One of the most critical decisions a winemaker or grower must make is when to pick the grapes. Unlike most fruits, grapes do not continue to ripen after being picked. If picked too early, the resulting wine can be tart and overly herbaceous. If picked too late, the wine can be "flabby" (low acidity) and oxidized. Choosing the optimal time to harvest is a challenging decision and involves balancing many different factors. Here, I will suggest some indicators that will help with the picking decision.
Seed and Stem Maturity
This is very easy to detect without the use of any fancy tools. Grape seeds will be green until fully ripe when they will turn brown. Once the seeds are ripe, they will turn brown and will more easily crush when chewed. A ripe seed should break apart easily when chewed. Stems should be brown or mostly brown at the time of harvest. Greens seeds and stems can introduce overly harsh tannins and herbaceous flavors that detract from the wine quality. This is especially important for red wines where the must will remain in contact with the seeds and a few stems.
Color
This is the second visual indicator but is not as simple as seed/stem maturity. At veraison, red grapes will turn from green to purple. As the grapes continue to ripen, the skins will turn a darker purple or some shade of purple/blue/black. For most red wine grapes, a cranberry-ish color will indicate that the grapes are not fully ripe. For white grapes, the color change will be more subtle. Grapes will change from bright green to green/yellow or green/orange. While it is important to keep an eye on the grape color, it is not a reliable indicator of ripeness unless used in conjunction with other sensory indicators and measurements.
Sugar
As grapes mature, the sugars increase thanks to photosynthesis. As harvest approaches, the sugars will increase very rapidly and should me measured often. For a red wine, a target between 23 and 26% sugar is common. For white wine, rosé, or native grapes 19-23% sugar will often be appropriate. A refractometer is used to measure sugar in grapes, with only a drop or two of juice being needed. Winemakers use the term "Brix" which is a measure of percent sugar by weight and is indicated on both refractometers and hydrometers.
Acidity
Grapes contain a combination of tartaric, malic and citric acid. pH is a quick and easy measure of the overall acidity in the grapes. As grapes ripen, the pH will gradually drop. The goal is to catch the grapes at the right point with the appropriate amount of acid. The target at harvest for a red wine is usually between 3.3 and 3.5. For a white or rosé wine, 2.9 to 3.3 is more appropriate. During fermentation the pH will generally rise by about .15 as citric and malic acid are metabolized by the yeast. If malolactic fermentation is performed, then another increase of .1 to .15 will occur as the malic acid converts to lactic acid. At bottling, a pH of 3.55 to 3.75 is common for red wine and 3.1 to 3.35 is common for white wine and rosé.
Another measure of acidity is TA or titratable acidity. This can be measured using a pH meter and a sodium hydroxide solution, as shown on my youtube channel. A good target for red wine is generally 6.5 to 7.5 grams per liter. White wine and rosé, can fall between 6.5 and 9 grams per liter. While this is a nice measurement, I generally focus on pH for harvest and adjustment decisions. TA can change dramatically during cold stabilization and is a less reliable indicator of final wine quality.
Taste
Going out and tasting the grapes is important. Most grapes will taste very good as they get close to optimum ripeness, so don't trust your taste buds solely. With enough practice and experience, some winemakers are able to reliably pick based on the taste and texture of the grapes. This is pretty awesome, but personally I recommend double checking with your instruments before committing to a harvest.
Each vineyard and site is different, so keep good notes on picking decisions. If you have enough grapes, it may be worthwhile to do a few picking trials where the grapes are picked at varying levels of maturity. With time and experience, you will be able to master your vineyard and make great wines.
Coming Soon: Target pH, TA, and Brix for common grape varietals. Stay tuned!
See supplemental video on my youtube channel here.
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