# Development of Korean Indigenous Low-Temperature-Tolerant Yeast for Enhancing the Flavor of _Yakju_
## 1 Introduction  
_Yakju_ is a genre of traditional Korean fermented liquor fermented with rice as the primary ingredient. It is characterized by a clean and intrinsic flavor by removing solids through filtration at the end of fermentation. _Yakju_ is fermented between 20 and 25 \({}^{\circ}\)C, and when the fermentation temperature is lowered, the pH is reduced to suppress harmful bacteria that spoil the liquor; organic acids and volatile aromatic compounds are also different from those produced by fermentation at room temperature [1]. Low-temperature fermentation prevents the loss of primary aromatic compounds and increases the synthesis of secondary aromatic compounds (ethyl and acetate esters) [2, 3, 4]. Furthermore, studies have reported that yeast in a low-temperature fermentation environment is influenced by several factors, such as protein transcription, cell membrane fluidity, RNA structure and enzyme activity, as well as aroma component-related reactions [5, 6]; therefore, when low-temperature fermentation is performed using a low-temperature-resistant strain, it is possible to prevent spoilage and create _yakju_ with a unique profile [7].  
The following microorganisms are involved in the fermentation of _yakju_: for saccharification of rice, fungi such as _Aspergillus_, _Rhizopus_, and _Mucor_ produce amylase to decompose starch into small molecules of saccharides [8], and then _Saccharomyces cerevisiae_ and othermicroorganisms work to produce ethanol and numerous organic substances [9; 10]. Depending on the yeast that is involved in the fermentation, the result shows significant differences, and the overall preference is affected because of its influence on the senses [11; 12].  
It is difficult to use most non-_Saccharomyces_ strains as ethanol-producing strains in liquor fermentation because of their high sensitivity to ethanol. Moreover, non-_Saccharomyces_ yeast has low primary metabolic efficiency, which limits fermentation performance; however, it can affect the organoleptic properties of the final fermentation product due to secondary metabolism [13; 14]. It is often used alone or through co-fermentation with _S. cerevisiae_ for liquor production [11; 13; 15; 16]. To date, non-_Saccharomyces_ strains, such as _Torulaspora_ spp. [15; 17], _Metschnikowia_ spp. [18; 19], _Pichia_ spp. [20], _Hanseniagpora_ spp. [21; 22; 23], and _Brettanomyces_ spp. have been investigated. Fermentation performed using controlled inoculum can improve the quality of the final product and can ensure better wine quality by utilizing multiple metabolic pathways of non-_Saccharomyces_ yeast [24]. Some yeasts have killer activity against spoiled microorganisms, which can be a biological inhibitor to prevent contamination by external microorganisms during fermentation [25; 26].  
In this study, to improve the flavor of low-temperature-fermented _yakju_, strains with excellent low-temperature tolerance and volatile aromatic compound production ability were first selected among -700 strains of yeast derived from native fruits and _nuruk_ (a traditional Korean fermentation starter). Then, using the selected strain, _yakju_ fermentation at 15 \({}^{\circ}\)C (single/co-culture) was performed, and the characteristics were analyzed.