In southern California where I live, work in the wine industry, and even take a stab at making my own wine, harvest seems to be coming in earlier and earlier. My family has been making home wine my entire life and most of my memories of harvest are centered squarely in fall. Of course it depends on what kind of wine you are making as to when the grapes are ready, white wines tend to come in off the vine much earlier in the season while something like big, dark Cabernet Sauvignon is going to come in close to the end. But I can remember getting grapes in mid November, wearing a coat and boots in our makeshift winery to help with punch downs and pressing. Now, its early September, in the high 90s in a very coastal town that is woefully ill equipped to deal with that kind of sustained heat, the locals are complaining, hardware stores are actually selling out of fans, sleep has become a difficult thing and the grapes are here. They are fermenting in days, up to temperature quickly since the surrounding air is just as hot, if not hotter than the heat their chemical reactions generate. I am now doing punch downs barefoot and in sun dresses and harvest is set to be pretty much finished, wine essentially made and in the barrel, by the end of this very hot September.
It’s possible that part of this is just me complaining, that living in a city that notoriously sticks to the mid 70s year round has spoiled me, and everyone else that lives here. And I will readily admit that is true, mid 90s and humid may not be the norm around here but it is certainly the weather a huge part of the country lives with a good portion of the year and I should probably just suck it up for these few, mildly miserable weeks. But aside from the complaining and the dreams of fall, actually sleeping under the covers, and wearing clothes with sleeves, these temperatures, though par for the course in many places, are simply not here. This heat wave and the early, fast harvest absolutely impacts the wine being made here. How exactly it is impacted isn’t really clear. It might not even be something good or something bad, but it is certainly something different.
But early or not, here is what is happening.
Up until now the whole focus has been on the vineyards, this is where that shifts. The moment the winemaker decides the grapes are ready to be picked many things happen all at once and everything happens fast. The grapes need to be picked quickly when they are ready. Once at the winery the grapes are, sometimes, sorted to remove any under ripe grapes, over ripe grapes, or any other debris that came in from the vineyard. Then the grapes are put through a crusher / de-stemmer to break open the skins of the grapes and remove the stems. Not all grapes have to be de-stemmed but most are. This mixture that comes from the crusher is a soupy mass of juice, skins, pulp, and seeds that is called the must. And this is the part of the process where red wines and white wines part ways.
For red wines the must is put into large containers or vats. Basically fermentation is the process of converting the natural sugars in the grape into alcohol. This process is facilitated by yeast. The grapes themselves have a thin coating of natural yeast that can start this process off; as well any space that has been an operational winery will have accumulated ambient yeasts from years of similar processes. Both of these yeasts would be able to start the fermentation process. But they are typically not that reliable and not very easy to control so most winemakers opt for adding in their own yeast. It is also important to note that different strains of yeast can illicit different flavors from the wine.
During fermentation, along with the alcohol both heat and carbon dioxide are produced. For fermentation to happen the temperature must rise to between 70° and 85°. Heat, along with the alcohol, helps to extract the color from the skins and tint the surrounding juice. However, the carbon dioxide, bubbling up, pushes all these skins to the top of the tank, forming a cap. Left alone this cap would just stay at the top and only the very top layer of the juice would be in contact with the skins. Therefore the cap is periodically broken down and re-emerged into the juice. This process is referred to as punching down and is usually done every few hours while the wine ferments. The more the juice can be in contact with the skins the better because this is where the wine receives most of its color, tannin, flavor and aroma.
White wines however are immediately pressed after they have been crushed and the juice is fermented without the skins. Instead of being placed in open top vats like red wines they are put into temperature controlled tanks so that they ferment at between 50°F and 65°F. With white wines the goal is not to extract color or tannin from the skins but to preserve the freshness and delicacy of the fruit, which is best achieved in a cool environment. However, some fuller bodied wines, most notably chardonnay, are barrel fermented. During this process the barrel is filled only ¾ of the way full to prevent the wine from foaming over during fermentation. During this type of fermentation the temperatures can rise to over 70°F and, in addition to the fruit flavors that develop, toasty, vanilla like flavors are pulled out of the wood.
During both process, after virtually all the sugar in the wine has been converted to alcohol the wine is considered to be dry and fermentation is over.