Microbial dynamics during barley tempeh fermentation

摘要

Tempeh is a traditional staple food in Indonesia mainly made from soybeans. Barley tempeh has been developed by adapting the soybean tempeh process. During soybean tempeh fermentation, Rhizopus oligosporus is dominant. However, other fungi and bacteria also occur, which may influence tempeh quality or even constitute a health risk. Introduction of food-grade lactic acid bacteria (LAB) and yeasts to tempeh fermentation may enhance tempeh nutritional and hygienic quality. The abilities of LAB and yeasts to grow together with R. oligosporus during barley tempeh fermentation and their possible effects on tempeh quality were studied. The LAB Lactobacillus plantarum and L. fermentum and the yeasts Saccharomyces cerevisiae, Pichia anomala and Kluyveromyces lactis could grow during tempeh fermentation and the yeasts even during cold storage. LAB and yeasts did not negatively affect the growth of R. oligosporus at an inoculation level 104 cfu/g, respectively, but at a higher inoculation level. Yeasts slightly increased the ergosterol contents of the final products, but did not have any obvious effects on amino acid, phytate and vitamin contents. The most abundant volatile compounds produced by R. oligosporus were ethanol, 2-methyl-1-butanol and 3-methyl-1-butanol on MEA and barleys, and 2-butanone on soybeans. The mushroom odor compounds, 3-octanone and 1-octen-3-ol, were only present in soybean and soybean tempeh. Different strains of R .oligosporus produced similar volatile profiles in similar substrates. LAB did not influence the volatile production by R. oligosporus. Mould growth was quantified by measurement of the ergosterol content, hyphal length determination, image analysis and real-time PCR. Ergosterol content and hyphal length were applicable for the quantification of R. oligosporus when co-inoculated with LAB. Image analysis was developed based on changes of colour, structure and number of grains of barley tempeh. The image data had a good correlation with the ergosterol contents and could be potentially used for online analysis of tempeh fermentation in industrial scale as no destructive sampling is required. A DNA extraction method that leaves barley grain particles almost intact were developed by adding extraction buffer with glass beads to frozen tempeh samples. Real-time PCR based on extracted DNA using species-selective primers was proven a fast and selective method for quantification of both R. oligosporus and yeasts during barley tempeh fermentation.