Measuring grape sugar
Almost all winemakers use a refractometer to help them determine when to harvest. There is a direct correlation between the amount of sugar present and the ability to make wine.
A refractometer is easy to use out in the vineyard and allows the winemaker to assess the ripeness of fruit. With most refractometers you must first calibrate the eyepiece, it’s super easy to do – instructions here. You can either use a refractometer that gives you an Oeschle reading or a Brix reading. Both scales indicate the sugar levels in the grape.
A graph of grape sugar over time is hugely useful as ripening is not linear. Of course weather affects ripening, but you can still see trends like that some varieties ripen faster but then sugar levels jump around so there is only limited benefit from leaving them on the vines for longer.
Grape Sugar:Titratable Acid (TA)
Acids give crispness, brightness and thirst-quenching qualities to wines and are essential components to balance a good wine. 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?
https://ohioline.osu.edu/factsheet/HYG-1434-11 — really good resource with graphs