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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
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