# Screening of Sourdough Starter Strains and Improvements in the Quality of Whole Wheat Steamed Bread
## 2 Results and Discussion
### Isolation and Identification of LAB and Yeast in Sourdough  
Observing the colony growth on YPD plate and MRS plate, we found 40 suspected strains of yeast and 40 suspected strains of LAB. A single bacterial colony was obtained by streaking pure culture; 15 strains of yeast and 20 strains of LAB were screened by colony morphology and electron microscope observation.  
After comparing the sequence homology of dominant strains in the gene bank, those with a sequence homology higher than 95% were selected (Table 2) to build the phylogenetic trees (Figures 1 and 2). Among the 15 strains of yeast and 20 strains of bacteria, three yeast species and six LAB species were identified: _Saccharomyces cerevisiae_ (six strains), _Wicherhamomyces anomalies_ (four strains), _Pichia kudriavzevii_ (three strains), _Lactobacillus jolnsonii_ (five strains), _Pediococcus pentosaccus_ (four strains), _Levililactobacillus brevis_ (three strains), _Lacticaseibacillus casei_ (two strains), _Lacticaseibacillus paracasei_ (two strains), _Pediococcus acidilactici_ (one strain), and _Acetobacter pasteurianus_ (four strains). _Saccharomyces cerevisiae_ is the dominant yeast in sourdough. _Wicherhamomyces anomalus_ is also a common yeast found in sourdough, which can produce substances to inhibit fungus growth during dough fermentation and bread preservation [19; 20]. It has been reported that steamed bread fermented by _Pichia kudriavzevii_ has higher consumer recognition than ordinary steamed bread [21]. _Lactobacillus jolnsonii_ has been proven to reduce the proliferation of Clostridium perfringens in the intestinal tract of poultry, and with its own envelope forms, _Lactobacillusijolnsonii_ plays an important role in inhibiting the proliferation of harmful bacteria [22]. _Levilactobacillus brevis_ can perform heterotypic fermentation and produce lactic acid, alcohol, organic acids, aldehydes, etc., which are important sources of the sourdough fermentation flavor [23]. In addition, _Acetobacter_ was isolated from samples in the Anhui province. The acetic acid produced by _Acetobacter_ metabolism is the characteristic flavor component of sourdough and can inhibit the growth of spoilage microorganisms.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Category** & **Strain** & **Results** & **Homology** & **Serial Number** \\ \hline \multirow{8}{*}{LAB} & L-1 & _Lactobacillus jolnsonii_ & 100 & L-11 \\  & L-2 & _Levilactobacillus brevis_ & 99 & MG5242 \\  & L-3 & _Lactobacillus jolnsonii_ & 99 & L-11 \\  & L-4 & _Lactobacillus jolnsonii_ & 100 & L-11 \\  & L-5 & _Pediococcus pentosaccus_ & 100 & SL4 \\  & L-6 & _Levilactobacillus brevis_ & 99 & S154 \\  & L-7 & _Pediococcus pentosaccus_ & 100 & ENM104 \\  & L-8 & _Levilactobacillus brevis_ & 99 & MG5242 \\  & L-9 & _Pediococcus pentosaccus_ & 99 & ENM104 \\  & L-10 & _Aectobacter pasteurianus_ & 99 & FV-24 \\  & L-11 & _Aectobacter pasteurianus_ & 99 & FV-24 \\  & L-12 & _Lacticoseibacillus paracasei_ & 100 & R094 \\  & L-13 & _Aectobacter pasteurianus_ & 99 & Ap-C92 \\  & L-14 & _Lactobacillus jolnsonii_ & 99 & MH8 \\  & L-15 & _Aectobacter pasteurianus_ & 98 & Ap-C92 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Identification results of yeasts and LAB.  
Figure 1: Phylogenetic tree of bacteria based on 168 rDNA gene sequence.