But you might of produced a wine that tastes like wet cardboard or, perhaps, cooked cabbage, again, the result of improper temperature control. The problem is that yeast produce heat during fermentation (sugar → alcohol). Without controlling this energy output, your ferment fail.
This chart will help you understand how heat impacts flavor, before and after. Temperature: The way the grape skin behave also means that reds and whites has different temperature needs. The higher fermentation temperature for reds extract color and tannins from the grapes’ skin, while the lower temp for whites is intended to retain the subtle fruity fragrances which would otherwise steam out during fermentation. You don’t want your Cabernet tasting like your Sauvignon Blanc, but neither do you want your white wine’s crisp lemony-citrus overtones lost by treating it like a big red. (The diagram makes clear that whites really need fifties while reds love the seventies.)
How Temperature Changes Wine Taste
Too hot, however, is the danger zone. In a small space, all of the heat that yeast gives off (it’s how they make their alcohol) builds up until the temperature gets well out of hand. A couple hours later it can be as much as ten degrees warmer then the air around it. At this point, when you hit sixty-fiveish on white wines and eighty-five-ish on reds, the yeast get stressed. Stressed yeast create fusel alcohols; flavors that smell like nail polish remover or some other solvent. They will also burn off those lovely aromas that you’re trying to retain. In summertime, active cooling methods is required to avoid bad-tasting wine, like an ice bath.
The other side of the equation. Cold temps. Presents another series of issues. Below forty-five degrees, yeast become dormant and stop working, but don’t die off yet. This causes what’s called a “stuck” fermentation. The sugar in your wine will not convert. This leads to a bacteria takeover, which can result in sweet, unstable wine destined for quick spoilage.
In order to restart a stalled-out cold ferment, it’s tough and typically requires warming the carboy slowly and introducing fresh yeast back into the mix. It helps if you can keep things consistent rather than aiming perfectly for each moment, as fluctuating temps is harder on the yeast than keeping something a little less-than-perfect-but-steady. If you don’t have a cellar with climate controls, it can be difficult to maintain temperatures in your home brewery as seasons change. The mild beginnings of spring quickly lead to unpredictable fall weather swings. Aggressive cooling tactics are needed in summer and heating wraps becomes necessary in winter to keep things alive.
At least you’ll have some number if you use a basic stick-on temperature strip. For the most accurate reading, digital probes inserted through the airlock provide internal temps. However, knowing the surrounding ambient temp will allow you to plan ahead and avoid making an expensive mistake down the road. Monitor this value twice daily when actively fermenting.
Your management of temperature depends on which yeast strain you choose as well. Certain strains is more tolerant of the cold than others. For example, Lalvin EC-1118 can handles high levels of alcohol and lower temperatures. It is a safe bet when making whites. Others love the warm environment because this brings out all their flavors (reds). Knowing which strain works best with your basement or garage’s climate reduces needless headache.
Growing wine is not just about growing, it’s about managing a biological ecosystem. So respect the biology. Watch the heat. Let the fruit shine. Those invisible degrees that you control while the bubbles come rising make all the difference between exceptional and mediocre. Warm up reds, keep whites cool, and see your craft evolve batch after batch.