A Comparative Analysis of Aroma Profiles of Soju and Other Distilled Spirits from Northeastern Asia

Abstract

The soju (Korean traditional distilled liquor) market is increasing worldwide. However, in contrast to well-explored distilled liquors, including baijiu (China) and shochu (Japan), soju is less investigated, with limited research on its aroma characteristics. To facilitate better understanding of the aroma characteristics of soju, this study aims to overview recent research on the flavor characteristics of soju and compare data with those of baijiu and shochu, well-established products in the market. Soju is generally made using rice and nuruk (a traditional Korean fermentation starter). Previous studies have reflected that the aroma characteristics vary with raw materials’ nutrition percentages, microbial taxa influenced in fermentation starters, and/or pressure reduction during distillation. The research on the aroma characteristics of baijiu, characterized by solid-state fermentation involving qu (a traditional Chinese fermentation starter), is focused on differences in regional characteristics of the flavor type. Research on the aroma characteristics of shochu has primarily demonstrated that the microbial community could contribute significantly to the development of specific aromatic compounds and/or attributes. Moreover, the association of the aroma characteristics of baijiu and shochu with their volatile compound development by the determination of selective ingredients has been examined. Understanding the current research progress can potentially facilitate the improvement in the aroma characteristics of soju.

1. Introduction

Soju (Korea), baijiu (China), and shochu (Japan) are the representative types of distilled liquor used in Northeast Asia. Historically, Korea, China, and Japan have been clustered in the same cultural region associated with various cross-cultural characteristics, including dietary culture and food technology. Distilled liquor is characterized by a clear appearance and a high alcohol content resulting from distillation. Its distinctive aroma depends on the characteristics of the ingredients and the distillation conditions. Various distilled liquors are available globally; however, this review focuses on the Northeast Asian distilled liquors soju, baijiu, and shochu, with special emphasis on their aroma characteristics.

In Korea, soju refers to both distilled and diluted liquors, but the production process of diluted soju was imported owing to liquor taxation laws during the Japanese colonial period [1]. The market strategy report of alcoholic beverages in Korea indicates the dominance of diluted soju in the domestic market [2]. In 2021, the market share of diluted soju amounted to USD 3096 million (40.1% of the total liquor market), with beer (41.0%) representing the majority of total sales with diluted soju, whereas traditional liquors represented only 1.1%. However, the share of traditional liquors has steadily increased with the recently increased popularity of traditional foods; an increase from KRW 40.0 to 94.2 billion (0.43% to 1.07% of the total) was noted, and distilled soju accounted for approximately 11.4% of all traditional liquors, with the share increasing from KRW 33,161 to 64,628 million (2017–2021) with an increased number of distilled soju manufacturing licenses (from 64 to 152 companies) [2].

As of 2003, the total liquor production in China amounted to 32 million tons, with baijiu accounting for 10.3% (CNY 55 billion); however, exports of baijiu were extremely low compared to the domestic market. Baijiu is classified depending on the fermentation process and/or the use of various raw materials. The trend of the baijiu market has changed according to the needs of younger consumers preferring cheaper baijiu; this led to changes in demand patterns in the liquor market, previously dominated by high-priced baijiu [3].

The shochu market accounts for USD 4.7 billion, representing 9.3% of the total liquor market (2019). Similar to the trend in China, most sales occur domestically. The sales of shochu have significantly expanded its consumption. It is characteristically consumed by mixing it with water or other beverages, leveraging its high alcohol content. Using a wide variety of raw materials, research on the flavor characteristics of shochu has been conducted extensively, leading to a better understanding of flavor characteristics, including the development of its flavor wheel [4,5].

Although the distilled soju market has seen significant growth, the limited understanding of its aroma characteristics hinders its further development compared to that of baijiu or shochu. The objective of this review was to analyze recent research on the aroma characteristics of distilled soju, which can potentially facilitate a better understanding of the characteristics of baijiu and shochu.

2. Previous Research on Aroma Characteristics of Distilled Liquor in Northeast Countries

Research on the flavor characteristics, including sensory science and instrumental analysis, of soju, baijiu, and shochu has been widely conducted in several countries. In Korea, most studies included instrumental analysis of soju reflecting aroma compound profiles using gas chromatography/mass spectrometry;

Table 1. VOCs previously identified in distilled soju.

Compound	Rice		Potato		Sweet Potato	Odor Threshold (mg/kg) [6]
	Atmospheric	Vacuum	Atmospheric	Vacuum	Atmospheric	Water	Ethanol/Water Solution
Alcohols
1-Propanol	O	O	O	O		8.5056	830
Ethanol	O	O	O	O		950	120
1-Butanol	O	O				0.4592	820
Isobutyl alcohol	O	O	O	O		6.5052	40
Isoamyl alcohol	O	O	O	O		0.004	56.1
Isohexanol	O					0.82–4.1	50
1-Hexanol		O				0.0056	8
n-Octanol				O		0.1258	10
Nonanol				O		0.0455
Nonen-1-ol				O		0.13
2-Methyl-1-butanol		O				0.0159	32
3,7-Dimethyl-6-octen-1-ol		O				0.062–2.2	0.1
3-Methylthio-1-propanol	O	O				0.12323	0.5
Benzyl alcohol	O	O				2.54621	159
Phenylethyl alcohol	O	O				0.56423	10
Undecanol					O	0.086–0.41
1-Dodecanol					O	0.016	1
2-Ethyl-1-hexanol		O				25.4822
Hexadecanol					O		>1.1
Acids
Acetic acid	O	O	O	O		99	26
Butanoic acid		O				2.4	10
4-Hydroxybutanoic acid	O
Propanoic acid		O				2.19	830
Aldehydes
Furfural	O	O				9.562	15
5-Methyl furfural		O				1.11	16
Benzaldehyde		O				0.75089	5
Benzeneacetaldehyde	O				O	0.0063	0.001
Hexdecanal
1-Tetradecanal					O
n-Valeraldehyde			O			0.012	0.11
Acetaldehyde		O		O		0.0251	10
Hexanal				O		0.005	0.07–0.1
2,4-Nonadienal		O				0.0001	0.0026
Esters
Ethyl formate		O				8.9–89
Ethyl acetate	O	O	O	O		0.005	7.5
n-Propyl acetate	O	O				2	65
Isobutyl acetate	O	O				0.025	3.4
Phenylethyl acetate	O	O				0.25	0.25
Ethyl butanoate		O				0.0009	0.02
Diethyl butanedioate		O					100
Ethyl butyrate	O	O				0.009	0.02
Isoamyl acetate	O	O		O		0.00015	0.03
Isoamyl propanoate		O				0.0086–0.043
Ethyl valerate		O				0.0058
Ethyl lactate		O				50–250	100
Dodecyl acetate				O
Ethyl caproate	O	O				0.0022	0.005
Methyl caprate				O		0.07
Ethyl 2-hydroxyisocaproate		O
Ethyl heptanoate	O	O			O	0.0019	0.3
Isobutyl hexanoate	O	O		O
Ethyl caprylate	O	O	O	O		0.0193	0.002
Isoamyl hexanoate	O	O				0.32	1.4
Propyl octanoate	O	O
Butyl octanoate		O			O
Ethyl 9-decenoate		O		O
Ethyl nonanoate	O	O		O		0.377
Isobutyl caprylate	O	O			O
Ethyl caprate	O	O	O	O		0.005	0.51
Isoamyl caprylate	O	O		O		0.07	0.6
Propyl decanoate	O	O		O
Isobutyl decanoate	O	O			O
Ethyl benzeneacetate		O
Methyl salicylate	O	O				0.04	0.071
Ethyl laurate	O	O	O	O	O	5.9	0.64
Isoamyl decanoate	O	O		O	O		>5.0
Ethyl myristate	O	O		O		4	494
Isoamyl laurate	O				O		100
Ethyl palmitate	O	O	O	O		2	>14
Ethyl palmitoleate	O			O	O		10
3-Methylbutyl hexanoate		O
3-Methylbutyl octanoate		O
Methyl 2-hydroxybenzoate		O			O
Ethyl benzeneacetate		O
Ethyl 3-methylbutyl butanedioate		O
Methyl hexadecanoate		O			O	>2
Ethyl-(E)-11-hexadecenoate					O
Ethyl octadecanoate	O				O		>0.5
Ethyl oleate	O				O		0.87
Ethyl linoleate					O		0.45
Propyl lactate	O			O	O
Butyl lactate				O			10
Diethyl phthalate			O
Bis(2-ethylhezyl)hexanedioate					O
Ethyl-4-decanoate					O
Ethyl pentadecanoate	O				O
Diethyl butanedoate					O
2-Hydroxy-methyl benzoate					O
Ethyl-3-heptenoate					O
3-Phenylethyl-2-propenoate					O
Ethyl-3-methylbutyl pentadecanoate					O
Ethyl undecanoate		O
2-Methyl decanoate					O	0.0043–0.0088
Ethyl 2-furoate		O					1
Ketones
2-Nonanone		O				0.041–0.082
Acetophenone	O					0.065
2-Dodecanone					O	0.042–0.083
Butyrolactone		O				>1	100
Trans-Whiskey lactone		O
Oaklactone		O
Acetals
1-(1-Ethoxyethoxy)pentane	O	O
Others
1-Ethyl-2,3-dimethylbenzene					O
1-Ethyl-3,5-dimethylbenzene					O
1,2,4,5-Tetramethylbenzene		O
2,6-Dimethyl-2,6-octadiene					O
1-Cyclohexylheptene					O
2-Pentadecyle-1,3-dicxolane					O
Phenol		O				58.58525	7.1
Eugenol		O			O	0.00071	0.005
2-Methoxy phenol		O				0.00084	0.03
Benzofuran		O
2-Acetylfuran		O				15.0252
2-Methylbenzofuran		O
2,4-Dimethylheptane		O
4-Methyloctane		O

References [7,8,9,10,11,12]. VOC, volatile organic compound.

lists the previously identified volatile organic compounds (VOCs) of soju. Most previous studies focused on rice-based soju, which is potentially attributable to the prevalence of soju derived from rice. The VOC profile of soju mainly indicates the presence of alcohols, acids, ketones, and a large proportion of esters. Additionally, VOC profiles differ with various raw materials and/or distillation methods. Furthermore, several studies have focused on the development of sensory lexicons in soju using descriptive sensory analyses to understand the flavor characteristics of soju (

Table 2. Sensory lexicon identified from distilled soju in previous studies.

Sensory Attributes		References	Sensory Attributes		References
Aroma	Brandy	[13,14]	Flavor/taste	Alcohol	[13,14,15,16]
	Sweet	[13,14,15,16]		Bitter	[13,14,15,16]
	Acetone	[13,14,16]		Sweet	[13,14,15,16]
	Alcohol	[13,14,15,16]		Fruit	[13,14,15]
	Fruity	[13,14,15,16]		Sour	[13,14,15,16]
	Gusu	[13]		Salty	[15]
	Sour	[14,15,16]		Mint	[15]
	Wine	[16]		Barley	[15]
	Yeast	[14,15,16]		Earthy	[15]
	Oak	[16]		Metal	[15]
	Earthy/woody	[15]		Yeast	[14]
	Nuruk	[14]	Texture	Body	[13,14,15,16]
	Sauce-like	[15]		Swallow	[13,14,15,16]
	Green grape	[15]		Persistence	[13,14]
	Pineapple	[15]		Pungent	[14,16]
	Barley	[14,15]		Astringent	[14,15,16]
	Metal	[15]		Cooling sensation	[14,15,16]
	Bleach	[14,15]		Tingling	[16]
				Spicy	[15]

). The commonly identified aroma attributes include sweet, alcohol, fruit, sour, and yeast, whereas flavors and tastes frequently include alcohol, fruit, bitter, sweet, and sour.

Previous findings based on other Northeast Asian countries are summarized in

Table 3. Previously reported aroma characteristics of baijiu and shochu.

Liquor	Raw Materials	Type	Objectives	Major Results	References
Shochu	Rice	Shochu	VOC enhancement	Ethyl caproate, ethyl lactate	[17]
			VOC synthesis	Ethyl caproate	[18]
		Barley shochu, awamori	Flavoromics focusing on roasted aroma	2-Furanmethanethiol	[19]
	Rice, sweet potato	Imo-shochu	Fermentation temperature control for sensory quality	Sweet/acidic taste	[20]
	Sweet potato	Sweet potato shochu	Flavor characteristic determination	Linalool, α-terpineol	[21]
	Sugarcane	Sugarcane shochu	Aroma profile differences by cultivar	MRPs (pyrazines and furans), β-damascenone, guaiacol	[22]
			Liming effect determination	pH-relevant volatiles	[23]
	Barley	Barley shochu	Aroma characteristic difference in koji	Ethyl lactate, 3-methyl-1-pentanol, ethyl benzoate, diethyl succinate, citronellol, 2-phenyl acetate	[24]
		Shochu, awamori	Major VOC threshold measurement		[25]
	Rice	Shochu	Effect of koji VOC on final product	Amino acids, volatile compounds, sensory lexicon	[26]
Baijiu	Rice	Rice-flavored baijiu	VOC profile	Key compound—ethyl lactate	[27]
			Major VOC threshold measurement		[28]
			Effect of rice cultivar on VOCs	Isopentyl acetate, ethyl hexanoate, benzeneacetaldehyde, phenethyl acetate, undecane-2-one, ethyl decanoate	[29]
		Sauce-flavored baijiu	Regional flavor characteristic determination		[30]
			Key VOCs and aroma lexicon development		[31]
			Regional-specific aroma characteristic determination		[32]
		Strong aroma baijiu	Regional chemosensory characteristic determination		[33]
			Correlation between sweetness and VOCs	Ethyl hexanoate, hexyl hexanoate, ethyl 3-methylbutanoate	[34]
	Qu, environmental elements	Strong aroma baijiu	Regional flavor profile difference-inducing factor	Fungal diversity differences by climate	[35]

MRP, Maillard reaction product; VOC, volatile organic compound.

. In China, research on the flavor characteristics of baijiu primarily focused on the manifestation of regional flavor characteristics; however, in Japan, most studies determined the aroma characteristics with various types of raw materials, as baijiu is categorized based on its flavor characteristics, which regionally vary owing to the variable fermentation procedures and/or starter cultures, while shochu is classified based on the type of raw material used. Moreover, research on the flavor characteristics of baijiu and shochu revealed the influence of fermentation conditions and yeast strains on the increase in specific VOCs that affect the overall quality of the spirit.

Some research groups have comparatively analyzed flavor characteristics with various raw materials used to produce soju; however, most have focused on determining the differences in VOC profiles induced by the different fermentation starters (

Table 4. Previous studies conducted on aroma characteristics of distilled soju.

Raw Materials	Objectives	Differences		References
Rice	VOC profile	Fermentation starter	Traditional/modified nuruk	[36]
		Distillation condition	Atmospheric/vacuum distillation	[36]
	Quality optimization	Commercial yeast	Domestic/commercial yeasts	[37]
	VOC profile	Fermentation starter	Traditional nuruk, koji	[38]
		Distillation condition	Atmospheric/vacuum distillation	[38]
	Liquor production properties	Yeast strain	Saccharomyces cerevisiae N4, N9 isolated from traditional nuruk	[39]
		Yeast strain	Saccharomyces cerevisiae 88-4 isolated from traditional nuruk	[40]
		Fermentation starter	Koji (Aspergillus kawachii, Aspergillus oryzae), traditional/modified nuruk	[41]
	VOC profile	Fermentation starter	Koji (Aspergillus luchuensis), traditional nuruk, crude amyloytic enzyme	[42]
	VOCs, sensory characteristics	Yeast strain	Saccharomyces cerevisiae 88-4 and GNIA2 (selective mutant) isolated from traditional nuruk	[7]
	Liquor production properties	Distillation condition	Atmospheric/vacuum distillation	[7]
			Atmospheric/vacuum single distillation, atmospheric continuous distillation	[43]
		Organic acid addition	Acetic, citric, lactic acid (0.3% w/v)	[44]
Sweet potato	Liquor production properties	Cultivar	Jinhongmi, Hobak	[10]
		Fermentation starter	Koji (Aspergillus awamori Nakazawa, Aspergillus kawachii, Aspergillus oryzae), modified nuruk	[10]
	VOCs, sensory characteristics	Cultivar	Yeonmi, Jeungmi, Shincheonmi, Shinwyeulmi	[10]

VOC, volatile organic compound.

). The knowledge of correlations between the VOC profiles and sensory flavor characteristics of distilled soju, as well as of the microfermentation and flavoromics-based mechanisms underlying VOC generation that influence the quality of soju, remains limited.

3. Aroma-Characteristic-Related Factors Involved in the Liquor Production Procedure

3.1. Raw Materials and Processing Methods

In Northeast Asia, the production of distilled liquor generally involves the fermentation of starchy grains by inoculating them using natural fermentation agents or a specific yeast strain. Saccharification of raw materials converts raw starch into an alpha-starch structure, which facilitates enzymatic breakdown, including saccharification and alcohol fermentation upon inoculation with the yeast strain [15,45]. Although similar methods are followed to produce grain-based distilled liquor in Northeast Asian countries, some significant differences induce regional differences in aroma characteristics.

3.1.1. Distilled Soju

In Korea, traditional soju is primarily produced from rice; however, the use of barley, wheat, and sweet potatoes has been reported. Wheat is rarely used to produce soju owing to its higher status as an edible grain compared to rice in the Korean Peninsula’s cultivation system. According to Korean liquor tax laws, soju is distilled using fermented starch-based materials (except in the continuous distillation process), e.g., grains; a fermentation starter (Guk); and water. Although the fermentation procedure of soju varies slightly in terms of fermentation period, temperature, and ingredient ratios, both the first and secondary mashes are typically produced using atmospheric pressure or vacuum distillation processes (Figure 1).

The characteristic aroma profiles of distilled soju produced using different raw materials were analyzed using instrumental and sensory analyses (Table 4). Woo et al. (2023) and Chin et al. (2024) determined that major VOCs of rice-derived distilled soju mainly comprise alcohol, esters, and aldehydes, which include various ethyl esters, acetaldehyde, methanol, furfural, acetic acid, N-propanol, isobutanol, n-butanol, isoamyl alcohol, 2-phenyl ethyl acetate, and 2-phenyl ethanol [7,46]. Woo et al. (2023) and Lee et al. (2012) reported several aromas of rice-derived distilled soju, which include alcohol, acetone, sour, sweet, wine, fruit, yeast, and oak; moreover, various flavors/(after)tastes include sweetness, sourness, bitterness, alcohol, tingling, astringency, cooling sensation, swallowing, and body [16,46].

A few studies on soju reported main ingredients other than rice; however, these reports demonstrated some differences in aroma characteristics. Hong et al. (2020) reported higher concentrations of some esters, including isoamyl acetate, ethyl octanoate, and phenylethyl acetate, as well as higher barley aroma and yeast flavor, detected through descriptive sensory analysis, in barley-derived soju than in rice-derived distilled soju [14]. Jeong and Seo (2012) determined the VOC profile of distilled soju produced via different distillation methods using potatoes and revealed the prevalence of esters and alcohols [9]. Furthermore, several studies have elucidated the differences in flavor characteristics based on the degree of polishing and/or cultivar of the raw materials. Kim et al. (2015) and Park et al. (2010) reported that the expression of VOCs in sweet potato-derived soju varies depending on the cultivar; decanoic acid and ethyl dodecanoate were more prominently expressed in the Hobak cultivar than in the Jinhongmi cultivar, while similar levels of other VOCs were detected. They also noted significant differences in the expression of monoterpene alcohols between sweet potato cultivars [10,47]. Furthermore, the brewing characteristics of rice cultivars and the correlation between the fermentation characteristics of fermented beverages and the degree of rice polishing (Makgeolli) have been explored [48,49,50]; however, to the best of our knowledge, no studies have focused on flavor characteristics.

3.1.2. Baijiu

The characteristics of baijiu vary greatly depending on the raw materials and fermentation methods used [51]; various grains (sorghum, wheat, corn, rice, and glutinous rice) are used as the raw materials. Figure 2 presents the general procedure for producing baijiu. The baijiu production method distinctively involves solid-state fermentation [51,52]. Solid fermentation differs from the general fermentation methods used in other Northeast Asian countries [52]. In the case of strong-aroma type baijiu, the most common type of baijiu, fermentation typically lasts for 30–90 days, during which saccharification and fermentation occur simultaneously [53]. It involves the use of qu (fermentation starters), and most raw materials remain solid throughout the fermentation and distillation stages [54]. In this strategy, lower temperatures are maintained during the steaming of raw ingredients and saccharification compared to other fermentation methods, which encourages the involvement of various microbes in fermentation. Liquid and liquid–solid combination fermentation methods have also been adopted for producing baijiu which are distinguished by the use of the main ingredient of mash and/or qu [54]. Moreover, these various types of fermentation methods influence flavor; baijiu is classified based on flavor characteristics, such as strong aroma, light aroma, sauce aroma, rice aroma, and mixed aroma induced by different production methods, the selection of fermentation starter, and/or fermentation regions [55,56].

The primary factor that influences the differences in the flavor characteristics of baijiu from other grain-based distilled liquors is considered to be its solid-state fermentation. Most research related to the production method of baijiu focus on the expression of aromatic compounds depending on the fermentation state [55,57]. Zhao et al. (2021) reported that the solid-state saccharification stage in the production of rice-flavor baijiu not only produces early alcohol that inhibits the contamination of mash but also reduces the number of short-chain fatty acids compared to the liquid-state saccharification method. This could be related to the generation of ester compounds that can contribute to the cheesy aromas of distilled liquor, together with the higher alcohol production in solid-state fermentation due to the higher protease activity [57]. However, previous studies examining the volatile compound profiles of soju and shochu also reported large quantities of various alcohols in their results as well as ester compounds (Table 1) [22,26,58]. Given that a variety of grains besides rice are used for the production of baijiu and shochu, no significant relationship between the fermentation state and formation of volatile compound profiles is observed; other factors contributing to the production of liquor may also influence the observed flavor characteristics.

3.1.3. Shochu

Figure 3 illustrates the general production process of shochu in Japan. Shochu is made from a wide variety of raw materials, including sweet potato, barley, rice, buckwheat, brown sugar, corn, and sake lees, while awamori refers to a specific type of shochu made in Okinawa Prefecture using restricted types of rice and koji fungus [59,60]. Similar to soju, based on distillation conditions, shochu is divided into diluted and distilled shochu by Japanese tax regulations [60].

The raw-material-associated variation in the aroma characteristics of shochu has been extensively investigated. Rice shochu is the most common type of this liquor; studies of rice shochu are mainly focused on the development of particular VOCs related to flavor quality [17,18,19]. Ethyl lactate and ethyl caproate, which contribute to the aroma characteristics of shochu, are considered major VOCs. Therefore, extensive research has been conducted on the production of these compounds in rice shochu under various fermentation conditions, using selective strains and production methods to enhance their formation [17,18]. Osafune et al. (2022) reported the differences in 2-furanmethanethiol development between barley-derived shochu and awamori; the production of 2-furanmethanethiol in barley shochu by atmospheric distillation is a potentially critical factor contributing to the generation of the characteristic roasted aroma of barley shochu [19]. Many raw materials are adopted for making shochu; various types of shochu contain different major VOCs; for instance, linalool and α-terpineol in sweet potato shochu [21], pyrazines and furans, β-damascenone, and guaiacol for sugarcane shochu [22] represent the VOCs of corresponding types of shochu. Moreover, Okutsu et al. (2016) demonstrated the association of the cultivation period of sweet potato with different concentrations of monoterpene alcohols, β-damascenone, rose oxide, and fatty acid esters in shochu, which is potentially attributable to the changes in physiochemical properties during cultivation [61]. Okutsu et al. (2023) reported that different amounts of amino acids in different cultivars affect levels of VOCs derived through the Maillard reaction (pyrazines and furans) in the final product [22]. As such, research trends of the flavor characteristic differences in shochu based on its raw ingredients are focused on the expression of specific aroma-active compounds. While dynamic variation in the aroma characteristics of soju with its raw materials is scarce because soju is predominantly produced with rice, broader diversity of aroma characteristics was observed in shochu due to its use of various ingredients such as sweet potatoes and barley.

Additionally, while studies in Korea (soju) mostly conducted the identification of volatile compound profiles and/or the increase/decrease in certain volatile compounds, research on shochu takes a more targeted approach to the formation mechanism of ingredient-specific, odor-active compounds which could impact flavor expression (Table 3). Because the overall production processes of soju and rice shochu share similar procedures for manufacturing, it is considered that the differences in aroma characteristics between soju and shochu are more likely influenced by the selection of ingredients and/or fermentation starters rather than production methods.

3.2. Fermentation Starters

The main difference between Western and Eastern liquors is the use of granule-based starters during fermentation, particularly in Korea, China, and Japan. In Western countries, fermentation generally involves a saccharification process to obtain soluble sugars before alcohol fermentation using endogenous enzymes expressed through grain germination, especially with malt. However, in Eastern countries, particularly Northeast Asian countries, fermentation starters comprising naturally or artificially inoculated fungi in granular forms are cultured to produce enzymes via the generation of microorganisms. The composition of microorganisms in the fermentation starters varies according to the procedure and/or raw materials used. Microorganism enzymes are involved in starch degradation while simultaneously facilitating alcohol fermentation. Consequently, these fermentation starters can significantly regulate the aroma characteristics of each distilled liquor.

3.2.1. Nuruk

Nuruk is prepared using various grains, such as wheat, barley, rice, and sweet potatoes, either alone or in combination with raw materials. Depending on its form, nuruk is mainly divided into wadded (Byeonggok) and scattered, spread-out (Sangok) forms and further varies based on its shape and size. The production of traditional nuruk involves natural fermentation, hosting a diverse microbial community, including various species of Saccharomyces, Pinchia, Aspergillus, Rhizopus, Mucor, Lactobacillus, and Leuconostoc. Yu et al. (1996, 1998) reported that the microbial composition of traditional nuruk can vary; 97 species of molds, 47 species of yeast, and 19 bacterial species have been reported to be involved [62,63], with Bacillus subtilis and Lactobacillus casei being the most common bacteria. Moreover, among lactic acid bacteria, Leuconostoc mesenteroides is commonly detected. Traditional nuruk can induce diversity in the sensory and physiochemical characteristics of the final products based on its production region/fermentation conditions (temperature and humidity), yet the reproducibility of the products is relatively low. In addition, the detection of high amounts of bacteria suggests an uncontrolled manufacturing system; therefore, the use of modified nuruk has been recently introduced to produce alcoholic beverages [64]. Modified nuruk generally involves only a single microbial inoculation; primarily, Aspergillus oryzae and Aspergillus luchuensis are used to adjust the brewing characteristics of the final products.

Because soju is generally made with rice, studies on the development of diverse aroma characteristics of soju tended to focus on the impact of the fermentation starter. As the production of soju has become industrialized, there is still insufficient quality characteristic information about the use of traditional nuruk in the final product, including aroma characteristics, due to heavy usage of koji in alcoholic fermentation and the natural fermentation method of traditional nuruk [65]. Therefore, modified nuruk has been developed to address these limitations. This shift is reflected in the research trends; most studies have focused on differences in the characteristics of distilled soju prepared using traditional and modified nuruk. Synthesis of VOCs, including alcohol, carbonyls, acids, esters, sulfur-containing compounds, and N-containing compounds, was significantly related to microorganisms in the fermentation starter profile, especially esters and fusel oils in the final soju products [38]. Lee et al. (2014) reported significant differences in the levels of several acids and alcohols; octanoic acid potentially related to the off-flavor of soju is more abundant in modified nuruk-based soju, whereas traditional nuruk soju showed higher levels of acetic acid (sour aromatics) and phenylethyl alcohol than the modified nuruk-based soju [38]. Moon and Cheong (2018) reported that traditional nuruk produced lower amounts of fusel oils and esters compared to modified nuruk inoculated with Aspergillus oryzae [41]. Several studies have isolated selected yeast strains to enhance the aromatic characteristics of traditional nuruk. Saccharomyces cerevisiae N4 and N9, which have high acid resistance, were isolated from traditional nuruk and induced higher alcohol production than commercial yeast, with differences in the development of volatile compounds, including i-BuOH, isobutanal diethyl acetal, ethyl caprate, and tetradecanoic acid [39].

3.2.2. Qu

Qu is a fermentation starter used in China that is widely used for the production of alcohol and various other fermented products. Jiuqu is a type of qu used in alcohol production. Jiuqu is primarily derived from barley, wheat, peas, cereal flour, and/or bran with variations and categorized as Daqu, Xiaoqu, Fuqu, and mixed qu based on usage and production methods/raw materials [45]. Similar to nuruk, qu undergoes natural fermentation involving microbes, including Sporolactobacillus, Clostridium, Mycobacterium, Flavobacterium, Candida, Pichia, Issatchenkia, Aspergillus, and Penicillium [66]. Daqu also serves as a raw material for liquor production, significantly affecting the aroma; in baijiu production, large proportions of Daqu are used, whereas Xiaoqu, made from rice, contains fewer types of microorganisms than Daqu, resulting in relatively lighter aromas. These two types of qu are sometimes mixed. Fuqu is produced by artificially inoculating Aspergillus on fermented bran to create various aroma profiles; however, the aroma characteristics are still unbalanced, necessitating further investigation [54].

As the selection of qu is highly related to its flavor, the aroma characteristics of qu are selected considering their influence on final products. Zhang et al. (2012) identified that strong-flavor Daqu contained N-containing compounds, alcohols, and phenolic volatile compounds, including a significant level of hexanal (green), phenylacetaldehyde (floral, rose), and 4-ethyl guaiacol (clove, unbalanced) [67].

Moreover, regional characteristics with flavoromics relationships were determined by identifying major VOCs in several types of baijiu [32,68]. While recent research of soju and shochu tended to focus on modification of the microbial community in fermentation starters and their contribution to changes in flavor characteristics, studies of baijiu were characterized by highlighting the influence of regional differences, noting that the the aroma profiles of specific types of baijiu and/or qu were influenced by regional characteristics. Recent studies were conducted on the variation in microbial composition in qu with regional differences and dominant microbial communities, including environmental factors related to the development of volatile compounds. A study on sauce-flavor baijiu identified 20 region-specific volatile compounds, although the descriptive sensory analysis results showed no significant differences in flavor across regions [32]. Together, regional variations in flavor profiles of strong-flavor baijiu were identified as being influenced by different patterns of microbial communities, especially fungal diversity, which is attributed to climatic differences across China [35].

This regional diversity of baijiu is considered to be more pronounced due to the geological characteristics of China compared to Korea or Japan. In Korea, while traditional soju varieties such as Andong soju, Igangju, and Munbaeju are considered regionally distinct types of distilled liquors, only their historical and cultural significance has been highlighted, with no critical emphasis on the regional factors affecting the generation of aroma characteristics. Similarly, in Japan, while Ryukyu’s awamori—made from local rice and black koji (Asp. luchuensis)—is recognized as a regional product, there has been no notable focus on the impact of regional characteristics on the aroma profile diversity.

3.2.3. Koji

Koji is generally prepared from rice, barley, or sweet potatoes using a single inoculation step. The fermentation processes in Korea and China commonly involve the use of various molds; contrastingly, the types of koji specifically depend on the single microorganism used as the inoculum: yellow koji (Aspergillus oryzae) is widely used in various fermented foods and alcoholic beverages. White koji (Aspergillus luchuensis mut. kawachii) and black koji (Aspergillus luchuensis) are solely used for shochu production. Aspergillus luchuensis produces citric acid at the early stages of fermentation, which prevents the growth of other bacteria and enhances the safety of the product. Although Aspergillus niger produces citric acid, its aroma characteristics are undesirable, leading to its exclusion from use [60].

Research of the aroma characteristics of koji mainly focused on the effect of koji on the final product of enzymatic synthesis by the microbial community in VOC production. Yuan et al. (2023) determined the effects of koji on the aroma characteristics of rice shochu and the differences in yeast strains related to the production of VOCs, and the use of Aspergillus oryzae for koji was reported to elevate alcohols and sweet-related free fatty acids with a less sour flavor in rice koji compared to other strains [26]. Moreover, dimethyl trisulfide and hexanal in koji can induce the synthesis of esters related to fruity aroma, and the formation of isovaleraldehyde, ethyl caprylate, ethyl caproate, and ethyl 2-methylbutyrate can influence specific aroma characteristics of rice shochu [58]. Research on the enhancement of specific VOCs by adjusting the microbial community profile in shochu mash fermentation was widely reported. Similar to other liquor-making procedures, the production of volatile compounds in shochu is related to the selected yeast strain [69]. Additionally, Yuan et al. (2015) and Tan et al. (2016) demonstrated the enhancement of ethyl caproate and ethyl lactate, which are related to the enhancement of the sensory quality of rice shochu, using a caproic acid-producing bacterial consortium and lactic acid bacteria [17,18].

Determination of the relationship between the microbial community of koji and development of VOC in shochu is highly influenced by the production method of koji—a single inoculation of a microbial strain. Both nuruk and qu contain complex microbial communities due to the naturally derived fermentation procedure, and the specific microorganisms have been identified to have a significant effect on the generation of volatile compounds and/or aroma characteristics.

The use of koji in soju production is also abundantly observed in the research field. Unlike studies in Japan, which were more focused on the flavor control with microfermentation for the expression of specific volatile compounds, studies of soju production often adopted koji as a control method. Studies have been conducted to determine the changes in volatile compound profiles in soju depending on vacuum/atmospheric distillation and the use of modified nuruk [36] and to identify quality changes in soju made from traditional/modified nuruk over various aging periods, employing koji in the production of soju as a control method [70].

3.3. Distillation Conditions

Distillation is a significant regulatory factor that influences the aroma characteristics of distilled liquor owing to the various odor-active VOCs produced by thermal reactions at different temperatures [1,71]. Aroma characteristic differences based on distillation condition were identified to have a major impact in soju. Atmospheric pressure distillation involves hot steam at 80–90 °C and the generation of VOCs in soju with atmospheric distillation, which become more varied with the increase in distillation time. Furthermore, several aldehydes, such as ethyl acetate, isoamyl acetate, ethyl alcohol, fusel oils, and higher fatty acid esters (ethyl palmitate and ethyl linoleate) are produced in large quantities in the primary stage; however, their levels decrease during distillation [1]. Several characteristics, including acid value, acetic acid, and β-phenyl alcohol, tend to become more pronounced as distillation progresses, and higher mineral contents and furfural level are expressed. The sensory characteristics of soju with atmospheric distillation included a mild taste with stronger characteristics due to these thermally induced secondary organic compounds [43,71].

Vacuum distillation allows indirect heating while maintaining low-pressure conditions to reduce the boiling point (Figure 4) [1,71], and the VOCs originating from heat are not expressed; therefore, the different VOC profiles can induce a characteristic aroma profile different from that of soju produced by atmospheric distillation. In particular, VOCs with low boiling points are released during the early to middle stages of distillation. A slight difference was identified between vacuum and atmospheric distillation because vacuum distilled compounds are mostly derived from fermentation rather than heat-induced changes [43,71]. However, several studies have demonstrated that VOCs with medium to high boiling points, which were released during the late stages of distillation, were more abundant in soju subjected to atmospheric distillation than in soju subjected to vacuum distillation, and isoamyl alcohol (whisky) and decanoic acid (sweet, nutty) were more abundant in soju subjected to atmospheric distillation than in soju subjected to vacuum distillation; this difference was attributed to the high boiling points of the compounds [38]. Consistent with previous reports, the 2-thiobarbituric acid value, a marker of oxidative deterioration, was observed, particularly in the later stages of atmospheric distillation process [72]. Moreover, furfural sharply increased in the late stage of distillation in atmospheric distillation, which is not detected in vacuum distillation. This is attributed to the thermally induced carbohydrate degradation during atmospheric distillation [12,43]. However, acid-related organic compounds show minimal differences between the two methods and are present in exceptionally lower quantities in vacuum-distilled soju because acetaldehyde is released in the early stages of distillation as a gas through the vacuum pump [1].

4. Conclusions

This review elucidates the recent research progress on the flavor characteristics of baijiu, shochu, and distilled spirits with cultural and technical similarities to soju, which potentially enhances the understanding of the aroma characteristics of soju.

Research trends on the factors influencing the development of flavor characteristics during the production of shochu has focused more on the identification of distinctive flavor characteristics differing according to the selection of main ingredients and on increasing the production of flavor compounds that positively affect the final product’s sensory qualities by the formation of a selective microbial community during koji production. In the case of baijiu, studies have mainly focused on identifying the flavor intensity and its characteristics based on the different types of the mash during fermentation, while studies on qu have focused on identifying the diversity of microbial communities across different regions. Although various studies of soju have been conducted on the differences in flavor characteristics according to raw materials and on identifying the microbial communities of nuruk as a fermentation starter, limited studies have been conducted on connecting these findings to the final product’s flavor characteristics. Unlike the research strategies of shochu and baijiu, the final products of which have diverse quality based on flavor profiles, studies of soju have not yet explored how current findings can be applied to the development of the final products. Moreover, research in the area of microfermentation aimed at improving the quality characteristics of soju is limited, together with the understanding of the development of aroma characteristics during fermentation. This review provides insights for future research on the flavor characteristics of soju.