Effect of Fermentation Processing on the Flavor of Baijiu - Journal of

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Cite This: J. Agric. Food Chem. 2018, 66, 5425−5432

Effect of Fermentation Processing on the Flavor of Baijiu Huilin Liu and Baoguo Sun* Beijing Advanced Innovation Center for Food Nutrition and Human Health, Beijing Key Laboratory of Flavor Chemistry, Beijing Technology and Business University, 11 Fucheng Road, Beijing 100048, People’s Republic of China S Supporting Information *

ABSTRACT: Baijiu, otherwise known as the national liquor of China, is a well-known distilled spirit that has been produced for over 2000 years. Baijiu is a clear and transparent fermented alcoholic beverage containing more than 1870 volatile compounds, including esters, alcohols, aromatics, ketones, heterocycles, nitrogenous compounds, acids, aldehydes, terpenes, sulfur compounds, acetals, and lactones. This perspective describes the baijiu production process, summarizes recent progress in the development and study of baijiu flavor chemistry, discusses the effect of fermentation processing on baijiu flavor, and presents novel development trends in the baijiu industry in the dual directions of flavor and health. Furthermore, the direction of research and development for modernization in the baijiu industry is discussed and proposed. KEYWORDS: baijiu, Chinese liquor, fermentation, flavor chemistry, developments

1. INTRODUCTION Baijiu, an ancient Chinese liquor, is the national liquor of China and possesses a unique position in traditional Chinese culture. Chinese liquor making has a 9000 year history, with reports as early as 7 millennia before Christ (B.C.).1 Baijiu is one of the oldest distillates in the world, with a 2000 year history dating from the Western Han dynasty (from 202 B.C. to 8 A.D.) according to the record of a distiller from the tomb of Haihunhou. Baijiu is a clear and transparent fermented alcoholic beverage with a high ethanol content ranging from 38 to 65 vol %.2 Baijiu is also the most consumed spirit globally, with over 13 billion liters consumed in 2016, representing a market value of approximately $97.4 billion. Therefore, baijiu constitutes a major part of the Chinese food industry. Baijiu is a world-renowned distilled spirit, alongside brandy, whiskey, vodka, gin, and rum. The raw material, production process, composition, and taste of baijiu are very different from those of other famous distilled spirits. Baijiu is normally prepared from sorghum alone or a mixture of corn, rice, wheat, peas, millet, and sorghum. For baijiu production, jiuqu, known as the foundation of baijiu, is often used as a fermentation starter3 and is mixed with grains to saccharify and ferment simultaneously, which produces ethanol and flavor compounds. The fermented mixture is then distilled under solid-state conditions to produce fresh baijiu liquor. Baijiu is rich in many flavor components, including organic acids (such as acetic, lactic, malic, tartaric, and linoleic acids), esters (such as ethyl acetate, ethyl lactate, and ethyl hexanoate), lactones, phenols, heterocycles, terpenes, and aromatic compounds.4 Furthermore, baijiu contains potential functional components, such as amino acids5 and peptides,6 which are beneficial to humans. A tetrapeptide (Ala-Lys-Arg-Ala) and tripeptide (Pro-His-Pro) have been successfully identified in sesame flavor-type baijiu and showed preventive effects against 2,2′-azobis(2-methylpropanimidamidine) dihydrochloride-induced oxidative stress in HepG2 cells and antihypertensive activity, respectively.6,7 The first economic history book from China, “Shi-Huo-Zhi” by Ban Gu, reported that “Jiu, an alcoholic beverage, has long been © 2018 American Chemical Society

used as a base for traditional Chinese medicine” since the Eastern Han dynasty, which is a good summary of the application of baijiu in medicine and healthcare.8 In the Qiaocheng district of Bozhou, a Gujinggong baijiu production region of China, the longevity of about 500 people that lived for more than 100 years has been partly attributed to the habit of drinking a little baijiu daily. In recent years, Xi et al. have shown that light and moderate alcohol intake might have a protective effect on all-cause and cardiovascular-disease-specific mortality in U.S. adults.9 Therefore, drinking baijiu in moderation might be beneficial to human health.10

2. BAIJIU PRODUCTION The production of baijiu is different from that of other distilled liquors because it combines the two distinctive processes of fermentation and distillation. During fermentation, conversions of starch to sugar and sugar to alcohol take place simultaneously. In the production of other distilled liquors, such as whisky and vodka, the above two reactions occur stepwise, or conversion of sugar to alcohol takes place alone, such as in rum production. Furthermore, most distillation processing is performed in the solid state during baijiu production, for which a special device, called the zeng (Figure 1), is often used. The zeng is a conical distiller (approximate dimensions: top diameter, 2 m; bottom diameter, 1.8 m; and height, 1 m) that is widely used for distillation. The zeng is uniquely designed according to the characteristics of solid-state fermentation, making it different from other distillation units. With scientific advances, the zeng material has been changed from wood to reinforced concrete or stainless steel, and the cooler has been changed from tianguo to straight to improve the cooling efficiency. However, the basic operation of artificial Received: Revised: Accepted: Published: 5425

February 6, 2018 May 8, 2018 May 11, 2018 May 11, 2018 DOI: 10.1021/acs.jafc.8b00692 J. Agric. Food Chem. 2018, 66, 5425−5432

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Journal of Agricultural and Food Chemistry

Figure 1. Traditional production processing of baijiu. Raw material is mixed uniformly with whole or powdered grains before adding hot water. The mixed grains are then fermented, and the husks are transferred to the zeng for distillation. The fresh liquor is then stored and aged in a pottery or stainless-steel vessel. After aging for several years, the stored liquor is blended to obtain differently graded products.

labor remains unchanged, with continuous feeding and discharging still not widely applied. The zeng is a special steaming tower in which, under constant heating conditions, the alcohol concentration of each layer is also constant. The concentration of volatile components decreases layer by layer in the lower section and increases layer by layer in the upper section. These volatile components are obtained from alcohol and aroma compounds in the fermented grains of by vaporization, condensation, and vaporization again, to achieve multicomponent concentration and extraction. The general scheme for traditional processing in baijiu production is shown in Figure 1. Typically, the raw material is mixed uniformly with whole or powdered grains before adding hot water. Cooking the starch granules in hot water causes them to further absorb, expand, rupture, and gelatinize. Meanwhile, the raw materials are sterilized, and some adverse volatile components are eliminated, under high temperature. When the temperature is 50−60 °C, amylase in the raw material decomposes starch into dextrin and sugar. When the whole raw material is cooked, limited sugar is generated by saccharification, but when the raw material is cooked after grinding, more sugar is generated. Some glucose will be converted to fructose and other ketones. Hexoses, such as glucose and fructose, can be dehydrated to produce 5(hydroxymethyl)furfural during high-pressure cooking, which then further decomposes to a keto acid and formic acid. This reaction process is irreversible and proceeds according to the first-order kinetic reaction formula. The mixed grains are then steamed with fermented grains and husks to prepare fresh liquor, also called base liquor. Distillation is the key step in improving baijiu flavor. The distillation efficiency depends upon the steam flow rate, distillation speed, water content, and

porosity of mixed grains. The baijiu is then stored and aged in a pottery or stainless-steel vessel for months or years depending upon the desired final product quality. Finally, the stored liquor is blended with water to reduce the alcohol content before bottling for sale. A repeated batch fermentation process is used for strong-aroma baijiu, as denoted by the red square in Figure 1, where the distilled grains are cooled to about 20 °C with fast air cooling before jiuqu is added and the mixture is placed in a fermentation pit to spontaneously ferment for about 1−3 months at 28−30 °C. The fermentation pit is a special container (approximate dimensions: length, 3.4 m; width, 1.8 m; and depth, 2.0 m) that is widely used for solid-state fermentation. The fermented grains at the bottom of the fermentation pit (∼1:4, v/v) are then distilled to obtain the spirit and discarded, with the same volume of fresh raw material added to repeat the batch fermentation. These distilled grains are also called the vinasse, which is not used further in baijiu production but taken as an important source of feed for pigs, cattle, poultry, and others. The process of light-aroma baijiu production includes only two fermentation processes, as denoted by the blue square in Figure 1, where the steamed raw material with jiuqu is added to a ground cylinder for the first fermentation of about 1 month and then the fermented grains are steamed to produce fresh liquor before the distilled grains, with new jiuqu added, are fermented again for about 1 month. After the second fermentation, these distilled grains comprise the vinasse. Traditional baijiu production methods have existed for thousands of years and mainly rely on the operation skill and experience of individuals. In recent years, some developments in modern biotechnology, automation, and electronic information technology have been applied in baijiu production to 5426

DOI: 10.1021/acs.jafc.8b00692 J. Agric. Food Chem. 2018, 66, 5425−5432

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ogy can easily provide baijiu with acceptable flavors and tastes and has many advantages, including higher fermentation productivity, higher end concentration of products, and higher product stability. However, solid-state fermentation technology has some problems, such as low mechanization, high costs, and long fermentation times, which result in low production efficiency. Semi-solid-state fermentation is used to prepare baijiu with rice and chi aromas in Guangxi and Guangdong provinces of China. This process uses rice as the raw material and jiuqu as the saccharification and fermentation agents, which are soaked in a distillation kettle or altar to achieve semi-solid-state fermentation.14 Liquid-state fermentation generally uses corn as the raw material to produce baijiu, with all processes performed in the liquid state,15 including saccharification, fermentation, distillation, and blending, to give a high production efficiency with simple operation and low material consumption, making it easier to achieve full automation. However, this process produces fewer flavor components, leading to a lower quality, because the only raw material, corn, is fermented with pure species. Liquid fermentation is an important direction for the development of baijiu production, but the problem of poor flavor compounds must be solved. 3.2. Fermentation Starters. Fermentation starters are another major factor affecting fermentation and, consequently, baijiu flavor and taste. Jiuqu contains a variety of microorganisms and enzymes that are essential to baijiu production as saccharification, fermentation, and aroma-producing agents. Fermentation starters are divided into daqu, xiaoqu, fuqu, and other jiuqu, according to the preparation process. Daqu is one of the oldest and most widely used starters in China and uses barley, wheat, or peas as raw materials. Under certain temperature and humidity conditions, Daqu can be formed into a brick shape by adding water, stacking, crushing, stirring, pressing, and cultivating in a Qu house, followed by turning and storage. Daqu is a naturally occurring microbiota in which the main microorganisms are molds, yeasts, bacteria, and actinomycetes, such as Mucor racemosus, Aspergillus niger, Thermomyces lanuginosus, Candida, Saccharomyces cerevisiae, Bacillus subtilis, lactic acid, and acetic acid bacteria.16 Daqu is divided into high-, medium-, and low-temperature daqu according to the cultivation temperature. High-temperature daqu is cultured at 60−70 °C and mainly used for sauce-aroma baijiu production. The main flavor compounds in hightemperature daqu include tetramethylpyrazine, guaiacol, phenylethanol, propanoic acid, 1,3-butanediol, methyl ester, and acetic acid.17,18 However, high-temperature daqu has a lower capacity for saccharification, liquefaction, and fermentation than low-temperature daqu. Medium-temperature daqu is formed at 50−60 °C and mainly used for strong-aroma baijiu production. It can be used to improve baijiu flavor by imbuing a more rich and full taste and producing more caproic acid and ethyl caproate. The main flavor compounds in mediumtemperature daqu include tetradecanoic acid ethyl ester, guaiacol, pyrazines, phenylethyl alcohol, and caryophyllene.17,19 Low-temperature daqu is heated to no more than 50 °C and mainly used for light-aroma baijiu production. Microorganisms are abundant in low-temperature daqu, resulting in a better fermentation capacity and higher yields of baijiu. A light flavor and pure taste are formed as a result of the large amounts of acetic acid, lactic acid, and alcohol compounds, in addition to, typically, hexyl acetate, ethyl acetate, phenylethyl alcohol, ethyl

improve traditional processing. As a result, semi-mechanized processes are widely used in baijiu production by large- and medium-sized enterprises to reduce labor intensity, improve the quality and flavor of baijiu, and reduce manual labor. However, this mode of production still faces a lack of control and standardization. Some enterprises have attempted to use fully mechanized processing for baijiu production, but this impacts the quality and flavor of the resultant baijiu. However, the development of baijiu processing technology is trending toward full mechanization, automation, and intellectualization, simultaneously, to ensure the quality and flavor of baijiu.

3. FERMENTATION PROCESSING OF BAIJIU Sorghum is the main raw material used in baijiu fermentation and contains starch (42.07−68.12%), protein (10.3−15.6%), lipids (3.6−5.2%), water (11.1−13.4%), fiber (1.5−2.7%), and ash (1.6−3.1%). Starch is changed into glucose by saccharification with Clostridium butyricum or amylases in fermented grains. Glucose is degraded to pyruvic acid via fructose 1,6biphosphate in the Embden−Meyerhof−Parnas (EMP) pathway by reaction with Saccharomyces cerevisiae. Pyruvic acid was converted to acetaldehyde by the removal of a carbon dioxide molecule in a reaction with decarboxylase. Acetaldehyde was then transformed into alcohol by reduction with ethanol dehydrogenase. Acid formation results from oxidation during the fermentation process. Ethanol forms acetaldehyde by oxidation and is further oxidized to acetic acid. At room temperature, lower fatty acids react with ethanol to form esters through enzyme catalysis. Because esterification is a reversible chemical reaction, when the highest esterification rate is reached, the esters will decompose into alcohols and acids, which plays a decisive role in the formation of baijiu flavor. Meanwhile, proteins in the raw materials are hydrolyzed into peptides and amino acids by protease catalysis. The aromatic components produced by fermentation mainly originate from the raw material, microorganisms, intermediates, or certain amino acids and tannins from sorghum. 3.1. Fermentation Types. The main fermentation process for baijiu, involving simultaneous saccharification and spontaneous fermentation in the solid state, is responsible for its rich flavor and good taste. This special fermentation process is usually performed in an underground fermentation pit for 1−3 months (strong-aroma baijiu), while some baijiu processes use stacking fermentation above ground before underground fermentation, such as Maotai and sesame-aroma baijiu. The pit mud in the fermentation pit, otherwise known as microbial gold, is a complex ecosystem that hosts diverse microbial communities, including bacteria, archaea, and fungi, which contribute to the production of characteristic flavor compounds, such as ethyl caproate.11 High diversity indices of about 16 phyla and 105 genera have been found in pit mud used to make Luzhou Laojiao baijiu.12 The fermentation of light-aroma baijiu is performed in ground cylinders that do not contain pit mud. Repeated batch fermentation is a complex process, as shown in Figure 1. According to the different fermentation methods, there are three main types of fermentation used in baijiu production, namely, solid-state fermentation, semi-solid-state fermentation, and liquid-state fermentation. Solid-state fermentation technology is commonly used to produce most well-known baijiu beverages with different flavors and characteristics, in which microbial cultures are grown on a solid matrix.13 Furthermore, solid-state fermentation technol5427


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Figure 2. Distribution of well-known baijiu with different aromas.

production is needed for mechanization and automation with modern microbiological and molecular biology technology. 3.3. Raw Materials. The raw materials for baijiu fermentation are mainly prepared from sorghum alone or a mixture of corn, rice, wheat, peas, millet, and sorghum.22 The flavor of baijiu is strongly influenced by its raw materials, with large differences noted when using different types of raw materials or the same raw materials from different regions. In general, sorghum is widely used as the raw material in baijiu fermentation to produce fragrance flavor and high liquor yields and is well-suited to baijiu brewing, owing to its nutritional composition and related physical properties.23 Sorghum is rich in inorganic elements and vitamins, which provides a foundation for the good growth and reproduction of microorganisms. Sorghum also contains tannins, which can be transformed into aromatic substances, such as syringic acid, to give a special aroma to baijiu. Rice contains less fat and cellulose than sorghum, which is too sticky to ferment easily after cooking and has a large ethyl hexanoate content that produces a soft sweet flavor and clean aftertaste.24 Corn contains more phytic acid, which is decomposed into inositol and phosphoric acid during fermentation. Inositol produces the sweet flavor in baijiu. Phosphoric acid can promote the production of glycerol in baijiu to give the liquor a sweet flavor. Owing to their high protein content, peas are not a good raw material for baijiu production. Peas are rich in vanillin and other phenolic compounds, which may contribute to the baijiu aroma. The fermentation of raw materials needs high contents of carbohydrates and certain proteins and tannins, which are suitable for the absorption and utilization of microorganisms. The raw material should also be easy to store and have an easily controllable moisture content to prevent spoilage. However, this does not mean that all high-starch materials are suitable for

alcohol, hexanal, (E)-2-octenal, (Z)-2-octen-1-ol, nonanoic acid, and pyrazines.16,17 Xiaoqu, which is widely used in the south of China, is another naturally occurring microbiota. However, xiaoqu is quite different from daqu and mainly uses rice with some herbs or clay as the raw materials. The main microorganisms in xiaoqu are molds and yeasts, especially Rhizopus sp., Rhizopus oryzae, Rhizopus peka, and S. cerevisiae. The light flavor is mild with rice, herbal, and fragrance aromas. The main flavor compounds include acetic acid, ethyl acetate, phenylethyl alcohol, and pyrazines.17,20 Xiaoqu produces a higher yield than daqu. Fuqu is another important fermentation starter with short production, excellent saccharification and fermentation capacity, and high utilization.21 Fuqu uses bran as the raw material, which is more economical than grains. The main microorganisms in fuqu are molds, such as R. oryzae and Aspergillus. These microorganisms are inoculated into the culture medium by artificial inoculation under optimized conditions. Other microbial enzymes, such as cellulase, amylase, glucoamylase, pectinase, and protease, in combination with daqu, xiaoqu, and fuqu can effectively improve the liquor fermentation yield, which saves labor and energy. Further studies have focused more on the interactions of good and pure strains as saccharification and fermentation agents for baijiu combined with traditional daqu, xiaoqu, fuqu, and pure cultures of multiple strains and commercial microbial enzymes or active dry yeast. This can compensate for flavor compound deficiencies in fermentations using a single strain. Meanwhile, new kinds of microbial enzymes or active dry yeast are needed for further research and development and to explore new full enzymatic brewing processes. Furthermore, jiuqu 5428


semi-solid-state fermentation

solid-state fermentation


rice aroma

feng aroma

te aroma

5429

semi-solid-state fermentation


mixed aromas


chi aroma

herbal aroma

fuyu aroma

solid-state fermentation and cylinder solid-state fermentation

stacking fermentation and solid-state fermentation

sauce aroma

laobaigan aroma


light aroma



strong aroma

sesame aroma

fermentation method

representative aroma

sorghum, wheat, barley, and peas

rice

sorghum, rice, wheat, corn, and glutinous rice sorghum

sorghum

sorghum, wheat, and corn

rice

sorghum

rice

sorghum

sorghum with or without glutinous rice, rice, wheat, and corn sorghum

raw material

high-temperature daqu

xiaoqu

daqu and xiaoqu

medium-temperature daqu and xiaoqu

medium-temperature daqu


medium-temperature daqu


xiaoqu


low-temperature daqu

medium- and low-temperature daqu

fermentation starter ethyl hexanoate, ethyl acetate, ethyl lactate, ethyl butyrate, ethyl valerate, ethyl heptanoate, hexanoic acid, butanoic acid, heptanoic acid, furfural, and phenylethyl alcohol

ethyl acetate, ethyl lactate, acetic acid, 2-methylpropanoic acid, β-damascenone, and terpenoids

ethyl hexanoate, hexanoic acid, 3-methylbutanoic acid, 3-methylbutanol, tetramethylpyrazine, ethyl 2-phenylacetate, 2-phenylethyl acetate, ethyl 3-phenylpropanoate, 4-methylguaiacol, and γ-decalactone β-phenethyl alcohol, ethyl acetate, ethyl lactate, ethylbenzenecarboxylate, and diethyl succinate ethyl acetate, ethyl caproate, and isoamyl alcohol more than 98% esters and alcohols among the total flavor compounds, especially ethyl acetate and ethyl caproate

Clostridium kluyveri, Burkholderia, Streptococcus, Lactobacillus, Lactobacillaceae, Saccharomyces cerevisiae, Saccharomycopsis fibuligera, Talaromyces, Pichia kudriavzevii, Aspergillus, Rhizopus, Eurotium, and Phanerochaete chrysosporium Pichia, Saccharomyces cerevisiae, Saccharomycopsis, Issatchenkia, Hanseniaspora, Torulaspora, Amylomyces, Lactobacillus, Streptpcoccus, Pediococcus, Aerococcus, Bacillus, Acetobacter, Endomycopsis, Candida, Hansenula, Saccharomyces, Geotrichum, Rhizopus, Absidia, Mucor, Aspergillus, Penicillium, and Monascus Aspergillus, Candida, Cryptococcus, Halomonas, Thermomyces, Thermoascus, Paecilomyces variotii, Zygosaccharomyces bailii, Saccharomyces cerevisiae, Pichia membranifaciens, Schizosaccharomyces pombe, Lactobacillus, and Bacillus

Lactobacillus, Bacillus, Brevibacilus, Sporosarcina, and Saccharomyces cerevisiae

Saccharomyces, Pichia, Galactomyces, Aspergillus, Brettanomyces, Cladosporium, Galactomyces, and Thermoascus

esters, alcohols, aldehydes, acids, ketones, hydrocarbons, pyrazines, aromatics, and phenolic compounds acids, alcohols, ethyl butanoate, and low content of ethyl lactate

butyric acid, ethyl hexanoate, caproic acid, dimethyl trisulfide, ethyl butanoate, 2-phenylethyl alcohol, 3-methyl butyrate, 4-methyl phenol, 4-methyl guaiacol, β-damascenone, ethyl pentanoate, (E,Z)-2,6-nonadienal, borneol, and fenchol (E)-2-octenal, 2-phenylethanol, 3-(methylthio)-1-propanol (methionol), diethyl 1,7-heptanedioate (diethyl pimelate), diethyl 1,8-octanedioate (diethyl suberate), and diethyl 1,9-nonanedioate (diethyl azelate) tetramethylpyrazine, and 2-pentyl furan

Saccharomyces cerevisiae, Acetobacter pasteurianus, Bacillus cereus, and Weissella cibaria Saccharomyces cerevisiae, Bacillaceae, Deinococcaceae, Microbacteriaceae, Micrococcaceae, Moraxellaceae, Nocardiaceae, Rhodobacteraceae, and Xanthomonadaceae Saccharomyces cerevisiae, Monascus, Rhizopus, Penicillium, Geotrichum candidum, and Actinomyces

Bacillus subtilis, Bacillus velezensis, Bacillus amyloliquefaciens, Bacillus licheniformis, Bacillus methylotrophicus, Saccharomycopsis fibuligera, Beauveria bassiana, Eurotium athecium, Eurotium amstelodami, Aspergillus versicolor, Thermoascus crustaceus, Mycocladus corymbiferus, and Paecilomyces variotii

Mucor, Rhizopus chinensis, Rhizopus oryzae, surface yeasts, Saccharomyces cerevisiae, and Candida utilis

ethyl pentanoate, ethyl hexanoate, 3-methylbutanal, methional, ethyl butanoate, 3-methyl-1-butanol, hexanoic acid, butanoic acid, trimethyl pyrazine, 2-acetyl furan, ethyl furoate, and methionol

Bacillus, Saccharomyces cerevisiae, Pantoea, Weissella, Lactobacillus, Rubellimicrobium, Halomonas, Thermoactinomyces, Laceyella, and Pediococcus

Rhizopus sp., Rhizopus oryzae, Rhizopus peka, and Saccharomyces cerevisiae

main flavor compound

dominant microorganism

Table 1. Representative Aromas and Characteristics of Baijiu

between those of sauce aroma (maotai) and strong aroma (luzhou)

fermented soybean-like with clean aftertaste

herb-like with sour aroma

soft sweet flavor and clean aftertaste sweet, mellow, and elegant aftertaste harmonious, strong, and light flavor combining fruity with sweaty, roasted sesamelike, and floral flavors soft mellow characteristics and a rich mouthfeel light, strong, and rice flavor traits

pure and mild flavor, mellow sweetness, and refreshing aftertaste soy sauce, with a full-bodied long-lasting aroma

fragrant flavor, soft mouthfeel, and long aftertaste

flavor characteristic

Journal of Agricultural and Food Chemistry Perspective


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Figure 3. Generalization of all factors influencing baijiu flavor formation (raw materials, fermentation, and distillation develop the flavor of baijiu; storage processing improves the flavor; and blending processing modifies the flavor).

tography (GC), gas chromatography−mass spectrometry (GC−MS), gas chromatography−olfactometry (GC−O), electronic noses, and colorimetric artificial noses. A total of 1874 kinds of volatile flavor compounds were identified in baijiu during this period, including 506 esters, 235 alcohols, 166 aromatics, 140 ketones, 132 heterocyclic compounds (including 77 furans), 129 nitrogenous compounds (including 74 pyrazines), 127 acids, 97 aldehydes, 78 hydrocarbons, 76 terpenes, 73 sulfur compounds, 44 acetals, 18 lactones, and 53 other compounds. We have listed and summarized odor thresholds of 217 aroma compounds of baijiu in Table S1 of the Supporting Information. A total of 217 kinds of aroma compounds were identified in baijiu, including 61 esters, 20 alcohols, 29 aldehydes, 18 ketones, 23 acids, 11 pyrazines, 6 furans, 13 phenols, 11 aromatics, 15 sulfur compounds, 4 lactones, and 6 other compounds. These different flavor compounds lead to special flavor types. Baijiu consists of four dominant aroma types, namely, strong, light, sauce, and rice aromas, from which all others derive. At present, there are 12 types of representative aroma, including the above four aromas and feng, te, sesame, laobaigan, fuyu, herbal, chi, and mixed aromas, which are distributed in different provinces of China (Figure 2). All Chinese regions produce baijiu, especially Sichuan, Guizhou, Shandong, Jiangsu, Anhui, Shanxi, and Henan provinces, which comprise more than 90% of total Chinese liquor output.26 Owing to the diversity of microorganisms and raw materials, baijiu compositions can vary greatly. These different flavor compounds lead to special flavor types. The representative aromas and characteristics of baijiu are listed in Table 1, which provides an overview of these 12 types of baijiu. Typically, the concentration of ethyl hexanoate is higher than that of other aroma compounds in strong-aromatype baijiu, making it the main flavor compound. Similarly, ethyl acetate, ethyl lactate, ethyl acetate and β-phenethyl alcohol, and ethyl propionate are the main flavor compounds in light-aroma-type, laobaigan-aroma-type, rice-aroma-type, and sesame-aroma-type baijiu, respectively. Among these, the strong-aroma type accounts for about 70% of total liquor production. Fenjiu in Shanxi province is a typical light-aroma baijiu that is also the original Chinese baijiu, while Maotai, a famous spirit, is a sauce-aroma-type baijiu. The flavor of baijiu is complicated and can be affected by the fermentation, distillation, aging, blending processes, raw

baijiu production, such as millet or buckwheat, which are in short supply. Multigrain brewing is complementary to baijiu fermentation, and its use is a development trend in baijiu production. Multigrain brewing requires that the various raw materials have reasonable ratios and relationships and that the scientific production process is adapted. Fermentation is an important process for taking advantage of microorganisms from jiuqu and the environment for baijiu production, in which starches or carbohydrates, proteins or amino acids, and lipids are transformed into ethanol and different flavor components through a series of biochemical reactions. The different fermentation methods use different microorganisms and enzymes, which contribute to the delicate balance and functions of stability, quality, and productivity, resulting in different metabolites in the fermentation process, which produce different baijiu flavors. Other new developments in the baijiu industry are that solid-state fermentation requires a combination with liquid fermentation, underground fermentation needs to be transferred to ground fermentation, natural fermentation needs artificial temperature control, and the control of large-scale mechanical equipment via automation and intelligentization will eventually replace manual labor. Traditional industrial techniques will be applied to achieve modernization, which could allow baijiu to be exported and become a more famous distilled beverage globally as a result of high-quality, inexpensive, and high-yielding production.

4. BAIJIU FLAVOR Baijiu is differentiated from other liquors because of its distinctive flavor, taste, and production process. Flavor substances in baijiu are endogenous, produced during jiuqu production, saccharification, fermentation, distillation, and storage processes. Owing to the diversity of microorganisms and raw materials, baijiu compositions can vary greatly. In 2007, Zhu et al. used two-dimensional gas chromatography/time-offlight mass spectrometry (GC × GC/TOFMS) to identify 528 compounds in Maotai.25 Recently, Jin et al. reported more than 1000 volatile compounds in baijiu.17 New compounds continue to emerge with more advanced analytical techniques. We have listed and summarized all volatile flavor compounds reported between 1990 and 2017. These compounds have been identified by techniques including spectroscopy, gas chroma5430


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(6) Wu, J.; Sun, B.; Zhao, M.; Zheng, F.; Sun, J.; Sun, X.; Li, H.; Huang, M. Discovery of a bioactive peptide, an angiotensin converting enzyme inhibitor in Chinese Baijiu. J. Chin. Inst. Food Sci. Technol. 2016, 16 (9), 14−20. (7) Wu, J.; Huo, J.; Huang, M.; Zhao, M.; Luo, X.; Sun, B. Structural characterization of a tetrapeptide from sesame flavor-type Baijiu and its preventive effects against AAPH-induced oxidative stress in HepG2 cells. J. Agric. Food Chem. 2017, 65, 10495−10504. (8) Huang, S. The First Economic History of ChinaShi-Huo-Zhi; China Economic Press: Beijing, China, 1991; Vol. 8, p 204. (9) Xi, B.; Veeranki, S. P.; Zhao, M.; Ma, C.; Yan, Y.; Mi, J. Relationship of alcohol consumption to all-cause, cardiovascular, and cancer-related mortality in U.S. adults. J. Am. Coll. Cardiol. 2017, 70, 913−922. (10) Sun, B. G.; Li, H. H.; Hu, X. M.; Sun, J. Y.; Dong, W. The development trend of healthy Baijiu. J. Chin. Inst. Food Sci. Technol. 2016, 8, 1−8. (11) Liu, M.; Tang, Y.; Zhao, K.; Liu, Y.; Guo, X.; Ren, D.; Yao, W.; Tian, X.; Gu, Y.; Yi, B.; Zhang, X. Determination of the fungal community of pit mud in fermentation cellars for Chinese strongflavor liquor, using DGGE and Illumina MiSeq sequencing. Food Res. Int. 2017, 91, 80−87. (12) Liu, M. K.; Tang, Y. M.; Guo, X. J.; Zhao, K.; Tian, X. H.; Liu, Y.; Yao, W. C.; Deng, B.; Ren, D. Q.; Zhang, X. P. Deep sequencing reveals high bacterial 302 diversity and phylogenetic novelty in pit mud from Luzhou Laojiao cellars for Chinese strong-flavor Baijiu. Food Res. Int. 2017, 102, 68−76. (13) Barrios-Gonzalez, J. Solid-state fermentation: Physiology of solid medium, its molecular basis and applications. Process Biochem. 2012, 47, 175−185. (14) Lu, Y.; Li, Y. J.; Huang, J.; Hu, Y.; Zhang, Z. Y. Classification, production process and nutritional value of Chinese liquor. Agric. Eng. Technol. 2007, 1, 21−24. (15) Zhang, H.; Chen, L.; Wang, J. F.; Wang, D. Y. Production techniques of liquid state fermented baijiu. China Pet. Chem. Stand. Qual. 2013, 24, 82. (16) Zheng, X. W.; Yan, Z.; Nout, M. J. R.; Smid, E. J.; Zwietering, M. H.; Boekhout, T.; Han, J. S.; Han, B. J. Microbiota dynamics related to environmental conditions during the fermentative production of Fen-Daqu, a Chinese industrial fermentation starter. Int. J. Food Microbiol. 2014, 182−183, 57−62. (17) Jin, G.; Zhu, Y.; Xu, Y. Mystery behind Chinese liquor fermentation. Trends Food Sci. Technol. 2017, 63, 18−28. (18) Wang, C. L.; Shi, D. J.; Gong, G. L. Microorganisms in Daqu: A starter culture of Chinese maotai-flavor liquor. World J. Microbiol. Biotechnol. 2008, 24, 2183−2190. (19) Wu, X. H.; Zheng, X. W.; Han, B. Z.; Vervoort, J.; Nout, M. J. R. Characterization of Chinese liquor starter, “Daqu”, by flavor type with 1 H NMR-based nontargeted analysis. J. Agric. Food Chem. 2009, 57, 11354−11359. (20) Gou, M.; Wang, H. Z.; Yuan, H. W.; Zhang, W. X.; Tang, Y. Q.; Kida, K. Characterization of the microbial community in three types of fermentation starters used for Chinese liquor production. J. Inst. Brew. 2015, 121, 620−627. (21) Zhang, W.; Wu, Z. Y.; Zhang, Q. S.; Wang, R.; Li, H. Combination of newly developed high quality Fuqu with traditional Daqu for Luzhou-flavor liquor brewing. World J. Microbiol. Biotechnol. 2009, 25, 1721−1726. (22) Wu, Y.; Liu, S.; Fan, X.; Yang, J.; Jiang, W.; Wang, D.; Li, N. Analysis of Aroma Components of Five Different Cooked Grains Used for Chinese Liquor Production by GC−O−MS. Food Sci. 2016, 37, 94−98. (23) Han, Y.; Song, L.; Liu, S.; Zou, N.; Li, Y.; Qin, Y.; Li, X.; Pan, C. Simultaneous determination of 124 pesticide residues in Chinese liquor and liquor-making raw materials (sorghum and rice hull) by rapid Multi-plug Filtration Cleanup and gas chromatography−tandem mass spectrometry. Food Chem. 2018, 241, 258−267.

materials, jiuqu, and environmental conditions during production (Figure 3). Therefore, the diversity in baijiu flavor arises from slight changes in production processing. In recent years, researchers have focused on investigating the microorganisms and flavor active compounds and improving baijiu production processes using modern advanced analytical techniques to identify additional unknown compounds in baijiu. Baijiu has been a favorite liquor in China for thousands of years, reflecting its cultural heritage and health benefits. We have focused on investigating baijiu production, fermentation, and flavor chemistry to summarize the typical flavor types of baijiu and compare fermentation methods, raw materials, fermentation starters, dominant microorganisms, main flavor compounds, and flavor characteristics (Table 1). Baijiu contains various flavor compounds, most of which are beneficial to humans. Therefore, development trends in baijiu production should be directed by both flavor and health. Another important direction for the development of baijiu research and production is the regulation of saccharification and fermentation processes and improving the contents of characteristic flavor substances and key health factors produced by functional microorganisms.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jafc.8b00692. Odor thresholds of 217 aroma compounds in baijiu (Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-10-68984768. E-mail: [email protected]. cn. ORCID

Huilin Liu: 0000-0002-7912-2611 Baoguo Sun: 0000-0003-4326-8237 Funding

This work was supported by the National Key R&D Program of China (2016YFD0400500), the Advisory Project of the Academician of the Chinese Academy of Engineering (2018XY-28), and the National Natural Science Foundation of China (31701567 and L1522022). Notes

The authors declare no competing financial interest.

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REFERENCES

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