Fermentation Process In Making Wines and Alcoholic Beverages
This process, whereby grape juice – or other juices containing certain yeasts – are converted into an alcoholic beverage, is both complex and simple. Those who are able to understand a little chemistry are advised to refer to one of the specialised studies of the subject. But for those who cannot, the following gives a basic guide to what goes on.
It was Louis Pasteur who was mainly responsible for the first exact studies of fermentation. Grape juice, left alone, ferments naturally, up to the stage that it reaches about 14% of alcohol, when it stops. Extremes of heat and cold can. Throughout, stop the wine yeasts working, just as yeasts in breadmaking will be stopped if the temperature of the kitchen is suddenly raised or lowered. Yeast varies according to different places, and there are different types of yeast. Certain wines, such as port, have their fermentation arrested by the addition of alcohol at a precise point – this also stops the yeasts working. But yeasts cannot work if they are weak or if there is insufficient sugar in the must on which they, or rather the enzymes they excrete, can act. This is why a certain amount of sun and warmth is important in regions where wine is to be made, as sun will make grapes fairly high in sugar whereas cold wi 11 make them too acid. Rain will also simply wash the bloom – which holds the yeast – off the grapes, so that fermentation in a wet year will be difficult. If fermentation is interrupted by some sudden climatic change, then it has to be started again and the result, in terms of wine, is seldom fine quality; more yeast may be added if the conditions are very difficult, or if existing strains are too weak.
Most wines have two fermentations or, one might say, their fermentation takes place in two stages. The first, usually tumultuous and rapid, takes place immediately the yeasts begin to work in the fresh must at vintage time. The fermentation slows down and then may cease as the weather gets cooler. The secondary fermentation may take place soon after the first – in some instances this is desirable, so that the wine may be completely ‘made’ as soon as reasonably possible – or else there may be a pause during the cool weather, and the secondary fermentation takes place in the spring of the year after the vintage. With certain sparkling wines, such as Champagne, the second fermentation in the spring is deliberately brought on so that the carbonic acid, which is the gas that creates the bubbles, may be controlled and contained within the bottle. In the ordinary way this gas is given off; which is why one is advised not to lean over a vat where wine is fermenting because of the danger of being overwhelmed by this happening. Breathing in the carbon dioxide can, at best, make one ill.
The exact type of fermentation will vary according to each type of wine and the year in which it is made. But while wines are in full fermentation, or are ‘working’, they cannot be tasted accurately and should not be drunk – as they then continue to ferment inside the drinker. If in doubt, a wine that tastes vaguely ‘beery’ or lacks its usual smell, and is vaguely fizzy in the mouth, should be rejected as fermenting; although on occasions people at tastings have been known to pour lavish praise on a wine in this condition! A very little petillance or a spritzig trait, however, can be a pleasant asset, indicating that a wine is young, vigorous and vivacious. Many great white wines are charming in this state, once they have been fully made, and there are some wines which are specifically made to accentuate and retain this type of secondary fermentation, such as the vinfou of the Jura, and vinho verde of north Portugal.
A wine may also suddenly show signs of fermenting again long after it should normally have ceased to do so. This is a matter for the specialist to deal with. And can be attributed to a number of causes. Anyone sure that this is what an otherwise matured wine is doing should ask advice and, if possible, return the wine as soon as may be. In its sealed bottle, to the source of supply. But one should not confuse the curious way in which many fine wines tend to go slightly out of condition at two periods of the year, with fermentation. In the spring at the flowering of the vine, and in the autumn at the vintage, the wine in the bottle may show that it is still. Mysteriously, linked to its sourceof life, the vine; and for a little while it may lose its bouquet and much of its taste and be at less than its best. This is not a type of fermentation.