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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 aerated 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.

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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. 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 forms; 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

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isolating the different micro-organisms, several culture media are employed so that all kinds may develop. We will not, however, treat of such analyses, as they do not usually occur in practice. The questions here arising usually tend only in one direction. In the analysis of water for brewery purposes, information is generally sought for as to the micro-organisms of the water which are detrimental to the working of the brewery; the noninjurious forms are of no importance in practice, and are therefore left out of consideration. The culture medium employed in the brewery, i.e., wort and beer, is used for such investigations. This simple principle had to be insisted on at one time by Hansen, as it was neglected by many workers.

The Hygienic Analysis of Water (after Koch) is performed with meat-extract peptone gelatine as culture medium; a certain quantity of the water is distributed in the latter and the number of colonies developed from it is determined. In analogy with this an air analysis is sometimes carried out as well by drawing the air over culture gelatine.

The object of these methods is to develop as many germs as possible. They are, therefore, also used to test the efficiency of a filter by subjecting a certain quantity of water to plate culture before and after filtering, and afterwards comparing the number of germs developed in the two cases.

Water Analysis for Brewery Purposes (after Hansen). A technical biological analysis of water for industrial purposes is made by sowing a certain quantity of the water in sterile wort, must, etc. An example of such an analysis of water for brewery purposes as carried out by Hansen may be described here. The questions to be answered are the following: How does the water behave towards wort and beer? How rich is it in such micro-organisms as can

develop in these liquids, and are there among them such species as can cause dangerous disturbances in practical operations?

From the analyses given below will be seen the differences which results may exhibit according as the one or the other of the above-mentioned methods is used. Hansen found by his analyses the following numbers: While cultures in beer always gave 0 growths and a simultaneous series of experi ments with wort 0, 0, 6·6, 3 and 9 growths per 1 c.c of water, he found when Koch's meat-extract peptone gelatine was used under the same conditions and with samples of the same water, 100, 222, 1,000, 750 and 1,500 growths per 1 c.c. of water. This shows that the Koch method is inapplicable to such brewery analyses.

The procedure is therefore as follows: When, e.g., the tap water in a brewery is to be analysed one begins by carefully cleaning the taps and tubing of the water supply, using all precautionary measures. The tap is then opened and the water allowed to run for some time, e.g., one hour, before samples are taken. This ensures the washing out of the piping. The difficulty here, as in all biological analyses, consists in getting an average sample. If the water sample is not for transmission, but is to be analysed on the spot, sterile Chamberland flasks can be used for this purpose; if, however, the water sample is to be despatched, sterile bottles with glass stoppers are employed, the sample being packed

in ice.

After the sample of water has been well shaken up, a small quantity is carefully withdrawn by means of a sterile pipette. The water is inoculated into sterilised wort and sterilised beer. In order to observe with greater ease the development of organisms that may be present it is best to employ a perfectly clear wort without sediment. Freudenreich flasks are the most suitable ones to use, as

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