yeast sample to be examined is still in beer, or if that is not the case, a little of the yeast can be placed in beer and then subjected to this temperature. Should living acetic acid bacteria be present these are quickly developed by this process. If it is required to determine in a yeast sample the bacteria which are capable of developing in wort, then of course wort is used instead of the liquids named above.

Testing the Contents of the Pure Culture Apparatus. The above methods for the biological analysis of yeasts are applied in breweries to test the contents both of the pure culture apparatus and of the fermenting vessels.

In the first case a sample of the top beer is taken from the pure culture apparatus at the close of the primary fermentation by the side tube j (see Figs. 50 and 51). Wild yeasts are then detected by means of the tartaric acid method, and bacteria in the manner already described.

Testing the Contents of Fermenting Vessels. - So far as the testing of the fermentation vessels is concerned, this may be confined chiefly to a microscopical examination of the fermenting wort, especially when the yeast produced is not to be used as pitching yeast. The appearance of the yeast species used will in general be readily recognised by daily practice, so that a foreign admixture will be detectable by mere microscopical examination. A special knowledge gained in this way is therefore of great advantage; but on the other hand it cannot be too strongly emphasised that a microscopical examination alone is not a perfectly reliable criterion, and this is especially the case when the culture yeast used is similar in appearance to a wild yeast (e.g., has more or less elongated cells). Foreign culture yeasts which may have found entrance in certain cases will, as a rule, not be recognisable; it is then even more necessary to apply physiological methods.

The sample is taken from the surface of the beer in a

sterile glass four to five days before the end of the primary fermentation. In the microscopical examination, the form of the cells is observed, and whether living bacteria, especially rod bacteria and Sarcina, are present. If it is wished to test for wild yeast, the sample is set aside till a yeast deposit has formed; the latter is placed on gypsum blocks and analysed according to the spore methods already described. The yeast is placed directly on the gypsum block because the wild yeast presumably present has just formed strong young cells at this juncture. At the same time a wort flask is inoculated with a little of the yeast, and the yeast generated here is employed next day in a new spore test. If wild yeast is found by this test, the yeast cannot be used for pitching. This holds also when Sarcina or other bacteria are present to an appreciable extent. Bacteria are always observed in the wort, but are usually dead. Wild yeast is likewise always found in practice in small amounts; when it cannot be detected by means of the ordinary spore method there is no reason for apprehension.

The table on page 139 is given as an example of the journal of a fermenting cellar.

Lindner's Drop Culture. P. Lindner, in examining for wild yeast, uses the "drop" culture. This consists in taking out a certain amount of beer by means of a pipette and distributing the contents of the pipette, drop by drop, (generally 50 drops) on the bottom and the cover of a Petri dish. It is thus known what quantity there is in these 100 drops. The Petri dish is placed in a thermostat at 25° C., or left at the temperature of the room. A development is visible in the drops after the lapse of several days. If the number of germs is too large, the liquid is diluted with wort to a suitable extent, before the dropping is performed. If it is wished to determine the principal kinds that are present, the upper dish is used, in the drops

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 flamed 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