Two different types of yeast are used by the brewers, one for ale production and another for lager beer.

Ale yeasts have much in common with distiller's and baker's yeast while lager yeasts seem to originate from an ancient species hybridization.

The purity of brewer's yeast is most precisely analyzed by DNA fingerprints.

Using this technique the yeast chromosomes are first separated according to size by gel electrophoresis. Chromosomal DNA bands are fluorescent in UV light when stained with ethidium bromide. Each of the analyzed strains exhibits a unique chromosomal DNA pattern (Fig. 1).

Since lager yeasts are closely related they are difficult to identify by this method alone. The detection of specific genes on DNA fragments derived from digested yeast chromosomes, provides an additional identification tool.

A combination of the two methods offers an easy way to distinguish a lager strain from all other yeasts. Fig. 2 shows an analysis of variants of a gene called URA3 in brewer's yeast. Furthermore individual lager strains can be differentiated by this method. The lager strains L1 and L2 are used for beer production.

In summary, DNA enable the brewer to recognize his own yeast and distinguish it from other brewer's strains and yeast contaminants.

Fig.1

Electophoretic analysis of chromosomes in five different brewer's yeasts. The chromosomal DNA molecules have been separated according to size. The yeast chromosomes range in size from 200 000 base pairs (0.2 Mbp) to 2 million (2 Mbp)

Fig. 2

Detection of the URA3 gene fragments on size-separated DNA from five Saccharomyces brewer's yeasts. Lager yeasts L1, L2 and L3 and L4 contain a long URA3 fragment IV together with one, two or none of the shorter fragments I-III. Ale strains (A) never exhibit band IV.