How to Understand Wine
How to Understand Wine: The Science of Yeast
What is yeast? Where does it come from? And why does it make a difference?
Alcoholic Fermentation
Wherever there is fermentation, there is yeast. Without yeast, there is no wine. Grapes have been grown in the wild for many years, but it was by chance in the Neolithic period that a yeast living on the grapes was activated around people, fermenting into an alcoholic drink which humans consumed for the first time.
Discovery of the Process
The human manipulation of yeasts to ferment grapes, wheats and other substances has been recorded as early as 7,000BC in Jiahu, China in the East and 6,000BC in Georgia in the West. Wine made from grapes was the first controlled fermentative process as grapes are one of the few fruits where yeasts live naturally on the skins of grapes (proven by Louis Pasteur). Yeasts are a type of fungal microorganism, and are naturally spread and carried by bees and wasps. They are the key catalyst in any fermentative reaction. Later on, the yeasts living on grapes meant that grapes themselves were added to mixtures of fruits, berries, honey and tree resins in order to kick-start the fermentation process.
When early man stored grapes in barrels or amphorae, the pressure and weight of the grape skins above would eventually split the grapes below. This causes the juices from the inside of the grapes to mingle with the yeasts on the skins. If the pile is heavy enough, oxygen is blocked from the grapes (which would hinder a fermentation) and enough grapes split that fermentation will occur. This is, in part, why foot-mashing grapes became popular later on. Foot-mashing causes the skins of grapes to break relatively gently. With higher pressures, the grape pips may be squashed, which releases a bitter and unpleasant juice.
Science of the Process
The juices mingling with the yeast, saccharomyces cerevisiae, cause the sugars in the grapes to come into contact with yeast. The yeast splits the initial long-chain sugars from disaccharides into monosaccharides…
…and then converts the short-chain monosaccharide glucose molecules to form ethanol (otherwise known as common alcohol) and carbon dioxide, which bubbles up to the surface. Elsewhere in wine, carbon dioxide is the gas trapped as bubbles in champagne (which happens as a bottle fermentation).
This fermentative process typically takes approximately 2 weeks, but during those two weeks the juices bubble furiously, with a thick layer of skins pressed up to the surface by the gases and heat coming off of the fermenting mix. At some point, the bubbling yeast was skimmed off the top and stored separately to enable man to make a whole host of other foods without grapes, such as beer and bread. At the end of two weeks, the skins will sink to the bottom of the juice which is now alcoholic and lies still.
Anaerobic Fermentation
The plug of skins prevents any oxygen reaching the juice below, making the fermentation occur without air (“anaerobic fermentation”). Anaerobic fermentation stops the juices from oxidising into something resembling vinegar and preserves the unspoilt taste and character of the grape. Pasteur found that inhibiting air also prevented the emergence of an acid byproduct, lactic acid, which is also the acid in gone-off milk.
In the New World during the C19th, newer techniques were developed which refined anaerobic fermentation, and preventing wine from encountering oxygen at almost every stage became the new goal. The process starts with winemakers allowing wines to sit in the open air for a few days while the liquid is being prepared for the fermentation. The short exposure to air encourages rapid reproduction and growth in populations of yeast. However, the steps following this process are strictly anaerobic so as to encourage ethanol production. Too much oxygen causes the ethanol formed to transform (“oxidise”) into acetaldehyde, or even further into acetic acid (also known as vinegar).
Acetaldehyde destroys the fruit character of wine and can make it smell stale, giving a brown tinge to the wine. To further prevent this, the restriction of oxygen is compounded by new equipment: copper vats are no longer used, as copper itself can help catalyse the oxidative breakdown of ethanol into acetaldehyde.
Yeasts
As for yeast itself — all yeasts used in wine are sub-species of that same saccharomyces cerevisiae that naturally occurs on grape skins. However, it has been discovered that different yeast strains have different characteristics and can impact taste differently, such as a sub-strain called Brettanomyces, or “Brett”. Brett is often said to carry the taste of mouse urine.
Winemakers now fear that by using their natural, unique yeasts found on their grapeskins they may end up with a case of Brett or worse, ending up with a spoiled batch. To that end, most winemakers will now gas the grapes before adding a lab-grown, bought commercial yeast. These yeasts are specifically chosen for factors such as stability, metabolic speeds and temperature tolerance.
