Harvesting: The most important decision for the winemaker

On of the most important decisions facing the viticulturist/winemaker is the timing of the grape harvest -- the so-called point of "optimal ripeness." As the starting point for the production of a quality wine, it is imperative that high-quality grapes be delivered to the cellar door. As noted by Wynboer, overripe as well as green grapes have a detrimental influence on the wine." As important as optimal ripeness is to the wine maker, there is no specific definition for the condition nor are there formalized mechanisms or technologies for determining its attainment. To further complicate matters, optimal ripeness will vary from winemaker to winemaker depending on (Bisson):

• the style of wine being pursued
• the working definition of quality
• variety
• rootstock
• site
• the interaction of variety, rootstock, and site
• season-specific factors
• viticultural practices
• downstream processing events and goals.

With a "definition" of optimal ripeness in place, we can then turn to the identification of the relevant parameters. Dami has divided the criteria into two broad classes: objective and subjective. The objective criteria revolve around the components of grape juice at maturity, with sugar, acid, and pH being the primary indicators based on (i) abundance and (ii) ease of measurement.

• Sugar concentration -- use of sugar concentration as the only measure of grape maturity ignores the contribution of a number of other elements to the perception of a quality wine. Also, there is not a one-to-one relationship between sugar and quality. According to Wynboer, Duplessis obtained quality differentials of between 8% and 27% with 4 years of Pinotage pressed at 22 °Brix.
• Titratable acidity – actual amount of acid in the wine. According to Dami should fall between 0.6 and 0.8 gm/100ml
• pH – measure of active acidity. Increases with increases in sugar concentration. According to Wynboer, not a reliable measure on its own. Should be between 3.1 and 3.3 for white grapes and 3.3 - 3.5 for red grapes
• Ratio between sugar and acid – Wynboer sees this as more significant than any of the preceding alone. Jackson sees this as a requirement for a commercial-grade operation in a temperate climate. Bisson points out its variability across cultivars and a lack of clarity as to whether optimal acid:sugar ratios coincide with optimal flavorant maturity.

 The subjective criteria identified by Dami are as follows: color; ease of removal of berries from pedicel; texture; aroma; and flavor. These tests should be conducted by the winemaker during vineyard walk-through. Of the objectives tests mentioned, the test for flavor is the most important. Bisson sees optimal maturity as assessable only by monitoring flavorants themselves but such a task is laborious and expensive (Dami, Jackson) and has to be approximated through tasting (Wynboer, Dami).
 
Given that picking at optimal ripeness is critically important to the production of a quality wine, and that there are no mechanical/technological aids available to assist in accomplishing that goal, winemakers have generally turned to vineyard sampling as a means of: (i) testing the progress of the various berry components; (ii) predicting the optimal pick time; and (iii) to get a sense of the must after harvest. Under ideal conditions, the winemaker would test the fruit and, when the optimal ripeness is determined to have been achieved, harvest the fruit of the entire vineyard. But things are not so simple. Grapes in a vineyard do not ripen evenly – asynchronous ripening – signaling different development paths. This is a problem for the winemaker as both underripe and overripe fruit provide the opportunity for production of a lower quality wine.  At the vineyard level, ripening differences could be due to (Jackson): differences in vineyard age and health; inter- /intra-block differences in soil structure, texture, nutrition, and moisture content; and, the length of the flowering period. Variation between berries can be a function of (Zoecklein); berry size; berry composition; number of seeds; seed size; and berry position.
 
The goal of the vineyard sampling activity is to collect a sample that, when processed, will be reflective of the must composition. In order to accomplish this goal, the sample has to be large, representative, and positionally aware such that previously described berry and vineyard variabilities are accounted for therein. The consensus approach (Dami, Heller, Zoecklein, Wann, Watson) to sampling design and execution is as follows:

• Devise the sampling scheme prior to entering the vineyard and, once there, stick with it
• Sampling can be either berry-specific or whole cluster. If berry, collect at least 200/block. If cluster, collect 10 – 20 clusters.
• Begin sampling about three weeks prior to the estimated pick date. Sample weekly, increasing frequency as you get closer in
• Avoid sampling from end rows and odd-looking vines
• Fruit should be selected from exposed as well as shaded locations at different heights and at opposite sides of the rows
• Sample in the morning rather than in the afternoon. Dami sees a 1 °Brix difference.

Avoid the sampling flaws identified by Watson:

• Collecting too few berries. According to Dami, two samples of 100 berries each are required to be within 1 °Brix of sugar at harvest
• Collecting riper, mature berries. Again, will not provide a true picture of the post-harvest must
• Sampling higher percentage of exposed berries. Neil points out that sun exposure of a berry, or the position of the cluster on the shoot, can influence maturity.

 

 References
 Bisson, Linda, “In Search of Optimal Grape Maturity,” Practical Winery and Vineyard Journal, July/Aug01.
Dami, Imed, “Determining Grape Maturity and Fruit Sampling,” Ohio State OAROC Extension.
Hellman, Edward, “How to Judge Grape Ripeness Before Harvest,” winegrapes.tamu.edu/grow/ripening.pdf
Jackson, Ronald S., Wine Science: Principles and Applications, 3^rd ed., Amsterdam: Elsevier, 2008
Nail, William, Collecting Berry Samples to Assess Grape Maturity
Roller, Ben, “Pre-fermentation cold maceration,” brsquared.org/wine/Articles/coldsoak.htm
Wann, Grady, Live presentation on optimal ripeness
Watson, Evaluation of Wine Grape Maturity
Wynboer, “Optimum Ripeness in Wine Grapes,” May 2000, wineland.co.za.
Zoecklein, Bruce, Enology Notes #143, apps.fst.vt.edu/extension/enology/EN/143.html

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