Hooo boy – here’s a goody. It is a fact that some wines do show a hot finish; no question. The question is, why. Most people assume the answer is because the alcohol content is high. To a first approximation this is a faulty assumption.

You can do the experiment yourself at home. Go to your wine store and buy five or six different bottles of, say, Chardonnay from different producers, but all with “high” alcohol contents, say, between 14.8% and 15.2%. Taste them blind. I guarantee some will be “hotter” than others.

“Alcohol” in wine is predominantly ethanol: the 2-carbon alcohol which is the primary by-product of the anaerobic metabolism of sugar by yeast. Pure ethanol does not taste “hot” – though it is astringent on the palate (if you were to drink ultra-pure ethanol your mouth would feel dry inside). However, the “alcohol content” of a wine may also include some isomers of higher alcohols – alcohols with 3 carbons or more in their structures – which DO taste very hot. And some of the oxidation products of alcohols (ketones) taste even hotter. In fact, it is the presence of very small quantities of higher alcohols and ketones – which are called “congeners” in distilled spirits – that make a wine taste hot.

So you are wondering why your hot wine has congeners in it. The answer is pretty simple – stressed fermentations. When yeasts are stressed, they start to pump out all sorts of junk, some of which are congeners. Yeasts are neurotic, metabolically speaking – their normal metabolism gets stressed by a whole laundry list of things: extremes of temperature, low nutrients, high sugar, high alcohol, competition with other micro-organisms (including other yeast), natural and man-made toxins, and more, ranging from the increasingly esoteric to the downright speculative.

So here is the indirect link between high alcohol and hot taste: yeast gets stressed out at the beginning of fermentation if the grapes are very ripe (cell biologists call this “substrate inhibition”), and then again at the end of fermentation by the high alcohol produced from high sugar levels (two factors here: end-product inhibition and cell membrane solubilization). Hurt at the beginning and hurt at the end. Double whammy. And if the fermentation sticks (stops before all the sugar is used up) the yeast used to restart the fermentation are stressed from the get-go – leading to a congener production trifecta.

Heaven help us if bacteria start growing at this point. Bacterial growth will further stress the yeast (quadruple toe loop) and the bacteria themselves are capable of churning out all sorts of crud – the very infernal quintessence.

I’m not saying this happens with every high-sugar fermentation, and I’ve already said that not every high-alcohol wine is loaded with congeners. Savvy winemakers can minimize fermentation problems by harvesting before the fruit is over-ripe (or artfully applying the garden hose if the sugar is really high), by inhibiting growth of spoilage organisms with sulfur dioxide at the crusher, by selecting sugar- and alcohol-tolerant yeast, adding vitamins, nutrients and yeast extracts to juice, controlling fermentation temperatures, adding oxygen, and waiting to inoculate with bacteria for malolactic fermentation until after all the sugar is gone.

But there are any number of winemakers out there who are cripplingly limited by their own philosophy. They must wait until the grapes are at 29° Brix to get the flavors they want, and would never use a garden hose. They won’t, or can’t (as in “organic” wine production) add anything to the juice. They live by the cult of “native” fermentation. Or some believe that they have to inoculate for malolactic before the end of primary fermentation to “get it done”.

I’m not being judgy and saying that any of these things is inherently bad, or good. What is certain is that these philosophical predilections can, and frequently do, result in wines with a “hotter” finish.

This blog was originally published July 2006 in John Kelly’s blog: “notes from the winemaker.