Saccharomyces cerevisiae, bringing us good things to eat and drink for thousands of years.
Making a quality red wine, one with pleasing fruity aromas and tastes, and an excellent complexity and balanced structure is a long and arduous task. All kinds of things can go wrong between the time that the grapes begin to emerge on the vines and the bottle is uncorked.
Fungal infections, such as powdery mildew (oidium), black rot and a host of others, menace the grapes throughout the growing season. Then there’s the problem of not enough water, too much water, and even frozen water, in the form of hail, that can quickly destroy the grape harvest.
And the most insidious threat of all, the one that winemakers must battle from the moment the grapes begin to ferment to when the wine is put in the bottle, is too small to see. Microorganisms, the bacteria and yeast that give wine much of its complexity and structure, and that take simple grape juice and turn it into something noble, are at once an ally and a menace.
Get the microorganism balance right, and you can have a very special wine. Get it wrong, and, well, you have half the contents of a good salad dressing.
Before Louis Pasteur set the stage for modern biology and biochemistry by debunking the myth of spontaneous generation, most wine had to be drunk in the season that it was produced. Pasteur showed that fermentation was not, as widely believed, a chemical process, but that it was instead a biological process carried out by microorganisms.
And for a century and a half, winemakers relied on two laboratory tools that Pasteur would immediately recognize—the microscope and the Petri dish, to identify and control the microorganisms that inhabit wine during its fermentation and aging.
You can buy a decent microscope for not much more than the price of a bottle of Second-growth Bordeaux, and anyone who has gotten through Biology 101 is probably capable of identifying yeast or bacteria in a wine sample in less than ten minutes. The lower limit of detection with a microscope is around 2,000 cells/ml. Microbes involved in altering fermentations for the worst, such as acetic acid bacteria and Brettanomyces yeast with its animal-like aromas, are often present at much higher levels than this, so they’re easy to spot.
And wine-related microbes can be grown, using different growth media, in a simple Petri dish. The problem is that it takes almost a week, and in that time the wine might already have turned into vinegar. Another problem is that a quick visual examination of the colonies growing in the Petri dish only tells you that microbes are present. They still need to be identified, and there might be several-hundred microbe cells in one colony, while a similarly-sized colony might only have several dozen cells in it.
Another important microbe monitoring system is the winemaker’s nose. You’ll often find winemakers sniffing around aging barrels and the spouts of wine fermentation tanks, searching for acetic acid (vinegar) odors or the characteristically sweet smell of ethyl acetate (similar to glues or nail polish remover).
In the past ten years, various molecular procedures, using scientific technology popularized on television crime-scene investigation programs, have begun to be used to study microbiological populations during winemaking. They allow the winemaker to identify the genetic diversity of different yeast and bacteria without having to go through the isolation and culture steps used in visual identification methods.
The polymerase chain reaction (PCR) technique used to amplify a piece of DNA, generating thousands or millions of copies of a particular DNA sequence, is as useful in identifying yeast or bacteria as it is in catching a criminal. The method uses thermal cycling (repeating heating and cooling) to first split the double-stranded DNA, opening them into two pieces of single-stranded DNA, and then pairing and copying this template sequence of DNA into increasingly exponential copies of the original, double-stranded DNA.
Originally, this was a very expensive, time-consuming process. Dr. Jean-François Gilis, who works in R&D at the Vivelys Group, a global company that provides grape and wine analysis technology, showed my oenology class a new yeast and bacteria detector that automates this entire amplification and detection process. The system is based on the Cepheid SmartCycler, a real-time PCR tester that delivers accurate and consistent test results in 20-40 minutes.
The SmartCycler can detect as few as 10 cells/ml, and it also can detect viable but non-culturable (VNC) microbes that are undetectable using microscopic or plating method detection. Even though the system is mobile—it can be housed in a box the size of an airline carry-on bag, most winemakers won’t want to spend €30k to buy one.
Gilis told us that an analysis test costs around €30, so it would seem worth the expense to test wine during critical fermentation points, such as at the beginning of the fermentation process, when there is a wide variety of yeast strains present; at the end of fermentation when the Saccharomyces cerevisiae yeast population is about exhausted, but some residual sugar remains in the wine (creating a bacterial-infection risk); and just before the wine is put into bottles.
There are 16 laboratories in France offering this type of analysis, and I imagine that most winemaking regions around the world have the same sort of capabilities.
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