Solid-substrate fermentation of soya beans to tempe : process innovations and product characteristics

Solid-substrate fermentations (SSF) are restricted by heat- and mass transfer limitations, which might result in unfavourable growth conditions. One way to prevent such conditions is by agitation of the substrate. In this study a Rotating Drum Reactor (RDR) was designed for the fermentation of soya beans with Rhizopus oligosporus. The aim of the study was to develop a process for the controlled fermentation of soya beans into a microbiologically safe and protein rich product.

The reactor and the measurement and control system enable an automatic control of the process. The most important process parameters are: rotation speed, substrate temperature, rotation frequency and the relative humidity.

A major disadvantage of RDR that has been cited in literature is the sensitivity of micro-organisms towards agitation. In our study we have shown that the fungal activity in a discontinuous RDR remained high up to 70 hours while in the traditional non-agitated systems fungal activity decreases already after 36 hours of incubation. During fermentation several enzymes, viz. lipases, proteases, phytases and carbohydrases are formed by R. oligosporus. Due to the enzymatic activity, changes in the chemical composition of soya beans were observed. At increasing temperatures a decrease in the total fat content was observed. It was also observed that the level of free fatty acids was lower than expected based on the decrease in glyceride bound fatty acids. This might be explained by the fact that R. oligosporus used fatty acids as a source of carbon. It was also observed that the firmness of the product in the RDR was significantly less compared to the non-agitated samples. In the RDR we observed increased activities of exo-proteases and glycosidases compared with the traditional non-agitated systems. It was shown that lactic acid (>0.05 % w/v, pH 4.2) delayed the germination of Rhizopus oligosporus.

There might be nutritional benefits from the the fermentation step in tempe manufacture through hydrolysis of soya bean cell walls, fats and proteins, making the product more easy to digest.