By Dr. Michela Centinari, Assistant Professor of Viticulture, Department of Plant Science

If just one adjective was chosen to describe the 2018 growing season to date, many of us might suggest ‘rainy.’ In many Pennsylvania regions, grape growers faced persistent rainfall for the majority of the summer. For example, in central PA, State College has had an accumulation of 29 inches (737 mm) of rainfall for the months of April through August.  Growers really had to be on top of their fungicide spray schedule and canopy management plans to minimize the risk of disease so that fruit will be healthy at harvest time. Recently, Bryan Hed and Jody Timer wrote blog posts that provided recommendations for late-season downy mildew control and insect problems. While the weather forecasted for harvest season is weighing heavily on the minds of many grape growers, a post-veraison task critical for a successful harvest is collecting grape samples to measure the progression of fruit maturity.

This article provides a brief review on what fruit ripeness parameters you should measure and how to collect berry or cluster samples to best assess fruit maturity. While this information could be particularly useful for new grape growers approaching their first vintage, experienced growers should review the information to ensure that they are using the best techniques for collecting representative fruit samples.

Harvest decisions

Grapes are typically harvested when they reach desired fruit quality parameters (e.g., sugar content, pH, flavor, color) which vary depending on the wine type or style the winemaker aims to produce. Grapes should be sampled periodically until harvest to monitor how parameters associated with fruit maturity (e.g., sugar, pH, organic acids, flavors) evolve through the ripening season. However, there are many other factors involved in selecting a harvest date, which may or may not directly relate to optimal fruit maturity. These factors include:

  • Fruit health condition (is the fruit deteriorating due to rot or other disease or insect damage?),
  • disease and insect pressure,
  • short and long-range weather forecasts,
  • available labor,
  • space available at the winery to process the grapes, and
  • type or style of wine that will be made.

What fruit ripeness parameters to measure

The evaluation of the overall fruit ripeness involves quantitative parameters (sugar content, pH, titratable acidity) but also measurements that go beyond analytical techniques(berry sensory analysis).

Quantitative measurements to determine grape ripeness:

The information reported below is adapted and summarized from the factsheet Determining grape maturity and fruit sampling written by Dr. Imed Dami, Ohio State University.

Sugars, organic acids, and pH are the primary indicators of technological or commercial grape maturitywhich is different from physiological maturity that occurs at or soon after veraison when seeds are ready to germinate.

Sugars: Sugars, specifically glucose and fructose, are the main soluble solids in grape juice. Sugar content is typically measured in degree Brix (°Brix); 1 degree Brix corresponds to 1 gram of sugar per 100 grams of grape juice. Desirable levels of sugar content are typically between 18 and 24ᵒBrix, depending on grape variety and wine style.

Sugar level is relatively easy to measure in the vineyard with a handheld refractometer ….