Yeasts are fungi, which play a vital role in wine production. The number of different species runs into tens or even hundreds of thousands, no one is quite sure. More than one may be used in fermenting the grapes, and this has a big influence on the wine produced. Yeasts produce aromatic compounds as well as alcohol and carbon dioxide.
What yeasts need to make them work
Yeasts need the right conditions to work their magic during wine fermentation:-
Temperature. They operate in a temperature range. Too low, and yeasts don’t function, too high and they are killed off. The grape juice may be actively heated or cooled to achieve this.
Nutrients.Sugar is the main one, but you need a source of nitrogen, which is provided by amino acids. Winemakers may add selected nutrients.
Alcohol level. Different yeasts work in different ranges of alcohol content.
Winemakers split yeasts into two main categories
‘Natural’ or ‘wild’ yeasts. These are the yeasts which are present in the vineyard, and stick to the grape skins. They are also on the wine equipment. These indigenous yeasts give the wine a local character, a sense of place. The term ‘terroir’ is often used to express this.
Cultured yeasts. These are developed to introduce particular characteristics to the wine, or to make fermentation more reliable. Usually, the indigenous bugs will be killed off with sulphur dioxide, and then the grape must/juice will be inoculated with the cultured yeast.
What can go wrong with a ‘wild yeast fermentation’
Problems with starting the fermentation. The grape must/juice will contain a range of bacteria as well as yeasts, and these compete for the nutrients. You may end up with acetic acid (vinegar) rather than alcohol.
Inconsistency. A number of different yeasts may be involved during a fermentation, with some dying off at quite low alcohol levels, and others taking over. If a different yeast takes over, you can end up with a different wine than you anticipated.
There is no right or wrong answer about the use of wild yeasts, it rather depends on the market you are aiming at. Cultured yeasts will help to produce a consistent product, which is going to appeal to the big brands. Small producers may well enjoy the variability.
Sparkling wines are more popular than ever, as they are lively and refreshing. How are they made, and what produces the bubbles?
Champagne cork and muzzle
Where do the bubbles come from?
Sparkling wines have carbon dioxide dissolved under pressure . When the bottle is opened, the pressure is released and the gas starts to evaporate. Bubbles form on irregularities on the glass, or material suspended in the wine.
What if there are no bubbles?
The main reason for bubble-free sparkling wine is dirty glasses. There is nowhere for the bubbles to nucleate.
What is the gas pressure within a bottle?
Champagne usually has a pressure of around 6 bar – 6 x atmospheric pressure – at room temperature. Which is why you get a mess if you open a warm bottle. The pressure decreases rapidly as the wine is chilled, which is one very good reason for serving cold Champagne. Other sparkling wines have a similar, or lower pressure.
Where does the carbon dioxide come from?
When sugar ferments in the presence of yeast, it produces alcohol and carbon dioxide. The more sugar you have, the more alcohol and carbon dioxide is produced.
In most sparkling wine production, grapes are fermented to a still wine. Sugar and yeast are added, and the second fermentation produces carbon dioxide for the sparkle.
In some rare cases there is just one fermentation, and the carbon dioxide retained, rather than allowed to evaporate.
The cheapest way of introducing the gas is to inject it into still wine, but this produces a coarse result.
Stopper to keep the fizz in a bottle
Producing bubbles using a second fermentation
Starting with a still wine, there are three main methods for producing the bubbles:-
The method used in Champagne, now generally known as the ‘traditional method’. Wine is put into a bottle, along with sugar, yeast and nutrients. This second fermentation produces carbon dioxide. The debris is removed, and the wine sold in the same bottle.
The transfer method, in which secondary fermentation is in a bottle, but the wine is then transferred to a vat, where the debris is filtered out. The wine is sold in a different bottle.
The Charmat or Tank Method, where the second fermentation is carried out in tanks. The wine is then filtered and bottled under pressure.
The quality factor
Quality sparkling wines undergo a number of processes and maturation to give the same depth and complexity as a fine still wine, but they also have the prickle of bursting bubbles on the tongue.