Kyoko Hiratsuka (1), Yasuhiro Fujita (1), Nobuyuki Hayashi (1); (1) Kirin Company, Limited, Yokohama, Japan

Yeast, Fermentation, and Microbiology

Recently, we expanded our beer product range to include “craft beers.” Many craft beers have unique properties, such as high pH, low alcohol content, low bitterness units (BU), and high residual fermentable extract, as well as including various ingredients. Incorporating these features in the brewing process tends to increase the microbiological risk. We are interested in identifying the beer-spoilage potential of yeasts and to differentiate between species that can grow in beer and those that do not pose a threat as beer-spoilage organisms with regard not only to our major brand of pilsner beer but also to our craft beers. Assessing the beer-spoilage ability of yeast isolates can be done by directly testing for growth in beer; in this time-consuming approach, however, the results are only known after several weeks. The aim of the present study, therefore, was to develop a more rapid and simple method to assess the ability of yeast to grow in beer. First, we referenced the hop-gradient agar plate method (Haakensen et al., 2009), which uses concentration-gradient agar plates to assess the ability of Lactobacillus and Pediococcus isolates to grow in beer. Next, we adapted the technique to assess the growth ability of yeasts. In searching for a method by which to differentiate between beer-spoilage and non-spoilage yeast, we explored the ability of yeast isolates to grow in varying concentrations of ethanol, different compositions of base medium, and other variable factors. Ultimately, we derived an optimal composition of medium and optimal cultivation conditions that can distinguish ordinary beer-spoilage yeasts from non-beer spoilage yeast by means of the degree of colony formation. This ethanol-gradient agar plate technique provides a rapid and simple solution to assessing the ability of yeast isolates to grow in beer, and provides new insights into “potential beer-spoilage yeast” that can grow only in products with low microbiological stability. By adjusting the assay conditions, we expect to apply this technique to assessments of spoilage potential in other alcoholic beverages such as wine, ready-to-drink beverages (RTDs), and so on.

Kyoko Hiratsuka is a researcher at the Integrated Beverage Analysis Center, Kirin Co., Ltd. She graduated from Tokyo University in 2009 with an M.S. degree in agriculture and joined Kirin Brewery. She worked at the Okayama brewery as a staff member in quality assurance and the brewing sections (2009-2013). Since 2014, she has been working within the Microbial Control Group in the Integrated Beverage Analysis Center, Quality Assurance Department in Kirin Co., Ltd.