Effect of fermentation processing on the flavor of Baijiu

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Effect of fermentation processing on the flavor of Baijiu Huilin Liu, and Baoguo Sun J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b00692 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 12, 2018

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Journal of Agricultural and Food Chemistry

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Effect of fermentation processing on the flavor of Baijiu

2

Huilin Liu, Baoguo Sun*

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Beijing Advanced Innovation Center for Food Nutrition and Human Health,

4

Beijing Key Laboratory of Flavor Chemistry, Beijing Technology and Business

5

University, No. 11, Fucheng Road, Beijing, 100048, China.

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*Corresponding Author: Tel.: (+86)10-68984768; Fax: 86-10-68984768; E-mail:

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[email protected].

ACS Paragon Plus Environment


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Abstract

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Baijiu, otherwise known as the national liquor of China, is a well-known distilled

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spirit that has been produced for over 2,000 years. Baijiu is a clear and transparent

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fermented alcoholic beverage containing more than 1,870 volatile compounds,

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including esters, alcohols, aromatics, ketones, heterocycles, nitrogenous compounds,

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acids, aldehydes, terpenes, sulfur compounds, acetals, and lactones. This perspective

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describes the Baijiu production process, summarizes recent progress in the

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development and study of Baijiu flavor chemistry, discusses the effect of fermentation

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processing on Baijiu flavor, and presents novel development trends in the Baijiu

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industry in the dual directions of flavor and health. Furthermore, the direction of

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research and development for modernization in the Baijiu industry is discussed and

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proposed.

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Keywords: Baijiu, Chinese liquor, Fermentation, Flavor chemistry, Developments


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1. Introduction

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Baijiu, an ancient Chinese liquor, is the national liquor of China and possesses a

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unique position in traditional Chinese culture. Chinese liquor-making has a 9,000-year

25

history, with reports as early as seven millennia before Christ (B.C.).1 Baijiu is one of

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the oldest distillates in the world, with a 2,000-year history dating from the Western

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Han dynasty (202 B.C. to 8 A.D.) according to the record of a distiller from the tomb

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of Haihunhou. Baijiu is a clear and transparent fermented alcoholic beverage with a

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high ethanol content ranging from 38 to 65 vol.%.2 Baijiu is also the most consumed

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spirit globally, with over 13 billion liters consumed in 2016, representing a market

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value of approximately $ 97.4 billion. Therefore, Baijiu constitutes a major part of the

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Chinese food industry.

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Baijiu is a world-renowned distilled spirit, alongside brandy, whiskey, vodka, gin,

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and rum. The raw material, production process, composition, and taste of Baijiu are

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very different from those of other famous distilled spirits. Baijiu is normally prepared

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from sorghum alone or a mixture of corn, rice, wheat, peas, millet, and sorghum. For

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Baijiu production, Jiuqu, known as the foundation of Baijiu, is often used as a

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fermentation starter,3 and is mixed with grains to saccharify and ferment

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simultaneously, which produces ethanol and flavor compounds. The fermented

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mixture is then distilled under solid-state conditions to produce fresh Baijiu liquor.

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Baijiu is rich in many flavor components, including organic acids (such as acetic acid,

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lactic acid, malic acid, tartaric acid, and linoleic acid), esters (such as ethyl acetate,

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ethyl lactate, and ethyl hexanoate), lactones, phenols, heterocycles, terpenes, and



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aromatic compounds.4 Furthermore, Baijiu contains potential functional components,

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such as amino acids5 and peptides6, which are beneficial to humans. A tetrapeptide

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(Ala-Lys-Arg-Ala) and tripeptide (Pro-His-Pro) have been successfully identified in

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sesame

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2,2'-azobis(2-methyl-propanimidamidine) dihydrochloride-induced oxidative stress in

49

HepG2 cells and antihypertensive activity, respectively.6,7 The first economic history

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book from China, ‘Shi-Huo-Zhi’ by Ban Gu, reported that “Jiu, an alcoholic beverage,

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has long been used as a base for traditional Chinese medicine” since the Eastern Han

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dynasty, which is a good summary of the application of Baijiu in medicine and

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healthcare.8 In the Qiaocheng district of Bozhou, a Gujinggong Baijiu production

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region of China, the longevity of about 500 people that lived for more than 100 years

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has been partly attributed to the habit of drinking a little Baijiu daily. In recent years,

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Bo Xi et al. have shown that light and moderate alcohol intake might have a

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protective effect on all-cause and cardiovascular disease-specific mortality in US

58

adults.9 Therefore, drinking Baijiu in moderation might be beneficial to human

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health.10

60

2. Baijiu Production

flavor-type

Baijiu

and

showed

preventive

effects

against

61

The production of Baijiu is different from that of other distilled liquors because it

62

combines the two distinctive processes of fermentation and distillation. During

63

fermentation, conversions of starch to sugar and sugar to alcohol take place

64

simultaneously. In the production of other distilled liquors, such as whisky and vodka,

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the above two reactions occur stepwise, or conversion of sugar to alcohol takes place


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alone, such as in rum production. Furthermore, most distillation processing is

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performed in the solid state during Baijiu production, for which a special device,

68

called the Zeng (Fig. 1), is often used. The Zeng is a conical distiller (approximate

69

dimensions: Top diameter, 2 m; bottom diameter, 1.8 m; and height, 1 m) that is

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widely used for distillation. The Zeng is uniquely designed according to the

71

characteristics of solid-state fermentation, making it different from other distillation

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units. With scientific advances, the Zeng material has been changed from wood to

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reinforced concrete or stainless steel, and the cooler has been changed from Tianguo

74

to straight to improve the cooling efficiency. However, the basic operation of artificial

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labor remains unchanged, with continuous feeding and discharging still not widely

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applied. The Zeng is a special steaming tower in which, under constant heating

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conditions, the alcohol concentration of each layer is also constant. The concentration

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of volatile components decreases layer by layer in the lower section and increases

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layer by layer in the upper section. These volatile components are obtained from t

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alcohol and aroma compounds in the fermented grains of by vaporization,

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condensation, and vaporization again, to achieve multicomponent concentration and

82

extraction.

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The general scheme for traditional processing in Baijiu production is shown in

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Fig. 1. Typically, the raw material is mixed uniformly with whole or powdered grains

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before adding hot water. Cooking the starch granules in hot water causes them to

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further absorb, expand, rupture, and gelatinize. Meanwhile, the raw materials are

87

sterilized, and some adverse volatile components are eliminated, under high



88

temperature. When the temperature is 50–60 °C, the amylase in the raw material

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decomposes starch into dextrin and sugar. When the whole raw material is cooked,

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limited sugar generated by saccharification, but when the raw material is cooked after

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grinding, more sugar is generated. Some glucose will be converted to fructose and

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other ketones. Hexoses, such as glucose and fructose, can be dehydrated to produce 5-

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hydroxymethyl furfural during high-pressure cooking, which then further decomposes

94

to a keto acid and formic acid. This reaction process is irreversible and proceeds

95

according to the first-order kinetic reaction formula. The mixed grains are then

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steamed with fermented grains and husks to prepare fresh liquor, also called base

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liquor. Distillation is the key step in improving Baijiu flavor. The distillation

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efficiency depends on the steam flow rate, distillation speed, water content, and

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porosity of mixed grains. The Baijiu is then stored and aged in a pottery or

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stainless-steel vessel for months or years depending on the desired final product

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quality. Finally, the stored liquor is blended with water to reduce the alcohol content

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before bottling for sale. A repeated-batch fermentation process is used for

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strong-aroma Baijiu, as denoted by the red square in Fig. 1, where the distilled grains

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are cooled to about 20 °C with fast-air cooling before Jiuqu is added and the mixture

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is placed in a fermentation pit to spontaneously ferment for about 1–3 months at

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28–30 °C. The fermentation pit is a special container (approximate dimensions:

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Length, 3.4 m; width, 1.8 m; and depth, 2.0 m) that is widely used for solid-state

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fermentation. The fermented grains at the bottom of fermentation pit (~1/4, v/v) are

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then distilled to obtain the spirit and discarded, with the same volume of fresh raw


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material added to repeat the batch fermentation. These distilled grains are also called

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the vinasse, which is not used further in Baijiu production, but taken as an important

112

source of feed for pigs, cattle, poultry, and others. The process of light-aroma Baijiu

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production includes only two fermentation processes, as denoted by the blue square in

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Fig. 1, where the steamed raw material with Jiuqu is added to a ground cylinder for

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the first fermentation of about one month, then the fermented grains are steamed to

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produce fresh liquor before the distilled grains, with new Jiuqu added, are fermented

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again for about one month. After the second fermentation, these distilled grains

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comprise the vinasse.

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Traditional Baijiu production methods have existed for thousands of years and

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mainly rely on the operation skill and experience of individuals. In recent years, some

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developments in modern biotechnology, automation, and electronic information

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technology have been applied in Baijiu production to improve traditional processing.

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As a result, semi-mechanized processes are widely used in Baijiu production by large

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and medium-sized enterprises to reduce labor intensity, improve the quality and flavor

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of Baijiu, and reduce manual labor. However, this mode of production still faces a

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lack of control and standardization. Some enterprises have attempted to use fully

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mechanized processing for Baijiu production, but this impacts the quality and flavor

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of the resultant Baijiu. However, the development of Baijiu processing technology is

129

trending

130

simultaneously, to ensure the quality and flavor of Baijiu.

131

3. Fermentation Processing of Baijiu

towards

full-mechanization,

automation,


and

intellectualization,


132

Sorghum is the main raw material used in Baijiu fermentation and contains starch

133

(42.07–68.12%), protein (10.3–15.6%), lipids (3.6–5.2%), water (11.1–13.4%), fiber

134

(1.5–2.7%), and ash (1.6–3.1%). Starch is changed into glucose by saccharification

135

with Clostridium butyricum or amylases in fermented grains. Glucose is degraded to

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pyruvic acid via fructose 1,6-biphosphate fructose in the Embden–Meyerhof–Parnas

137

(EMP) pathway by reaction with Saccharomyces cerevisiae. Pyruvic acid was

138

converted into acetaldehyde by the removal of a carbon dioxide molecule in a reaction

139

with decarboxylase. Acetaldehyde was then transformed into alcohol by reduction

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with ethanol dehydrogenase. Acids formation result from oxidation during the

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fermentation process. Ethanol forms acetaldehyde by oxidation and is further oxidized

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to acetic acid. At room temperature, lower fatty acids react with ethanol to form esters

143

through enzyme catalysis. As esterification is a reversible chemical reaction, when the

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highest esterification rate is reached, the esters will decompose into alcohols and acids,

145

which plays a decisive role in the formation of Baijiu flavor. Meanwhile, proteins in

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the raw materials are hydrolyzed into peptides and amino acids by protease catalysis.

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The aromatic components produced by fermentation mainly originate from the raw

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material, microorganisms, intermediates, or certain amino acids and tannins from

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sorghum.

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3.1 Fermentation types

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The main fermentation process for Baijiu, involving simultaneous saccharification

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and spontaneous fermentation in the solid state, is responsible for its rich flavor and

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good taste. This special fermentation process is usually performed in an underground


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fermentation pit for 1–3 months (strong-aroma Baijiu), while some Baijiu processes

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use stacking fermentation above ground before underground fermentation, such as

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Maotai and sesame-aroma Baijiu. The pit mud in the fermentation pit, otherwise

157

known as microbial gold, is a complex ecosystem that hosts diverse microbial

158

communities, including bacteria, archaea, and fungi, which contribute to the

159

production of characteristic flavor compounds, such as ethyl caproate.11 High

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diversity indices of about 16 phyla and 105 genera have been found in pit mud used to

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make Luzhou Laojiao Baijiu.12 The fermentation of light-aroma Baijiu is performed in

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ground cylinders that do not contain pit mud. Repeated-batch fermentation is a

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complex process, as shown in Fig. 1. According to the different fermentation methods,

164

there are three main types of fermentation used in Baijiu production, namely

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solid-state fermentation, semi-solid-state fermentation, and liquid-state fermentation.

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Solid-state fermentation technology is commonly used to produce most

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well-known Baijiu beverages with different flavors and characteristics, in which

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microbial cultures are grown on a solid matrix.13 Furthermore, solid-state

169

fermentation technology can easily provide Baijiu with acceptable flavors and tastes,

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and has many advantages, including higher fermentation productivity, higher

171

end-concentration of products, and higher product stability. However, solid-state

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fermentation technology has some problems, such as low mechanization, high costs,

173

and long fermentation times, which result in low production efficiency.

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Semi-solid-state fermentation is used to prepare Baijiu with rice and chi aromas in

175

Guangxi and Guangdong provinces of China. This process uses rice as the raw



176

material, and Jiuqu as the saccharification and fermentation agents, which are soaked

177

in a distillation kettle or altar to achieve semi-solid-state fermentation.14

178

Liquid-state fermentation generally uses corn as the raw material to produce

179

Baijiu, with all processes performed in the liquid state,15 including saccharification,

180

fermentation, distillation, and blending, to give a high production efficiency with

181

simple operation and low material consumption, making it easier to achieve full

182

automation. However, this process produces fewer flavor components, leading to a

183

lower quality, because the only raw material, corn, is fermented with pure species.

184

Liquid fermentation is an important direction for the development of Baijiu

185

production, but the problem of poor flavor compounds must be solved.

186

3.2 Fermentation starters

187

Fermentation starters are another major factor affecting fermentation and,

188

consequently, Baijiu flavor and taste. Jiuqu contains a variety of microorganisms and

189

enzymes that are essential to Baijiu production as saccharification, fermentation, and

190

aroma-producing agents. Fermentation starters are divided into Daqu, Xiaoqu, Fuqu,

191

and other Jiuqu, according to the preparation process.

192

Daqu is one of the oldest and most widely used starters in China, and uses barley,

193

wheat, or peas as raw materials. Under certain temperature and humidity conditions,

194

Daqu can be formed into a brick shape by adding water, stacking, crushing, stirring,

195

pressing, and cultivating in a Qu-house, followed by turning, and storage. Daqu is a

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naturally occurring microbiota in which the main microorganisms are molds, yeasts,

197

bacteria, and actinomycetes, such as Mucor racemosus, Aspergillus niger,


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Thermomyces lanuginosus, Candida, Saccharomyces cerevisiae, Bacillus subtilis,

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lactic acid, and acetic acid bacteria.16 Daqu is divided into high-temperature,

200

medium-temperature, and low-temperature Daqu according to the cultivation

201

temperature. High-temperature Daqu is cultured at 60–70 °C and mainly used for

202

sauce-aroma Baijiu production. The main flavor compounds in high-temperature

203

Daqu include tetramethylpyrazine, guaiacol, phenylethanol, propanoic acid,

204

1,3-butanediol, methyl ester, and acetic acid.17,18 However, high-temperature Daqu

205

has a lower capacity for saccharification, liquefaction, and fermentation than

206

low-temperature Daqu. Medium-temperature Daqu is formed at 50–60 °C and mainly

207

used for strong-aroma Baijiu production. It can be used to improve Baijiu flavor by

208

imbuing a more rich and full taste, and by producing more caproic acid and ethyl

209

caproate. The main flavor compounds in medium-temperature Daqu include

210

tetradecanoic acid ethyl ester, guaiacol, pyrazines, phenylethyl alcohol, and

211

caryophyllene.17,19 Low-temperature Daqu is heated to no more than 50 °C and

212

mainly used for light-aroma Baijiu production. Microorganisms are abundant in

213

low-temperature Daqu, resulting in a better fermentation capacity and higher yields of

214

Baijiu. A light flavor and pure taste are formed due to the large amounts of acetic acid,

215

lactic acid, and alcohol compounds, in addition to, typically, hexyl acetate, ethyl

216

acetate, phenylethyl alcohol, ethyl alcohol, hexanal, (E)-2-octenal, (Z)-2-octen-1-ol,

217

nonanoic acid, and pyrazines.16,17

218

Xiaoqu, which is widely used in the south of China, is another naturally

219

occurring microbiota. However, Xiaoqu is quite different from Daqu, and mainly uses



220

rice with some herbs or clay as the raw materials. The main microorganisms in

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Xiaoqu are molds and yeasts, especially Rhizopus sp., Rhizopus oryzae, Rhizopus

222

peka, and Saccharomyces cerevisiae. The light flavor is mild with rice, herbal, and

223

fragrance aromas. The main flavor compounds include acetic acid, ethyl acetate,

224

phenylethyl alcohol, and pyrazines.17,20 Xiaoqu produces a higher yield than Daqu.

225

Fuqu is another important fermentation starter with short production, excellent

226

saccharification and fermentation capacity, and high utilization.21 Fuqu uses bran as

227

the raw material, which is more economical than grains. The main microorganisms in

228

Fuqu are molds, such as Rhizopus oryzae and aspergillus. These microorganisms are

229

inoculated into the culture medium by artificial inoculation under optimized

230

conditions.

231

Others microbial enzymes, such as cellulase, amylase, glucoamylase, pectinase,

232

and protease, in combination with Daqu, Xiaoqu, and Fuqu can effectively improve

233

the liquor fermentation yield, which saves labor and energy.

234

Further studies have focused more on the interactions of good and pure strains as

235

saccharification and fermentation agents for Baijiu combined with traditional Daqu,

236

Xiaoqu, Fuqu, and pure cultures of multiple strains and commercial microbial

237

enzymes or active dry yeast. This can compensate for flavor compound deficiencies in

238

fermentations using a single strain. Meanwhile, new kinds of microbial enzymes or

239

active dry yeast are needed for further research and development, and to explore new

240

full-enzymatic brewing processes. Furthermore, Jiuqu production is needed for

241

mechanization and automation with modern microbiological and molecular biology


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technology.

243

3.3 Raw materials

244

The raw materials for Baijiu fermentation are mainly prepared from sorghum

245

alone or a mixture of corn, rice, wheat, peas, millet, and sorghum.22 The flavor of

246

Baiju is strongly influenced by its raw materials, with large differences noted when

247

using different types of raw materials, or the same raw materials from different

248

regions. In general, sorghum is widely used as the raw material in Baijiu fermentation

249

to produce fragrance flavor and high liquor yields, and is well suited to Baijiu

250

brewing owing to its nutritional composition and related physical properties.23

251

Sorghum is rich in inorganic elements and vitamins, which provides a foundation for

252

the good growth and reproduction of microorganisms. Sorghum also contains tannins,

253

which can be transformed into aromatic substances, such as syringic acid, to give a

254

special aroma to Baijiu. Rice contains less fat and cellulose than sorghum, which is

255

too sticky to ferment easily after cooking, and has a large ethyl hexanoate content that

256

produces a soft sweet flavor and clean aftertaste.24 Corn contains more phytic acid,

257

which is decomposed into inositol and phosphoric acid during fermentation. Inositol

258

produces the sweet flavor in Baijiu. Phosphoric acid can promote the production of

259

glycerol in Baijiu to give the liquor a sweet flavor. Owing to their high protein content,

260

peas are not a good raw material for Baijiu production. Peas are rich in vanillin and

261

other phenolic compounds, which may contribute to the Baijiu aroma.

262

The fermentation raw materials need high contents of carbohydrates, and certain

263

proteins and tannins, which are suitable for the absorption and utilization of



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microorganisms. The raw material should also be easy to store and have an easily

265

controllable moisture content to prevent spoilage. However, this does not mean that

266

all high-starch materials are suitable for Baijiu production, such as millet or

267

buckwheat, which are in short supply. Multigrain brewing is complementary to Baijiu

268

fermentation, and its use is a development trend in Baijiu production. Multigrain

269

brewing requires that the various raw materials have reasonable ratios and

270

relationships, and that the scientific production process is adapted.

271

Fermentation is an important process for taking advantage of microorganisms

272

from Jiuqu and the environment for Baijiu production, in which starches or

273

carbohydrates, proteins or amino acids, and lipids are transformed into ethanol and

274

different flavor components through a series of biochemical reactions. The different

275

fermentation methods use different microorganisms and enzymes, which contribute to

276

the delicate balance and functions of stability, quality, and productivity, resulting in

277

different metabolites in the fermentation process, which produce different Baijiu

278

flavors. Other new developments in the Baijiu industry are that solid-state

279

fermentation

280

fermentation needs to be transferred to ground fermentation, natural fermentation

281

needs to artificial temperature control, and the control of large-scale mechanical

282

equipment via automation and intelligentization will eventually replace manual labor.

283

Traditional industrial techniques will be applied to achieve modernization, which

284

could allow Baijiu to be exported and become a more famous distilled beverage

285

globally due to high-quality, inexpensive, and high-yielding production.

requires

combination

with

liquid

fermentation,


underground

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286

4. Baijiu Flavor

287

Baijiu is differentiated from other liquors because of its distinctive flavor, taste,

288

and production process. Flavor substances in Baijiu are endogenous, produced during

289

Jiuqu production, saccharification, fermentation, distillation, and storage processes.

290

Owing to the diversity of microorganisms and raw materials, Baijiu compositions can

291

vary

292

chromatography/time-of-flight mass spectrometry (GC×GC/TOFMS) to identify 528

293

compounds in Maotai.25 Recently, Jin et al. reported more than 1,000 volatile

294

compounds in Baijiu.17 New compounds continue to emerge with more advanced

295

analytical techniques. We have listed and summarized all volatile flavor compounds

296

reported between 1990 and 2017. These compounds have been identified by

297

techniques

298

chromatography-mass spectrometry (GC-MS), GC-olfactometry (GC-O), electronic

299

noses, and colorimetric artificial noses. A total of 1,874 kinds of volatile flavor

300

compounds were identified in Baijiu during this period, including 506 esters, 235

301

alcohols, 166 aromatics, 140 ketones, 132 heterocyclic compounds (including 77

302

furans), 129 nitrogenous compounds (including 74 pyrazines), 127 acids, 97

303

aldehydes, 78 hydrocarbons, 76 terpenes, 73 sulfur compounds, 44 acetals, 18

304

lactones, and 53 other compounds. We have listed and summarized odor thresholds of

305

217 aroma compounds of Baijiu in Table S1. A total of 217 kinds of aroma

306

compounds were identified in Baijiu, including 61 esters, 20 alcohols, 29 aldehydes,

307

18 ketones, 23 acids, 11 pyrazines, 6 furans, 13 phenols, 11 aromatics, 15 sulfur

greatly.

In

including

2007,

Zhu

spectroscopy,

et

al.

gas

used

two-dimensional

chromatography


(GC),

gas

gas


308

compounds, 4 lactones, and 6 other compounds. These different flavor compounds

309

lead to special flavor types. Baijiu consists of four dominant aroma types, namely

310

strong, light, sauce, and rice aromas, from which all others derive. At present, there

311

are 12 types of representative aroma, including the above four aromas and feng, te,

312

sesame, laobaigan, fuyu, herbal, chi, and mixed aromas, which are distributed in

313

different provinces of China (Fig. 2). All Chinese regions produce Baijiu, especially

314

Sichuan, Guizhou, Shandong, Jiangsu, Anhui, Shanxi, and Henan provinces, which

315

comprise more than 90% of total Chinese liquor output.26 Owing to the diversity of

316

microorganisms and raw materials, Baijiu compositions can vary greatly. These

317

different flavor compounds lead to special flavor types. The representative aromas

318

and characteristics of Baijiu are listed in Table 1, which provides an overview of these

319

12 types of Baijiu. Typically, the concentration of ethyl hexanoate is higher than that

320

of other aroma compounds in strong-aroma-type Baijiu, making it the main flavor

321

compound. Similarly, ethyl acetate, ethyl lactate, ethyl acetate and β-phenethyl

322

alcohol, and ethyl propionate are the main flavor compounds in light-aroma-type,

323

Laobaigan-aroma-type, rice-aroma-type, and sesame-aroma-type Baijiu, respectively.

324

Among these, the strong-aroma type accounts for about 70% of total liquor

325

production. Fenjiu in Shanxi province is a typical light-aroma Baijiu that is also the

326

original Chinese Baijiu, while Maotai, a famous spirit, is a sauce-aroma-type Baijiu.

327

The flavor of Baijiu is complicated and can be affected by the fermentation,

328

distillation, aging, blending processes, raw materials, Jiuqu, and the environmental

329

conditions during production (Fig. 3). Therefore, the diversity in Baijiu flavor arises


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330

from slight changes in production processing. In recent years, researchers have

331

focused on investigating the microorganisms and flavor active compounds, and

332

improving Baijiu production processes using modern advanced analytical techniques

333

to identify additional unknown compounds in Baijiu.

334

Baijiu has been a favorite liquor in China for thousands of years, reflecting its

335

cultural heritage and health benefits. We have focused on investigating Baijiu

336

production, fermentation, and flavor chemistry to summarize the typical flavor types

337

of Baijiu and compare fermentation methods, raw materials, fermentation starters,

338

dominant microorganisms, main flavor compounds, and flavor characteristics (Table

339

1). Baijiu contains various flavor compounds, most of which are beneficial to humans.

340

Therefore, development trends in Baijiu production should be directed by both flavor

341

and health. Another important direction for the development of Baijiu research and

342

production is the regulation of saccharification and fermentation processes, and

343

improving the contents of characteristic flavor substances and key health factors

344

produced by functional microorganisms.

345

AUTHOR INFORMATION

346

Corresponding Authors

347

*Tel.: (+86)10-68984768. Fax: 86-10-68984768. E-mail: [email protected].

348

Funding

349

This work was supported by the National Key R&D Program of China

350

(2016YFD0400500), the Advisory Project of the academician of the Chinese



351

Academy of Engineering (2018-XY-28), and the National Natural Science Foundation

352

of China (No.31701567 and No.L1522022).

353

Notes

354

The authors declare no competing financial interest.


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FIGURE CAPTIONS

431

Fig. 1. Traditional production processing of Baijiu. Raw material is mixed uniformly

432

with whole or powdered grains before adding hot water. The mixed grains are then

433

fermented and the husks are transferred to the Zeng for distillation. The fresh liquor is

434

then stored and aged in a pottery or stainless-steel vessel. After aging for several years,

435

the stored liquor is blended to obtain differently graded products.

436

Fig. 2. Distribution of well-known Baijiu with different aromas.

437

Fig. 3. Generalization of all factors influencing Baijiu flavor formation (raw materials,

438

fermentation, and distillation develop the flavor of Baijiu; storage processing

439

improves the flavor; and blending processing modifies the flavor).



440


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Table 1. Representative aromas and characteristics of Baijiu.

441 Represe ntative aroma

Fermentation methods

Raw materials

Fermentation starters

Dominant microorganisms

Main flavor compounds

Flavor characteristics

Strong aroma

Solid-state fermentation

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

Mediumand Low-temperature Daqu


Sorghum

Low-temperature Daqu

Ethyl hexanoate, Ethyl acetate, Ethyl lactate, Ethyl butyrate, Ethyl valerate, Ethyl heptanoate, Hexanoic acid, Butanoic acid, Heptanoic acid, Furfural, Phenylethyl alcohol. Ethyl acetate, Ethyl lactate, Acetic acid, 2-Methylpropanoic acid, β-Damascenone, Terpenoids.

Fragrant flavor, soft mouthfeel, and long aftertaste.

Light aroma

Clostridium kluyveri, Burkholderia, Streptococcus, Lactobacillus, Lactobacillaceae, Saccharomyces cerevisiae, Saccharomycopsis fibuligera, Talaromyces Pichia kudriavzevii, Aspergillus, Rhizopus, Eurotium, 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, Monascus.


Pure and mild flavor, mellow sweetness, and refreshing aftertaste.


Sauce aroma

Stacking fermentation and Solid-state fermentation

Sorghum

High-temperatur e Daqu

Aspergillus, Candida, Cryptococcus, Halomonas, Thermomyces, Thermoascus, Paecilomyces variotii, Zygosaccharomyces bailii, Saccharomyces cerevisiae, Pichia membranifaciens, Schizosaccharomycespombe. Lactobacillus, Bacillus.

Rice aroma

Semi-solid-st ate fermentation

Rice

Xiaoqu

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

Feng aroma


Sorghum


Lactobacillus, Bacillus, Brevibacilus, sporosarcina, Saccharomyces cerevisiae.

Te aroma


Rice

Medium-tempera ture Daqu

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


Ethyl hexanoate, Hexanoic acid, 3-Methylbutanoic acid, 3-Methylbutanol, Tetramethylpyrazine, Ethyl 2-phenylacetate, 2-Phenylethyl acetate, Ethyl 3-phenylpropanoate, 4-Methylguaiacol, γ-Decalactone. β-Phenethyl alcohol, Ethyl acetate, Ethyl lactate, Ethyl benzenecarboxylate, Diethyl succinate. Ethyl acetate, Ethyl caproate, Isoamyl alcohol. More than 98% esters and alcohols among the total flavor compounds, especially Ethyl acetate and Ethyl caproate.

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Soy sauce, with a full-bodied long-lasting aroma.

Soft sweet flavor and clean aftertaste.

Sweet, mellow, and elegant aftertaste. Harmonious, strong, and light flavor.

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Sesame aroma


Sorghum, wheat and corn


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

Laobaig an aroma

Solid-state fermentation, cylinder,

Sorghum

Medium-tempera ture Daqu

Saccharomyces cerevisiae, Acetobacter pasteurianus, Bacillus cereus, Weissella cibaria.

Fuyu aroma


Sorghum, rice, wheat, corn and Glutinous rice

Medium-tempera ture Daqu and Xiaoqu

Saccharomyces cerevisiae, Bacillaceae, Deinococcaceae, Microbacteriaceae, Micrococcaceae, Moraxellaceae, Nocardiaceae, Rhodobacteraceae, Xanthomonadaceae.


Ethyl pentanoate, Ethyl hexanoate, 3-Methylbutanal, Methional, Ethyl butanoate, 3-Methyl-1-butanol, Hexanoic acid, Butanoic acid, Trimethyl pyrazine, 2-Acetyl furan, Ethyl furoate, Methionol. Esters, Alcohols, Aldehydes, Acids, Ketones, Hydrocarbons, Pyrazines, Aromatics, Phenolic compounds. Acids, Alcohols, Ethyl butanoate, Low content of ethyl lactate.

Combining fruity with sweaty, roasted sesame-like, and floral flavors.

Soft mellow characteristics and a rich mouthfeel.

Light, strong, and rice flavor traits.


Herbal aroma


Sorghum

Daqu and Xiaoqu

Saccharomyces cerevisiae, Monascus, Rhizopus, Penicillium, Geotrichum candidum, actinomyces.

Chi aroma

Semi-solid-st ate fermentation

rice

Xiaoqu

Mucor, Rhizopus chinensis, Rhizopus oryzae, Surface yeasts, Saccharomyces cerevisiae, Candida utilis.


Butyric acid, Ethyl hexanoate, Caproic acid, Dimethyl trisulfide, Ethyl butanoate, 2-Phenylethyl alcohol, 3-Methyl butyrate, 4-Methyl phenol, 4-Methyl guaiacol, Beta-damascenone, Ethyl pentanoate, (E,Z)-2,6-Nonadienal, Borneol, Fenchol. (E)-2-octenal, 2-Phenylethanol, 3-(Methylthio)-1-propano l (methionol), Diethyl 1,7-heptanedioate (diethyl pimelate), Diethyl 1,8-octanedioate (diethyl suberate), Diethyl 1,9-nonanedioate (diethyl azelate).

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Herb-like sour aroma.

with

Fermented soybean-like with clean aftertaste.

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Mixed aromas


Sorghum, wheat, barley, and peas


Bacillus subtilis, Bacillus velezensis, Bacillus amylo-liquefaciens, Bacillus licheniformis, Bacillus methylotrophicus, Saccharomycopsis fibuligera, Beauveria bassiana, Eurotium athecium, Eurotium amstelodami, Aspergillus versicolor, Thermoascus crustaceus, Mycocladus corymbiferus, Paecilomyces variotii.


Tetramethylpyrazine, 2-Pentyl furan.

Between those of sauce aroma (Maotai) and strong aroma (Luzhou).


442


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40x22mm (600 x 600 DPI)



252x256mm (150 x 150 DPI)


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1180x424mm (95 x 95 DPI)


Effect of fermentation processing on the flavor of Baijiu

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