Yeasts are single celled fungi of the class Ascomycete.There are about 15 species of yeasts naturally occurring on grape skins, but most of these cannot survive in must as they are sensitive to ethanol and sulphur dioxide and require high levels of oxygen to grow. However, some of these, such as Kloeckera apiculata and Hanseniaspora uvarum can start the alcoholic fermentation off, but these are quickly dominated by the Saccharomyces yeasts, particularly S. cerevisiae var cerevisiae and S. cerevisiae var bayanus.

The development of yeasts during fermentation will normally go in four phases:

• Lag period – yeast acclimatises to the must (sugar concentration, SO₂, pH etc).
• Exponential growth – yeast multiple, and 50-70% of sugar converted in short time.
• Stationary phase – yeast stop multiplying, nutrients reduced, yeast inhibited by alcohol.
• Decline phase – severe nutrient and sugar shortage; yeasts ‘poisoned’ by high levels of alcohol.

Yeast selection

Formerly, winemakers relied on wild yeast and the practise of making yeast starters from juice (sulphited to eliminate wild yeast and promote wine yeast growth) which were inoculated into the must after several days of spontaneous fermentation.

Today, the majority of wineries use Active Dry Yeast (ADY) made from pure cultures of natural strains selected for specific attributes. The benefits of using ADY are:

• Predictable and controllable fermentations
• Rapid fermentation onset
• Ferment highly clarified juice more easily
• Absence of undesirable flavours and aromas (e.g. hydrogen sulphide production)
• Efficient sugar conversion to alcohol
• Decrease in the risk of stuck fermentations
• Tolerance to higher sugar levels (e.g. S. cerevisiae var bayanus)
• Moderate glycerol production levels
• Low volatile acidity (acetic acid) production
• Predictable flocculation character (especially for sparkling wines)
• Flavour ‘enhancement’ by increasing levels of thiols, higher alcohols and esters and by breaking down bonds with sugar molecules
• De-acidification

The choice for winemakers will be based on the wine style required and the condition of the vintage.

Preparation & inoculation

White juice is normally inoculated with ADY at a concentration of 10-30 g/hl (or 5-10 x 10⁶ cells/ml of juice).

To inoculate a 50 hl tank, a recommended method would be to:

• Add 1000 g of ADY to 10-20l of water at 40 ºC
• Leave for 15-20mins
• Stir yeast through water
• Add 10l of juice or must and leave for a further 15 minutes to acclimitise.
• Stir thoroughly to aerate

N.B. the yeast ferments actively and needs a much bigger bucket than you think!

This addition can be done on each tank, or actively fermenting must can be used to inoculate a new tank. It is important to note that during fermentation, the volume of the must will expand considerably and some foam may form, so it is important not to over-fill the tanks or barrels.

Nutrition

Highly clarified musts and musts from rotted vintages may be low in yeast nutrients and so fermentations may be sluggish and stop prematurely. Over-clarified musts will also suffer from higher concentrations of dissolved carbon dioxide and a lack of particles for the yeasts to cling on to. Yeasts that are deprived on nitrogen (as ammonium) will break down amino acids dissolved in the must, and this releases hydrogen sulphide (H₂S), which has a ‘bad eggs’ smell.

If the fermentation is sluggish or if large amounts of hydrogen sulphide are generated, it might be beneficial to add diammonium phosphate (DAP) at a dose of 200 mg/l in order to increase levels of yeast available nitrogen (YAN). This should only be done during early stages of fermentation. In later stages, aerating the fermenting must may be beneficial
Ammonium sulphate is also permitted, and this liberates both ammonium and sulphur dioxide. Rotted vintages may also benefit from the addition of 1 mg/l thiamine.