The Study On Lactic Acid Bacteria And Yeast By Industrial Producing Soybean Paste

Soybean paste is a traditional fermentation soybean product. Fermented soybean paste madewith such materials as wheat and soybean. Soybean paste has been popular more than one thousandyears because it has pleasure aroma, taste, nutrition and function. Microorganism play veryimportant role in the processing of soybean paste. Microorganism found in koji almost alwaysbelongs to fungi species, Aspergillus oryzae and/or Asp. Sojae. The koji mold has the ability toutilize starch, oligosaccharides, simple sugars, organic acids and alcohols, etc as carbon sources,and protein, amino acids and urea, etc as nitrogen sources. Halophilic yeast and lactic acid bacterialalso play an important role in developing flavour during fermentation of soybean paste, particularlyin the later stage of the fermentation. Traditionally, the mixture of boiled soybean and wheat flourmade cuboid or elliptoid shape and kept in an incubation room to make koji. The organisms comefrom either the previous batch or environment in which they are natural present. The inoculumgenerally contains selected flora of salted tolerant yeasts and bacterial capable of growing underanaerobic conditions. The dominant microorganisms are yeast Zygosaccharomyces Rouxii andTorulopsis sp and certain lactic acid bacteria such as Tetragenococcus Halophilus. Since pureculture of these microorganism speeds up fermentation and reduce the influence of weed yeast andbacteria, its use in commercial soybean paste preparation has been popular in recent years. In thispaper we focus on Tetragenococcus Halophilus, Zygosaccharomyces Rouxii and TorulopsisCandida, to produce commercial used lactic acid bacteria starter and dried yeast that can be appliedin industrial producing soybean paste.1. Tetragenococcus Halophilus can be grown up at 12% NaCl, and Zygosaccharomyces Rouxiiand Torulopsis Candida can be grown up at 18% NaCl.2. Tetragenococcus Halophilus is aerobic in MRS in culture medium, with growth optimagenerally at pH 6.5, a temperature of 34℃, a dissoluble oxygen of 15%, a inoculation concentrationof 3%, a stir speed of 120rpm, and the incubation time of 28h. At this optimum cultural condition,the active cell number of Tetragenococcus Halophilus was 1.22×10¹² cfu/mL. The culture organismwas centrifugalized 20 min at 5000rpm. The bacterial concentrate was mixed with 10% skim milkpowder and 5% glycerin, and was dried by vacuum freezing. The active cell number of driedTetragenococcus Halophilus was 5.58×10¹⁰ cfu/g., and the survival rate of the dried bacteria was73.28%. 3. The culture media of Zygosaccharomyces Rouxii and Torulopsis Candida contained raw soysauce, glucose, and (NH₄)₂SO₄. It had been proved that 0.05% and 0.07% K₂HPO₄ in the culturemedia benefit the growth of Zygosaccharomyces Rouxii and Torulopsis Candida respectively. Theoptima fermentation condition of Zygosaccharomyces Rouxii was that controlled dissoluble oxygenof 50%, a pH of 3.5, a inoculation concentration of 8%, a stir speed of 120rpm, a temperature of25℃, a incubation time of 32h. The active cells number of Zygosaccharomyces Rouxii was1.07×10¹¹/mL. The optima fermentation condition of Torulopsis Candida was that controlleddissoluble oxygen of 60%, a pH 3.5, a inoculation concentration of 6%, stir speed of 120rpm, atemperature of 25℃, and a incubation time of 36h. The active cells number of Torulopsis Candidawas 1.55×10¹¹/mL. The culture organism was centrifugalized 20 min at 5000rpm. The yeastconcentrates were washed by 2.5% NaCl several times, and the concentrates were mixed withspan60 and tween80 (1:1). The mixture was dried to 8% moisture content at 40℃. The dried yeastpowder was dissolved in 12Béwort at 36℃(15min), and then the yeast solution was incubated at25℃, 120min. The survival rate Zygosaccharomyces Rouxii and Torulopsis Candida were 91.70%and 91.07% respectively.4. In order to determine the optimal proportion of three strains, the three strains wereinoculated into soybean paste. Based on the effect of single strain on the quality of soybean paste, aL₉(3³) orthogonal experiment design was conducted to study the proportion of TetragenococcusHalophilus, Zygosaccharomyces Rouxii and Torulopsis Candida. The results were thatTetragenococcus Halophilus was inoculated into soybean paste in the 11th day, and the inoculationconcentration was 10⁶cfu/g. Zygosaccharomyces Rouxii and Torulopsis Candida were inoculatedinto soybean paste in the 14th day, however, the inoculation concentration of both was 10⁵ cfu/g and10⁶cfu/g respectively. 24 volatile flavor compounds were detected from soybean paste by GC-MS.The taste, chemical and physical characteristic and flavors, of that fermented by Aspergillus oryzae,yeast and lactic acid bacteria, were better than that only fermented by single Aspergillus oryzae.