A-2046A: Enumeration and Size Distribution of Yeast Cells in the Brewing Industry

Introduction Breweries, such as those operated by Anheuser Busch, Inc., utilize the Coulter Principle for their yeast cell enumeration and sizing. This technology uses the ZTM Series instrument models and the Multisizer 3. This document will focus on the Z Series models: the Z1 Single Threshold, the Z1 Dual Threshold, and the Z2 Analyzer. Yeast cells are analyzed during the fermentation process. Carbohydrates in the wort, which is the water containing sugars from the grains, are converted by the yeast strain into alcohol, carbon dioxide, and numerous by-products. There are several stages in this process, during which the yeast cells are analyzed. The first, and very critical stage for cell enumeration is during the pitching process. Pitching is the introduction of the live yeast culture into cooled wort to begin the fermentation. It is important to deliver the proper cell concentration for a successful fermentation. In addition, the yeast concentration will affect beer taste. A typical pitch concentration is 30.3 x 106 cells/mL. Yeast counts using either a Z1 single or dual threshold can be used to determine the initial cell concentration. If size distribution is being measured, a Z2 model is necessary. Often, samples are taken during the fermentation process. During this time, the yeast cells may be budding. The Z Series instruments will enumerate a budding cell as a single unit. The third stage for sampling is at the end of the fermentation process. A sample analyzed at this time is highly likely to have proteinaceous material present. To accurately measure these samples, a Z2 analyzer is needed. The size-distribution histogram may show a bi-modal size population. One peak is the yeast cell population; the other, flocculated protein (Figure 1). To remove the protein material, simply add several drops of 2 M sodium hydroxide (NaOH) to the yeast sample, mix, and analyze on the Z2.