Soju Distillation Methods

Korean soju is a clear distilled spirit often compared to vodka or sake. It is one of the most popular alcoholic beverages in Korea and is enjoyed in bars, restaurants, and at home.

The soju distillation, or, if I can say, choosing the distillation method used, determines the type of soju being produced, which can be vastly different in taste, alcohol strength, and quality. To someone new to the world of soju, it can be unclear how it is possible under one product name to have two completely different beverages. To try to answer that question, I think we should look in a bit more detail at the type of distillation methods being used and how they influenced the overall flavour of the soju.

Traditionally, soju was made from rice, but today, it is typically made from sweet potatoes, wheat, or barley. Making soju involves fermenting the base ingredient and then distilling the resulting liquid to increase its alcohol content.

Distillation

Distillation is an ancient technology for separating liquid mixtures into their components by applying heat and taking advantage of the differences in their boiling points. It is performed to concentrate ethanol and desirable aroma compounds, as the goal is to obtain the best balance between the congeners present.

In simple terms, distillation is the process in which a liquid is vaporized (turned to steam), recondensed (turned back into a liquid), and collected in a container.

Different types of distillation exist, such as simple, fragmental, and steam distillation. The one we are concerned with is called fractal distillation, and it is used in the production of alcohol.

Fractal distillation is a process of extracting essence through boiling and condensing by utilizing different boiling points of a previously fermented liquid consisting of ethanol (the ethyl alcohol found in alcoholic beverages) and water.

 Fractional distillation can also be vacuum distillation, which lowers the boiling points of the compounds.

In simple distillation, water has a lower boiling point than ethanol, in which the ethanol remains liquid while all the water boils out of the solution.

The water boils at 100ºC, and the ethanol boils at 78.4ºC, but the actual separation of ethanol from the water is between the two boiling points.

We have to keep in mind, though, that not all of the extracted alcohols are suitable for drinking, as some of them can be quite harmful to one’s health or introduce undesirable aromas.

There are three main parts in the final phase of the distillation process:

Heads, Hearts and Tails. All three are based on different boiling points of the various alcohols in the fermented product.

The distillers are after the liquid in the Heart section of the distillation. Depending on the final product (vodka, whisky, brandy, etc.), they discard the alcohols (methyl alcohol, acetone, etc.) in the head section (lower boiling point) and keep some from the tail section.

The Tails usually made 35% of the last run of the alcohol distillation. They can be recognized by sight and smell and have a burnt flavour. The tails contain a high percentage of fusel oils produced by alcoholic fermentation.

The separation of the different alcohols is not as straightforward as it seems. The distiller decides on the type of distillation, where to cut, and what kind of still to use: continuous columns, Single pot, or Hybrid stills.

The quality of the distilled soju is greatly influenced by the raw materials used, the method of converting starch into sugar, and the quality and type of the distillation method after fermentation.

Traditionally, distilled soju is made using a pot still at atmospheric pressure, but currently, modern vacuum distillation is widely applied and gaining more popularity in the industry.

The traditional method of making soju with atmospheric distillation uses a device called Sojutgori; on Juju island, it is called Gosori.

In atmospheric pressure distillation, not only the flavour components other than alcohol but also the flavour components generated by fermentation from the yeast (a kind of protein) or fermented by fermentation due to the high temperature in the distillation process are changed by high heat.

It has the advantage of producing rich and diverse tastes and aromas depending on the manufacturing method, materials, and shape of the still. Still, at the same time, it is difficult to maintain quality compared to vacuum pressure distillation.

As a result, it has many unpleasant odour components and the concentrated flavour of the original base fermentation product.

The soju produced this way requires a long aging process to remove the unpleasant odour. ¹

The vacuum distillation method is a method of distilling performed at a low-temperature distillation (60 ° C or less) by intentionally lowering the evaporation point and applying a low pressure without direct heat using a stainless-steel concentrator. It has the advantage of maintaining highly volatile aroma components with less heated and burnt flavours of the distilled liquor.*

Flavours other than the flavour components of the fermented product are not captured/released, thus making it possible to produce a distilled soju that is relatively comparable to the fermentation flavour base, creating a distilled product with a purified taste compared to the atmospheric distillation soju.

Steps of Soju Distillation

Soju is a distilled spirit traditionally made from fermented grains. It is prepared by fermenting raw starch with a fermentation agent (nuruk) and then distilling the fermented base.

It is distinguished by a distillation type: a diluted soju is produced using a multi-column continuous still, and a distilled soju is still distilled in a single pot.²

Fermentation

The first step in soju making is preparing the fermentation starter – nuruk.

• Nuruk is a traditional fermentation starter meant to saccharify the rice. ( a wheat base starter of microorganisms growing on malted wheat cake (nuruk) –  It helps break the grains’ starches into fermentable sugars. It is made from ground wheat, rice, and barley and placed into a cloth-lined container. It is then compacted into a mould to create a wheat cake. The wheat cake is then removed from the mould and set aside to ferment so that yeast and other microorganisms can grow. This process, which takes around three weeks, solidifies the cake by removing the moisture. After this period, the disc of Nuruk is mashed in a mortar until powdery.

It also contains many microorganisms that start different types of fermentation, including alcohol and beverages like kombucha or boza.

The primary raw materials used to produce nuruk vary widely according to the production area and the characteristics of Korean traditional nuruk, not so much on the commercialized one, and it depends on the geographical and climatic diversities of their production area.

The type of Nuruk being used has a significant role in the flavour profile of the final drink.

The next step is to steam white rice in large batches and allow it to cool to the proper temperature. This step is essential because the soju may taste bitter or sour if the rice is still too warm when proceeding to the next step. When the rice is thoroughly cooled, it is mixed with crushed wheat and a little water to make a mash, which is a bit dry. Then, the mixture is transferred to a crock or brewing vessel, stirred, and allowed to ferment for more or less 12 days.

The mash is called “wonju”. As the wonju is left to ferment, it separates and settles into two layers. 

The top layer, called cheongju or yakju, is golden and clear. The bottom layer is a cloudier mixture called takju, where most rice sediments settle. After a while, the top layer is separated, and it becomes the base for the distillation into soju, while the bottom is diluted with water and bottled as makgeolli. The bottom layer that becomes makgeolli is left unfiltered, and the remaining rice sediments contribute to makgeolli’s cloudy appearance and milky texture.

The mixture looks like rice porridge resulting from the fermentation of rice starch converted to sugars. The process is similar to how beer is made but without hops.

In beer and whisky, saccharification is triggered using germinated grains (malting enzymes); soju is done by fungal saccharification enzymes (nuruk microorganisms). The quality is affected by the method of manufacturing yeast, the amount used, and the fermentation method.

The fermentation process is also a base for creating another famous Korean beverage, makgeolli (a byproduct of soju fermentation before distillation). It has a low alcohol content of between 6 and 8 percent, with a sour and refreshing taste.

• Makgeolli is fermented with nuruk, which supplies fungi (such as Aspergillus oryzae and Rhizopus oryzae) and yeast-like like Saccharomyces. It uses the conversion of starch into sugar by utilizing acids or enzymes like amylolytic fungal species such as Saccharomyces cerevisiae.
  
• Makgeolli can be made from nonglutinous (Oryza sativa L.) and glutinous (‘sticky’) rice. Some purple glutenous rice wines have a Sherry character.
  
• Which rice to use for making soju is up to the producers; for instance, the non-glutenous rice produces a higher level of high alcohols (more aromas), but at the same time, the presence of excess higher alcohols (more than 10 g/L) has been reported to be related to hangover and severe headache.³

A recent study⁴ showed that the formation of higher alcohols in rice wine fermentation using different rice cultivars an extremely strong correlation between the starch in rice and higher alcohols in rice wine. Less higher alcohols were found in rice wines made from the five glutinous rice cultivars with less starch content than the other five rice cultivars and therefore suggested that the selection of rice with less starch content would help control the formation of higher alcohols in the rice wine industry.

The same studies also raised the question of whether all glutinous rice cultivars were merited to be more suitable for making rice wine than nonglutinous rice. These findings suggested that the selection of rice cultivars instead of rice type, that is, glutinous or nonglutinous, was more correlated with the quality of rice wine (Wang et al., 2014; Xie et al., 2016). 

Nuruk, Koji, and Qu

Nuruk, Koji, and Qu/Jiuqu are fermentation starters integral to the production of alcoholic beverages (soju, shochu, and baijiu), and even though they have the same purpose, they are different in their composition, the flavours and aromas they contribute to the finished product.

Another main difference between nuruk and koji is that the latter is produced using a specific type of mould. At the same time, the composition of the former microorganism depends on the local environment’s unique characteristics.

In Nuruk preparation, the cake matures at a precise temperature until mould forms, resulting in a unique blend of microorganisms that reflect the local terrain and environment. This showcases Korea’s distinct cultural practices in alcohol production.

Nuruk produces an extensive range of flavours and aromas, including fruity, floral, earthy, and spicy notes.

Koji is a mould used to produce sake, also known as Japanese rice wine. It is made by inoculating cooked rice with the Aspergillus oryzae mould, which then grows on the surface of the rice. The koji mould enzymes convert the starches in the rice into sugars, which are then fermented to produce sake. The koji mould is carefully cultivated to produce the desired enzymes and flavours.⁵

Jiuqu, or qu, is a dried fermentation starter for traditional Chinese alcoholic beverages. It consists of mould, yeast, and bacteria crushed up and mixed with damp grains to ferment them.

There are two types of qu, known colloquially as ‘big qu’ and ‘small qu.’

• Big Qu – Daqu
  • It is usually a combination of cereal grains, though vegetables are sometimes included. The resulting flavours are pretty complex and can lead to very high-end baijiu. The cereal grains are mixed with water until a paste-like consistency is formed. Once ready, they are shaped into bricks weighing around 7 lbs each. These bricks are stacked in a warm, enclosed room and left alone for up to two months.

• Small Qu – Xiaoqu
  • is made from cooked rice rather than grains. This creates a more aromatic baijiu, as sometimes the manufacturers also add a variety of herbs. The process of creating small qu starts the same, with the addition of water. As opposed to bricks, this mixture is rolled into small balls. 

Different types of qu will make very different-tasting variations of baijiu. The majority of baijiu brands will use daqu for its unique taste. A lighter, more aromatic baijiu may rely on xiaoqu, though.

The traditional nuruk is similar to qu or “big qu” regarding the cultivation method and its use in fermentation as a saccharification enzyme. ⁶.

One of the soju producers, Tokki soju, uses a slightly different approach to Nuruk preparation; according to them, their method is more similar to koji preparation.

Qu is different than what we personally use, because qu has wild yeast strains and bacteria attached to it. We only want aspergillus oryzae in our nuruk for conversion purposes and consistency. So, in our case, our nuruk is more similar to koji production, except koji is cultivated on whole rice and nuruk uses milled wheat cakes.

^{_{https://www.alcademics.com/2020/07/getting-nerdy-about-soju-production-sticky-rice-nuruk-fermentation-multi-abvs-korean-production.html}}

Distilled Jeungryusik Soju

The distillation of Soju in Korea produces two very different flavours and alcohol content products, distinguished by distillation type: Jeungryusik Soju and Hiseoksik Soju.

Jeungryusik Soju is a traditional drink that distills rice wine in one step. The quality of distilled soju depends on the type of raw material and the method of converting starch into sugar.

Hiseoksik Soju is made by diluting the rectified spirit; a diluted soju produced by diluting a multi-column continuous still creates a colourless and tasteless spirit with no taste or aroma of the original alcohol remaining. The spirit is then diluted with water, and sweetener is added.

The only objective way to compare them is to examine the differences in their distillation processes and extrapolate the perceived flavour difference in the final products.

The earliest stills used by the Arabs and eastern Asians were small glass or earthenware pot stills of the alembic type. It was simple in shape and usually had a distillation kettle for heating a fermented product, and at the beginning, it did not use flowing water as a coolant. In later times, the scientists (alchemists) added a cooling tube to the alembic vessel for condensing the distillation vapours back into a liquid, which eventually led to an apparatus known now as a pot still, used predominantly not just by the Soju distillers, but also by whisky, brandy, and rum producers.

The traditionally distilled soju uses a sojutgori, which helps retain the taste and aroma of the original liquor. No sweeteners are added, and it has an excellent taste and aroma. However, the raw material and strong burn odours can adversely affect the taste due to unstable temperature control and cooling structure.⁷

Right after the fermentation, the rice mixture is roughly strained, producing a milky, cloudy liquid known as makgeolli or takju. Once it is filtered, it becomes cheongju, a base for Soju distillation.

This distillation process is usually single-staged and is inefficient in removing impurities, leaving behind many congeners. It produces a more flavoured distillate and adds character to the final distilled product.

Type of Distillation Process

Another way to distill traditional soju is using a hybrid pot. Hybrid stills technically combine two types of stills, column and pot, and can be run as both.

In a pot-type distillation system, fermented grain is boiled to vaporize the alcohol. The alcohol-water vapours then flow through a distillation column to bring about concentration.

One of the producers using this ype of stills is Brandon Hill, the distiller behind the Tokki Soju; according to him, the way they use the hybrid still is:

We do a two phase distillation in our process where we use both styles for our soju. First, is the pot stripping run and then cleaned up and finished with the column spirit run.

Technically, the soju fermentation base is closer to beer than wine.

One of the advantages of this pot distillation process is its simplicity. It does not require a constant supply of fermented base and provides a straightforward equipment system. Cooking, fermentation, and boiling for distillation are carried out in the same vessel.
This procedure may aid in sterilizing equipment between successive batches since cooking and fermenting in the same vessel tends to heat-sterilize.⁹

Vacuum distillation

Traditional distillation evaporates the volatiles at temperatures between 170 and 200 degrees Fahrenheit, so the liquids and flavouring ingredients are cooked. It changes the quality of many aromas, especially more delicate, flowery, and fruity citrus.

 That’s where vacuum distillation is beneficial for extracting more delicate aromas. A vacuum distillation pump pulls air and vapours out of the distilling vessel, reducing air pressure and causing the volatiles to evaporate at a lower temperature. The higher the vacuum, the colder the temperature at which alcohol and aromatics can be distilled, and the less cooked the resulting flavours are.

This method lowers the pressure above a liquid to less than its vapour pressure. This allows the most volatile liquids to be selectively boiled off and distilled. Vacuum distillation is particularly useful if the temperatures required for a fluid to boil at atmospheric pressures are hot enough to damage sensitive molecules.

One of the producers utilizing this technology is Hwayo’s Soju. They make an artisanal premium rice soju (a clear spirit distilled from rice) that is vacuum distilled for optimal aroma and flavour and matured underground for three months in traditional Korean Onggi pots (microporous earthenware known for their breathability).

According to them, these are the steps their premium soju goes through.

Compared with traditional distillation, this method produces an alcohol with an aroma much closer to the original base.

Sensory and Volatile Profiles of Korean Commercially Distilled Soju

Our sensory perceptions of taste and aromas are highly subjective experiences greatly influenced and defined by our brain’s ability to recognize and construct a complete sensory picture of a particular beverage’s flavour.

In our everyday lives, we consciously or subconsciously judge the quality of the drinks and food we consume. Yet, we cannot fully discover all the underlying subtle aromas due to our limited ability (for most people) to sense all the aromas.

Profiling volatile compounds is an essential step in determining the flavour of distilled spirits, and objective chemical analysis can help discover the volatile profile of an object of interest, such as soju.

In a study done by Jung-Min Hong, Tae-Wan Kim, and Seung-Joo Lee and published in MDPI, volatile compositions and sensory characteristics of 11 commercially distilled soju samples were investigated using headspace solid-phase microextraction (HS-SPME) with gas chromatography-mass spectrometry (GC-MS) and sensory descriptive analysis.

Table 1. Information about the 11 commercially distilled soju samples.

Fifty-nine major volatile compounds, consisting of 32 esters, 10 alcohols, 2 acids, 5 aldehydes, 3 ketones, one hydrocarbon, one furan, 2 phenols, and 3 miscellaneous compounds, were identified. From the principal component analysis (PCA) of volatile data, MSJ made by atmospheric distillation showed a clear distinction in volatile compositions compared to that of other samples made by vacuum distillation.¹⁰

Volatile compounds in rice-distilled soju aged in pottery, oak, or stainless steel tanks for 18 months were isolated by headspace solid-phase microextraction (HS-SPME). The soju samples aged in oak showed higher concentrations of ketones, aldehydes, and miscellaneous compounds.

A total of 32 esters were detected and were among one of the largest classes of detected compounds. All the detected esters were previously found in various alcoholic beverages. The major compounds detected were isoamyl acetate, ethyl decanoate, ethyl octanoate, ethyl decanoate, and ethyl dodecanoate. Esters are produced by yeast during fermentation and affect the fruity flavours of alcoholic beverages.

The ethyl octanoate and ethyl decanoate levels in MSJ were much higher than in the other samples. Ethyl octanoate is well known to show wine, brandy, and fruity aromas; ethyl dodecanoate has an oily brandy aroma and is an essential flavour component in distilled liquor.

The level of alcohol compounds depends on the raw material, fermentation conditions, and distillation technique. Among the 10 alcohol compounds identified, isobutyl alcohol (sweet and musty aroma), 1-hexanol (mild, sweet, green aroma), 3, 7-dimethyl-6-octenol (rose-like aroma), and 2-phenylethanol (rose, honey-like aroma) were detected in more than 7 samples.

Acids are considered important contributors to flavour. Octanoic acid (unpleasant flavour) was detected in eight samples, and decanoic acid (unpleasant flavour) was detected in four samples.
Acid is an important component because even a small amount in combination with alcohol will form esters. The samples made with atmospheric distillation, such as MSJ, showed higher levels of these acids than those of the other samples. As also reported in volatile compounds of spirits by different distillations and filtrations, the sample with the atmospheric distillation showed higher levels of esters and alcohols than other samples with vacuum distillation.

The study found that the main difference in the volatile compounds depended on the distillation method.
MSJ produced by atmospheric distillation had many longer chain esters, contrasting with the other samples made by vacuum distillation.

There were also differences in sensory characteristics based on the raw material, distillation, and aging methods, which greatly affected the volatile and sensory attributes of distilled soju samples.

Another study using a different scientific approach focused on the volatile compounds influenced by the storage and aging process was published on the Mdpi site. For more details on the Analysis of Volatile Compounds in the Soju study, check the attached file or visit the link above.

The steps they used in their experiment are illustrated in the graphic below.

Hiseoksik Soju

This is currently the cheapest, best-selling, and most popular type of soju. It is a mass-produced commercialized manufacturing process selling millions of bottles. It has a 95-percent ethyl alcohol base, water, flavours, and sweeteners, similar to how flavoured vodka products are made.¹¹

Is the diluted soju better than the traditional one? I’m not sure, as defining what flavour is superior is highly subjective and dependent on many external factors such as price, availability, perceptions, marketing, etc.

The Hiseoksik soju is produced by the fermentation of sugars from various sources, including barley, maize, potato, rice, sweet potato, tapioca (dangmil in Korean), and wheat.

It is prepared by diluting a purified neutral alcohol produced by continuous distillation, and it is a cheap and efficient method of making an alcoholic beverage. The undiluted spirit is highly rectified and has few congeners, like vodka aging, which is not required.

At the same time, it has the disadvantage of not containing the original flavour of fermented liquor.

After the purified neutral alcohol is produced by continuous distillation, it is diluted on average to 15–35% ABV and sweetened with additional flavouring additives.

Many flavoured soju contains a wide range of sweeteners (honey, maple syrup, and non-carbohydrate sweeteners such as aspartame, stevioside, or xylitol) and flavourings.

Stevioside is a carbohydrate-free, natural sweetener derived from Stevia Rebaudiana, a South American plant. Xylitol, a sugar alcohol, is produced by reducing the carbohydrate xylose.

Despite its popularity in Korea and many varieties, soju is still relatively unknown in North America. However, that is beginning to change as soju is gaining a reputation inside the Asian market as well as in North America, where it’s being advertised as a modern alternative to standard spirits like vodka.

As Korean culture continues to benefit from worldwide recognition, it seems in all likelihood that soju, particularly the distilled one, will continue gaining popularity. Its unique taste and cultural significance make it an attractive choice for adventurous drinkers seeking something extraordinary.

1. https://patents.google.com/patent/KR101576097B1/en
2. https://patents.google.com/patent/KR101576097B1/en
3. https://www.frontiersin.org/articles/10.3389/fmicb.2022.978323/full
4. https://www.frontiersin.org/articles/10.3389/fmicb.2022.978323/full#B28
5. https://sojusociety.com/difference-between-korean-nuruk-and-japanese-koji/
6. https://www.alcademics.com/2020/07/getting-nerdy-about-soju-production-sticky-rice-nuruk-fermentation-multi-abvs-korean-production.html
7. https://doi.org/10.1002/fsn3.3099
8. based on Kim, U.-s., Kim, J.-h., Kim, T.-W., &
   Choi, J.-h. (2023). Analysis of distillation characteristics via
   CFD (computational fluid dynamics) of Korean traditional
   ‘Sojutgori’ and study on structure for distillation efficiency
   enhancement. Food Science & Nutrition, 11, 590–598. https://
   doi.org/10.1002/fsn3.3099
   20487177, 2023, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/fsn3.3099 by Cochrane Canada Provision, Wiley Online Library on [01/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
9. https://www.extension.purdue.edu/extmedia/ae/ae-117.html
10. https://www.mdpi.com/2304-8158/9/9/1330
11. In 2020, HITEJINRO sold around 95.3 million soju cases, almost threefold the amount sold by the world’s No. 2 distilled spirit brand, Philippines’ Ginebra, which sold about 31.2 million cases last year.. https://www.smartblend.co.uk/blog/2022-market-analysis-on-south-korean-soju