The Effects of Harvesting Methods and Crop Numbers of Top-Fermenting Yeasts on the Flavor of Wheat Beer

Abstract

Cropping yeast is prevalent in many commercial breweries due to routine brewing patterns and cost savings. One of the biggest challenges of cropping yeast in wheat beers is the impact of cropping methods on yeast performance and on beer quality and sensory characteristics. The objective of this study was to evaluate the ability of top-fermenting yeasts, when re-pitched to subsequent worts, to reach fermentation objectives and to assess their effects on wheat beer flavors. With the increase in crop numbers of the top-fermenting yeasts, the surface wrinkles on the older cells were deepened as observed using scanning electron microscopy (SEM), and the cell-to-cell adhesion tended to be more pronounced. In comparison to the freshly propagated yeasts, fermentation by older cropped yeasts resulted in significant increases (p < 0.05) in the levels of turbidity (up to 20%), lactic acid (up to 182%), total acids (up to 79.0%), residual sugars (up to 25%), isobutanol (up to 33%), and isoamyl alcohol (up to 24.2%) in the final wheat beers of the current study. Finally, sensory evaluation revealed that the sourness of the final wheat beers was more pronounced with older yeast crops. These results showed that re-pitching with later crops of top-fermentation yeasts was not optimal for the quality of the final wheat beers. Better methods of re-pitching top-fermenting yeasts are suggested to be cropping from the surface of young beer after primary fermentation or directly pitching the fermenting wort for the subsequent brew.