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What is Korean alcohol or sool?

*This is a modified version of my Master's thesis introduction.

Traditional alcohol is widely consumed throughout the Korean peninsula. In fact consumption of soju, a distilled drink from rice, is so high that it is one of the world’s highest selling distilled spirit. If you’ve ever been to Korea then you’ve probably already heard of soju as well as makgeolli, a milky alcoholic drink made from rice. Almost every expat living in Korea has a tale of late night adventures with friends that started with bowls of makgeolli or shots of soju. Both can easily be found in any convenience store. However, these convenience store finds only skim the surface of Korean alcohol, or ‘sool’, as it’s known in the traditional brewing community. There are many other types of sool waiting to be discovered. These are cultural treasures that are the result of decades of family tradition as well as trial and error. When you look deeper into the Korean alcohol brewing community and learn more about the painful history of sool, it is clear to see what potential Korean alcohol has to being embraced by the world.

Korean alcohol is made with three ingredients; rice, water, and a fermentation starter. While the fermentation of rice has been practiced widely throughout Asia the production of Korean sool is unique. Oftentimes people reach for well-known alcohols to compare it too. This is why many people know sool as rice wine or rice beer since these are easier for people to understand. People will also try to compare sool to another well-known alcoholic rice beverage, sake. However, as you will see, the production process is distinctly different from that of wine, beer, sake, and other rice alcoholic beverages starting with its fermentation starter.

Fermentation starters have been used throughout Asia to ferment rice. These fermentation starters typically consist of filamentous fungi and yeasts. In China the fermentation starter is Hong Qu or daqu, in Thailand it is loog-pang, in Vietnam it is banh men, and in Japan it is koji (Limtong et al., 2002; Lv et al., 2012). With the exception of koji all of these fermentation starters are made through a natural inoculation process giving them a diverse microbiome.

In Korea the fermentation starter used to make sool is called nuruk (누룩). This fermentation starter contains filamentous fungus and yeasts that are important to the fermentation process (Song et al., 2013). These organisms have been widely studied and have an impact on the organic acids and volatile organic compounds that contribute to the flavor and scent of sool (Jung et al., 2014; Lee et al., 2012; Park et al., 2013). In an effort to improve the quality of traditional Korean alcohol, single isolates of these organisms have been studied extensively (Park et al., 2014; Yang et al., 2013). Based on this research some manufacturers use what is known as ipguk (입국), which uses a single species of fungi in the fermentation process and is more similar to koji than traditional nuruk. Nuruk comes in many different varieties and can be made from wheat, rice, or barley. Differences in location and regional manufacturing practices are believed to give nuruk different microbial profiles. Nuruk is integral to the fermentation process and one of the major contributors to the final flavor of sool.

Nuruk is important because it provides the major fungi and yeasts that are important to the production of alcohol from rice. This is a two-step process that first requires the starches in rice to be converted to sugars and then a second process where those sugars are converted to alcohol. In some ways these steps are similar to beer brewing. Those familiar with beer brewing will know the importance of the malting process where grains are soaked to activate enzymes, within the grains, called alpha-amylases. These amylases convert the grain’s starches into sugars which are later converted to alcohol by yeasts (Linko et al., 1998). The first step in the sool production process is similar because it also uses alpha-amylases to convert starches to sugars however these amylases come from fungi found in nuruk rather than from the rice grain. It is also similar to sake brewing, however sake uses one species of Aspergillus and sool utilizes many different species of fungi.

The second step of the fermentation process is also similar to beer and wine brewing. In the production of beer and wine often commercialized yeasts are used to convert simple sugars into alcohol (Jolly et al., 2006). However, sool uses the yeasts that are naturally found in nuruk for this process. The unique thing about sool is that these two processes are happening at the same time. As starch is being broken down into sugar it is simultaneously being converted into alcohol. Once the alcohol concentration becomes too high for the yeasts they die but the enzymes that convert the rice to sugars are still work to make the sool sweeter. This aspect of the brewing process helps to set it apart from beer, wine, and sake brewing practices.

Another unique aspect of making sool is the multi step fermentation process. During fermentation rice is added in stages. At the beginning of the process rice, water, and nuruk are combined to make something called a mitsool (밑술) (Kim et al., 2012). This is allowed to ferment for 2-3 days before more rice and/or water is added to create a dotsool (덧술). More rice can be added 2-3 days later in a second dotsool. The addition of rice can be stopped at any stage of this process. If only a mitsool is made the alcohol is called a danyangju (단양주) (Won, 2013). If one dotsool is added it is called an Eeyangju (이양주) or ‘two-stage’ fermentation. If a second dotsool is added it is called a samyangju (삼양주) or ‘three-stage’ fermentation and so on (Fig 1-2). More and more rice can be added depending on the recipe. Some can have as many as 12 steps however this is rare. Most recipes don’t go over tree steps. The number of fermentation steps is one of the defining characteristics of Korean sool that is important for the brewer however after filtration there are even more ways of categorizing sool that might be more familiar to consumers of sool.

After the rice fermentation is complete and the product roughly filtered, something called takju is produced (Won, 2013). Takju is a cloudy white liquid and the word comes from the Korean word meaning turbid or cloudy (Note: traditional brewers will argue that the word beakju (백주) or “white” liquor should be used instead since they consider them to be more white than cloudy (Ryu, 2014)). Takju is considered a unique category of sool that is wide and includes the very popular, makgeolli. Makgeolli is a low alcohol takju that can be made in many different ways such as diluting takju after filtration or by using special fermentation methods (Ryu, 2014). If takju is allowed to settle, it will separate into two parts, a clear liquid top layer and a bottom layer of fine white sediment. According to the Korean liquor tax law if the clear upper layer is made with ipguk the drink is referred to as cheongju and if it is made with nuruk it is referred to as yakju. However traditional brewers disagree with these categories and instead use other categorical systems. They say, if the clear layer is produced with only rice, water, and nuruk it is considered cheongju. If the clear layer is thought to have medicinal qualities through the addition of other ingredients such as herbs, flowers, roots, or even animals then the liquor is considered yakju rather than cheongju (Ryu, 2014). This makes more logical sense since the Korean word for medicine is ‘yak’. (Traditional brewers also have many other disagreements with the Korean liquor tax law however that might be another story for another time.) The clear upper layer can be consumed as is or it can be distilled to create soju and many master brewers specialize in infusing their soju with different medicinal herbs. Currently there are a few hundred different types of traditional Korean sool available on the market however at one point in history there was over a couple thousand different types of sool.

Once you look more closely the story of sool is amazingly interesting and complex. There is so much history to unpack and uncover and I hope to do that here. There has been an increase in recognition of traditional sool within Korea since I started doing my masters work on the fungi in nuruk. There are more comprehensive Korean alcohol bars and restaurants that offer a wider selection of Korean sool and more sool sommeliers on the scene. I hope that this space can be a resource to anyone who wants to know learn about and enjoy sool.


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