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. We have developed a reactor-engineering model for red wine
fermentations that includes the fundamentals of fermentation kinetics, heat transfer, diffusion,
and compressible fluid flow. To develop the heat transfer component of the model, the heat
transfer properties of grapes were experimentally determined as a function of fermentation
progression. COMSOL was used to solve all components of the model simultaneously utilizing a
Finite Elements Analysis (FEA) approach. Predictions from this model were validated using
prior experimental work. Model prediction and experimental data showed excellent agreement.
The model was then used to predict spatial profiles of active yeast cell concentration and ethanol
productivity, as well as liquid velocity profiles. The model was also used to predict how these
gradients would change with differences in initial nitrogen concentration, a key parameter in
predicting fermentation outcome in nitrogen-limited wine fermentations. After validation, this
model was applied to examine how fermentor design (e.g. scale and aspect ratio) would affect
fermentation mixing, temperature control, and chemical gradients. Along these lines,
temperature control and mixing were also evaluated for concrete eggs using the same model.
Finally, a preliminary model for phenolic extraction from skins and seeds was developed and
validated using experimental data. This led to an analysis of phenolic release of tannins from
grape seeds that we are currently pursuing. We are now in the process of the next step in
modeling—combining the two models for fermentation dynamics and phenolic extraction to be
able to predict and control phenolic profiles in finished red wines. In the seven months since this
grant began, we have been highly productive having published two papers, submitted a third that
is under review, and will be submitting a fourth paper within the next month. We have also
presented this work at various extension venues around the state.