Chinese Baijiu and Whisky: Research Reservoirs for Flavor and Functional Food

Baijiu is a traditional spirit with high reputation in the Chinese community, and whisky, on the other hand, is a renowned spirit in Western culture, with both contributing a major proportion to the consumption and revenue in the global spirit market. Interestingly, starting with similar raw materials, such as grains, diverse production methods lead to different organoleptic profiles. In addition, such enormous attention they attract renders them as a crucial part in food and the related industry. Therefore, great efforts are made in improving product quality and optimizing production processes, such as flavor enhancement, facility development, and deep utilization of byproducts. Given the huge impacts and great involvements of these spirits in the general food industry, research focusing on either spirit is of referential significance for other relevant fields. With the aim of facilitating such collaboration, this review discusses the current research status, in a comparative manner, of both spirits in respect to key production processes-oriented sensory and flavor analysis, deep utilization of byproducts, and spirit-derived functional food investigations. Finally, the internal correlations based on the abovementioned criteria are identified, with research prospects proposed.


Introduction
Spirits, or liquors, make up essential parts of the global alcoholic drink industry, or even the broader food industry. Specifically, spirits, such as whisky, brandy, rum, vodka, gin, tequila, and Chinese Baijiu, are of great popularity. Starting from different raw materials, such as grains (whisky, vodka, gin, Chinese Baijiu), fruits (brandy, rum, gin, tequila), vegetables (vodka, gin), and sugars (vodka, rum, gin), each spirit embraces unique production techniques leading to distinctive flavor profiles. Among them, whisky and baijiu share the most similarities in terms of raw materials (grains) and production processes (saccharification, fermentation, distillation, aging, and blending) and are both popular and symbolic alcoholic drinks in both Western and Eastern cultures.
Baijiu, also known as Chinese national liquor, is a distilled spirit originating from China. Embracing a history of 2000 years, baijiu enjoys a high reputation in the Chinese community. It ranks at the top on the global spirit-consuming list, with~7.5 billion liters consumed in China in 2021 [1], and therefore, the baijiu industry, as a part of the food sector, plays a key role in the Chinese national economy. Made from diverse grains as raw materials, such as sorghum, corn, barley, and rice, and different production methods, baijiu is known for various aroma types, of which the symbolic ones are strong, light, sauce, rice, and mixed flavors [2]. Unlike most globally renowned alcoholic beverages, production of baijiu is featured with solid-state fermentation prior to distillation, with Figure 1. Baijiu flavor wheel, adapted from the Chinese National Standard of Terminology of Ba sensory evaluation. Explanations of certain phrases: Qu (fermentation and saccharification ag comprised of mixtures of micro-organisms; detailed explanation available in Section 3.1), Jiao ( mas derived from specific fermentation vessels, which are the habitat of mixtures of micro-org isms and grains, characteristic aromas in strong-aroma baijiu), Jiang (sauce aromas derived fr unique production techniques characteristic of sauce-aroma baijiu), Mi (aromas derived from ba made from rice instead of sorghum), Chen (aromas derived from baijiu aging), Jiuhai (a specific ag vessel; detailed explanation available in Section 3.2).
As for whisky, the major differences compared with baijiu lie within certain prod tion steps, namely, fermentation, and aging. Various versions of flavor wheels have b generated according to the whisky types, as well as the purpose. For malt whisky, sens descriptors are categorized into eight groups, including flavors derived from fermen tion, distillation, and maturation ( Figure 2). Grain-based aromas originally arise from r materials and are further modified by fermentation and distillation, while fruity and flo aromas are mainly derived from fermentation and distillation. Peaty character is co monly imparted by malt kilning and is a symbolic scent of many malt whiskies. Fe and sulphury compounds come in during distillation, and the aromas evolve as the d tillation progresses. Such characters may as well be mellowed and transformed dur aging in good wooden vessels. In addition, maturation in various styles of barrels br Explanations of certain phrases: Qu (fermentation and saccharification agent comprised of mixtures of micro-organisms; detailed explanation available in Section 3.1), Jiao (aromas derived from specific fermentation vessels, which are the habitat of mixtures of micro-organisms and grains, characteristic aromas in strong-aroma baijiu), Jiang (sauce aromas derived from unique production techniques characteristic of sauce-aroma baijiu), Mi (aromas derived from baijiu made from rice instead of sorghum), Chen (aromas derived from baijiu aging), Jiuhai (a specific aging vessel; detailed explanation available in Section 3.2).
As for whisky, the major differences compared with baijiu lie within certain production steps, namely, fermentation, and aging. Various versions of flavor wheels have been generated according to the whisky types, as well as the purpose. For malt whisky, sensory descriptors are categorized into eight groups, including flavors derived from fermentation, distillation, and maturation ( Figure 2). Grain-based aromas originally arise from raw materials and are further modified by fermentation and distillation, while fruity and floral aromas are mainly derived from fermentation and distillation. Peaty character is commonly imparted by malt kilning and is a symbolic scent of many malt whiskies. Feints and sulphury compounds come in during distillation, and the aromas evolve as the distillation progresses. Such characters may as well be mellowed and transformed during aging in good wooden vessels. In addition, maturation in various styles of barrels bring in woody and winey aromas, which are typical aging characters of whisky. Other whiskies, on the other hand, such as Bourbon, of which the dominant ingredient is maize, is endowed with flavor profiles with subtle differences. For instance, unlike maize, wheat, and malt, which bring in sweet notes to the spirits, rye typically imparts spicy character. For Bourbon whiskey, flavors categorized as spices, sweet, and earthy are generally present, acting as distinctive notes for Bourbon compared with malt whisky. and malt, which bring in sweet notes to the spirits, rye typically imparts spicy character For Bourbon whiskey, flavors categorized as spices, sweet, and earthy are generally pre sent, acting as distinctive notes for Bourbon compared with malt whisky. Moreover, alcohol content (alcohol strength) is also a crucial parameter of spirit prod ucts, not only as a physicochemical property, but also an interactive factor with flavor and sensory profiles. For whisky, 36-50% ABV (Alcohol by Volume) covers most products where 40-43% ABV is most often seen in markets. On the other hand, wider alcoho strength ranges apply for baijiu, from 18 to 78% ABV, where 38-55% ABV is more com mon. Ethanol is an essential factor for aroma perception in the headspace above the spirit and sensorial effects brought by different ethanol contents are worth studying. However Moreover, alcohol content (alcohol strength) is also a crucial parameter of spirit products, not only as a physicochemical property, but also an interactive factor with flavor and sensory profiles. For whisky, 36-50% ABV (Alcohol by Volume) covers most products, where 40-43% ABV is most often seen in markets. On the other hand, wider alcohol strength ranges apply for baijiu, from 18 to 78% ABV, where 38-55% ABV is more common. Ethanol is an essential factor for aroma perception in the headspace above the spirit, and sensorial effects brought by different ethanol contents are worth studying. However, such reports are limited, therefore presenting a potential research gap for future research. Nevertheless, a recent study on the impact of whisky dilution on aroma perception indicated that the usual consumption behavior of adding water to whisky does make a difference. Specifically, dilution over 80% whisky/20% water may significantly reduce the differences among various styles, and yet, differences were observed between Scotch and American styles as further dilution continued. This study provided crucial insights into interactions of spirit dilution and aroma perception and was of referential value for further investigations for both spirits.

Key Processing Steps Associated with Flavors
As has been mentioned, the production methods of both spirits are similar in general, including the processing of raw materials, fermentation, distillation, and aging. However, as traditional alcoholic beverages in different cultures with long histories, both baijiu and whisky possess complex production procedures (Figure 3), of which a few candidate processes make huge differences in the styles, flavors, and quality of the final products. interactions of spirit dilution and aroma perception and was of referential value for further investigations for both spirits.

Key Processing Steps Associated with Flavors
As has been mentioned, the production methods of both spirits are similar in general, including the processing of raw materials, fermentation, distillation, and aging. However, as traditional alcoholic beverages in different cultures with long histories, both baijiu and whisky possess complex production procedures (Figure 3), of which a few candidate processes make huge differences in the styles, flavors, and quality of the final products.

Microbial Diversity Involved in Saccharification and Fermentation
Grains, rich in starch as carbohydrates, are the starting point of making both spirits, and yet diverse flavor profiles are obtained from them. Saccharification and alcoholic fermentation are two of the major steps that make the difference. For whisky production, these two steps happen in a stepwise manner. In most cases, such as Scotch whisky, saccharification is initiated by endogenous enzymes in malts, including α-amylase and βamylase. Potent enzymatic activities are induced in malted barley by steeping, kilning, milling, and mashing. During the mashing process, starch-degrading enzymes work together to produce maltose from starch. Specifically, α-amylase helps with the liquefication of the gelatinized starch molecules, while β-amylase proceeds further with hydrolysis, yielding maltose [9]. In addition, other enzymes, such as carboxypeptidases responsible for the production of amino acids, are also developed in the starchy endosperm. The

Microbial Diversity Involved in Saccharification and Fermentation
Grains, rich in starch as carbohydrates, are the starting point of making both spirits, and yet diverse flavor profiles are obtained from them. Saccharification and alcoholic fermentation are two of the major steps that make the difference. For whisky production, these two steps happen in a stepwise manner. In most cases, such as Scotch whisky, saccharification is initiated by endogenous enzymes in malts, including α-amylase and β-amylase. Potent enzymatic activities are induced in malted barley by steeping, kilning, milling, and mashing. During the mashing process, starch-degrading enzymes work together to produce maltose from starch. Specifically, α-amylase helps with the liquefication of the gelatinized starch molecules, while β-amylase proceeds further with hydrolysis, yielding maltose [9]. In addition, other enzymes, such as carboxypeptidases responsible for the production of amino acids, are also developed in the starchy endosperm. The abovementioned enzymes facilitate the production and extraction of fermentable sugars, amino acids, vitamins, and minerals from malts, so that the following alcoholic fermentation can readily occur. For whiskies made from other grains, including corn, rye, and wheat, of which amylolytic enzymes are limited in the raw materials, malted barleys are normally added to provide the "diastatic power", thus producing fermentable sugars. For instance, a typical Bourbon mash contains~70% corn, 15% rye, and 15% malted barley, of which starch-degrading enzymes are present, with amino acids and small assimilable peptides contributed by other cereals. Commercial enzyme preparations may be applied for certain types of whiskies, of which malt is absent from the mash bills. Such examples include Canadian rye whisky, where milled rye with maize and/or wheat are used as raw materials, and enzymes, like amylases with different temperature tolerance, β-glucanases, and amyloglucosidases, are added at different stages of mashing to ensure the optimal release of fermentation ingredients.
Fermentation occurs after mashing when the cereal/malt worts are cooled down, and yeasts (typically Saccharomyces cerevisiae) are inoculated. Yeasts consume sugars and nitrogenous compounds during alcoholic fermentation, yielding ethanol, CO 2 , and secondary metabolites, including volatile compounds, and such compounds play a crucial role in the flavor profiles of both new-make spirits and final products. As is known, yeastderived aroma profiles are species/strain-dependent, and therefore, each distillery possess their unique yeast selections to produce whiskies of individual styles. The application of "non-conventional" yeasts, mainly non-Saccharomyces yeasts, which is a common practice in winemaking, is gradually attracting attention, and distinctive flavors are obtained for these trials [10,11]. In addition, since boiling is absent prior to fermentation, the non-sterile environments after wort production enable the presence of diverse local microflora, such as lactic acid bacteria (LAB). Among them, species of Lactobacillus are normally in dominance due to their tolerance of high alcohol content and acidic and anaerobic environments. Other surviving LABs include Lactococcus, Pediococcus, Streptococcus, and Leuconostoc. Typically recognized as spoilage in beer brewing, the limited occurrence of bacterial cultures and wild yeasts in whisky production is gradually deemed related to the improved flavor quality of the new-make spirit [12]. If present, they can produce lactic and acetic acid that can be esterified into ethyl lactate and ethyl acetate, imparting fruity, sweet, and creamy characteristics [13,14]. Additionally, γ-dodecalactone contributing the sweet and fatty flavors is also produced by LABs [14,15]. Therefore, lactic acid fermentation is sometimes desirable in distilleries. For instance, during whiskey production in the United States, spent mash (processed grains) can be added back, as a small portion, into the wort prior to fermentation, allowing the growth of lactic acid bacteria [16]. However, the occurrence and prosperity of the bacterial community during fermentation is crucial, since early growth may lead to poor ethanol yields. Alcoholic fermentation for whisky production generally lasts for 2-3 days, producing a final ethanol concentration of~6-8% in the fermented worts/mashes, which are ready for the following distillation [9].
Unlike whisky, sorghum is one of the most popular raw materials in baijiu production, followed by corn, rice, wheat, millet, and peas. All these grains are rich in starch, and therefore, similar to whisky production, conversion of starch into fermentable sugars is an essential step. And, in fact, saccharification and fermentation are deemed the most critical for baijiu production, responsible for producing thousands of flavor compounds that make up the complex aroma profiles of baijiu. Solid-state fermentation, of which simultaneous saccharification and fermentation occur in a solid mixture, is a characteristic step in baijiu production responsible for the rich and complex flavors in final products. In addition, semi-solid-state and liquid-state saccharification and spontaneous fermentation are both typical, though less common, in baijiu production, and are reported as producing spirits with less aromatic profiles, yet with higher fermentation efficiency.
Qu, as the traditional saccharifying and fermentation agent, comprises tremendous amounts of microbes and enzymes, and can be divided into daqu, xiaoqu, fuqu, and others. Daqu is a brick-shaped solid mixture of grounded barley, wheat, peas, and various indigenous microbes, such as molds (Mucor racemosus, Aspergillus niger, Thermomyces lanuginosus), yeasts (Saccharomyces cerevisiae, Candida), bacteria (Bacillus subtilis, lactic acid bacteria, acetic acid bacteria), and actinomycetes [4]. Different cultivation temperatures result in corresponding daqu types, including high-(60-70 • C), medium-(50-60 • C), and low-temperature (~50 • C) daqu, leading to various flavor characteristics [3]. High-temperature daqu is typically applied to produce sauce-aroma baijiu and has been reported in relation with the production of phenylethanol, 1,3-butanediol, acetic acid, propanoic acid, methyl ester, guaiacol, and tetramethylpyrazine [4]. Medium-temperature daqu contributes a fuller flavor and taste and is often used in the production of strong-aroma baijiu, rich in phenylethanol, caproic acid, tetradecanoic acid ethyl ester, ethyl caproate, guaiacol, pyrazines, and caryophyllene [4]. Low-temperature daqu, on the other hand, comprises larger microbiota, leading to a higher fermentation efficiency than the two other types. Such an agent is typically used in the production of light-aroma baijiu and is responsible for a pure flavor and taste, of which phenylethanol, (Z)-2-octen-1-ol, hexanal, (E)-2-octenal, nonanoic acid, acetic acid, lactic acid, hexyl acetate, ethyl acetate, and pyrazines are characteristic flavor compounds resulting from low-temperature daqu [4]. Apart from daqu, xiaoqu and fuqu are two major saccharification and fermentation agents used in baijiu production, resulting in diverse flavor characters. Xiaoqu is composed of rice, herbs, and sometimes clay as the raw materials, and rich in indigenous microorganisms, mainly molds and yeasts, such as Rhizopus sp., Rhizopus oryzae, Rhizopus peka, and S. cerevisiae [4]. Baijiu produced by xiaoqu is known for light and fragrant aromas. Unlike those that harbor naturally occurring microbes, fuqu, starting from bran as the raw material, is made by deliberate inoculation of molds under optimal conditions, and therefore, is known for the best saccharification and fermentation capacity [3].

Distillation and Aging
Baijiu distillation is another characteristic feature in baijiu production, which is different from that of whisky. Solid-state fermentation is commonly seen in baijiu-making, and therefore, the key feature in distillation is to distill ethanol and flavor compounds out of solids, namely, fermented grains. Water vapor is, therefore, applied as the agent to bring out these compounds from the solids. Zeng, a huge pot where fermented grains are distilled, is typically used for the distillation vessel, especially for solid-state fermentation. Fermented grains are placed as the top layer in zeng, and water is heated from the bottom of the vessel, underneath fermented grains, resulting in the rise of water vapor to pass through the fermented solids and bring volatile compounds, such as ethanol and esters, out of it. These compounds are further condensed, producing spirits with plentiful compounds of flavor and other benefits. Zeng is more like a special steaming tower, in that fermented grains are placed layer by layer in zeng, and therefore, volatile compounds are vaporized and condensed repeatedly as they pass through each layer, achieving multi-distillation. The alcohol contents of new-make baijiu vary and is as high as 75% ABV.
Whisky distillation, on the other hand, however, is more well-known and has been extensively reviewed and discussed. Pot distillation and continuous distillation are major methods, and similar to baijiu, timings for heads, hearts, and feints selection are critical for both spirits' production.
Aging, however, is also where the differences lie, and is critical for quality improvement for both spirits. New-make baijiu is described with odors like green, harsh, peppery, and raw cardboard [17]. For baijiu, aging typically happens in various vessels, including pottery jars, jiuhai (Mare Nectaris, a traditional aging container woven from vines and coated with hemp paper, animal blood, and lime), and stainless-steel vessels. During aging, various chemical reactions occur within the liquid, such as reduction, oxidation, esterification, hydrolysis, and the Maillard reaction, and compounds of low boiling points con-tributing negative flavor properties are eliminated by evaporation through micropores [18]. Oxidation and esterification reactions were confirmed by experimental investigations. During baijiu aging, the ester concentration increases due to the conversion of alcohols into their corresponding acids and esters, while a constant balance of ester levels is obtained via esterification [19]. Studies, with the help of density functional theory and molecular dynamics simulation, suggest that for strong-aroma type baijiu, the activation energy of acid hydroxyl-oxygen protonation was higher than that of ester alkyl-oxygen protonation, therefore explaining the increased level of acids and decreased ester concentration during aging in pottery jars [20]. In addition, the sorption effect is also proposed, especially when aged in pottery jars, and trials, like applying pottery chips, have been conducted, confirming the potential sorption effects [21]. Since pottery jars are reusable, such pottery materials are identified as vectors responsible for transferring flavor compounds into newly distilled baijiu and, therefore, accelerating the aging process [21].
Aging in wooden casks plays a crucial role in determining the flavors and styles of whisky. For many production regions, it is mandatory for new-make whisky spirit to be matured in wooden (mainly oak) containers for a period of time before being ready for markets. In Scotland, whiskies are required to be aged in oak barrels for at least 3 years, and it is not rare to find 12-year-old or even older products. The flavors and textures imparted to the spirits are time-and wood-dependent, leading to a grand selection of aging vessels for the worldwide distilleries. For Scotch whisky, it is a common practice to employ used barrels for maturation. For instance, the Sherry cask, which has previously been used for Sherry wine (a type of fortified wine) aging, is frequently applied by many whisky producers including Scotch whiskies for complex flavor extraction. And, such used barrels previously containing other alcoholic products, such as port (a type of fortified wine), madeira (a type of fortified wine), and Tokaji (a type of sweet wine made from grapes infected with noble rot), are popular choices for many producers. On the other hand, however, Bourbon whiskey only allows for the use of new oak barrels for maturation, while used Bourbon barrels can be reused by other production regions for more flavor blending and creation. Investigation aiming at characterizing different barrels revealed the chemical signatures of two popular options, Bourbon and Sherry casks [22]. Bourbon casks are identified by compounds, including flavonols, oligolignols, and fatty acids, whereas for Sherry casks, polyphenol glycosides, such as quercetin-glucuronide, myricetin-glucoside, and carbohydrates, can be used for discrimination [22].
Similar to some types of baijiu, whisky aging exposes the liquid to a micro-oxygen environment, owing to the permeation effect of the wood staves [23]. And, therefore, chemical reactions, such as oxidation, occur, enabling whisky stabilization and flavor formation. However, compounds transferred from wood into spirits and the following fusion are other major reasons shaping the characters. The frequently used oak wood normally comprises cellulose (45-50%), hemicellulose (22-25%), lignins (23-32%), and extractables (mainly acids, carbohydrates, and phenolic compounds, 3-10%) [24]. Oak woods of different origins are reported with dissimilar extractable properties. For instance, European oak is reported with higher amounts of phenolic compounds and extractable solids than American oak, whereas the latter is normally known for its greater contribution of oak flavor to liquids [24]. Sensory-directed study coupled with instrumental analysis identified ellagitannin vescalagin, castalagin, grandinin, roburins A-E, and 33deoxy-33-carboxyvescalagin as the major compounds contributing to the astringency from the European oak (Quercus alba L.) [25]. The migration of extractable carbohydrates is elucidated in Bourbon whiskey [26]. Recalcitrant cellulose and hemicellulose are liberated from the barrel staves, and meanwhile, the spirit expands into and contracts from the wood, therefore forming a complex, which may be associated with the characteristic flavor and palate [26].

Volatile Compositions of Baijiu and Whisky
With diverse sensory profiles identified for both spirits, it becomes a crucial task to reveal the scientific nature, namely, volatile compositions, to thoroughly understand the flavor and sensory characteristics. Such knowledge may, therefore, play an instructive role in quality improvement during production. Similarities are observed for both baijiu and whisky based on volatile components, while differences in terms of quantities and compositions contribute greatly to their distinctive sensory characters. For better data interpretation, a heatmap ( Figure 4) was hereby generated in terms of the occurrences of key aroma compounds reported in baijiu and whisky, while a correlation network ( Figure 5) based on volatile compositions was proposed for both spirits [5,8,18,25,. Data were obtained from various brands and styles of both spirits, with diverse sample extraction methods and identification methods applied in different studies. Such extraction methods include direct injection (DI), liquid-liquid extraction (LLE), liquid-liquid microextraction (LLME), solid-phase extraction (SPE), solid-phase microextraction (SPME), stir bar sorptive extraction (SBSE), and simultaneous distillation and extraction (SDE), and identification methods include GC-MS, GC-O-MS, GC-FID (flame ionization detector), GC × GC-TOFMS, GC-PFPD (pulsed flame photometric detection), GC-QTOF-MS, and UPLC-MS. For both spirits, factors such as alcohol strength, style, and production of origin, play crucial roles in volatile compositions and flavor profiles, making it complex to assess flavor similarities and differences. And, therefore, markers of significant representativeness are valuable for spirit comparison. Key aroma compounds, which are further identified in these studies by analyzing volatile concentrations and sensory contributions by means of the molecular sensory science approach, were included as such crucial markers for comparison due to their significant contributions to sensory properties ( Figure 4). Specifically, the general methods involved in molecular sensory science study include identification and quantification of volatile compounds, aroma extract dilution analysis (AEDA) coupled with GC-O, aroma reconstitution, and omission tests coupled with sensory evaluation. And, such a comprehensive process renders it as a valid method for the identification of key aroma compounds, and therefore, comparisons accordingly help minimize the effects brought by other variables. Though both start with grains as raw materials, low similarities (~25%) in terms of key aroma compounds were observed in the baijiu and whisky ever reported, which may be explained by the diverse production methods of each spirit. On the other hand, a correlation network was proposed by analyzing volatile compositions available from the literature for both spirits ( Figure 5). Surprisingly, certain styles of both spirits showed relevance, such as Bourbon whisky vs. most baijiu, and sauce-flavor baijiu vs. most whisky. To further specify the similarities and differences, detailed discussions are hereby made for common and crucial volatile compounds, including alcohols, esters, aldehydes, acids, and sulfur compounds.

Alcohols, Esters, Aldehydes
Esters are crucial volatile groups in almost all alcoholic beverages, endowing the liquids with diverse aroma characters, such as fruity, floral, sweet, and milky. Previous research illustrates the significant proportions of these compounds in both baijiu and whisky. For instance, a total of 510 types of esters were identified in baijiu [50], and such systematic investigations are also encouraged for whisky flavor research. The most common esters encountered in baijiu are ethyl esters, followed by methyl esters, propyl esters, butyl esters, amyl esters, hexyl esters, and lactones. Similarly, ethyl esters are the major ester groups as key aroma compounds present in whisky. Ethyl acetate and ethyl lactate are the most frequently observed esters in baijiu, and are present in all 12 aroma types of baijiu. Furthermore, all these aroma types share several other esters, including ethyl butyrate, ethyl valerate, ethyl caproate, ethyl heptanoate, ethyl caprylate, ethyl nonanoate, ethyl decanoate, propyl caproate, ethyl 2-methylpropionate, ethyl 2-methylbutyrate, ethyl 3-methylbutyrate, 3-methylbutyl caproate, ethyl phenylacetate, and diethyl succinate. Differently, ethyl 3methylbutanoate is commonly identified as a major aroma-active compound in whisky, with whisky lactone being the characteristic ester in whisky.

Alcohols, Esters, Aldehydes
Esters are crucial volatile groups in almost all alcoholic beverages, endowing the liquids with diverse aroma characters, such as fruity, floral, sweet, and milky. Previous research illustrates the significant proportions of these compounds in both baijiu and whisky. For instance, a total of 510 types of esters were identified in baijiu [50], and such systematic investigations are also encouraged for whisky flavor research. The most common esters encountered in baijiu are ethyl esters, followed by methyl esters, propyl esters, butyl esters, amyl esters, hexyl esters, and lactones. Similarly, ethyl esters are the major ester groups as key aroma compounds present in whisky. Ethyl acetate and ethyl lactate are the most frequently observed esters in baijiu, and are present in all 12 aroma types of baijiu. Furthermore, all these aroma types share several other esters, including ethyl butyrate, ethyl valerate, ethyl caproate, ethyl heptanoate, ethyl caprylate, ethyl nonanoate, ethyl decanoate, propyl caproate, ethyl 2-methylpropionate, ethyl 2-methylbutyrate, ethyl 3-methylbutyrate, 3-methylbutyl caproate, ethyl phenylacetate, and diethyl succinate. Differently, ethyl 3-methylbutanoate is commonly identified as a major aroma-active compound in whisky, with whisky lactone being the characteristic ester in whisky.
Esters are produced via various reactions, including microbial metabolic synthesis and spontaneous chemical reactions. In addition, a few esters are ingredient-derived [50], which are applicable for both spirits owing to the similar ingredients used for production. Esters are produced via various reactions, including microbial metabolic synthesis and spontaneous chemical reactions. In addition, a few esters are ingredient-derived [50], which are applicable for both spirits owing to the similar ingredients used for production. Nevertheless, a group of esters, whisky lactones, is unique to whisky under the influence of aging in oak barrels. For both spirits, microbial enzymatic esterification accounts for the major source of ester compounds. Such a reaction is catalyzed by enzymes, including carboxylic acid hydrolase, lipase, esterase, and cutinase [51]. Due to the traditional manufacturing method, a complex microbial network is identified in baijiu production, thus harboring numerous microorganisms with versatile metabolic abilities, including esterification. A fungus, Monascus purpureus, recognized for high ester-producing ability, was isolated from baijiu fermentation starter. And, the key enzyme, a carboxylesterase, was identified to synthesize fatty acid esters [52]. Such an environment not only provides various microbes for complex aroma production, but acts as a microbial niche for screening microorganisms of potential benefit.
For whisky, it is noted that long-chain ethyl esters play a crucial role in shaping diverse aromas, though with similar compound compositions [53]. These compounds are produced from ethanol and medium-chain fatty acids during yeast fermentation. The number of the carbon numbers in the aliphatic chain of ethyl esters can be as many as  [53]. Research indicated that such hydrophobic esters, such as ethyl octanoate, ethyl decanoate, and ethyl dodecanoate, make up a great proportion of whisky headspace, are present at a significant quantity, and lead to key contributions to whisky aroma profiles [55,56].

Acids
Acids, in both volatile and non-volatile forms, are mainly produced during fermentation, the results of microbial metabolism. Specifically, they are produced by acetyltransferase systems present in different microorganisms. For instance, byproducts including fusel alcohols are synthesized during ethanol production, and such alcohol compounds are further transformed to corresponding acids by alcohol dehydrogenases and aldehyde dehydrogenases. Examples include 2-methyl-1-propanoic acid and 3-methyl-1-butanoic acid. Additionally, for baijiu, Acetobacter aceti and lactic acid bacteria (LAB), responsible for the production of acetic acid and lactic acid, respectively, are commonly occurring microbes during fermentation. On the other hand, however, LAB may also be present in whisky fermentation in some cases. The late phase of lactic fermentation is mostly desirable in producing flavor-positive compounds, and yet, comprehensive mechanisms therein remain fully elucidated [57].
These compounds are extensively studied in many alcoholic drinks, including baijiu and whisky, from the perspective of flavor contribution. In addition, acids are also important compounds of health potential that are discussed in further details in the following sections. In baijiu, acids are frequently detected as metabolites of the complex microbial network involved in baijiu-making, whereas such compounds are rarely reported as key flavor compounds in whisky. Acids are present in baijiu at large ranges, from the highest of 65% for strong-aroma to as low as 9% for buckwheat-aroma. Major acids in baijiu include acetic acid, propionic acid, butanoic acid, hexanoic acid, pentanoic acid, heptanoic acid, nonanoic acid, octanoic acid, decanoic acid, 2-methylpropionic acid, and 3-methylbutanoic acid [58]. On the other hand, 3-methylbutanoic acid present in Tennessee whiskey is one of the few odor-active acids reported in whisky [28]. Despite the fact that acids are not as influential as other aroma compounds, such as esters, in determining volatile profiles of these spirits, yet their role as precursors of ester formation is non-neglectable. For instance, short-chain ethyl esters, which are responsible for fruity and floral characters, are formed via chemical reactions of short-chain acid compounds and ethanol. Furthermore, aroma-neutral acids can be transformed to flavor-active acids via chemical reactions during fermentation, including 2-methylpropanoic acid, 3-methylbutanoic acid, sorbic acid, lactic acid, and benzoic acid.
In addition, non-volatile acids play crucial roles in shaping the overall flavor characters by matrix effects, such as masking or synergistic effects. A study aiming at reconstituting the signature flavors of Laobaigan-aroma baijiu revealed the significance of these acids in altering the volatile profiles of the spirit matrices [44]. Matrices containing both non-volatile acids and odor-active compounds are judged as having elevated characters of fruity, acidic, floral, jujube, and grain, yet with reduced sweety and alcoholic notes, compared with those containing only odor-active compounds [44]. Specifically, lactic acid showed additive or synergistic odor effects on ethyl lactate and ethyl acetate by decreasing olfactory thresholds significantly [38]. Though acids are not among the top focus lists of whisky researchers, similar flavoromics-based interactive studies are encouraged to extensively depict the flavor map, as well as flavoring mechanism of whisky.

Sulfur Compounds
Sulfur compounds are a group of flavor substances crucial to both spirits. They can be a double-edged sword and offer pleasant aromas at low concentrations, yet make negative contributions at high levels. Certain sulfur compounds are identified as the key aroma compounds characteristic of some aroma-type baijiu. And, such examples include dimethyl sulfide and benzothiazole, responsible for smoky and toasty notes in strong-aroma baijiu [2]. Therefore, targeted research was conducted extensively for thorough understanding of the distribution of these sulfur-containing volatile compounds in final spirit products, as well as obtaining rational management through production for a more desirable flavor profile.
In whisky, sulfur compounds evolve throughout the whole production process. Dimethyl sulfide (DMS) is reported to be formed during the malting process. Specifically, DMS is produced during the kilning process from S-methyl methionine, which is originally formed from methionine during barley germination [59]. The higher the kilning temperature, the more DMS is produced. The following fermentation step leads to more production of sulfur compounds (Table 1), many of which derive from the methionine biosynthesis pathway. Similarly, for baijiu production, volatile sulfur compounds are mainly produced during fermentation, as well. And, yet, unlike the germination process for whisky, microbial hydrolysis of proteins naturally occurring in raw materials, such as sorghum, typically takes place for baijiu production, yielding sulfur-containing amino acids as precursors. Such compounds are further converted to various sulfur volatiles, including methional and dimethyl trisulfide [60]. In addition, the complex microbial system involved in baijiu production enables the production of other sulfur compounds, such as benzothiazole [61]. Moving forward, distillation is also a critical step in managing sulfur contents in the spirits, especially for whisky production. Copper is commonly acknowledged as the suitable material for the distillation apparatus, making it possible for undesirable sulfur removal, according to the longstanding industry practice. However, further investigations suggest the possibility of formation of sulfur volatiles by catalysis of copper compounds. Specifically, methyldisufanylmethane (dimethyl disulfide, DMDS) and methyl trisulfanylmethane (dimethyl trisulfide, DMTS) are proposed to be produced more in the presence of copper, with precursors being methanethiol, and methanethiol and hydrogen sulfide, respectively [62][63][64]. Finally, levels of certain sulfur compounds decrease during the maturation process for whisky. And, such compounds include DMDS, DMS, and DMTS [62]. The mechanisms therein remain unsolved, while the absorption effect brought by the inside charcoal layer of the oak may be one of the possibilities, which requires further scientific explanations.
Volatile sulfur compounds can be undesirable sometimes, especially for whisky. And, therefore, efforts are made for sulfur elimination. Physical treatment, with no chemical reactions involved, is recognized as a crucial solution as downstream management. Among them, charcoal or activated carbon are commonly used, though at the expense of potentially losing desirable components, due to the lack of selectivity in absorption. Different activated carbon materials were trialed, such as coal and coconut shell-based activated granular carbons, and reduced levels of sulfur volatiles were noted based on organoleptic characters. Further study indicated the possible relationship of pore size, contact duration, and contents of sulfur compounds is required [65].

Compound Name Odor Description Whisky Baijiu References
Methyl benzyl sulfide - Gasoline, leather, roasted, smoky, and rubber -√ : such compound has been reported in the respective spirit; -: such compound has not been reported in the respective spirit.

Instrumental Analysis Methods
Volatile detection is one of the most crucial methods for thoroughly understanding the flavor profiles of both baijiu and whisky. However, challenges are present due to the following reasons: a. the complex matrix various products offer; b. the generally broad and vague reference parameters for certain volatiles based on the literature, such as the retention index, thus potentially leaving out novel compounds; c. the relatively high detection thresholds of many instruments for trace compounds.
Many challenges were addressed throughout the development of instrumental analysis, thus rendering it a useful tool for decoding the chemical composition of different spirits. For instance, specific detection methods for sulfur-, nitrogen-, and phosphorous-containing volatiles are utilized, including the nitrogen chemiluminescence detector (NCD), nitrogen phosphorous detector (NPD), sulfur chemiluminescence detector (SCD), and flame photometric detector (FPD). In addition, other applicable extraction and detection methods are devised and summarized in Table 2.
Despite analytical technologies advancing greatly, a fixed and mature methodology is yet absent for most types of baijiu and whisky. And, such conditions in turn encourage further technical improvement of volatile analysis to expand the identification scope, as well as the detection level of aroma compounds, especially those present at trace levels, yet potentially crucial to sensory profiles.
For baijiu, various extraction methods coupled with qualitative and quantitative approaches were trialed and revealed the occurrence of more than 1, 900 flavor compounds. And, these extraction methods include direct injection (DI), liquid-liquid extraction (LLE), liquid-liquid microextraction (LLME), solid-phase extraction (SPE), solid-phase microextraction (SPME), stir bar sorptive extraction (SBSE), and simultaneous distillation and extraction (SDE). Similarly, a few of the abovementioned extraction methods are applied in whisky flavor analysis, such as SPME and LLE. On the other hand, however, solventassisted flavor evaporation (SAFE) was also reported as one of the techniques used during aroma profiling. Such a method may help with the extraction of non-volatile compounds or compounds with lower boiling points migrated from oak barrels into whisky.
The following analytical processes are also customized based on the detection requirements of various volatile groups and spirit types. Common techniques include gas chromatographic mass spectrometry (GC-MS), ultra-high-performance liquid chromatography (UPLC), and two-dimensional gas chromatography time-of-flight mass spectrometry (GC×GC-TOF-MS). In analyzing particular compounds, such as compounds at trace levels, customized detection techniques are applied, such as nuclear magnetic resonance spectrometry (NMR), GC-NCD, GC-SCD, GC-NPD, GC-FPD, UPLC-high resolution mass spectrometry (UHPLC-HRMS), and UHPLC-quadrupole orbitrap HRMS (UHPLC-Q-Orbitrap HRMS).
Since volatile compositions are not in direct correlations with sensory characters due to the various thresholds and complex sensorial interactions between each compound, identifying the key aroma compounds contributing the crucial organoleptic properties is therefore of great importance. The molecular sensory approach, which incorporates instrumental analysis with sensory study, becomes increasingly popular for both baijiu and whisky research. Such a systematic methodology starts with the identification and quantification of volatile compounds, followed by recognizing their aroma contributions, via instrumental-coupled sensorial analysis. In identifying odor active values (OAVs), along with the conduction of aroma extract dilution analysis (AEDA), aroma omission, and reconstitution analysis, volatile compounds of crucial organoleptic contributions are identified for both spirits, which practically makes it possible for future potential flavortargeted quality improvement.

Health Potentials
Both spirits share a common dilemma where large quantities of byproducts are obtained during the production process. Specifically, two byproducts are involved in the production of whisky, namely, draff and pot ale. Draff is the solid waste (wet grains) and is rich in carbohydrates, fiber, and proteins, whereas pot ale refers to the liquid waste from the distillation, rich in proteins and potentially copper, if copper still is used. Similarly, two major byproducts are identified from baijiu production, as well, jiuzao and huangshui, which are solid and liquid wastes, respectively. Jiuzao is the spent grains after distillation rich in proteins and carbohydrates. Huangshui is the viscous liquid produced from solidstate fermentation and, therefore, contains numerous trace compounds generated during complex fermentation. Currently, the solid wastes derived from both spirits' production are mainly treated as animal feed and biomass combustion, while liquid wastes are discharged as waste water after minor treatment. However, in accordance with global carbon neutrality, such waste management is no longer suitable, and therefore, efforts are being made in exploring reusable solutions to bring out the full potential of these byproducts.
To begin with, the hidden values of these products are investigated thoroughly. Though utilizing similar raw materials as a start, various follow-up procedures for either spirit making, i.e., saccharification and fermentation, can potentially lead to diverse major components of health benefits. For baijiu, both jiuzao and huangshui are identified as valuable pools for potentially valued extractables, such as peptides and polysaccharides. Essential activities, such as antioxidant, anti-inflammatory, and immunomodulatory capacities, were identified within these compounds via in vitro assays (Table 3). Similarly, phenolic compounds, as a major group of compounds in grains, were studied in both whisky draff and baijiu jiuzao, where antioxidant capacity, to various degrees depending on specific compounds, was identified (Table 3). Rather than simply treated as feedstocks and fuels, such materials can be fully utilized for their biological potential, and further function-based investigation is encouraged. Subsequent exploitation may include production of functional foods or adding back to spirits, the latter of which requires additional research on their influence on the overall flavor profiles.

Within the Spirits
In addition to the byproducts, the health and functional values within the spirit are worth studying, as well. Such a topic is controversial since ethanol is deemed risky to the human body, and yet, both baijiu and whisky are mixed solutions containing thousands of trace compounds, therefore providing potential values, or mitigating the harmfulness ethanol brings. One of the major advantages that is generally acknowledged is the pleasure and psychological comforts it brings after proper intake. Further in-depth research targeting both physiological and psychological benefits are conducted for comprehensive explanation. Specifically, health potentials are discussed in respect to compounds by categories within the spirits and broad health values based on clinical surveys.

Acids and Esters
Acids and esters are crucial flavor compounds present in both spirits, of which esters are widely identified as key odor compounds contributing to characteristic flavor profiles. Their occurrences in both spirits being elucidated, many of these compounds are investigated for their bioactive benefits.
However, it should be noted that certain health potentials of the abovementioned compounds are investigated under in vitro assays, and that ethanol matrix-based experiments are required for more comprehensive results. In addition, the detoxification or mitigation of ethanol damage of these compounds is also among the top research interests.

Phenolic Compounds
Phenolic compounds are one of the groups well known for their antioxidant capacity. For both spirits, these compounds are derived from grains as raw materials-microbial bioconversion for baijiu during fermentation, and extraction from oak barrels for whisky maturation. Diverse research approaches are trialed, including in vitro and in vivo assays and clinical investigations, to identify their potential health benefits from a broader perspective.
Phenolic compounds identified in baijiu include guaiacols as the major research target, such as 4-ethylguaiacol and 4-methylguaiacol. These compounds, as well as vanillin, presented cytoprotection against abnormal oxidative stress via in vitro analysis [91]. Research demonstrated that such phenols showed a protective capacity by gene modulation, especially the expression levels of antioxidative enzymes [91]. One of the compounds, 4-ethylguaiacol, was examined, demonstrating an anti-inflammatory effect via in vitro assay [67]. Such an effect may be explained by the inhibition on the activation of inflammasome and the production of inflammatory cytokines [67].
In whisky, human-based clinical trials were conducted, in which phenolic compounds are treated as a whole for health-related investigations. Consumption of whisky, instead of unmatured new-make spirit, significantly elevated the plasma total phenol content and antioxidant activity in healthy male participants [92]. In addition, individual groups of phenolic compounds are also investigated. Phenolic syringaldehyde, one of the key odorants in whisky, was studied and validated for its pharmaceutical values via molecular docking and subsequent in vivo and in vitro assays [93]. Health potentials, such as anti-oxidation, anti-inflammation, and anti-diabetes, are identified with syringaldehyde as the individual target, and therefore, such exploration of composite effects in the ethanol matrix is highly encouraged for validation. Further biological benefits other than anti-oxidation are identified among other phenolic compounds. Several characteristic whisky polyphenols, including ellagic acid, gallic acid, lyoniresinol, syringaldehyde, syringic acid, vanillin, hydroxymethyl furfural, sinapylaldehyde, vanillic acid, coniferylaldehyde, and protocatechuic acid, are proved as having anti-melanogenic capacity via in vitro analysis [94]. Similarly, certain phenolic fractions, especially fractions over 10, 000 molecular weight, were identified as having SOD (superoxide dismutase)-like capacity [95]. Among them, ellagic acid, gallic acid, and lyoniresinol were elucidated as the key contributors against melanogenesis [94]. Ellagic acid, on the other hand, was examined as a crucial compound for eliminating ethanol-induced damage in the stomach [96]. In vivo assays on rats demonstrated the lower irritation to gastric mucosa from whisky compared with pure ethanol, which may be due to the presence of ellagic acid, under its scavenging mechanism against oxygen and hydrogen radicals [96]. For aged whisky, where phytophenols are obtained in contact with oak barrels, four phenolic compounds, including caffeic acid, vanillin, syringaldehyde, and ellagic acid, were examined, demonstrating the ability to reduce the acetaldehyde level in the blood of mice [97]. The potential mechanism therein was explained by the inhibition of the liver alcohol dehydrogenase (ADH), thus eliminating alcohol metabolism. And, therefore, alcoholic consumption in the presence of the abovementioned phenolic compounds may have a preventive effect on alcohol-related diseases.

Other Compounds
There are other compounds than the abovementioned groups that were identified as having potential health-related benefits, especially in baijiu. Representative components include sulfur-and nitrogen-containing compounds, such as pyrazines and peptides. Tetramethylpyrazine, one of the most abundant pyrazines identified in baijiu [6,98], has long been recognized as pharmaceutically active, which was originally identified from a traditional Chinese medicine, Ligusticum chuanxiong [99]. And, therefore, numerous targeted investigations were conducted, with its potential benefits addressed as follows: positive effects on cardiovascular and cerebrovascular diseases; in relation with anticancer and diabetic complications; protective effects on the liver and kidneys; beneficial to neurodegenerative diseases [99].
In addition, another group of nitrogen-containing complex molecules, peptides, were studied. Similar to those identified from byproducts during production, peptides were discovered in baijiu spirits, as well. Polypeptides, including peptides (Ala-Lys-Arg-Ala, Pro-His-Pro) identified from sesame-flavor baijiu and peptides identified from Qingke baijiu (Val-Val-Thr-Gly-Val-Gly-Gly-Gln, Leu-Pro-Val-Gly-Pro, Leu-Leu-Ser-Pro-Pro, Phe-Pro-Leu-Gln-Pro-His-Gln-Pro), were investigated for their health-related functions via in vitro assays. Effects of these peptides include cytoprotective capacity against oxidative stress and inhibition of ACE and, therefore, potentially helpful in cardiovascular diseases [100][101][102]. However, these studies focused primarily on in vitro tests, with pure compound applied as the research target. And, therefore, more in-depth research in the context of ethanol is required to further elucidate their potential benefits.

Overall Effects
With the diverse compounds identified in both spirits being elucidated for their intrinsic health values, demands for the integrated effects in the ethanolic context greatly increase for both theoretical and industrial reasons. Several studies were carried out on either animal models or human volunteers, with certain aspects investigated for health potentials or mitigations of ethanol-induced damages.
Milder alcoholic liver injuries were identified in baijiu-treated animal models (mice and rats) compared with pure ethanol treatment, and such an effect is more prominent in aged baijiu rather than fresh [103,104]. In addition, aged baijiu showed a more complex interaction with gut microbiota and the subsequent serum metabolome, shedding light on further investigation of the mechanism related to liver injury [103]. An in vivo study proved the potential link between gut microbiota regulation and alcoholic liver damage mitigation [105]. Correlations among gut microbiota, volatile production, and serum metabolites (i.e., bioenergy-related, anti-inflammatory-related) were noted with mice models, and conclusions were obtained that gut microbiota and host metabolism play key roles in regulating the relief of alcoholic liver damage [105]. For whisky, similar alleviation effects were observed for gastric mucosal injury. A protective activity for gastric mucosa, potentially due to the presence of congeners in whisky, was recorded in rat models, and yet, such protection followed a dose-dependent manner and was inhibited at higher doses (i.e., 150 mg/kg body weight) [106].
Apart from ethanol-induced damage mitigation, observations focusing on other human-related aspects were made, as well. A cross-sectional study conducted in Southern China (Guangdong province) investigated the relationship between baijiu consumption and dental health-dental caries, in particular. A protective effect of baijiu consumption against dental caries was obtained in middle-aged and elderly people via statistical analysis [107]. However, with the solid scientific explanation therein unsolved, further related research elucidating the correlations and mechanisms is encouraged.

Conclusions and Outlook
Both baijiu and whisky receive great attention and acceptance in their respective cultures. Known as the best-selling alcoholic beverage globally, baijiu is mainly consumed by the Chinese community. On the other hand, however, whisky, one of the most popular distilled spirits worldwide, enjoys its fame in diverse regions. In addition, given the established culture of spirit consumption, whisky is booming in China in recent years. Such a phenomenon, along with the unstoppable globalization, enables the potential cooperative development of these two ancient and renowned spirits. However, despite the similar raw materials as ingredients, the diverse production procedures lead to various organoleptic and compositional properties.
Based on this review, the similarities and differences in terms of flavors and potential health benefits between these two spirits are summarized comprehensively. In addition, the investigation approaches applied for both spirits are of great importance for any relevant products. Examples include the molecular sensory method used in identifying the key aroma compounds in baijiu and whisky. Tailored and targeted analysis techniques for trace compounds offering potentially significant organoleptic properties in these spirits are also in great need for further development. On the other hand, given the similar byproducts obtained for both spirits during production, future research is encouraged on more diverse utilization of similar compounds to meet the zero-carbon strategy for sustainable and responsible development. Finally, the potential health benefits, though controversial in the context of alcoholic beverages, should not be neglected for investigation, especially with such a huge consumer market. Many studies, mainly focusing on in vitro assays, were conducted as pilot investigations, shedding light on future in-depth research. Most importantly, the functional compounds identified in spirits should be evaluated in the alcoholic context at proper contents for validation. Furthermore, the synergistic effects are also encouraged for thorough investigation. Such understanding will be of guiding significance for baijiu and whisky producers to produce healthier products to meet the increasing demands for healthy food by major consumer markets.

Conflicts of Interest:
The authors declare no conflict of interest.