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culture on a gypsum block are of course made; in fact, the characteristics of the yeast are investigated. It may happen that the growths in the flasks which contain pure cultures of the required species are nevertheless somewhat varied, although they belong to the same species. We are here confronted by individual differences, which may always be met with, and we must now make a choice from among these growths also.

After preserving some of the pure culture (on the preservation of yeasts, see page 114) partly in saccharose solution and partly in the dried condition on cotton wool, the procedure is as follows:

Four or five 1 litre Pasteur flasks, containing ordinary aërated but sterilised wort from the brewery, are inoculated from the flask containing the perfectly pure culture; these are set away at the temperature of the room, and in a week they will contain a considerable yeast sediment; four such flasks will generally be sufficient, the fifth being really a reserve culture. After pouring off the beer, the yeast of each of these flasks is introduced into a Carlsberg vessel charged with about 7 litres of brewery wort. After one week as much yeast sediment will have formed in these vessels as is necessary to prepare stock yeast for 1 hectolitre. of wort in the brewery. A vessel of 14 hectolitre volume is then set up in the fermenting cellar; this is sterilised by means of a gas flame and charged with a hectolitre of aërated brewery wort. The contents of the four Carlsberg vessels are then poured into this vessel. If the partially fermented wort in the vessels is not to be added also, it is previously drawn off. In the latter case it is advisable to let the flasks stand a little longer, about ten days, so that the yeast may sink more completely to the bottom. If the brewery is some distance from the laboratory it will be always necessary to draw the beer off beforehand. The

flasks mentioned above (see page 119) are then used for transporting the thin yeast liquid. The hectolitre of wort mentioned is fermented under the normal conditions of the brewery. If the result is satisfactory the yeast is introduced into practical working.

It is conceivable that a brewery may have operated continuously with a mixture of different species of culture yeast, and that the combination of the latter has formed the character of the product. It is on no account advisable to isolate these species separately and then to employ a mixture of them as stock yeast, for the relative proportions of the species cannot be controlled during fermentation. Besides, it would be far too much trouble in practice.

We have mentioned before that a pure culture yeast of this kind introduced into practice can only keep sufficiently pure in the fermenting vessels for a certain length of time. The resisting powers of the different races against wild yeasts and bacteria is extremely varied. It is, therefore, necessary from time to time to introduce fresh quantities of pure culture yeast into the brewery. There are, however, cases where a race has remained pure in the ordinary fermenting vessels of practice for more than a year without having been renewed.

The Hansen-Kühle Pure Culture Apparatus.-In order to have ready at hand the requisite amount of pure culture yeast, Hansen, in conjunction with Kühle, has constructed a pure culture apparatus for the continuous production in mass of absolutely pure yeast. A description of the apparatus is given below:

The apparatus (Fig. 50) consists of three principal parts, viz., an air pump, A, with an air reservoir, B, a fermentation cylinder, C, and a wort cylinder, D. The air pump receives the air through a filter and pumps it into the reservoir which is provided with a manometer and safety

valve. The air can pass from the reservoir through the communicating pipes, which are provided with outlet taps for condensed water, through the cotton wool filters, g and m respectively, into the fermentation or wort cylinder.

The following parts are connected with the fermentation cylinder, C: (1) a doubly bent side tube, c, which opens under

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FIG. 50.-Hansen-Kühle Pure Yeast Culture Apparatus.

water in a vessel, d; (2) a glass tube, f, with marks (10, 20, 79), which indicate the amount of liquid in the cylinder; taps are fitted at e and h; (3) a stirring apparatus, b, to stir up and distribute any bottom yeast formed in the cylinder; (4) an outlet cock, l, for beer and yeast; (5) a short side tube, j, provided with rubber tubing and glass stopper; the pure

culture of yeast previously prepared in the laboratory is introduced through this tube, the side tube of the flask being connected with it in a flame in the ordinary way; (6) a pair of windows, a, set at an angle to one another, which are only added if requested, being best omitted as they are superfluous. Lastly there is sometimes a water jacket (Fig. 51) fitted round the cylinder when the apparatus has to be set up in a spot where the temperature must be regulated.

Connected with the wort cylinder, D, are: (1) a doubly bent side tube, n, opening under water in the vessel, o; (2) the stopcocks, q, r and s; s is in communication with the pipe through which the boiling hot wort is passed from the brewery into the cylinder; (3) a sprinkling ring and a cold water pipe, p, for cooling the wort. The sprinkling ring is perforated with small holes on the side next to the cylinder. The cylinder is fitted into a box which is provided with an outlet tube, t, and which receives the cold water flowing through the sprinkling ring. In Fig. 51 a water jacket is employed instead of this ring; it is more efficient but also more expensive. The wort and fermentation cylinders are

connected by means of the pipe, k.

The outlet cock, l, is fitted so that infection from outside is obviated during tapping. The construction may be seen from Fig. 52 in which the cone valve is shown shut. The arrows give the direction of flow when the stopcock is screwed open. The construction of the lower part of the stirring apparatus, b, is given in Fig. 53.

When the apparatus is fitted up, it is first tested to see whether it is air-tight. For this purpose steam is led into the cylinder through the pipe, k, all other cocks being shut. As soon as it is found that the apparatus is steam-tight, it is sterilised by means of steam, all the cocks being opened in turn and closed after the steam has passed through for some time. When the apparatus is sterilised, the wort

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FIG. 51.-Hansen-Kühle Pure Yeast Culture Apparatus.

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