In California, Fall is an exciting time of year for winemakers as it is grape harvest season and a time for new beginnings.  Most harvests in the Northern Hemisphere occur in the months of August, September, and October – while harvests in the Southern Hemisphere typically occur in February, March, and April.  In California, some early-ripening varietals, such as Sauvignon Blanc, Chardonnay, Muscat, and Pinot Noir, may be harvested in late August.  On the other hand, some late-ripening varietals, such as Cabernet Sauvignon, Cabernet Franc, Zinfandel, and Syrah, may be harvested in early November. Year to year climate variations will also affect timing of the harvest season, possibly causing a later than usual, or earlier than usual harvest.

The timing of the grape harvest is one of the most important viticulture decisions affecting the winemaking process.  The quality of the grapes at harvest establishes the potential quality of the wine early in the winemaking process.  Good winemaking practices may help offset some deficiencies in grape quality but cannot fully make up for deficiencies in grape flavor when the harvesting is done too early or too late.  As the saying goes, “You can make bad wine from good grapes, but you can’t make good wine from bad grapes”.

So, how is this important harvest timing determined?  Viticulturists, or grape growers, will often work with the winemaker and establish their targets for grape sugar, acid, and pH levels, along with goals for different flavor and phenolic components.  Grape color, berry size and texture can also be taken into consideration for harvest timing. These targets may vary from varietal to varietal as well as vary to each winemakers’ stylistic choice.

As grapes reach maturity, their sugar level rises, and this sugar has long been the standard for determining harvest timing.  Grape sugar measurements are done using a refractometer and are recorded in “Brix” (Bx), with a typical Brix measurement falling between 21^oBx and 25^oBx at harvest.  Warmer climates, such as in California, will obtain higher Brix levels, while cooler climates in Europe may struggle to obtain the minimum ripeness and Brix level.  Since during fermentation yeast convert sugar into alcohol, a higher grape or juice Brix level has potential for higher alcohol in the finished wine.

At the same time that sugar levels are rising closer to harvest, acid levels begin to fall.  Acid levels in wine are very important as they are the “backbone” of wine, giving it balance and structure, as well as being the key component in food and wine pairing.  The major acids in wine are tartaric and malic acids, with minor amounts of citric and succinic acids. Tartaric acid is nearly unique to grapes and has the most impact on wine flavor, so measuring of tartaric acid, known as “TA” (titratable acidity) is a key measurement in grape ripeness.  White wines typically have a TA level between 7 g/L to 9 g/L, while red wines typically have a TA level between 6 g/L to 8 g/L.

The pH level in the grapes is also measured, and has somewhat of an inverse relationship to the “TA”.  Low pH numbers indicate that there is a high concentration of acids in the grape juice (or wine).  So, as the grapes are ripening, sugar levels are rising with acid levels falling, resulting in an increase in the pH level.  If the pH level is too high, that may be a sign that the grapes are overripe and may impact the quality of the wine.  Most wines will have a pH reading between 3 and 4 on the pH scale.  White wines typically have a lower pH and a higher TA, giving it “crispness”, while red wines typically display “roundness” with a higher pH and a lower TA.

Measuring for the targeted grape properties begins with systematic berry sampling as the harvest season approaches – this is a collection of about 100-200 berries from different grape clusters within the varietal block to be harvested.  Care must be taken to obtain a truly representative sample from the entire harvest block.  Berries should be selected from different rows in the vineyard, sun exposure, location on the vine, and from different areas within the cluster.  Sampling usually begins about 3-4 weeks before the anticipated maturity date.  As harvest approaches, Brix levels often rise about 2^o Bx per week so sampling at regular intervals is important.  After also evaluating berry color, size and flavor, the sample is processed so that sugar, acid and pH tests can be performed on the juice.  Often, seed ripeness is also evaluated as an indicator of grape maturity.  Brown seeds are considered mature while green, un-ripe seeds may impart undesired bitterness in the wine.

Once the targeted grape properties have been reached, the harvest is done quickly and usually during the cool evenings or early mornings so that heat does not encourage the start of early fermentation.  Harvesting can be done by hand or mechanical harvesters, both having their pros and cons but both serving to get those ripe grapes from the vineyard into the winery for the winemaking process to begin.

Making the decision to harvest grapes is a critical decision in the winemaking process and is dependent on many factors requiring precise timing.  So much more can be written on the process of winemaking after the grape harvest.  I encourage you to visit the OCWS Winemaker group at: https://ocws.org/winemakers-activities/  to learn more about winemaking.

• Irene Scott

Wine Education Chair