Jordan Beaver, Cristina Medina-Plaza, Anita Oberholster,* David Block, Nick Dokoozlian, Ravi Ponangi, and Tom Blair
*University of California Davis, 1 Shields Ave, Davis, CA 95616 (aoberholster@ucdavis.edu)

Fermentation temperature and ethanol concentration are two factors that greatly impact the extraction of phenolics from grape skins and seeds during the making of red wine. Additionally, it has been shown that grape polyphenols adsorb to the cell walls of grape skins thereby preventing their extraction into the finished wine. Considering these phenomena, it is likely that the kinetics and extent of polyphenol adsorption to grape skin cell walls are impacted by both temperature and ethanol. Therefore, the effects of temperature and ethanol concentration on grape proanthocyanidin (PA) adsorption to grape skin cell wall material (CWM) were investigated. Adsorption experiments were conducted at various temperatures (15 and 30°C), ethanol concentrations (0 and 15% (v/v)), and starting concentration of PA (500, 1000, and 1500 mg/L). A full factorial design was implemented to investigate the impact of each variable and their interactions. PA were exposed to CWM in small, bench-top experiments, and sequential sampling was used to analyze the kinetics of the adsorption reactions. Qualitative analysis of the PA solution, using gel permeation chromatography and phloroglucinolysis, was also conducted to investigate preferential adsorption of specific molecular weights or subunit composition of PA. The extent of adsorption was shown to be dependent on both temperature and ethanol concentration. Isothermal modeling was applied for each temperature-ethanol condition and equilibrium constants were calculated. Elucidating impact of polyphenol adsorption to grape-derived cell walls will ultimately increase our understanding of the limitations of phenolic extraction in the making of red wine, and thereby enhance the winemaker’s ability to predict and control the chemical and sensory properties of the finished product. 

Funding Support: E&J Gallo Winery