Microorganisms produce the metabolites that form flavour substances during the brewing of Baijiu [24]. The difference of microbial species and community structures affect the formation of different flavour [2]. Therefore, there is an increasing interest in revealing the characteristics of microbial community and the important influence of changes of microbial community on Baijiu brewing [25].
Bacillus in starter can secret various hydrolases including amylases, proteases, and lipases for macromolecular hydrolysis and produce specific flavour compounds improving the sensory experience and confer health benefits, such as acetate, higher alcohols, diacetyl, pyrazines and aromatic compounds (Zhang et al., 2017). Moreover, cooperation Bacillus and Aspergillus produce more aromatic compounds [10]. Pantoea bacteria is an important heterotrophic bacterium with metabolizing and brewing. Although the abundances of Acetobacter and Gluconobacter are not high during brewing, they can produce precursor acetic acid of ethyl acetate contributing positively to the flavour, which is the main flavour substance of xiaoqu Baijiu (Wang et al., 2018b).
Lactobacillus, Weissella, Pediococcus, Leuconostoc and Lactococcus belong to Lactic acid bacteria (LABs). The increase trend of Firmicutes in the process of brewing Baijiu was mainly related to the increase in abundance of Lactobacillus [26, 27]. LABs, facultative anaerobic or anaerobic bacteria [28], have important role in regulating biological structure in Baijiu brewing. LABs also predominate in sauce-flavour, strong-flavour, and light-flavour [26, 11, 6] during the brewing stage. Lactic acid and acetic acid, produced by lactic acid bacteria, can react with ethanol to produce ethyl lactate and ethyl acetate, which are important flavour substances in xiaoqu Baijiu. [23]. But too much ethyl lactate will bring a undesirable flavour in light-flavour Baijiu. Lactic acid, produced with the increase of LABs, reduced the pH in fermented grains influencing community structure [23] and inhibited the propagation of other spoilage bacteria [29]. A negative correlation between LABs and other prokaryotic genera were observed during the brewing process [23]. LABs, also possessing different enzymes such as glucosidase, esterase and proteases, played a critical role in the formation of volatile compounds in fermented foods [30, 31]. The type, quantity and dynamic change of LABs are very important for improving Baijiu taste. LABs may be introduced from the environment of xiaoqu Baijiu brewing workshop during sorghum cooling, because no LABs added in xiaoqu. The abundance of LABs was very high on the surface of ground and tools in the brewing workshop [32].
Molds and yeasts are mainly responsible for saccharification and alcoholic brewing to produce Baijiu [33]. Rhizopus and Aspergillus are considered as important functional microorganisms in many fermented foods [13]. The relative abundance of Aspergillus and Rhizopus decreased due to the continuous increased concentration of acids and alcohols. Rhizopus oryzae has high amylase production capacity, which can degrade starch in sorghum, and is widely used in koji and Baijiu brewing [34]. It affects Baijiu flavour by produce glycerol, lactic acid, enzymes and volatile compounds, such as ethanol, 2-methyl-1-butanol, and 3-methyl-1-butanol [35]. Aspergillus produce a wide spectrum of proteolytic and other lytic enzymes, playing an important role in saccharification of starch and protein hydrolysis in sorghum [36]. Aspergillus can also promote the synthesis of some esters, improving the Baijiu quality.
Saccharomyces, having strong ethanol brewing capacity under anaerobic conditions, is responsible not only for the alcohol yield but also for the flavour and taste feature of xiaoqu Baijiu [13, 37]. Saccharomyces also interact with other microorganisms such as mold and bacteria, improving the quality of xiaoqu Baijiu. Saccharomyces could tolerate anaerobic and high ethanol concentration. Therefore, relative abundance of Saccharomyces increased in the xiaoqu Baijiu brewing process. Issatchenkia, abundant in soil and fermented grains, can produce ethanol and ethyl acetate with acid resistance, high temperature resistance and ethanol resistance. It is a functional fungal in the brewing process of Baijiu, and has been reported in many Baijiu Daqu and fermented grains with different flavour types [32, 38, 39].
Baijiu could maintain its character due to the similar microbial communities of different batches in the same period. The difference of microbial communities in different workshops or periods may cause the different flavour profile even same processing procedures were used [25]. The quantity and species of microorganisms such as bacteria, yeast and molds in fermented starters, more conducive to the yielding of the flavour compound, are the key factors to produce high-quality Baijiu [40]. They were altered due to the change the micro-environment, such as reduction of substrate, decrease of pH, increase of alcohol concentration, anaerobic situation, biotic and abiotic interactions [41, 42].
Xiaoqu and xiaoqu Baijiu are produced in an open environment [1]. Microorganisms is derived not only from the starters, but also the environment encountered in the process of material pretreatment [3, 43], such as water, machine, tools, ground and air in material pretreatment room. Therefore, the quality of Baijiu depends on well-balanced microbial community from the environment and starter. Microorganisms such as Lactobacillus, Bacillus, and Saccharomyces were supposed to be introduced into sorghum from brewing workshop environment during cooling and entered into the alcoholic brewing stage [32]. Lactobacillus and Saccharomyces were the predominant bacterial and fungal genera at the later stage of brewing, respectively. They are closely related to the formation of various flavours during xiaoqu Baijiu brewing.