Development of a prediction tool for phenolic extraction in red wines as a function of winemaking practices and fermentor design

Red wine fermentations are performed in the presence of grape skins and seeds to ensure extraction of color and other phenolics. The presence of these solids results in two distinct phases in the fermentor, as the solids float to the top to form a “cap.” Modeling of red wine fermentations is, therefore, complex and must consider spatial heterogeneity to predict fermentation kinetics and phenolic extraction. We have developed a reactor-engineering model for red wine fermentations that includes the fundamentals of fermentation kinetics, heat transfer, diffusion, compressible fluid flow, and extraction of phenolics (anthocyanins, skin tannins, and seed tannins).

COMSOL was used to solve all components of the model simultaneously utilizing a Finite Elements Analysis (FEA) approach. Prediction of phenolic concentration gradients and temperature gradients from this model were validated against measurements in 2000 L pilot fermentations. Model prediction and experimental data showed excellent agreement for anthocyanin and tannin concentrations and distributions over the course of fermentation.

After validation, this model was applied to examine how fermentor design (e.g. scale and aspect ratio) and operational decisions (temperature set point, pump over frequency) would affect phenolic extraction rates, relative concentrations of skin to seed tannins, and distribution of phenolics throughout the fermentor in the absence of cap management. These results were a follow up to 2018-2019’s work, where the model was used to explore fermentation dynamics and temperature control in red wine cylindrical fermentors and white wine concrete egg fermentors. Example findings include optimization of skin tannin extraction via cap management, with 1x/day pump overs being found superior to both no cap management and 8x/day pump overs, a finding made possible via the combined spatial fermentation-extraction model.

Our results have opened up two exciting avenues of further investigation. The first is applying our reactor engineering models to isothermal fermentation process acceleration, where wine fermentation process cycle time could be greatly decreased by the judicious application of yeast nutrients throughout the fermentation, maximizing yeast biomass. This would greatly improve productivity in existing wine fermentors and lower the capital cost of new winery equipment. The second is the application of COMSOL extraction models to external grape pomace extraction columns, allowing for the fine-tuning of phenolic profiles in the end wine, potentially in a much more rapid fashion than in-tank extraction.

In the nineteen months since this grant began, we have been highly productive having published six primary research papers, along with a review of wine fermentation process modeling. We have also published three papers in cooperative works stemming from this grant. We have presented this work at various extension venues, as well as technical conferences throughout the nation.