# Screening of Sourdough Starter Strains and Improvements in the Quality of Whole Wheat Steamed Bread
## 1 Introduction  
Steamed bread is one of the traditional staple foods, accounting for over 40% of wheat products in China. It is usually prepared from wheat flour, microbial starter, and water. When compared to traditional sourdough, steamed bread made with commercial dry yeast has a plain flavor, poor aroma, and low overall sensory quality. However, when compared to refined wheat flour, whole wheat flour contains higher levels of vitamins, minerals, dietary fiber, antioxidants, and other phytochemicals such as carotenoids, flavonoids, and phenolic acids [1, 2]. Studies have shown that the regular consumption of whole-grain foods can effectively reduce the risk of chronic diseases, such as obesity, cardiovascular and cerebrovascular diseases, and diabetes [3]. However, the presence of insoluble polysaccharides in bran and the physical structure of bran particles affect the formation of the gluten network and water distribution of dough, which further affect the texture of the product [4]. Therefore, how to produce high-quality whole wheat flour products is an urgent problem to be solved at this stage. Sourdough is a traditional starter to produce fermented products made from flour. In Europe and other countries, sourdough is usually used to make baked bread, especially traditional sourdough bread and rye bread [5]. In China, sourdough is also called \"Liaomian\" and \"Jiaozi\", which are mainly used to make traditional fermented wheat products such as steamed bread and buns [6, 7]. Traditional sourdough involves a complex micro-ecological environment, mainly including yeasts, lactic acid bacteria, and acetic acid bacteria [8]. The sourdough with a multi-strain mixed fermentation system develops aunique flavor and nutritional quality [9]. To date, more than 70 kinds of lactic acid bacteria and 25 kinds of yeast have been identified in the sourdough system [6]. Both the strains have positive impact on the flavor, taste, and nutritional value of products [10]. The quality of traditional sourdough fermentation is unstable between batches, because the microbial flora is complex and susceptible to endogenous or exogenous factors [11]. Moreover, most of the traditional starters are naturally fermented, and they are produced by households in workshops. The sanitary conditions are poor, and they may contain some miscellaneous bacteria or even harmful bacteria. It is thus difficult to apply to industrial production. Therefore, the separation of dominant strains from Chinese traditional sourdough (CTS) has gradually attracted attention in recent years. [PERSON] et al. [12] fermented steamed bread with different mixing ratios of lactic acid bacteria, and studied the effect on the specific volume, crumb texture, volatile profile, and other aspects of steamed bread quality. [PERSON] et al. [13] collected sourdough from traditional locations and selected excellent strains with phytate degrading ability to make whole wheat bread, and the results showed that the whole wheat bread prepared by _Kluyveromyces aestuarii_ possessed the highest porosity percentage and the lowest hardness among all the tested strains [13]. In the steamed bread fermentation process, yeast and lactic acid bacteria played syneristical roles in giving the sourdough fermented product a unique texture and flavor [7], and the taste in the process is more mellow and delicate, as well as the nutritional value of the products being improved, especially in terms of protein quality and mineral utilization [14]. Since the sourdough fermentation technology has a positive effect on imparting a special flavor to steamed bread, improving its texture, as well as enhancing its nutrition, sourdough fermentation technology can be used to improve the organoleptic quality in whole wheat steamed bread. Against this background, the aim of this study was to enhance the organoleptic quality of whole wheat steamed bread by the microbial strains isolated from CTS.