Abstract
The colour of red wine is largely determined by the concentration of anthocyanins that are extracted from grape skins during fermentation. Because colour is a key parameter in determining the overall quality of the finished product, understanding the effect of processing variables on anthocyanin extraction is critical for producing a red wine with the desired sensorial characteristics. In this study, the effect of convective conditions (natural and forced) on the mass transfer properties of malvidin-3-glucoside (M3G) from pre-fermentative grape solids was explored at various liquid phase conditions representing stages of fermentation. A mathematical model that separates solid and liquid phase mass transfer parameters was applied to experimental extraction curves, and in all cases, provided a coefficient of determination exceeding 0.97. Calculated mass transfer coefficients indicated that under forced convective conditions, the extraction process was controlled by internal diffusion whereas under natural convection, both internal diffusion and liquid-phase mass transfer were relevant in determining the overall extraction rate. Predictive simulations of M3G extraction during active fermentation were accomplished by incorporating the current results with a previously developed fermentation model, providing insight into the effect of a dynamic liquid phase on anthocyanin extraction.
Highlights
Monomeric anthocyanins existing in pH-dependent equilibrium forms are among the largest class of phenolic compounds present in young red wines
(266 g L−1 sugar), middle (133 g L−1 sugar, 7% ethanol), and end point (14% ethanol) of a red-wine fermentation were chosen to determine the changes in mass transfer parameters over the course of a simulated fermentation
For each of the forced and natural convective conditions, three different liquid solutions differing in concentrations of sugar, ethanol, and water were chosen to emulate extractive conditions representative of the start, middle, and end of a red wine fermentation process for a wine finishing at 14% v/v of ethanol
Summary
Monomeric anthocyanins existing in pH-dependent equilibrium forms are among the largest class of phenolic compounds present in young red wines. In their cationic flavylium form, these compounds are directly responsible for the red colour in young red wines and over time interact with other (typically colourless) organic compounds, leading to long term stability of colour in older wines. During the red winemaking process, anthocyanins are extracted from the skins into the fermenting liquid through a multi-stage solid-liquid mass transfer process. Due to the vast array of reactions and interactions, a distinct
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