“Grapes are different from many other fruits in that they do not continue to ripen after being picked from the vine”
Unlike apples or bananas, you can’t let a bunch of grapes ripen in a cold room.
The moment of picking is therefore crucial to make a good wine. Too early and the wines will lack concentration, will have astringent tannins and will be too acidic; but picking too late will give wines that are too alcoholic and not enough acidity or tannic structure.
It is only by choosing the right day that the resulting wine will be perfectly balanced. In addition, it is very difficult to determine the maturity of a grape just by looking at it.
Each grape variety has its own ripening timescale and each wine-growing site has characteristics, such as water retention or tilt towards the sun, that affect this ripening. It is therefore important to get to know each vineyard parcel, through the measurement of maturity and historical harvest data.
We assess the maturity of the pulp by measuring the sugar content using a refractometer. It’s a smart but simple device that measures the refraction of light through grape juice. The more sugar there is in the juice, the more the light is bent. The refractometer translates this flexion on a scale which gives us the specific density of the juice.
There are several scales used to measure specific gravity (just like there are Celsius and Fahrenheit scales for temperature). In France, the Baumé scale is used, which has the advantage of giving a figure which coincides by coincidence with the potential alcohol in the juice. For example, a juice with a Baumé of 13 degrees will give a wine with 13% alcohol after fermentation.
We measure the acidity of the juice using two methods. The first is to use a pH meter, similar to those used to measure the pH of a swimming pool. White wines should have a pH between 3.1 and 3.4 and red wines between 3.5 and 4.0. Lower pH means higher acidity.
The second method is to titrate the juice against a known concentration of sodium hydroxide. I’m sure we can all remember the headlines from chemistry lessons in school. We add a few drops of indicator to 10 ml of juice and measure the amount of sodium hydroxide needed to change the color of the indicator. This tells us the amount of acid in the wine in terms of grams / liter and is known as titratable acidity (TA).
The amount of acid is not the same as the pH, so knowing both helps the winemaker see how the acid in the juice changes and possibly how acidic the wine will taste.
To measure phenolic maturity, most winegrowers trust their own taste buds by chewing the skins of grapes as they take samples. However, sophisticated laboratory equipment can provide numbers to gauge the amount and type of tannins in juice (and several other things as well), for winemakers who know how to interpret the results.
However, no vineyard is homogeneous. Different vines will have clusters of varying maturity and the grapes in that cluster will also be different. In general, the more clusters a vine grows, the more variation there will be. Winegrowers prune their vines in winter and perhaps remove excess fruit in summer to try to manage variation while still having a profitable yield.
Terroir also affects variation as fertile soils and more rainfall will result in more and larger bunches of grapes. This is why historically the best wines come from terroirs that limit the yield, instead of producing the most fruit.
“We cannot know the maturity of the whole vineyard until we have picked the grapes”
You have to take samples to see how the vineyard is maturing and predict when it will be at its peak. Each year, the harvest date will change depending on the weather.
We can get a rough idea of its precocity or its delay between the flowering date and the veraison date (when the grapes change color). When we are less than two weeks away from the planned harvest, we start taking samples from the vines on a regular basis. I take samples from my vines every three days.
There are several sampling methods that aim to give a good representation of the entire vineyard from a small number of samples. I am a method that consists of walking through a few rows of vines, stopping every 20 steps to pick only five grapes from a cluster that seems typical of this vine. Once I have about 100 grapes I use them to make a juice sample.
By looking at the rate of change in sugar and acidity, and comparing them to previous vintages, I can predict when the grapes will be perfect. However, temperature and precipitation will distort ripening. Warmer days accelerate it. The rain will initially dilute the berries, causing both sugar and acidity levels to drop. A few days after the rain, the vines generally benefit from a boost in ripening.
Not only do we have to take the weather into account when estimating the ripening rate, but we also need to look at the forecast to make sure that there will be no rain on the day we want to pick. Picking in the rain will dilute the juice quite a bit. Picking after a rain will usually allow the vine to absorb water into the grapes, balancing maturity.
However, too much rain will swell and dilute the grapes and, if persistent, lead to botrytis rot. Heavy rains during the harvest can therefore lead to a bad vintage for this region because the winegrowers were forced either to pick too early to avoid the rain, or to pick diluted and possibly rotten fruit.
There are ways to compensate for not picking at optimum maturity. In the north of France, cellars are allowed to add a certain amount of sugar to the juice (chaptalisation) while in southern regions they are allowed to add tartaric acid if the grapes are overripe. The acidity can be reduced by adding calcium or potassium carbonate.
Adding water to juice or wine is prohibited in France (although perfectly legal and often practiced in the New World). However, none of these practices makes it possible to obtain a wine as good as a wine made from grapes picked at perfect maturity.
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