Deciding when to pick grapes for wine can be challenging. There are many factors involved, but luckily two of the most important are easy to measure; sugar and acid. As the harvest approaches, sugar levels will increase, while acids will decrease to a manageable level. If you are lucky, sugar and acid will be where you want them both at the same time. If you are not so lucky, you can try to get as close as possible while minimizing trade offs. Depending on the variety that you are growing, the optimal sugar and acid levels will vary. This article is intended to provide some target numbers to get you into the ballpark for each popular grape variety.
At Harvest vs At Bottling:
The pH will generally rise .15 to .3 during primary fermentation and malolactic fermentation. To achieve the target balance at the time of bottling, the grapes will need to be picked with a bit more acid to compensate for this loss.
Grapes will be picked with very high sugar levels, but this does not mean they will be sweet. For a dry wine, nearly all of this sugar is converted to alcohol. More sugar means more alcohol. If you know the target alcohol number, divide it by .57 to get an estimate of the percent sugar needed to achieve the target.
Targets vs Reality
The targets provided are a great starting point if you are unfamiliar with the variety. There are great wines that are made outside of these target windows though. The winemaker style and the climate are driving factors that would lead one to make a wine that doesn't meet the norm for that varietal.
Target Values for Red Wines:
Cabernet Sauvignon
Harvest pH 3.3-3.4, 24-26.5° Brix, .6-.7 TA
Final pH 3.6-3.7
Merlot:
Harvest pH 3.2-3.4, 23-25.5° Brix, .65-.8 TA
Final pH 3.55-3.65
Cabernet Franc:
Harvest pH 3.2-3.4, 23-25.5° Brix, .65-.8 TA
Final pH 3.55-3.6
Malbec:
Harvest pH 3.2-3.4, 23-26° Brix, .65-.8 TA
Final pH 3.55-3.65
Pinot Noir:
Harvest pH 3.2-3.3, 22-25° Brix, .65-.8 TA
Final pH 3.5-3.55
Zinfandel:
Harvest pH 3.3-3.45, 24-28° Brix, .6-.75 TA
Final pH 3.65-3.75
Grenache:
Harvest pH 3.3-3.45, 25-27° Brix, .6-.75 TA
Final pH 3.65-3.75
Petite Sirah:
Harvest pH 3.3-3.5, 25-27° Brix, .6-.7 TA
Final pH 3.6-3.75
Tempranillo:
Harvest pH 3.3-3.5, 24-27° Brix, .6-.7 TA
Final pH 3.65-3.8
Sangiovese:
Harvest pH 3.2-3.4, 23-26° Brix, .6-.75 TA
Final pH 3.65-3.8
Normal Range for Dry Red Wine:
Harvest pH 3.2-3.4, 22-27° Brix, .6-.7 TA
Final pH 3.55-3.7
Target Values for White Wines, Native Wines, and Rosé
Riesling:
Harvest pH 2.9-3.2, 20-24° Brix, .7-.9 TA
Final pH 3.1-3.4
Gewurztraminer:
Harvest pH 2.9-3.2, 20-24° Brix, .7-.9 TA
Final pH 3.2-3.4
Sauvignon Blanc:
Harvest pH 2.9-3.3, 20-24° Brix, .7-.9 TA
Final pH 3.2-3.4
Pinot Griggio:
Harvest pH 2.9-3.2, 20-24° Brix, .7-.9 TA
Final pH 3.2-3.4
Chardonnay:
Harvest pH 3.0-3.3, 22-25° Brix, .7-.9 TA
Final pH 3.3-3.45
Concord:
Harvest pH 2.9-3.3, 14-19° Brix, .8-1 TA
Final pH 3.2-3.45
Niagara:
Harvest pH 2.9-3.3, 14-18° Brix, .8-1 TA
Final pH 3.2-3.45
Rosé:
Harvest pH 2.9-3.3, 18-23° Brix, .7-.1 TA
Final pH 3.2-3.5
How to Measure Sugar and Acid in The Field
The easiest way to measure sugar in the field is with a portable refractometer. They work by bending light through the solution and are handy because they only require a drop or two of juice to take a reading. Keep in mind, they are only accurate, if the juice is not already fermenting and does not already contain alcohol. Perfect for when you are popping a few berries with your fingers. Make sure to take several readings and average them before committing to picking.
As for acids, I prefer to focus on getting pH right and and using the TA for reference only. To measure the pH in the field you will need a good portable pH meter that is capable of reading to two decimal points. Nice pH meters will include a replaceable probe and three point calibration. To calibrate, you will need pH buffer solution.
What to do when the Sugar and Acid are Out of Whack
Sometimes things just won't be optimal in the field, especially if you are not in the perfect climate zone. You do have some options to dial things in though. If acids are too high, you can choose a yeast strain that moves the pH needle a little more than usual like Lalvin 71B. Encouraging Malolactic Fermentation by adding an an aggressive strain of malolactic bacteria will help bring the pH up. CH35 will handle most situations. Adding water to dilute the must is another method. In more extreme cases, calcium carbonate or potassium bicarbonate can be used to reduce the acid.
If the sugar is too high, water can be added to bring it down. Most commercial wineries will use reverse osmosis to avoid losing flavor concentration. If the sugar is too low, table sugar can be added in a process called chaptalization.
A great solution is blending when you can. If you have a wine that is out of balance, you can bring it around by blending with a wine that compliments it. At the end of the day, you are just trying to make the best possible wine from the grapes you have.
Final Thoughts
Some things to keep in mind when working outside of the box are; Tannin and acid work together on the palate. If your acid is high (low pH), you are going to want to keep your tannin at a lower level or it may be overpowering. Sugar and alcohol go hand in hand. Dry wines do better with a little more alcohol because of the subtle sweetness it provides. Sweeter wines, generally have lower alcohol.
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Thank you, this is immensely helpful. I used the target information above to create a one-page spreadsheet for quick reference.