# Screening of Sourdough Starter Strains and Improvements in the Quality of Whole Wheat Steamed Bread
## 2 Results and Discussion
### Selection of Starters  
#### 2.3.1 Selection of Strains  
Due to the fermentation capacity and acid production capacity, three yeast strains, six LAB strains, and one acetic acid bacteria strain were isolated in this study. The gas-producing abilities of different yeast strains are shown in Figure 3A. The fermentability of yeast strains was evaluated by the drainage rate in 0-4 h as the index of fermentability. The speed and gas production of yeast are comprehensively listed in Figure 3A. The low homology in the early identification of strains Y-4 and Y-11 were not included in this study. Finally, three strains with relatively good fermentation capacity were selected from 13 yeast strains for subsequent experiments, which were Y-2 (_Saccharomyces cerevisiae_), Y-6 (_Pichia kudriuzevii_), and Y-9 (_Wicherhammovces anomalies_). _Saccharomyces cerevisiae_ proved to be a superior starter due to its good gas production and low pH tolerance [24]. _Wicherhammovces anomalous_ is the second most frequently isolated yeast in traditional sourdough fermentation [19].  
The acid-producing capacity of 16 LAB strains was investigated by measuring the change in fermentation broth pH. Organic acids such as lactic acid and acetic acid metabolized by bacterial could reduce pH, and more flavor could be added when sourdough was used as a fermentation agent of whole wheat flour [9]. The pH results of fermentation liquid are shown in Figure 3B. Considering the subsequent fermentation in combination with the same yeast, six LAB of different genera with relatively good acid-producing capacities were selected for subsequent experiments: L-1 (_Lactobacillus johnsonii_), L-2 (_Levilactobacillus brevis_), L-5 (_Pediococcus pentosaceus_), L-12 (_Lacticaseibacillus paracasi_), L-16 (_Lacticaseibacillus casei_), and L-18 (_Pediococcus acidilactici_).  
Figure 2: Phylogenetic tree of yeast based on ITS gene sequence.  
Acidification can increase the solubilization of proteins under acidic conditions because it has a positive net charge and also can affect the viscoelastic behavior of the dough and the activity of cereal and bacterial enzymes [25]. The above four strains of acetic acid bacteria L-10, L-11, L-13, and L-15 were relabeled as A-1, A-2, A-3, and A-4, respectively. The four strains of acetic acid bacteria were all _Acetobacter pasteurii_, but their acid producing abilities were different. A-1 was selected for subsequent experiments (Figure 3C). The most prominent manifestation of acetic acid bacteria is the production of acetic acid, which can reduce the pH and inhibit the growth of fungus. The data showed that acetic acid bacteria were relatively common in starter cultures, but this had been overlooked in past studies [24].  
Figure 3: (**A**) Gas production rate of yeasts (mL-h\({}^{-1}\)). (**B**) Rate of fermentation broth pH change of LAB (h\({}^{-1}\)). (**C**) Rate of fermentation broth pH change of acetic acid bacteria (h\({}^{-1}\)). Different superscript letters (a–g) above bars indicate significant difference (\(p<0.05\)).