and textures of sufu products. Hence, it is critical to understand the transformations of protein components during manufacturing (Cheng et al, ; Han, Ma, et al, ; Li et al, ). Figure shows the DH of SEWS during processing, with the changes in the total nitrogen and amino nitrogen contents.…”

“…Sufu , alternately known as furu or toufuru , is a traditional East Asian fermented soybean product (Cheng, Hu, Li, Saito, & Yin, ; Li, Yu, & Chou, ). Sufu has a unique flavor and soft cheese‐like texture, and is typically consumed as a side dish or an appetizer (Chou, Ho, & Tsai, ; Han, Cao, Rombouts, & Nout, ; Han, Rombouts, & Nout, ).…”