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of which the colonies are crowded at the lowest point, or are intimately mixed. All the drops are now touched with the finger, which has been cleaned and Alamed previously, in order to obtain an average sample, which is then examined microscopically. To distinguish normal bottom yeast from wild yeast, the property is taken advantage of that the former aggregates in flakes, whilst the greater part of the wild yeast cells distribute themselves like dust in the drop. But since, as Lindner mentions, a part of the wild yeast also forms flocks, and the growths of culture yeasts on the other hand can assume a dusty appearance, other distinguishing features must be sought.
From Lindner's description of his method for analysing yeast in the brewery, it is seen that he draws his conclusions from the microscopical appearance of droplet cultures in hollow glass slips; this may be of use so long as it is remembered that cells of one and the same species, even when they are cultivated side by side in drops of liquid, often yield growths, the cells of which are so different as to seem to belong to several species. Under these circumstances culture yeasts may also develop cells with an appearance similar to one of the wild yeasts and vice versa. Each characteristic is, as is known, subject to variation, but this is especially so with regard to the form of the cells. This analysis, when combined with the ordinary microscopical one, is of advantage to the practised specialist, whose eye for the form and general appearance of the yeast cells has been specially trained.
9.—Hansen's Test of the Stability of Beer in Cask.
Samples from lager casks are taken by boring holes in the cask. The place is first cleaned with spirit, and a try-cock
similarly cleaned is placed in the hole. Portions of the beer are drawn off from the upper, middle and lower layers, samples being taken from these three different parts of the liquid in order to get, as nearly as possible, an average sample. The weak point in this and similar analyses consists in the taking of the samples, which must give a correct average if the analysis is to be of any value. The beer analysis described in the following is given chiefly for the purpose of testing the stability, a point of great importance to the brewer ; what concerns him is to have some idea how the beer will behave after the lapse of a certain time. Hansen has published the following description of the test : The beer is drawn off in sterile white glass bottles, which are then closed with sterile corks and placed away in a dark cupboard at the temperature of the room. As soon as the samples are taken, their smell, taste, clarification and colour are noted. It is also noted how long it takes to form an appreciable deposit, and further, how the latter behaves: whether it distributes itself easily through the liquid on shaking, so that this becomes turbid and opaque, or whether it forms flocks which quickly sink to the bottom without substantially affecting the transparency. Changes of this kind are caused by the presence of micro-organisms. If the liquid becomes gradually turbid and decolorised without having been shaken, disease bacteria are present. However, this takes place but seldom after the pure culture system is introduced. On the other hand a yeast sediment always forms after a certain time even in the best beer, and may arise from culture yeast or from wild yeast, but is most frequently due to a mixture of the two. Hansen has shown that wild species of yeast can produce diseases at this stage. For the rest, when speaking of stability, reference is made only to the formation of yeast sediment and not to bacterial diseases. A yeast species which gives a stable beer is, in this sense, one which not only increases to a small extent in the finished beer, but is able at the same time to suppress its rivals during fermentation. The latter obtains either because it is in a better condition than its competitors to suit itself to the conditions of nutrition and especially to take up the oxygen, or because it gives off, during its multiplication, products which act as poisons.
Hansen's work has shown that samples from the upper layers of lager casks produce a yeast sediment more quickly than those taken from other parts. It is to be remarked here that beer under brewery conditions is strongly aërated when drawn off (when not drawn off under pressure of carbonic acid); but this does not happen when these test samples are taken, and this materially affects the increase of the yeast. Hansen's work showed further that it is necessary to keep the samples at the temperature of the room and not at 25° to 27° C., since the yeast sediment is produced sooner in the former case than in the latter. Moreover, the varying conditions in practice will be naturally of great importance in such tests, and therefore do not admit of the establishment of a general rule. It is therefore necessary as regards separate breweries to fix upon a standard which is obtained by experiment and analysis, and which is not changed so long as the same yeast is used, and so long as there are no great changes introduced during the manufacture of the beer.
In practice the following procedure is adopted when the production of a well-stored lager beer is in view: A sample is drawn about two months after the beer is casked. In doing this, white sterile bottles with sterile corks are employed. They are set away in darkness at the room temperature and observed several times in the course of a fortnight. It is noted how soon a sediment forins; if the sediment is considerable, its constituents are investigated. If the stability does not appear to be satisfactory, a sample may be taken before the beer is drawn off, and this is treated in the same manner. When the drawing off begins, samples are again taken from the same casks, but in ordinary bottles, not sterilised ones. They are treated as by the retailers, being, for example, shaken up. Otherwise the examination is the same as that given above.
The table given on page 144 will serve as a specimen of the data entered in a lager cellar journal.
10.—The Biological Analysis of Water, Air and Soil.
In biologically examining water, air and soil, it is of fundamental importance to deal with an average sample; otherwise the analysis has little value. This is, however, very difficult, and in addition to this the organisms in water, air and soil vary considerably, with respect to number and species, with the time of year; it is therefore necessary to perform a large number of analyses at different times in order to obtain a knowledge of the actual flora of the microorganisms and their relative proportions. Hansen's investigations mentioned in the next section may be cited as an example of such a series of analyses, which had as the object of investigation the circulation in nature of saccharomycetes, and especially of Sacch. apiculatus, and, above all, to discover what organisms are present in the air at different times of the year.
Principles of the Technical Analysis of Water, Air and Soil. The manner in which a biological analysis of water, air and soil should be carried out depends upon the object of the analysis. A chief principle is the separation of the germs in sterile water and seeding from the mixture. If the question is to find all the species of microorganisms present in a sample, the undertaking will be a very difficult one, especially as regards bacteria. For