Screening And Fermentation Properties Of Yeasts Isolated From Chinese Sourdough

Chinese steamed bread (CSB), as a popular fermented food, mainly consists of flour, water and starters (sourdough starters). Key steps for CSB-making include mixing, fermentation, kneading, dough segmenting, ring and steaming. Starters, consisting of yeast and lactic acid bacteria, greatly affect the sensory and final quality of CSB. The present study deals with the roles of yeasts isolated from traditional sourdough products collected from the Inner Mongolia of China in affecting the fermentation of CSB dough according to their growth and physiological characteristics. It stressed the effects of the selected yeast strains alone or in combination with Lactobacillus paralimentarius 412 on sourdough's fermentation, CSB's texture and aromatic formations. The following are main results.1. Five strains isolated from sourdough, namely Sy11, Sy12, Sy13, Sy13, Sy21 and Sy22, were identified as Saccharomyces cerevisiae according to their microbiological, morphological, physiological and biochemical characteristics. Among them, strains Sy12 and Sy22 fermented dough very well. The two strains were homology (100%) to Saccharomyces cerevisiae D53 in 26S rDNA D1/D2 sequence analysis. Thus, they were named for Saccharomyces cerevisiae Sy12 and Sy22 based on phenotypic and genotypic characteristics.2. The optimal conditions for the cultivation of strains Sy12 and Sy22 included an initiation pH at 5.0, inoculation size of 6%, and incubation time for 12h at 30C. Strain Sy22 and lactobacilli strain 412 grew together when they were co-incubated in sourdough. After 12h fermentation, pH in dough decreased from 5.58 to 3.85. Fermentation ability, expressed in dough volume, kept above 1.65 until 12h-incubation. Presence of L. paralimentarius 412, besides strain Sy22, promoted the acidification and fermentation of dough.3. Increasing the initial inoculation of strain Sy22 started qucikly dough fermentation, and shortened its fermentation time. However, no correlation between the dough acidification and the inoculation size of strain Sy22 was observed. The optimum inoculation size of strain Sy22 was 107CFU/g in the presence of strain 412. The initial inoculant of strain 412 affected the fermentation ability of the dough, acidifying dough very fast. 4. DY (Dough Yield) affected the fermentation rate of dough. An un-completed fermentation was observed when DY was less than 140. However a high DY that was above 180 leaded in an instability in dough fermentation. DY that was 160 was recognized to be best to favor dough fermentation. Addition of carbohydrates to dough promoted fermentation, and shortened the fermentation time. Inclusion of maltose and sucrose favored dough fermentation better than those of glucose and fructose did. The presence of amylase and protease started dough fermentation very quickly.5. Use of the selected strain Sy22, in combination with strain 412, produced good CSB products both in its sensory quality and physical properties. To make good quality CSB products, an extended fermentation time, from 12h to 24h, was required for the manufacture of good dough.