Yeast are very complex little creatures. They're a single celled fungus, and the type that is primarily responsible for alcoholic fermentation is called saccharomyces cerevisiea. The inner workings of a yeast cell are much like those of our own cells, with similar internal structures that have great names like mitochondria, golgi bodies, and endoplasmic reticula. They really aren't that different than we are on a cellular level. The major differences are the way in which they get energy from their food source, and what they produce as a result. In a very basic sense, fermentation is how yeast get energy to live.
As important as grapes are themselves in wine aroma, the bulk of the aromas in wine arise through the fermentation process, that is to say, yeast take what is present in the grapes and convert it into aroma and flavor.
The major aromatic compounds that come from yeast action are organic acids, higher alcohols, esters and aldehydes. As yeasts go about the business of producing alcohol, it's not a linear process of sugar in alcohol out-- there are a number of steps that the sugar goes through and at each step an intermediary is produced. Some of these go on to become aroma compounds rather than alcohol. In addition, sometimes the alcohol reacts with one of the intermediary products to form an aroma compound. It's all part of a living system, and living systems tend to be pretty complex.
The major compounds that yeast are resposible in wine are volatile fatty acids, higher alcohols, esters, and sulfur compounds. With many of these compounds, the effect of their presence can be either pleasant or unpleasant depending quite a bit upon their concentration in the wine.
Volatile fatty acids include compounds like acetic acid, which is essentially vinegar, and octanoic acid, which can taste sweet and buttery, and they are the products of two enzyme systems in yeast. They can range from oily to rancid to sweet and back again.
Higher alcohols are formed through the interaction of amino acids and sugars, and can account for such aromas as marzipan, fresh grass, and flowers.
You may remember from our discussion on grapes that esters are responsible for general fruity aromas in wine. Yeast produce these through sugar byproducts-- from a reaction between alcohol and a compound called acetyl-CoA, or through an interaction between amino acids and alcohol. They can give the wine aromas like banana, violet, and pear.
Unfortunately, yeast also can produce sulfur compounds that can add an unsavory note to wine. If you've ever had a wine that smelled like rotten eggs, burnt ruber, onion, or boiled cabbage, you know what I'm talking about. Those are yeast-produced aromas, and they're all sulfur compounds. Luckily, properly managing the fermentation process can ensure that yeast don't produce these in levels that are detectable.
This is obviously a very compressed version of how yeast contribute to the aromas in your glass of wine, but barring getting too deeply into metabolism, I hope it has helped illuminate the important role that yeast have to play not only in producing the alcohol in wine, but in the development of the aroma and flavor itself.
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