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Sesame flavour baijiu: a review

2020; Wiley; Volume: 126; Issue: 3 Linguagem: Inglês

10.1002/jib.614

2050-0416

Wenqing Zhang, Jinglei Li, Zhiming Rao, Guanru Si, Xian Zhang, Gao Chuan-qiang, Ming Ye, Zhou Ping,

Polyamine Metabolism and Applications

Journal of the Institute of BrewingVolume 126, Issue 3 p. 224-232 Review articleFree Access Sesame flavour baijiu: a review Wenqing Zhang, Wenqing Zhang orcid.org/0000-0003-3515-6489 Engineering Research Centre of Bioprocess, School of Food and Biological Engineering, Hefei University of Technology, 230009 Hefei, Anhui, ChinaSearch for more papers by this authorJinglei Li, Jinglei Li Engineering Research Centre of Bioprocess, School of Food and Biological Engineering, Hefei University of Technology, 230009 Hefei, Anhui, ChinaSearch for more papers by this authorZhiming Rao, Zhiming Rao The Key Laboratory of Industrial Biotechnology, Ministry of Education, School of Biotechnology, Jiangnan University, 214122 Wuxi, Jiangsu, ChinaSearch for more papers by this authorGuanru Si, Guanru Si Research Institute of Jiangnan Small Pit Brewing Technology, 242000, Xuanjiu, Xuancheng, Anhui, ChinaSearch for more papers by this authorXian Zhang, Xian Zhang The Key Laboratory of Industrial Biotechnology, Ministry of Education, School of Biotechnology, Jiangnan University, 214122 Wuxi, Jiangsu, ChinaSearch for more papers by this authorChuanqiang Gao, Chuanqiang Gao Research Institute of Jiangnan Small Pit Brewing Technology, 242000, Xuanjiu, Xuancheng, Anhui, ChinaSearch for more papers by this authorMing Ye, Corresponding Author Ming Ye yeming123@sina.com Engineering Research Centre of Bioprocess, School of Food and Biological Engineering, Hefei University of Technology, 230009 Hefei, Anhui, ChinaCorresponding author: Ming Ye, Engineering Research Center of Bioprocess, School of Food and Biological Engineering, Hefei University of Technology, 230009, Hefei, Anhui, China. Email: yeming123@sina.comSearch for more papers by this authorPing Zhou, Ping Zhou Research Institute of Jiangnan Small Pit Brewing Technology, 242000, Xuanjiu, Xuancheng, Anhui, ChinaSearch for more papers by this author Wenqing Zhang, Wenqing Zhang orcid.org/0000-0003-3515-6489 Engineering Research Centre of Bioprocess, School of Food and Biological Engineering, Hefei University of Technology, 230009 Hefei, Anhui, ChinaSearch for more papers by this authorJinglei Li, Jinglei Li Engineering Research Centre of Bioprocess, School of Food and Biological Engineering, Hefei University of Technology, 230009 Hefei, Anhui, ChinaSearch for more papers by this authorZhiming Rao, Zhiming Rao The Key Laboratory of Industrial Biotechnology, Ministry of Education, School of Biotechnology, Jiangnan University, 214122 Wuxi, Jiangsu, ChinaSearch for more papers by this authorGuanru Si, Guanru Si Research Institute of Jiangnan Small Pit Brewing Technology, 242000, Xuanjiu, Xuancheng, Anhui, ChinaSearch for more papers by this authorXian Zhang, Xian Zhang The Key Laboratory of Industrial Biotechnology, Ministry of Education, School of Biotechnology, Jiangnan University, 214122 Wuxi, Jiangsu, ChinaSearch for more papers by this authorChuanqiang Gao, Chuanqiang Gao Research Institute of Jiangnan Small Pit Brewing Technology, 242000, Xuanjiu, Xuancheng, Anhui, ChinaSearch for more papers by this authorMing Ye, Corresponding Author Ming Ye yeming123@sina.com Engineering Research Centre of Bioprocess, School of Food and Biological Engineering, Hefei University of Technology, 230009 Hefei, Anhui, ChinaCorresponding author: Ming Ye, Engineering Research Center of Bioprocess, School of Food and Biological Engineering, Hefei University of Technology, 230009, Hefei, Anhui, China. Email: yeming123@sina.comSearch for more papers by this authorPing Zhou, Ping Zhou Research Institute of Jiangnan Small Pit Brewing Technology, 242000, Xuanjiu, Xuancheng, Anhui, ChinaSearch for more papers by this author First published: 06 July 2020 https://doi.org/10.1002/jib.614Citations: 6AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract Sesame flavour baijiu is an important style of Chinese liquor (Baijiu). However, the characteristic flavour substances of sesame flavour baijiu and associated microbial metabolic mechanisms are unclear. In recent years, research on sesame flavour baijiu has grown. This review focuses on current studies on the brewing process, the diversity of microorganisms involved in fermentation and the important aroma compounds in sesame flavour baijiu. This review will be helpful in understanding the brewing process of sesame flavour baijiu, but also in highlighting measures for improving quality. © 2020 The Institute of Brewing & Distilling Introduction Chinese liquor or baijiu is a transparent and highly alcoholic (38-65% abv) beverage. It is traditionally fermented and distilled product that is enjoyed by people in China and other East Asian countries (1). Baijiu is a liquor with a long history, and is one of the six most popular distilled spirits in the world together with brandy, whisky, vodka, rum and gin (2). The brewing materials, production processes and aroma components of baijiu are different from these other distilled liquors (see Table 1 for comparison). Baijiu uses sorghum, wheat, rice, corn and other grains as raw materials and the starter microbial culture ‘qu’. Baijiu is commonly prepared using multi-microbe solid state fermentation and a solid state distillation, with ageing in clay pots and then stainless steel containers (Table 1) (3). The saccharification and fermentation of baijiu occur simultaneously. Qu, the starter culture, provides the enzymes and microorganisms for saccharifying and fermenting the starch in the raw materials. The aroma compounds of baijiu are mainly esters, alcohols, acids, aldehydes, ketones and other substances in low concentration. These odorants are mainly produced by microorganisms during fermentation, in contrast to the flavour compounds of brandy and whisky which are formed during storage in oak barrels (1, 2). Baijiu is divided into 12 flavour types according to the different raw materials and production processes used: strong (nong in Chinese), light (mild, qing), sauce (jiang), sesame (zhima), rice (mi ), chi, dong, nongjiang, feng, te, laobaigan, and fuyu (4-7). Among them, the special rice flavour and chi flavour baijius are produced through semi- solid state fermentation (2). Table 1. Differences between the production processes for the six main distilled beverages (1-4) Whisky Brandy Rum Vodka Gin Baijiua Raw materials Barley Grape Sugar cane juice, molasses Potato, wheat, barley, molasses, rye Barley, rye, oats, juniper berries Sorghum, wheat, corn, rice Saccharification - fermentation Saccharification - fermentation Fermentation Fermentation Saccharification - fermentation Saccharification - fermentation Simultaneous saccharification and fermentation Fermentation method Liquid state fermentation Liquid state fermentation Liquid state fermentation Liquid state fermentation Liquid state fermentation Solid state fermentation Fermented microorganism Pure yeast culture Yeast culture Pure culture of yeast and butyric acid bacteria Pure yeast culture Pure yeast culture Mixed culture of microorganisms from daqu, fuqu, pit mud, tools, environment Distillation method Liquid state distillation Liquid state distillation Liquid state distillation Liquid state distillation Liquid state distillation Solid state distillation Distillation equipment Pot Stills Pot Stills Distillation kettle Distillation kettle Distillation kettle Zeng Aging in Oak barrel Oak barrel Oak barrel - - Clay pot and stainless steel tank a Sesame flavour baijiu as a typical example of baijiu. Sesame flavour baijiu, one of the twelve types of baijiu, was established after the founding of the People’s Republic of China and is the most recent flavour type of baijiu. The sesame flavour was first found in Jingzhi baijiu in the 1950's (8-10) and classified as a new type in the 1960's. Subsequently, the light industry standard (QB/T 2187-1995) and the national standard (GB/T 20824-2007) on sesame flavour baijiu were published in 1995 and 2006, and then officially established (10). The flavour characteristics of strong, light and sauce flavour baijiu were absorbed into the production process for sesame flavour baijiu, as well as modern technological factors. Sesame flavour baijiu possesses the properties of ‘purity’ and ‘elegance’ from light flavour baijiu, those of ‘softness’ and ‘fullness’ from strong flavour baijiu, and those of ‘refinement’ and ‘nuance’ from sauce flavour baijiu (10, 11). Comprehensive sensory evaluation has shown that the typical aroma of this baijiu is a unique aroma of roasted sesame-like flavours. Fuqu, the combination of bacteria, yeast and moulds, prepared by modern biotechnology and the traditional starter daqu are used for fermentation in the production of sesame flavour baijiu. Compared with the other flavour types of baijiu, the use of Fuqu is an innovative improvement in traditional baijiu production (1). Indeed, in the field of baijiu research, sesame flavour baijiu has been attracted much attention (12-17). However, with the increasing difficulty and complexity of the production process, the retail price of sesame flavour baijiu has also increased. The main production areas of sesame flavour baijiu are concentrated in the Shandong, Jiangsu, and Anhui provinces of China. The largest manufacturers are: Jingzhi, Baotuquan and Bandaojing in Shandong; Meilanchun, Jinshiyuan and Yanghe in Jiangsu; and Xuanjiu and Gujinggong in Anhui (4, 18). Despite this, the brewing process for sesame flavour baijiu is not fully understood and the key aroma compounds are not clearly identified. Further, the relationship between the flavour compounds, the brewing process and the microorganisms in the production of sesame flavour baijiu production have yet to be resolved. Accordingly, this review summarises the published research on the brewing process, the microorganisms and the important aroma compounds of sesame flavour baijiu so as to better understand the characteristic flavouring components and the interactions with the brewing process and associated microorganisms. Brewing process The production process of sauce flavour baijiu is the basis of the process for brewing sesame flavour baijiu (18). This production process combines the process strengths of the three basic flavours (sauce, strong, and light flavours) of baijiu to form the process style (19). Figure 1 outlines the brewing process for sesame flavour baijiu. Overall, there are eight major steps: preparing the raw material; moistening the grains; steaming the grains; mixing with starter cultures; stack fermentation; solid state fermentation; distillation; and fresh liquor storage (20). Figure 1Open in figure viewerPowerPoint The brewing process for sesame-flavour baijiu Raw materials The raw materials used for brewing sesame flavour baijiu consist of grains, bran and rice husks (1, 21) with sorghum being the most commonly used grain for baijiu brewing (22). The sorghum is crushed into 4-6 flakes, such that less than 20% pass through a 20-mesh sieve. Rice and wheat are also sometimes used for producing sesame flavour baijiu. If wheat is used, it is crushed into 2-4 flakes, but rice is not crushed. After mixing the crushed sorghum and wheat, water is added at 80-85°C (Figure 1). This is mixed evenly and stacked for 1-2 hours. Rice and glutinous rice are then added with further mixing. The grains are then moistened by trapezoidal stacking for 2-3 hours. Depending on the characteristics of each type of grain, different processing and moistening techniques are used to optimise the cooking, gelatinisation and fermentation stages (2, 23). The use of bran enhances the protein content in the brewing materials, which helps to form the flavour of sesame flavour baijiu (10). As a by-product of wheat flour processing, bran is rich in protein (13-16%). Rice husk is used as a filler to adjust looseness and accordingly influences the oxygen availability, acidity and moisture of the fermented grains (or ‘Zaopei’ in Chinese). Starter culture – qu, daqu and fuqu Qu is important for baijiu fermentation and flavour formation (24-26). It plays three main roles: providing flavours formed during the qu preparation process; providing enzymes and microorganisms for saccharification, fermentation and producing flavours; and contributing unused starch and protein from the qu preparation process as raw materials for fermentation (27). Compared with the other flavour types of baijiu, the qu used for brewing sesame flavour baijiu is more complicated, as it contains both daqu and fuqu (10). Daqu is a qu used for brewing traditional baijiu and is named after its brick shape, with dimensions of typically 30 × 20 × 10 cm. Daqu uses wheat, barley and peas as raw materials and is prepared by culturing microorganisms from the environment in an open system (27, 28). Depending on the maximum temperature that the culture reaches during preparation, daqu is divided into three types: high temperature (60-70°C), medium temperature (50-60°C) and low temperature (40-50°C) (22, 29)). High temperature daqu is mainly used in the production of sauce flavour baijiu with medium temperature daqu used for strong flavour baijiu (30). High temperature and medium temperature daqu are used for sesame flavour baijiu production to increase its fullness and elegance. However, the bricks of daqu have to be broken up before they can be used. Fuqu the other starter culture in sesame flavour baijiu fermentation is named after bran (Fupi in Chinese) as the raw material (31). Fuqu is prepared by culturing moulds (Aspergillus kawachii), yeasts (Saccharomyces cerevisiae, Hansenula and an ester producing yeast) and bacteria (Bacillus subtilis, Bacillus amyloliquefaciens and Bacillus licheniformis). Fuqu can be divided into three types: fungal, yeast and bacterial fuqu. The microorganisms used for fuqu preparation are selected for their contribution to sesame flavour baijiu including enhancing the level of enzymes (eg amylase, glucoamylase and protease) or for contributing aroma compounds such as tetramethylpyrazine, guaiacol and furan (20, 32). Compared with the other types of baijiu, fuqu allows the production of sesame flavour baijiu of high quality in a stable process (5). The bran in fuqu can also increase the protein content of the raw material for brewing sesame flavour baijiu (10). Stacked fermentation Sesame flavour baijiu fermentation is in two parts - stacked fermentation followed by solid state fermentation (Figure 1). After mixing with daqu and fuqu, the zaopei is stacked in a ladder shape, typically to a height of 55-65 cm. The solid state fermentation is carried out in a pit, under anaerobic conditions. Stack fermentation lasts for 2-3 days at an initial temperature of 20-25°C. The stacked zaopei are mixed after 24 hours, then left stacked until a significant number of white spots (yeast) appear on the surface. The temperature in the stack reaches 50-53°C at this point and the fermentation ends. The higher temperature during stack fermentation promotes the formation of heterocyclic aroma compounds through the Maillard reaction (10, 20) which are typical of sesame flavour baijiu. Stack fermentation also plays an important role in producing sauce flavour baijiu. Unlike sesame flavour baijiu, no fuqu is added to the zaopei before stack fermentation with the microorganisms being enriched from the environment. Accordingly, stack fermentation takes longer for sauce flavour baijiu than for sesame flavour baijiu (5). Solid state fermentation After stack fermentation, the zaopei is mixed, cooled to 22-24°C, then placed in a pit for solid state fermentation for 40 to 45 days (Figure 1, Figure 3) (20). Solid state fermentation for baijiu follows a traditional process. After the zaopei is loaded into the pit, it is covered with a layer of food grade plastic film then sealed with mud to maintain an anaerobic environment. Once in the pit, it is difficult to control zaopei fermentation until the end of the process. Therefore, before it enters the pit, it is important to manage the zaopei parameters with a temperature of 22-24°C, starch content of 16-20%, a moisture content of 50-55% and an acidity between pH 1.8-2.8. The fermentation temperature of the zaopei in the pit increases slowly during the first few days, reaching 36°C on the seventh day then maintaining this temperature for 4 to 5 days, before slowly decreasing (2). During the early stages of fermentation, the increase in temperature is caused by saccharification and growth of microorganisms. During the middle fermentation stage (at the highest temperature), the starch content decreases rapidly, with the formation of ethanol and acid. During the later stages of fermentation, the rate of saccharification and microbial activity decreases along with the temperature (1, 2). The aromatic compounds, especially esters, are mainly generated during this period. The pit used in sesame flavour baijiu production is similar to that used for other styles of baijiu. The typical sizes of the pit are length 3 m; height 2 m; and width 1.7 m (Figure 3). The bottom of the pit is covered with mud, with the side walls made of brick (10). The mud contributes the brewing microorganisms, traditionally prepared by inoculating the aged mud in normal fermentation pits (34). These days, the mud is usually prepared by inoculating functional bacteria, which produce caproic acid, methanogen and butyric acid (35-37). The aged mud that has been repeatedly in contact with zaopei, is important for baijiu fermentation and its flavour formation (38, 39). Stones and mud can also be used to make the side walls of the pit (Figure 3B). Of these three materials, mud is the most suitable habitat for microorganisms, followed by brick and stone. The pit used for sesame flavour baijiu is different from the strong flavour baijiu (the bottom and the side wall are both made of mud), the sauce flavour baijiu (the bottom is mud and the side wall is stone) and the light flavour baijiu (ceramic cylinder, with no mud) (10, 40, 41). The pit for sesame flavour baijiu can be regarded as a combination of various baijiu brewing approaches. Therefore, the aroma of the fresh liquor distilled from zaopei in each layer of the pit is different (12, 20). The bottom layer (zengguo, section Z8, Figure 3C) is influenced by the mud with the fresh liquor with this layer of zaopei having a high concentration of ethyl hexanoate to produce strong flavour baijiu (38). The fresh liquor from zaopei in the middle layer (Z3-Z7, Figure 3C) has a high levels of ethyl acetate to produce light flavour baijiu. The fresh liquor from zaopei in the upper layer (Z1-Z2, Figure 3C) has a strong aroma and sauce flavour to produce sauce flavour baijiu (10). The zaopei in the different layers are separately distilled, and the fresh liquors obtained are collected and stored separately. Distillation After fermentation, the fermented grains (zaopei) is distilled using solid state distillation equipment or zeng (2). The batch distillation tower is designed according to the characteristics of the zaopei. Typically, its upper diameter is 2 m; bottom diameter 1.8 m; and height 1 m and is usually divided into two parts: the steamer (Zengguo in Chinese) made of stainless steel or wood together with the condenser (Figure 2). The condenser is either water cooled or air cooled. The zengguo is filled with the zaopei then hot steam is introduced at the base (Figure 2) (33). The aroma components and alcohol in the zaopei are repeatedly vaporised then condensed, so that they are continuously concentrated and extracted, and eventually form the fresh liquor in the condenser (2). To achieve effective distillation, the zaopei is placed in layers when loaded into the steamer which can take about 45 minutes. The pressure of the hot air is 0.05-0.07 MPa. The fresh distilled liquor is produced at a rate of 1.2-1.5 kg/min at a temperature of 30-35°C. The zengguo can also be used for steaming raw grains. For example, after solid state fermentation, the zaopei can be divided into batches in the zengguo then distilled as the top down layer in a pit (2, 33). Figure 2Open in figure viewerPowerPoint Schematic diagram of distillation equipment or zeng Storage and recycling of fermented grains According to its position in the pit, the zaopei for sesame flavour baijiu can be divided into two types: surface fermented grains (Z1, Z2, Figure 3C) and fermented grains (Z3-Z8, Figure 3C). The fresh distilled liquors from the surface fermented grains are stored separately. The fresh liquors are usually aged in clay pots for 2-3 years then stored in stainless steel tanks for a further 2-3 years (Table 1, Figure 1). The long storage time is very important for forming the soft style of sesame flavour baijiu by volatilising substances with a low boiling point, promoting hydration reactions between water and ethanol, and achieving a balance of aroma substances (10). Figure 3Open in figure viewerPowerPoint The pit for production of sesame flavour baijiu. A: the side wall of the pit made of brick; B: the side wall of the pit made of stone and mud; C: zaopei in the pit (8 Zeng sections Z1-Z8); D: the distribution of zaopei in the pit for the next round of fermentation. [Colour figure can be viewed at wileyonlinelibrary.com] After distillation, the surface fermented grains cannot be used for brewing baijiu and are discarded (Figure 1) for animal feed (2). The fermented grains (Z3-Z8, Figure 3C) are divided into two parts after distillation (Figure 1). One part (Z3, Z8, Figure 3C) is distilled then cooled after which daqu, fuqu, bran and rice husk are added to the surface fermented grains for the next round of fermentation of sesame flavour baijiu (Figure 1, Figure 3D). The other part of the fermented grains (Z4-Z7, Figure 1, Figure 3C) are distilled then cooled after which daqu, fuqu, bran and rice husk together with treated grains are added to the fermented grains in the next round of fermentation for producing sesame flavour baijiu (Figure 1, Figure 3D). Therefore, some sections of zaopei are recycled when producing sesame flavour baijiu to promote the utilisation of the raw materials, to accumulate more aromas and to stabilise the fermentation process. This processing method is derived from that used for producing strong flavour baijiu (33, 40). After distillation, the zaopei and the moistened grains are mixed and steamed, which helps to increase the purity of the sesame flavour baijiu and also to transfer the odour of grain into the zaopei. This processing method is derived from that used for producing light flavour baijiu (10, 42). Microbial diversity Baijiu is brewed with a variety of microorganisms in an open environment. Starch, protein and other macromolecules in the raw materials are converted into ethanol and flavour compounds during the process. The microorganisms used to make baijiu are mainly derived from the starter (daqu and fuqu), the pit mud, the process environment and the tools that are used (43). The starter contributes the fungal communities involved in baijiu fermentation, while the environment and tools contribute the bacterial communities (44), emphasising the importance of fermentation in baijiu brewing (45). Here, recent studies are reported on the microbial diversity of sesame flavour baijiu fermentation including the stack and solid state fermentation. During the process of stack fermentation, the loading of yeast and bacteria increase while those of mould decrease. The starch in the raw materials is converted into sugar by the enzymes in the starter which are then used for microbial growth and metabolism. As the temperature of the zaopei increases, the growth of moulds is suppressed with an increase in heat resistant Bacillus in the fuqu. Yeasts grow well in an aerobic and slightly acidic environment and grow on the surface of the zaopei during the stack fermentation. The anaerobic, acidic and high temperature environment created inside the stack provides the conditions for enzyme action, thermal degradation and non-enzymatic chemical reactions. Large numbers of bacteria increase the activity of proteases in zaopei, promoting the degradation of protein, and producing nitrogen containing heterocyclic compounds which are aroma precursors for sesame flavour baijiu. The distribution of the microflora in zaopei can be altered by the local environment (10, 20). Xu et al. (46) studied changes in microbial biomass and community structure during the stack fermentation of sesame flavour baijiu by analysis of phospholipid fatty acids. They showed that the duration and temperature of stack fermentation had a marked effect on the biomass content with bacteria the dominant microbial flora. Cao et al (47) analysed the prokaryotic and eukaryotic microbial communities during the stack fermentation for sesame flavour baijiu using Illumina high-throughput genetic sequencing. Bacillus, Weissella and Acetobacter bacteria were the dominant prokaryotic genera (> 71.9%) at the beginning of stack fermentation (0-6 hours). After 20 hours, Weissella decreased and the proportion of Acetobacter and Lactobacillus increased significantly. At the start of stack fermentation, the dominant eukaryotic microorganisms were Aspergillus (72.8%), Pichia (13.4%) and Saccharomyces (8.9%). After 6 hours, Aspergillus decreased, while Pichia and Saccharomyces gradually increased becoming the dominant flora during stack fermentation. Cui et al (48 also confirmed that Bacillus was the most dominant prokaryote during stack fermentation with Lactobacillus, Acetobacter and Pediococcus increasing during the late stages. Aspergillus, Pichia, Saccharomyces and Candida were the dominant eukaryotic microorganisms. During the solid state fermentation of sesame flavour baijiu, the temperature of the zaopei is a direct indication of microbial activity and an important factor in changing the microbial community (49) as different microorganisms have different optimal temperature ranges. When the temperature is 28-30°C, yeasts grow most rapidly, with optimum fermentation occurring at 31-35°C. As the temperature rises to 37°C, yeast metabolism become compromised and above 40°C the yeasts die. In contrast, the optimal temperature range for the growth of bacteria is 35-37°C. In the early stages of solid state fermentation (0-10 days), the temperature of zaopei in the pit is low, the oxygen concentration and nutrient availability is high (4). After a adaptation period (0-2 days), microorganisms begin to proliferate rapidly (48, 49). At this stage, metabolism is predominately aerobic and exothermic, resulting in the zaopei temperature increasing. As fermentation progresses, the rate of oxygen consumption increases, and metabolism becomes progressively more anaerobic. Sugars are fermented to produce ethanol and carbon dioxide, generating heat which raises the temperature of the zaopei to its highest point (1, 2, 4, 49). From the initial stage of solid state fermentation to the top temperature phase, the temperature rises gradually, reaching optimal values for the growth and metabolism of diverse microorganisms (49-51). If the temperature rises too quickly or the top temperature is too high, microbial fermentation will be compromised. During the period of stability (10-15 days), heat generation and dissipation in the pit are in a dynamic equilibrium such that the temperature of the zaopei is maintained close to the top temperature with concomitant production large amounts of ethanol (2, 4, 48-51)). In the late stages of fermentation (> 15 days), the microecological environment in the pit deteriorates as the nutrient content of the zaopei decreases alongside the accumulation of ethanol and organic acids (1, 2, 49). Substantial numbers of microorganisms die, such that only acid tolerant, anaerobic bacteria remain active in the zaopei (48, 49). Accordingly, microbial activity diminishes and heat production decreases. At the same time, organic acids and corresponding esters are produced (2, 4, 49). The esterification reaction is endothermic, which gradually lowers the temperature of the zaopei in the pit. Li (50) found that the dominant genera during the solid state fermentation process for sesame flavour baijiu were Lactobacillus, Bacillus, Pichia, Saccharomyces, Aspergillus, Wickerhamomyces and Geotrichum. The solid state fermentation of sesame flavour baijiu was divided into two stages with the first stage when the temperature in the pit increased from 22°C to the highest temperature (35 ± 2°C), when the rate of ethanol synthesis and reducing sugar consumption were at the highest. During this stage, acidity was the environmental factor influencing the succession of microbial communities (51). The second stage was defined as when the temperature in the pit decreased from the top temperature to 26 ± 1°C, and the acidity increased at its highest rate. During this stage, ethanol was the major environmental factor influencing the microbial communities (50, 51). Additionally ethanol and acidity act synergistically on the development and changes in the microbial community (50). During the first stage, Bacillus (relative abundance, 54.1%) was the dominant microbial genus of the prokaryotic microbial communities which also included Lactobacillus (17.9%), Staphy