Characterization and product innovation of sufu - a Chinese fermented soybean food

 

Over the centuries, Chinese people have consumed soybeans in various forms of traditional fermented soybean foods. Sufu ( Furu ), a cheese-like product originating in China, is one of the most popular fermented soybean foods in China, and is becoming popular in Chinese shops all over the world. It is made by fungal solid-state fermentation of tofu followed by salting and ripening in dressing mixture containing various ingredients. Several types of sufu can be distinguished according to processing method or colour and flavour.

High levels of surviving but inactive bacterial endospores and concomitant high numbers of culturable mesophilic aerobic bacteria were found in all process stages of sufu production as well as in commercial sufu. Most samples contained only low levels (< 10 ³cfu/g) of Bacillus cereus , whereas no Enterobacteriaceae were detectable in any of the commercial and experimental sufu products. From a microbiological safety point of view, sufu products are stable and safe when they are produced under the conventional conditions.

Phylogenetic relations based on sequencing of genomic DNA-its-1-4 regions of collected fungal starters and of relevant control strains indicate that the genera Mucor , Actinomucor and Rhizopus form distinct and homogeneous clusters. Most Mucor and Actinomucor spp., especially Actinomucor elegans (the most frequently used starter) cannot grow well over 30°C. Rhizopus oligosporus has similar growth and enzyme production abilities as A. elegans and could thus offer an alternative for the latter during the hot summer season.

NaCl strongly affects the changes of microflora, textural properties and the hydrolysis of protein and lipid in sufu. Low-salt sufu (7-8% salt content) is qualified to be termed "finished product" after only 45 d of ripening, which takes more than 3 months for conventional red sufu with over 11% salt content. However, sufu will spoil during the ripening stage at salt contents of 5% or lower.

SDS-PAGE profiles showed that after 60 d of ripening, all protein subunits had disappeared in sufu with 8% salt content, which indicates that most proteins were degraded into peptides and amino acids. Consequently, a large amount of free amino acids, notably glutamic acid, were found in matured sufu.