Submitted to: Oregon State University Extension Publications
Publication Type: Popular Publication
Publication Acceptance Date: February 11, 2011
Publication Date: February 18, 2011
Citation: Tarara, J.M., Lee, J. 2011. An introduction to environmental influences on ripening in grapes: focus on wine grapes and phenolics. Oregon State University Extension Publications. Available: http://www.extension.org/pages/Environmental_Factors_Affecting_Ripening. Technical Abstract: Manipulating grapevines to improve fruit quality predates much of our knowledge of grape and wine chemistry. For millennia, humans have coaxed and battled the natural growth habit of the grapevine to make the plant agriculturally productive. Farming goals are to confine this rapidly growing plant into manageable rows, at reasonable heights for harvesting; to redirect the grapevine's propensity for vegetative growth; and to produce as much fruit as possible that meets market standards for fruit quality. Historically, quality in wine grapes was loosely defined by sugar ripeness, the concentration of soluble solids in the fruit and the sugar:acid balance. These attributes are critical to successful fermentation and a balanced wine. With advances in analytical chemistry over the last half century, the number of chemical attributes that we associate with fruit and wine quality has grown. Phenolics are responsible for major sensory properties in grapes and wine. The grower's management of vegetation, or the canopy, indirectly influences grape quality by determining solar radiation exposure of the fruit. Solar radiation influences fruit maturity attributes and phenolic profiles by driving berry temperature, which can exceed air temperature by as much as 10 to 15C when berries in mid-summer. The dependence of total soluble solids and organic acids on temperature has been documented at least since the 1890's, in progressively finer detail by cultivar and climate. Among phenolics, the biophysical role of flavonols is absorption of UV radiation; they also impart anti-microbial properties in response to wounding. Anthocyanins impart dark color (red, purple, black) to berry skins and thus, to red wine. Condensed tannins impart astringency to grapes and wine. In temperature-control experiments in the field, we found flavonols to be insensitive to temperature. Short periods of high temperature extremes during ripening, for example 5 to 26 hours above 40C, were detrimental to anthocyanin accumulation. Condensed tannin profiles were not straightforward.