Around late August in the Northern Hemisphere, grapes start to make their way into cellars. If they’re white, they start to be pressed, or if they’re red they start to be loaded into tanks to macerate. Thus begins a long series of steps that will turn juice into wine. There will be hoses and pumps, transferring liquids from one tank to the next. There will be pieds de cuve, fermentation starter packs full of nutrients and yeast. There will be big plungers, used to punch down the floating skins in a red wine tank in order to extract more color pigments, tannins, and aromas into the fermenting wine. And before winter, the reds will be put in barrels, ready to go into the long stretch of hibernation we call “aging.”
All of this might lead to good vintage. But that is dependent on one one factor alone: the grapes. A good vintage means, quite simply, good grapes.
As one winemaker from Alsace told me this fall, “If the grapes are grown well, my job in the cellar is trivial.” Sharing his sentiments is Stéphane Ogier of the Northern Rhône’s Côte-Rôtie. “The wine is made in the vineyard. We can do nothing without this basis.” These winemakers understand that the output is only as good as the input; no amount of Mega Purple or chaptalization can make low quality grapes into high quality wine.
What drives high quality grapes?
Certainly, the grape variety must be a good fit for its soil and climate. If the variety tends to be very vigorous, it should be placed in low fertility soils to balance this vigor. If it tends to have very low vigor, it should be placed in nutrient rich soils to give the vine enough energy to produce high quality fruit. Concerning climate, the goal should always be for the grapes to achieve ripeness as late as possible in the growing season in order to achieve varietal aroma development. For instance, if a grape tends to ripen quickly, such as Chardonnay or Pinot Noir, it should be planted in a cool climate in order to try to draw out the ripening period and make sure those secondary metabolites are developed.
Assuming that the plant material is well chosen for the climate and soil, grape quality variation is mostly influenced by two different effects: the vintage effect and the terroir effect. The vintage effect is the variation in the quality and typicity of a wine from the same vineyard year after year due to weather. The terroir effect is the variation in the quality and typicity of wines based on where they are grown (i.e. according to their terroir).
The Vintage Effect
When considering the same wine producer who is growing grapes on the same vineyard year after year, the variation in vintages will be predominantly from the weather. Here are the five main weather parameters that influence the quality of grapes, and thus the finished wine.
Spring frost is a major determinant of the quality of the season’s harvest. If frost occurs after budburst, it can damage the tissue of the developing bud. This can make the buds unviable, meaning that they won’t continue to develop. If this is the case, sometimes the secondary buds will develop. These buds will produce only about 25% of the fruit that the primary buds would have produced, and the fruit will be of lower quality. But remember, as I wrote in my blog on the fundamentals of vineyard science, these buds were intended to enter the game in the following growing season. So to top it off, spring frost could also impact the next vintage because the current vintage is “stealing” the buds intended to be developed for the next year.
In the early summer period between flowering and fruit set, it is crucial that the vine get enough water. This is because water stress at this stage can lead to poor fruit development, lowering the harvest quantity and quality.
But a little water deficit around the period of véraison, when the berry skin starts to change color, can actually be a good thing, since this stress signals to the vine that it should work on creating ripe, attractive berries for seed dispersers if it wants to spread its genes. This accelerates its sugar accumulation and switches on the grape’s secondary metabolism, which produces those key elements to wine quality, like phenolics (tannins, anthocyanins, and flavonoids) and aroma precursors. However, it’s important to note that summer water stress is, in general, more tolerated by red varieties than by white varieties.
Once the berry starts to ripen, it’s important for it to be dry. We want a dry harvest — not only because picking in the rain is no fun — but because mold at this stage in the game is devastating to wine quality.
Also, while irrigation can be used in some countries, like the US, it is forbidden in other countries, like France. Thus, the timing of rain is extra important where irrigation is unpermitted.
Temperature is another key factor that leads to a good vintage. In the spring, the air temperatures around the vines need to remain around 10°C for three days in a row in order for budburst to occur. After budburst, it’s extremely important that the temperatures don’t drop below freezing (see point 1 on spring frost).
Grapes grow in regions around the world whose growing season temperatures are between 13–21°C (55–70°F). Prolonged temperatures above 30°C (86°F) are problematic, and photosynthesis can stop all together when temperatures exceed 35°C (95°F). If photosynthesis is reduced or halted, this would drastically reduce the sugar accumulation in the berries, leading to an unripened, withered grape.
Beyond sugar accumulation, it can impact the development of phenolic compounds responsible for color pigmentation in red grapes. One study has shown that high temperatures around 35°C, applied one week before veraison, reduced anthocyanin concentrations in the skin of the Cabernet Sauvignon to less than half of what accumulated at 25°C.
And in general, higher temperatures lead to more jammy, cooked fruit aromas and flavors rather than herbal or vegetal aromas. One study tried to mimic what Bordeaux wines would taste like in 2050 due to higher temperatures driven by climate change. It found that the wine was denser and more bitter, with overripe fruit aromas.
Sunlight also drives good vintages. The right amount of sunlight is crucial for the sensory profiles of the berries, and thus the finished wines. One study showed that shaded Syrah grapes made wines of a less intense color, lower flavor compounds, and lower phenolics, leading to a less tannic and less fruity wine.
For white varieties, higher exposure to sunlight can actually induce a natural sunscreen, carotenoids, which break down into volatile aromas. In Riesling, for example, higher sunlight in the vineyard can lead to the breakdown of carotenoids into norisoprenoids, leading to a stronger petroleum smell. In moderation, this odor is a desirable trait of Riesling, but in excess, it can lead to an unpleasant medicinal smell.
It’s also been shown that for Sauvignon Blanc berries, higher sunlight exposure before véraison can lead to lower levels of methoxypyrazines, the family of aromas responsible for the green bell pepper or pea pod smell. In other words, shaded berries lead to a wine marked by vegetal aromas.
A bad vintage could also be due to crop damage brought on by hail. Summertime hail is caused by drafts of wind that push raindrops higher into the atmosphere where temperatures are colder, causing the rain to freeze and drop as hail. For grapevines, hail is disastrous. In moderate hail storms, it can damage the fruit, leading to bruising or open wounds that are more susceptible to disease and pathogen infestation. In extreme hail events, the vines can be totally stripped of their fruit or flowers, leading to a lower harvest and loss of money. Nets can be used to cover vines and prevent hail damage, but these nets can be costly and time consuming to put up.
While the winemaking surely plays into the final product, what happens in the vineyard limits the winemaking’s effect. The best winemaker can do little in the cellar to turn low quality grapes into high quality wine. There are many vineyard practices that can lead to higher quality grapes, but sometimes, a little bit of luck is needed when facing Mother Nature and her weather whims.