# Nutrition, Flavor, and Microbial Communities of Two Traditional Bacterial Douchi from Gansu, China
## 1 Introduction  
There are many types of fermented bean products, including natto, miso, tofu, douchi, sufu, cheonggukjang, doenjang, kanjang, meju, and tempeh [1]. Many microorganisms are involved in the production process of fermented bean products, and some bean products require additional inoculation of specific microorganisms for fermentation. During the fermentation process, microorganisms can produce various enzymes, including amylase, protease, and lipase, which play an important role in the quality and flavor formation of fermented bean products [2, 3]. The types of microorganisms in traditional fermented bean products are influenced by various factors, including fermentation materials, processing personnel, production environment, etc. [4].  
Douchi is a traditional fermented bean product that is widely used as a compliant in China due to its unique flavor and taste [1]. Douchi is generally divided into two types: fungal-type douchi and bacterial-type douchi. According to the difference in inoculated microorganisms, fungal-type douchi can be divided into _Apergillus_-type douchi, _Mucor_-type douchi, and _Rhizopus_-type douchi. During the fermentation of bacterial-type douchi with no artificial inoculation of microorganisms, naturally occurring microorganisms, which are mainly bacteria with a small amount of fungi, play a role in fermentation. Douchi is fermented by microorganisms that help to retain the inherent nutritional components of soybeans [2]. Douchi is rich in nutrients and beneficial amino acids, as well as various vitamins, minerals, and physiologically active substances required by the human body. It alsohas medicinal value and a unique flavor [3]. The formation of flavor compounds in douchi is mainly through three main pathways that are closely related to the microbial community. First, the microbial community directly metabolizes and processes large compounds into smaller flavor components. Second, the microbial community produces enzymes related to metabolism that degrade large compounds into small flavor components. In addition, microorganisms themselves can produce precursors of flavor compounds [1;2]. Therefore, the flavor of douchi mainly comes from aroma components (volatile compounds) and flavor substances (mainly free amino acids). The direct contribution of FAAs to flavor is limited, but they are precursors for the formation of other flavor compounds [4]. FAAs can decarboxylate to form amines and carbon monoxide and deaminate to form ammonia and \(\alpha\)- keto acid (further converted into flavor compounds such as aldehydes, alcohols, and acids) [5]. For example, tryptophan generates indole pyruvate through the action of transaminases, which further decompose and metabolize to benzaldehyde [6].  
Longnan and Qingyang douchi are two typical bacterial-type douchi from Gansu Province in northwest China, which are made of soybeans. These two kinds of douchi are very popular with the local people. There have been numerous reports on the nutrition and flavor of _Aspergillus_-type douchi, _Mucor_-type douchi, and _Rhizopus_-type douchi [7;8]. However, there are few reports on the nutrition and flavor of bacterial-type douchi [9; 10]. Therefore, this study focused on the traditional fermented bacterial-type douchi in Longnan and Qingyang and compared their nutritional components and flavor substances. In addition, it analyzed the correlation between flavor and microorganisms to provide theoretical support for the quality control and evaluation of bacterial-type douchi.