PURE YEAST CULTURE.

By pure yeast is meant a yeast derived from a single cell by methods excluding the possibility of infection.

If yeast is mixed with the wort in the brewery and allowed to ferment in open tubs, it is evident that bacteria, wild yeasts and mycoderma that may be present in the air are apt to fall into the fluid, where they have opportunities to develop and subsequently settle, in part, in the yeast sediment.

The Old Way.--In former years, no special pitching yeast was employed, the wort being simply left to spontaneous fermentation. Frequently the fermenting tubs were not cleaned out, and the new wort was pumped on the old sediment. This operation being repeated a number of times, the sediment would acquire a certain characteristic composition. If the beer turned out satisfactory, the yeast was not thrown away when the tubs were cleaned, but was transferred to other vessels. In that way the barm gradually grew up. What this barm really was, no one knew.

Investigation of Yeast.-Inquiry into the nature of this body grew active with the improvement of the microscope, which about the beginning of the nineteenth century became an important factor in scientific research. Being applied to yeast, this mass was found to be composed of countless numbers of very small cells, each of which constitutes a living being very prolific in multiplication and endowed with the peculiar power of inciting fermentation.

PASTEUR'S PURE YEAST.

The full import of these investigations was not realized until Pasteur startled the scientific world with his classical experiments which demonstrated conclusively that the yeast mass is mixed

with large numbers of cells still smaller than those of the yeast proper, and that these minute cells were bacteria. He described several varieties or species of these micro-organisms as being capable of causing beer diseases, and he advised brewers to seek to keep them out of their yeast, for which purpose he devised a method to remove them or make them harmless. This method consisted in treating the yeast with tartaric acid, which killed the bacteria and resulted in what was, in a sense, a pure yeast, that is, comparatively free from bacteria.

Pasteur's pure yeast, however, never acquired any practical importance. This was due to the fact, not at first understood, that this mass, while consisting practically altogether of yeast without any appreciable admixtures of ferments of other classes, was by no means of uniform composition, but contained different varieties of yeast, many of which, present in large quantities, were just as dangerous to beer as the bacteria, although in different ways. These yeasts were afterward called wild yeasts. They are able to produce certain beer diseases, as turbidity and offensive odor and taste.

HANSEN'S PURE YEAST.

Wild yeasts were first found to be the causes of beer disease by Hansen, who traced turbidity in certain Danish beers to their presence.

Hansen set himself to discover means to produce a yeast that should be absolutely free from any admixture of wild yeast, and came to the conclusion that the only way to produce such a yeast with absolute certainty was to develop the yeast from a single cell under conditions that excluded the possibility of infection. This is what is known as Hansen's pure yeast. (For methods of preparing pure cultures see "The Brewer's Microscopical Laboratory.") For obtaining pure cultures of yeast Hansen's moist chamber method is preferable.

Selecting the Type.-The peculiar character of a beer yeast is due mainly to that variety which preponderates in the yeast, and among the pure cultures obtained from the propagation of a number of individual cells taken from such yeast it is natural that a majority should have the characteristics of the original yeast. Each pure culture is examined first as to the degree of attenuation, whether high or low, next as to clarification, whether rapid

or slow, and also as to the taste which it imparts to the beer. If among the pure cultures are found several that show the same degree of attenuation and the same clarifying power and taste of beer that was observed in the original yeast, the conclusion will be justified that these are the ones that exert the desired influences, and it will be proper to select one of them for propagation. Propagating the Yeast.-The yeast type that has been thus selected is propagated as described in connection with the pure yeast apparatus, until a sufficient quantity has been developed to start a fermenter in the brewery with it. Since it is not always certain that the first fermentation will take a strictly satisfactory course, it is advisable to finish the beer from this fermenter and judge the yeast by the character of this beer when finished. If the beer gives satisfaction, the yeast is introduced for permanent use.

Before this is done, a standard culture is prepared for future reference. A few drops of the yeast are placed in a vial with a sterilized 10 per cent sugar solution, and kept in a dark place. In this way the yeast can be kept unchanged for years.

Advantages of Pure Yeast.-The great advantage of pure yeast in brewing operations consists mainly in the fact that the brewer has at all times at his disposal the same identical yeast type. Consequently, he is able, other things being equal, to produce a beer of constant, uniform character. Even if the yeast should become infected or deteriorate from any other cause, a fresh batch of the identical original yeast can be developed in a few weeks from the reserve culture, and a yeast of the same properties as was possessed by the first lot be once more introduced, the reserve culture having been derived from the same original cell as the first lot of pitching yeast.

Pure yeast, however, is valuable in other ways also. It is a rule that admits of general application, that micro-organisms of one species will crowd out organisms of another species contained in the same nourishing liquid, the more effectually, the greater their relative number. In the same way, a pure yeast containing no foreign organisms, is much more resistant to disease and infection than a common mixed yeast. A pure culture yeast can be infected only by uncleanliness or by germs contained in the air, while common brewers' yeast is in itself a most prolific hotbed of infection, being frequently contaminated with bacteria, wild yeast

and mycoderma, which spring into action at slight changes of temperature or composition of yeast food, while a pure yeast will adapt itself more readily to such changed conditions.

PURE YEAST APPARATUS.

An apparatus for the development of pure culture yeast was devised by Hansen. His own description, from "Practical Studies in Fermentation," follows:

HANSEN'S APPARATUS.

As shown in the accompanying illustration, the apparatus consists of two main portions and the connecting tubes, namely: The fermenting cylinder C, and the wort cylinder D. Air pump and air holder are not shown in the drawing.

The pump A is driven by machinery and draws the air through a filter in order to effect a preliminary purification. The air-holder B is provided with a pressure-gauge and a safety valve. It is charged with air under a pressure of 1 to 4 atmospheres. The pipes must be fitted with cocks at suitable points for removing the water which collects in them. This is of especial importance in the case of the pipe between the air-holder B and the filters g and m. These are best united by metal tubes with the air pipes. If metal tubes are used, they should naturally possess some degree of elasticity and must be so arranged that the filters can be readily fitted and disconnected.

Through the top of the fermenting cylinder C passes a stirrer b, the lower end of which is fitted with two blades, one carrying a sheet of rubber cut in such a way that when rotated it comes into contact with both the bottom and the sides of the cylinder. From the top there passes a doubly bent tube c, and by opening its cock, connection is made with the inside of the cylinder. The lower free end of the tube dips under water in the vessel d.

A little below the top is a horizontal tube e provided with a cock, and by means of which the inside of the cylinder is connected with the vertical glass tube f. This is connected at its upper end with the filter g and at its lower end with a second cock and similar horizontal tube h to that described above.

The top mark on the glass tube is 31.3 in. from the bottom of the cylinder, the next 8 in. and the lowest 4 in. from the bottom of the cylinder. When filled to the top mark, the cylinder holds about 12 barrels. The glass tube is fixed into the cocks