Grape Harvest Preparation – Facts for Fancy Fruit

Grape Harvest Preparation

The grape harvest will get started in the southern part of the state in the next couple of weeks. Most varieties are slightly behind normal this year. In Lafayette, early varieties are at the start of veraison and should be ready to harvest in four to five weeks. We generally harvest early varieties such as Brianna, Edelweiss and Prairie Star about the third week of August and many other early varieties starting the first week of September. Lately, the weather has been good for fruit ripening as the cooler conditions will improve fruit quality. Fruit quality overall is very good with minimal disease problems (for the most part).

With wine grapes, all fruit of a given cultivar is typically harvested from the vineyard or block at a single time to coordinate winery activity and to reduce costs. It is important to plan carefully so that the harvest date coincides with the optimum fruit quality.

Fruit quality is comprised of several factors, most importantly sugars, organic acids, and pH. Other factors such as phenolics, anthocyanins, aroma and flavor compounds are also very important to wine quality. And of course, freedom from rots is an important consideration. Unlike some other fruits, grapes do not continue to ripen after harvest. Consequently, it is important to harvest grapes at the peak of quality and with the desired parameters for the intended use.

Most vineyards have some degree of variability in soil type and drainage, sunlight exposure, wind, insect and disease pest, nutritional status, etc. Fruit from different parts of the block, from adjacent vines, as well as from different parts of the same vine can vary. Much of the variability is reduced with proper vineyard management, e.g. cluster thinning, shoot thinning, canopy management, etc.

As harvest nears, it is very important to monitor grape chemistry. Growers should sample weekly leading up to harvest with a protocol to collect a representative sample of fruit from the entire vineyard. This can be a sample of 200 berries per block collected from vines randomly, but with emphasis on collecting berries from the top, middle and bottom of clusters, and from exposed and shaded clusters. Some growers prefer to collect a sample of  5 to 10 whole clusters per block rather than individual berries to capture the variability within clusters. Often sampling occurs from a few select “cardinal” vines in a block, chosen for their average performance overall. Whatever approach is used, be sure to compare your sampling results to the actual final harvest juice parameters at the press to determine the accuracy of your sampling. Most of the time pre-harvest samples tend to overestimate the level of fruit maturity, but not always.

Wine grape growers should have the ability to measure sugar content (with a refractometer), titratable acidity and pH (with a pH meter and burette). Equipment and supplies to measure these parameters can be purchased for about $500-1,000. Each of these factors is important for determining proper harvest time, but none alone can accurately estimate overall fruit quality. It is the balance of sugars, acids and juice pH that is important to the winemaker. And of course, there are the subjective qualities of seed and skin maturity, tannins, anthocyanins, flavors, aromas, etc. The Berry Sensory Analysis method addresses the evaluation of these more subjective factors such as skin, pulp, and seed maturity. More needs to be done to adapt the method for use with our Midwest varieties, but as a descriptive tool, it can be an excellent way for growers to go beyond the basics of sugar, acid, and pH. Work with your winemaker/buyer on harvest decisions as much as possible.

As harvest nears every grower begins to worry about what can go wrong. Birds, raccoons, deer, turkeys, etc can all take a toll. More importantly, berry skin cracking from rain, bird pecks, and bee damage can lead to sour rot caused by yeasts and vinegar spoilage bacteria. The vinegar (acetic acid) leads to high volatile acidity levels in the wine. We experience major problems with sour rot in wet years. The lack of rain recently means that we have not had any problems this year. Let’s hope the weather continues to cooperate. Growers need to closely monitor for development of sour rot, especially if rains occur near harvest, and take measures to reduce the spread by managing fruit flies and microbial organisms. Ultimately it may be necessary to develop a strategy to minimize harvest of rotted clusters. A pre-harvest walk through the vineyard block should identify any clusters with sour rot and those lagging in ripeness. In most cases, late clusters will never catch up to the rest, and will only reduce the overall quality of the crop at harvest. Pre-harvest is a good time to drop any undesirable fruit. Don’t expect your harvest crew to sort as they pick. Go through beforehand and eliminate the guesswork.


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