3.1.1.c Monitoring the progression of technological maturity

Determining Potential Alcohol By Volume (PABV)

Measuring the sugar content of the grapes is essential because it directly determines the alcohol content of the future wine, and the latter is regulated by the specifications for each appellation. As ripening progresses, the berries become more concentrated in sugar. In the event of heavy rain, it is sometimes possible to observe a temporary decrease in sugar concentration due to dilution. The most commonly used tools to measure sugar levels are refractometers and densimeters. The results can be expressed in various measurement units depending on the instruments used. The most common units are density in g/l (usually called density even though it is not), Baumé degree and Brix degree (Balling). Once the result has been obtained, it has to be converted to the corresponding potential alcohol content (the official value approved by the EU is that 16.83g/L of sugar is required to form 1% by vol. of alcohol). Measurements can be taken during the véraison period and then as regularly as possible as maturity and the desired alcohol content approaches. Very high sugar concentration can have a negative impact on the alcoholic fermentation process. At maturity, sugar content varies between 160 and 250 g/l.

Total acidity (TA) and pH

The total acidity also known as titratable acidity is the sum of its titratable acidities when titrated to a pH of 7 by adding a standard alkaline solution. More simply, the total acidity is the total of all organic acids present in the must and wine. Following the total acidity is important because it determines the total amount of tartaric, malic and citric acid. Potentiometric and bromothymol blue titration are the official methods for quantifying total acidity.

It is essential to monitor the pH as the grapes begin to ripen. The pH increases during ripening and continues to increase during the winemaking process. A pH that is too high can lead to a higher microbiological risk as the environment is more favourable to microbial development. Furthermore, a high pH makes SO2 less effective. The official method of quantifying pH is a measurement of the potential difference between two electrodes immersed in the must. A pH meter is easy to use and portable. If the technician and/or the winemaker do not have this tool, oenological laboratories can take these measurements and provide them in a monitoring report.

Weight of 100 berries and skins/juice ratio

After a sample is taken, before any analysis that requires pressing or crushing, 100 or 200 berries must be weighed to establish a potential harvest weight and thus adjust the yield estimate. The weight of 100 berries enables the impact of rainfall and the water supply to be monitored. This measurement can be facilitated by using an abacus-type device with 100 places.

Malic acid

Malic acid is an organic acid, like tartaric acid. However, following véraison, malic acid does not behave at all like tartaric acid. Levels of tartaric acid undergo very small variations after véraison, but malic acid can break down and decrease significantly in certain weather conditions, particularly warm temperatures. Malic acid content drops rapidly after the véraison, reaching levels of 1 to 7 g/l in the must at maturity. The winegrower must therefore follow the breaking down of this acid closely during the ripening process for all types of red wine and certain white wines. Malic acid concentration will affect the malolactic fermentation. Generally speaking, MLF is facilitated between concentrations of 1.5 and 4 g/l of malic acid.

Measurements via an FTIR system are common because they are fast and do not require reagents, but enzymatic methods are also common for greater precision. When these tools are unavailable to the winemaker or technician, sending samples and/or musts to a laboratory is a reliable and fast alternative for monitoring malic acid during ripening.