Alberto Arduino, Cristina Medina-Plaza, Anita Oberholster, and David Block*
*University of California, Davis, 595 Hildgard Lane, Davis, CA, 95616 
(deblock@ucdavis.edu)

Grape phenolics are compounds of primary interest for winemaking, as they are responsible for the main sensory characteristics and important physicochemical interactions with other molecules. These compounds are mainly extracted from the solid phase of the berry – skins and seeds – throughout alcoholic fermentation. The extent and rate of extraction depend strongly on temperature, alcohol concentration, and juice-mixing procedures. Cell wall (CW) composition of grape skin also been plays a key role in the net extraction of grape phenolic compounds. We proposed a mechanistic model for phenolic extraction that includes CW composition. This study aimed to use spectrometers to non-destructively measure CW composition and, integrating this technology into existing engineering models, predict grape phenolic extractability. Twenty-one grape samples were collected from eight different sites in Northern California. Studied varieties were Cabernet Sauvignon, Pinot noir, Petite Sirah, Zinfandel, and Barbera. Each grape set was microfermented in triplicate at 24°C. Commercial yeast strain EC1118 was inoculated. Sugar concentration was adjusted to 25 Brix, titratable acidity to 6 g/L tartaric acid, and yeast assimilable nitrogen to 250 mg/L. Grape and wine phenolic compositions were analyzed by reversed-phase high-performance liquid chromatography. CW material was isolated from berry skins and characterized: proteins, cellulose, and uronic acid were determined spectrophotometrically and lignin, gravimetrically. Intact berries were then scanned with Ocean Insight Flame and NirQuest+2.2 spectrometers (Ocean Insight, Orlando, FL), covering wavelengths from 350 to 2200 nm. Preliminary results show a correlation between CW composition and phenolic extractability and between variety and site-specific variability for extractability and CW composition. Recently, other groups have successfully detected CW components of fruits using spectrometry. We expect spectrometry data to show a correlation with CW composition and phenolic extractability of grapes.

Funding Support: E. & J. Gallo Endowed Chair in Viticulture and Enology, Robert Lawrence Balzer Scholarship, David E. Gallo Educational Enhancement Fund, Richard and Saralee Kunde Scholarship Fund, Horace O. Lanza Scholarship, Margrit Mondavi, Wine Institute Award, Knights of the Vine Scholarship Fund