The heat is on!

With the heat waves we have been experiencing the past view weeks, everyone is asking how this will affect the harvest…when a sudden rise in temperature occurs the grapes are forced ripe and the sugar content rises.

In South Africa we typically harvest from February to April.

During this period, growers closely monitor the development of the grapes. At the most basic level, as the berries ripen, they become sweeter. The acidity levels decrease and the sugars, which will be fermented into alcohol, increase; the more sugar in the grapes, the higher the potential alcohol level of the wine. In addition, in red grapes, the skin color begins to intensify in darkness. Flavour compounds develop, showing more fruit and fewer vegetal characteristics, and the tannins—compounds found in skins, stems and seeds that contribute to texture and structure—soften.

The weather has a tremendous impact on how the grapes in a given year will behave in a bottle of wine. For starters, the ideal weather for growing grapes includes a winter that is cool with good moisture. However, once spring hits heavy moisture is “discouraged” and throughout the summer cool nights with moderate days is the goal. During the actual harvest, wineries are praying for dry weather to bring the grapes home.

Heat waves shorten the ripening time of the grape leading to pronounced sugar ripening rather than phenolic ripening (a sudden increase in heat causes the sugar levels in the grape to rise faster than usual). This results in lower acid levels and hence higher pH levels. On the other hand phenolic ripeness increases with more time on the vine. However grapes need to be picked at the correct degree balling (a measure of sugar levels in the grape) in order to ensure the consequent alcohol level is where it needs to be.

Acids are very important structural components of wine. If a wine is too low in acid, it tastes flat and dull. If a wine is too high in acid, it tastes too tart and sour. Usually, the winemaker can easily manipulate the acidity.

Grapes grown in warmer climates have lower acidity than grapes grown in cooler climates.

The warmer the climate the higher the sugar content of the grapes.

In summary, warmer climates result in high sugar and low acid whereas cooler climates result in low sugar and high acid.

Winemakers may test grape juice, from a sampling of grapes across different parcels of a vineyard, in a lab to check pH and Brix (a measurement of sugar) to help them determine how ripe the grapes are. But they’ll also head into the vineyards regularly—sometimes daily—to taste and examine the grapes in the weeks leading up to harvest. They’re checking for what's referred to as phenolic maturity or physiological ripeness—gauging the intensity and character of flavours and the quality of the tannins. They'll look at skin thickness, berry texture, seed colour and texture and whether the stems have turned from green to brown. Ultimately, winegrowers are seeking a good balance between the sugars, acidity, tannins and flavour compounds.

Wineries want to get the grapes to crush, where the grapes are not “smashed” but “gently split” so that the juice starts to flow, as quickly as possible. Great pains are taken to escalate the process, while keeping the grapes from becoming too warm during the transport from vineyard to the crusher. For example, many grape varietals are cut from the vine in the cool, early morning hours.

If you harvest too soon, you will probably end up getting a wine too low in alcohol content (there won't have been enough sugar to convert to alcohol). These wines will be "thin." If you delay harvest, there may be too much sugar, which leads to too low acid content. This also affects the taste (and the aging possibilities) of the wine.

Depending on the grape variety, region and wine style, the ripening process can last anywhere from 30 to 70 days. Some grapes, like Tempranillo, ripen quickly. Others, like Petit Verdot, ripen long after other varieties are being transformed into wine. As for regions, cooler climates, like Hermanus, typically have longer ripening cycles, while hotter ones, like Robertson, have shorter cycles.

As harvest nears, growers follow weather reports very carefully to stay ahead of sudden changes. Heat waves, excessive rain and even frost can ruin a crop. A year's worth of hard work can be wiped out by a hailstorm days before picking. Sometimes a grower is forced to pick early rather than risk losing his crop or needs to leave grapes hanging longer than expected to dry out and regain balance after heavy rain.

The date of harvest is rarely ever the same from one year to the next, so winegrowers must call it as best they can. Pick too early, and tannins may be "green," or bitter and underdeveloped. Pick too late and the sugar levels may get too high, resulting in a flabby, unbalanced wine.

Finally, the style of wine being made also influences the time of harvest. In sparkling wines, high acidity is desirable, so the harvest is early. In dessert wines, by contrast, it's the sugar that counts and, thus, a late harvest.