Effects of temperature and relative humidity on growth and enzyme production by Actinomucor elegans and Rhizopus oligosporus during sufu pehtze preparation

Sufu is a Chinese soybean cheese obtained after maturation of solid-state mould-fermented tofu. Ambient temperatures of 30–35 °C during the summer season prohibit the use of the usual starter Actinomucor elegans. We compared the properties of the latter with a potential alternative starter Rhizopus oligosporus that could be used at higher temperatures. The effects of temperature and relative humidity on growth rate of Actinomucor elegans and Rhizopus oligosporus were optimum at 25 °C at RH 95–97%, and 35 °C at RH 95–97%, respectively. Yields of protease (108 U/g pehtze), lipase (172 U/g) and glutaminase (176 U/g) by A. elegans were maximum after 48 h at 25 °C and RH 95–97%, and for α-amylase (279 U/g pehtze) and α-galactosidase (227 U/g) at 30 °C and RH 95–97% after 48 and 60 h of incubation. Highest protease (104 U/g pehtze), and lipase (187 U/g) activities of R. oligosporus were observed after 48 h at 35 °C and RH 95–97%, while maximum α-amylase (288 U/g pehtze) and glutaminase (187 U/g) activities were obtained after 36 h at 35 °C and RH 95–97%. Maximum α-galactosidase activity (226 U/g) by R. oligosporus was found after 36 h at 30 °C and RH 95–97%. It is concluded that R. oligosporus is a potential alternative to A. elegans as sufu pehtze starter during hot seasons.