262 
Transactions of the Society. 
required quantity of yeast might be obtained, starting from a 
single cell. It was then found that certain species of yeast were as 
much to be feared as many of the bacteria, and that the number of 
microscopic enemies with which the brewer had to contend was 
greater than had been imagined. Thus, a species of yeast known 
as Sacch. Pastorianus I produces a nauseous bitter flavour and a 
disagreeable smell ; Sacch. Pastorianus III , and Sacch. ellipsoideus 
II , persistent turbidity ; Sacch. anomalus , a marked fruity flavour ; 
Sacch . ilicis, a disagreeable bitter flavour ; and Sacch. fcetidus , 
stench. 
In the course of his work Hansen found that morphological 
characters were frequently useless for the purpose of distinguishing 
between one species and another, for not only did many of these 
resemble one another somewhat closely in appearance, but the 
shape of any one species varied within wide limits, depending 
chiefly upon the conditions under which it had been grown. This 
method of obtaining pure cultures when used in conjunction with 
certain other methods of differentiation, such as the behaviour of 
the yeasts towards certain selected carbohydrates, and the optimum 
temperatures for ascospore and film formation, has enabled zymo- 
technologists to isolate and describe many distinct species, of some 
of which numerous varieties are known. It should be said, how- 
ever, that only a few of these are of industrial importance. 
Although the use of “ pure ” — that is, single-cell — yeast has 
been widely adopted on the Continent, attempts to introduce it into 
English breweries has met with much less success. One reason 
for this is that the conditions obtaining in most English breweries 
are such as to result in the production of a definite type of yeast,, 
which gives the exact class of beer required, and which can without 
resort to pure culture methods be kept practically pure — that is,, 
free from bacteria and foreign yeasts within the limits necessary 
for successful working. 
It will be seen, therefore, that the main endeavour of the 
brewer must be to maintain his pitching yeast in a pure condition 
in the above sense — that is, he must take steps to protect it from 
infection with bacteria on the one hand, and from the intrusion of 
pathogenetic species of yeast on the other. 
When it is remembered that these organisms — both bacteria 
and yeasts — may not only be derived from the plant, but are 
always present to a greater or less extent in the air, and that in 
the brewing of beer very large volumes of a highly susceptible 
liquid have to be dealt with, some idea of the difficulties of the 
modern brewer may be formed. The cooling and refrigeration of 
the wort in purified air, the scrupulous cleansing of all vessels 
with which the wort or beer comes into contact, and the systematic 
testing of materials are all necessary if disaster is to be avoided. 
In the majority of the larger breweries there are laboratories in 
