The importance of yeasts in determining the composition and quality of wines

Abstract

From the wine makers' view point at present the advantages of the use of selected yeasts are the rapid and predictable onset of fermentation, its evenness and completion and the absence of undesirable aromas and flavours. However it is now apparent from our work that wine yeasts differ considerably in other characteristics which can also be of oenological importance.
Compounds formed by yeasts which can directly affect the flavour of wines include hydrogen sulphide and mercaptans, iso-amyl alcohol, and ethyl and amyl acetate. Othex· aroma materials which are not reminiscent of pure compounds, but which affect wine quality, can also be produced. These may confer a quality improvement or, in the case of spoilage yeasts, a reduction in quality. Compounds which indirectly affect the quality of wines are those which bind sulphur dioxide and prevent it from carrying out its antioxidant and germicidal functions. The most important compounds in this connection in normal wines are acetaldehyde, pyruvic and a-ketoglutaric acid, and the amounts produced are influenced by the yeast strain.
It is also possible to exercise control over reduction in acidity during the fermentation by controlling the amount of 1-malic acid metabolised by choice of a suitable yeast strain. Likewise some control is possible over the quantities of ethanol and higher alcohols produced and the prevention of off-flavours resulting from fermentation.
One of the most important results of our work has been the prevention of hydrogen sulphide formation in wines. This formation is basically a microbiological reduction of elemental sulphur or sulphur-containing compounds during fermentation, and the control has been to carry out the fermentation with a yeast which is unable to produce hydrogen sulphide. Such yeasts exist naturally and may be selected on the basis of suitable laboratory tests. These yeasts need to have other desirable attributes such as high ethanol tolerance, the ability to carry out a regular and complete fermentation and to be able to grow fast enough to dominate the indigenous microflora.
As a result of ·our investigations the use of selected yeasts has become widespread in Australian wine making, and the strain of yeast is now regarded as one of the controllable factors necessary to make wines of high and consistent quality.