Evaluation of Cell Concentration Profiles and Mixing in Unagitated Wine Fermentors

Abstract

Because no external source of agitation is normally applied to wine fermentations, gradients of total insoluble solids and yeast cell concentration naturally exist as a function of depth in the fermentor. In order to understand how these gradients may affect the fermentation kinetics, it is necessary to be able to measure them in a systematic and accurate way. For two 1200-L fermentations of different white grape juices, we measured the total insoluble solids (yeast and grape solids) concentration at 1-foot intervals over the depth of the tank during the entire course of the fermentation using an immersible solids concentration probe. This probe measures optical density and gives a linear response with cell concentrations up to approximately 5 g dry cells/L and an increasing, but nonlinear, response at concentrations up to 100 g/L. With this approach, we found that the yeast concentration was below detectable limits in the bulk juice within 17 hours after inoculation, and that all solids were concentrated in the sediment layer. Subsequent cell growth began from the bottom of the fermentor and rose during the course of the fermentation, becoming uniform only as cell growth ceased and carbon dioxide production increased. Small-scale experiments have helped to elucidate this behavior, including the influence of yeast-solids interaction, incomplete yeast hydration, and potential nutrient gradients present in fermentors. The solids concentration probe may be useful in evaluating cell growth in industrial fermentations and in developing strategies for more rapid initiation of cell growth.