Distribution of Yeast Cells, Temperature, and Fermentation By-Products in White Wine Fermentations

Abstract

Yeast inoculation from the tops of tanks is common in white wine fermentations. Our objective was to examine whether inhomogeneities occur in tanks following yeast addition from the top with and without brief stirring after inoculation. We investigated whether inhomogeneities affect fermentation kinetics and formation of acetaldehyde, acetic acid, and pyruvate in Riesling juices with low turbidity. Samples and temperatures were taken at multiple tank heights in pilot-scale (105 L) and industrial-scale (2500 and 7000 L) tanks to investigate the distributions of temperature, yeast, and fermentation products in tanks of different volumes and height-to-diameter ratios. When the tank was not stirred following inoculation, cell counts were higher in the upper tank section for multiple days. Fermentation started earlier at the top of the tank than at lower levels, as indicated by specific gravity measurements. The top and bottom of the tanks differed by up to 8.6°C and 0.040 in specific gravity. The inhomogeneities lasted four days in the 7000 L tanks. These results suggest slow yeast sedimentation and poor mixing in the downward direction during the early fermentation phase. During vigorous fermentation, all parameters were homogenously distributed. Brief stirring after yeast inoculation provided uniform conditions during the early fermentation phase, except for temperature differences due to natural stratification. Stirring also shortened the fermentation duration. The final pyruvate concentration was significantly lower when the yeast was stirred in, presumably due to better nutrient availability throughout the tank, reducing sulfur dioxide demand. No differences were observed in acetaldehyde and acetic acid concentrations. When yeast is not stirred in following inoculation, tank sampling valves and temperature sensors do not reflect the entire tank during the early fermentation phase, which will negatively affect process control.