A New Approach for Detecting Spoilage Yeast in Pure Bottom-Fermenting and Pure Torulaspora Delbrueckii Pitching Yeast, Propagation Yeast, and Finished Beer

Abstract

Beer quality is highly dependent upon yeast condition. Consequently, breweries should consider the quality of the pitching yeast. Spoilage yeasts such as wild types of Saccharomyces cerevisiae, S. cerevisiae var. diastaticus, and some non-Saccharomyces yeasts occur occasionally in pitching yeast and as secondary final-product contaminants. They negatively impact pitching yeast fermentation and beer flavor. In this study we used a novel device to measure the pressure in small-scale fermentation vessels and to detect five spoilage yeast strains of S. cerevisiae var. diastaticus in four pure pitching yeast strains of the brewing yeast Torulaspora delbrueckii. We investigated a method to detect the five spoilage yeasts in pure pitching yeast of four S. pastorianus strains. Pitching yeast was chosen for its high cell density, activity, adaptability, and therefore stress resistance, and because it could produce false positive results in the detection method. Cultivation in the vessel at 37°C in YM broth inhibited all the pitching yeasts but increased the spoilage yeast. Real-time polymerase chain reaction validated the method. Low spoilage yeast concentrations of 10 cells/mL and contaminated industrial samples were reliably detected. Minimal time was needed to prepare the sample and detect spoilage yeasts.