3? 
thoroughly the progress that has been made in 
this respect. 
So familiar has the idea of the existence of 
bacteria become to the mind that some effort is 
required to realise the state of things fifty years 
ago, when brewing was carried on in total ignor- 
ance of the very existence of these ubiquitous 
organisms. At that comparatively recent date 
even the facts that yeast was a living organism 
and that fermentation was correlated with its 
growth and activity had only just been finally and 
definitely proved by Pasteur and were by no means 
universally accepted. 
The recognition of these facts has placed in 
the hands of the brewer the means of instituting 
a rigorous system of control, which must be 
regarded as one of the most important applications 
of scientific method to the art of brewing. It is 
now recognised that infection by “disease” 
organisms is the cause of many of the troubles 
which beset the brewer, and, further, that these 
infecting organisms may either be bacteria or 
“wild” yeasts. 
As in medicine, so in brewing, prevention is 
better than cure, and the brewer’s first line of 
defence is the avoidance of infection from 
external agencies, such as dust, unsterilised casks, 
bottles, etc. This application of the elementary 
principles of bacteriology, simple as it may 
appear, has worked nothing less than a revolu- 
tion in brewing practice, for we read that in 1865 
the breweries in Burton were habitually shut down 
during the summer months owing to difficulties 
which we now know were due _ to infection. 
Another weapon in the hands of the scientific 
brewer is the system of “forcing ’’—incubating a 
sample of the beer for several days—whereby early 
information is obtained as to the stability of the 
beer and the presence or absence of dangerous 
infection. Even if infection has occurred and the 
dreaded Saccharobacillus pastorianus, which 
sours the beer by producing lactic acid from 
sugar, has increased to a dangerous extent, the 
yeast can be freed from it by subculture in the 
solution containing o'r per cent. of tartaric acid, 
which was originally proposed by Pasteur for this 
purpose. This purified yeast can then be used 
with perfect safety for pitching purposes. The 
statement made by Hansen that this treatment 
encouraged the growth of wild yeasts to a danger- 
ous extent has not been realised in British practice. 
The hops used in the production of beer pro- 
vide a natural prote ction against bacteria, but not 
against “wild” yeasts, and it is to the undesired 
intrusion of these widely spread organisms that 
many faults of flavour and clarification are due. 
These wild yeasts grow chiefly on fruits, passing 
the winter and spring, as shown by Hansen: in 
the soil, and are therefore very abundant in dust 
during the late summer and autumn. They pass 
into the wort or beer in precisely the same manner 
as bacteria, but there find a friendly instead of an 
antagonistic medium, and they withstand the acid 
treatment which is fatal to bacteria. In the 
presence of a healthy culture yeast the wild yeasts 
only develop very slowly, but in their absence— 
NO. 2446, VOL. 98] 
NATURE 
| a stock yeast, 
[SEPTEMBER 14, 1916 
as, for example, when the beer becomes infected 
after racking—they may increase rapidly and 
cause serious trouble. The explanation of this 
inhibitive action of the culture yeast on the growth 
of the wild yeast is due to Dr. Horace Brown, 
who found that the growth of,all yeasts is con- 
ditioned by the amount of oxygen which is taken 
up by the cells before fermentation commences. 
This is shared between the large amount of culture 
yeast and the small infection, and, since the 
quantity of yeast present only increases some five- 
' fold during a large-scale fermentation, no great 
increase in the infecting organism is possible. If, 
however, infection occurs after racking, when only 
a small proportion of primary yeast is left, and if, 
at the same time, air, as is usually the case, 
obtains access to the beer, all the conditions for 
a well-developed yeast trouble are at hand. To 
be forewarned is to be forearmed, and, the con- 
| ditions which conduce to contamination having 
| been ascertained, 
the brewer is in a position to 
avoid these as completely as possible. 
A further refinement, chiefly valuable wig 
applied in addition to the most scrupulous “ bac- 
teriological ” cleanliness throughout the brewery, 
is the use of air which has been freed from 
organisms by filtration. The wort, after having 
been boiled, is cooled and aerated. During these 
processes, before the addition of the yeast, there 
is a manifest danger of infection by air organisms, 
which is largely avoided by the use at this stage 
of filtered air. As already pointed out, however, 
the main source of danger in actual practice i$ 
the infection of the beer after the primary fer- 
mentation has been completed. : 
One of the chief triumphs claimed for the appli- 
cation of scientific method to brewing is the pro- 
duction and use of pure yeast cultures, by which, 
it is maintained, greater constancy of result is 
obtainable. Following the principles laid down 
by Hansen, such cultures are derived from a 
single cell which is isolated under the microscope, 
and serves as the origin of the whole of the yeast 
employed. This system is largely used abroad, 
but has not been adopted to any great extent in 
this country, partly owing, no doubt, to the 
national inertia, but partly to the different con- 
ditions prevailing in the top-yeast breweries. 
This system undoubtedly keeps the pitching yeast 
free from contamination, but is, of course, in 
itself no protection against the subsequent in- 
fection by wild yeasts or bacteria, which, as we 
have seen, is the most frequent cause of trouble. 
It is, moreover, doubtful, as pointed out by Dr. 
Brown, whether a single-cell culture can be ex- 
pected invariably to reproduce all the qualities of 
which represent the statistical 
average of the properties of an immense number 
of cells, all differing slightly from each other. 
From quite a different side the investigations of 
the past half-century on the action of diastase on 
starch, both in the barley grain and in the mash- 
tun—investigations in which Dr. Brown himself 
has taken a leading part—have also largely con- 
tributed to the scientific control of the brewery. 
The mechanism of the processes of malting and 
