My wine warmed up in shipping. Is it still OK?

Like most wineries, with our tasting room closed the last two months we've seen a surge in phone and e-commerce wine orders. Some of these customers are regulars in ordering wine, but we know that for a lot of them, this is a new thing. For the first six weeks of the quarantine, we benefited from mostly cool weather around the country, but the last two weeks have warmed up and we've begun our annual weather watch to make sure that our shipments to you go out and arrive when the weather is as cool as possible.

Unfortunately, summer is coming, and there will soon be swathes of the country that will be hot for months. Does this mean you can't order wine? And while it always ships out cool, if it does warm up in transit, is it damaged? I've received a handful of questions about this in the past couple of weeks, and, following my own advice from 13 years ago, it seems like that means it's a good subject for a blog.

First, it's important to know what is happening when a wine warms up, what the warning signs are that your wine might be affected, and if so, what your options are. I'll take those in turn.

What happens chemically as wine warms up
The short answer is not much... unless oxygen gets into the bottle. And then, look out. In general, a wine bottle is a sealed environment, and unless that seal is compromised (see below) the difference between what goes on chemically at 55°F and 90°F is pretty minimal. Long-term, the storage of wine at higher temperatures is likely to age a wine more quickly, as the slow, gradual chemical processes that break down a wine's tannic structure and fruit, while producing deeper, earthier flavors speed up like most temperature-sensitive chemical reactions. But wine often gets up in the 80°F-90°F range during fermentation without negative effects, and over the course of a few hours, unless it gets really hot, your concerns shouldn't be the chemistry as much as the physics. Read on.

What happens physically as wine warms up
Wine is a liquid, typically about 85% water, 13% ethanol, and 2% all the other things that give it color and flavor. That means that physically, it acts more or less like water. And water, like most substances, expands as it gets warm. Unlike most substances, it also expands when it gets really cold (below 4°C) and expands significantly when it freezes (as everyone knows if they've forgotten a bottle of wine in the freezer and had it break). The graph below (from Wikipedia, by Klaus-Dieter Keller) shows the density of water at various temperatures.

Of relevance for your wine in transit (hopefully) is what's going on between normal storage temperature (say, 55°F/13°C) and what might happen if a package were riding around on a hot day and got up to 95°F/35°C. You can see that the density of water would change, on a sliding scale, from about .999 kg/L to about .994 kg/L. That may not sound like much, but it results in an increase in volume of about half a percent. That changes 750ml of wine to 753.75ml. Since the head space (the space that's filled with air between the cork and the liquid) is typically between 4ml and 7ml, the expansion of the liquid would pressurize that air. Although air is quite compressible, eventually it will seek to escape, and there's only one way out of a bottle: past the cork, either by pushing the cork out the neck of the bottle, or by squeezing wine (or air) between the cork and the bottle. Neither is good news, because in either case the cork's seal has been compromised.

Know the warning signs for overheated wine
There are two things to look for. Either will let you know you have a problem. The first is to see if you can find evidence of any wine that has escaped past the cork. Typically, that will leave a sticky residue on the outside of the neck of the bottle. You may also be able to smell the wine. Once this has happened, the trail of wine on the outside of the cork provides a pathway for oxygen that's outside the bottle to make its way into the bottle at a rate far faster than in an uncompromised seal. Once oxidation starts, the clock is ticking. It's usually not hard to miss, though white wines can be a little less conspicuous:

No seeping wine? That's good. The next thing to check is to see whether the expansion of the liquid has pushed the cork up into the capsule. That's not as bad as seepage, but it's not good. Typically it does weaken the seal of the cork, and often wine has seeped up around the cork even if it hasn't made its way all the way outside the bottle. Plus, if the air in the bottle gets pushed out past the cork by expanding wine, once the liquid cools, its contraction creates a vacuum inside the bottle, which tends to suck air back in past the cork. That introduces new oxygen (not good) and further compromises the seal (also not good). In the below photo, the foreground cork has pushed enough to make me suspicious, while the one in the background is clearly concave:

What to do next
If your wine arrives with signs of heat damage (pushed corks or leaking wine) snap a photo and let whoever sent it to you know as soon as you can. Most wineries and wine shops will replace it, no questions asked. Then stash the wines in your fridge and plan to consume the bottles in the next few days. Even if the oxidation process has begun, it's a temperature-sensitive reaction and will be slowed by cooler temperatures (this is why it's always smart to put an opened but unfinished bottle in the fridge to preserve it). And unless it was hot for a long time, the wine will probably be fine to drink in the short term (like a week or two). But at this point, it's not a candidate for aging.

If your wine arrives warm but you don't see any signs of seepage or pushed cork, you're probably OK. Get that wine cooled down as soon as you can, because the seeping could be happening invisibly, and the sooner you reduce the wine's volume the better. When you eventually do open it, the thing to look for is notes of oxidation. This article on Wine Folly does a great job of describing the symptoms: browning of red wines or darkening of whites, and aromas more like sherry or nuts. If you find that, don't feel bad about requesting a replacement. Even without visible symptoms, if a cork's seal is compromised and has been leaking oxygen for months or years, the wine had no shot.