Acids give crispness, brightness and thirst-quenching qualities to wines and are essential components to balance a good wine. Acid has an arguable optimum level of 6-8 g/L for reds, 6-9g/L for whites and 11-13 g/L for sparkling. It’s time to harvest when the acid comes closest to these optimums at the same time that sugar comes closest to optimum level of around 22 Brix. In many cases optimums are not be reached, so there are other rules of thumb for judging readiness of harvest (use Brix:TA Ratio as described below).
Titratable acidity is a measure of total acid in the grape juice expressed as the tartaric acid content. To do a TA test you neutralise a set amount of juice with an accurately measured alkaline solution. You need two 20mL syringes, one for juice and one for the alkaline solution. Mark them separately. Plus a beaker specifically for this.
It is advised to do the TA test twice and then average the results. If the results differ by more than 0.05 percent start again. Use same berries for determining TA that you used for determining sugar content.
When assessing grape ripeness Jon Pollard, vineyard manager at Gusbourne Estate, recommends you pick and test a minimum of 6 bunches per varietal-clone combination, and increase that number based on the variability of ripeness within a varietal-clone block.
It is also very helpful to graph how acidity is changing as the weeks go on, as most of the time ripening isn’t linear (Sectormentor does this on-the-go with our Ripeness Report). There are often clear trends for different varietal/clone combos over multiple years (despite varying weather conditions) – e.g. some will ripen faster but then acid levels jump around so there is limited benefit from leaving them on the vines for longer.
Grape Sugar:Titratable Acid (TA)
There is a direct correlation between the amount of sugar present and the ability to make wine. Optimal Brix and TA levels are not always achieved, so the Brix:TA Ratio is often a more appropriate measure in cooler climates.
Brix:TA Ratio measures ripeness. Balance between acidity is a basic concept in judging the quality of grapes. UC Davis researchers found wine most in balance when the Brix:TA ratio is between 3:1 and 3.5:1 (assuming you are measuring TA in g/L) of course for a sparkling wine this is more like between 2.1:1 and 2.6:1. So as your grapes are ripening, you take weekly measurements of Brix and TA from veraison. You will see the values are gradually getting close to the optimum, with the TA level dropping and Brix reading increasing. For example, maybe grapes are at 16 Brix and 12 acid. Brix:TA ratio is about 1.3:1 – nowhere near harvesting ready. But then sugar content had developed to 19.5 Brix and 9 TA, ratio is now 2.1:1 and if it’s for sparkling wine then you can start harvesting.
It can be good practice to let the grapes hang after they first reach this optimum. That is the art and speciality of the winemaker and vineyard manager to decide, it all depends on how quickly the ripening happened, was there a short heat wave and maybe the grape flavour isn’t fully mature? You can also measure and visualise pH and the ripening process using the Ripeness Indicator Tool on your Sectormentor web app.