as have for long been cultivated in fermentation industries, though only recently in a systematic manner and under a certain amount of control. As the consumption of beer is now extremely widespread, culture yeasts will, of course, be met with in nature; most yeast species, however, which occur either on fruits or in the earth belong to so-called wild yeast species. Culture yeasts are, like wild yeasts, partly top yeasts and partly bottom yeasts. Their use depends on certain special qualities for which they are prized, e.g., in breweries, when they give a stable beer, a good clarification, a particular flavour, etc. A yeast which gives a stable beer must not only multiply comparatively slowly in the finished beer, but must also be able to suppress rival yeasts during fermentation. The immediate cause why certain species are more pre-eminent in the latter respect may, in some cases, be sought in the fact that they can fit themselves to nutrimental conditions, and especially can assimilate oxygen more powerfully than their rivals; in other cases it is due chiefly to them giving off substances during multiplication which act as poisons on the intruders.

Since the demands made regarding taste and other qualities vary so widely the choice of available species and races must necessarily adjust itself accordingly, whether it be for wine manufacture, or for breweries, distilleries or pressed yeast factories; thus the number of species and races introduced into practice continually increases. In the first two branches of industry the demands are more for clarification, taste, smell and stability; in the latter a large production of alcohol and great multiplying power are more particularly sought.

6.-Injurious and Stimulating Influence of Chemical and Physical Factors.

Influence of Chemical Factors. With saccharomycetes, as with all other organisms, the decomposition products act as

a poison to the organism in question. Here it is the alcohol formed, and the organic acids, among other things, which exercise an injurious influence on their growth. The deleterious effect of carbonic acid, however, does not appear to be great.

As regards the action of organic acids, the experiments of Hansen on the effect of tartaric acid have already been mentioned (see p. 135). He found that beer yeasts die off very quickly if they are cultivated in a sugar solution to which tartaric acid has been added, an effect not produced on wild yeasts.

The stimulating effect of antiseptics on yeast has been studied by earlier investigators, e.g., by Biernacki and Schulz (antiseptics in general), by Hayduck (sulphuric and lactic acids) and by Heinzelman (salicylic acid). Agents were thus found, by means of which not only could the development of bacteria be prevented, but the energy of fermentation also increased. Now and then experiments in practice were also instituted. In this direction Effront has quite recently recommended the use of fluorides. Hydrofluoric acid is, however, a strong poison for yeast which can hardly withstand 1 to 2 grams per hectolitre; but, by adaptation, the dose can be raised to 200 grams. A yeast habituated to such large quantities of hydrofluoric acid possesses indeed but feeble powers of budding, although it has great fermenting power. The method is only applicable in distilleries. According to Holm and Jörgensen's experiments, this addition of fluorides hastens the development of Mycoderma, and, at the same time, Bacterium aceti is not thereby destroyed. In mixtures of brewery yeast with wild yeast the same authors state that the development of the wild yeast. is favoured, the effect being thus the same as that of tartaric acid. This method also is, therefore, quite inapplicable to the "purification" of beer yeast.

Sulphurous acid, corrosive sublimate and several other substances are likewise strong yeast poisons. Thus alcoholic fermentation is, according to Yabe, prevented by the following solutions: Phenol 1: 200, resorcin 1:100, pyrogallol 1:50. Bokorny gives: sulphuric acid 1: 5,000, potassium hydroxide 1 : 5,000, potassium permanganate 1: 10,000, chlorine 1: 10,000, and iodine 1: 10,000. Siebel found that if beer is treated with a solution of formalin (40 per cent. solution of formaldehyde) in the proportion 1 : 10,000, neither yeast, Mycoderma nor bacteria develop; in solutions of 150,000, yeast and Mycoderma develop but not bacteria.

Influence of Physical Factors. As regards the influence of physical causes, yeast cells do not usually withstand 50° to 60° C. moist heat, but die off between these two temperatures. Hansen found that strong young cells of Sacch. ellipsoideus II. die after five minutes' heating in distilled water at a temperature between 54° and 56° C. Old cells of the same species could withstand a temperature of 60° C. for five minutes under the same conditions without being killed. Quite ripe spores of the same species, which had been partially dried on a gypsum block for about a week, withstood five minutes heating in water at 62° but not at 66° C. Similar experiments were made with Sacch. cerevisia I.; strong young cells could withstand five minutes heating at 52° C. under the same conditions, but not at 54° C., and spores treated in the same manner as those of the foregoing species could withstand five minutes heating at 58°, but not at 62° C. Yeast cells are said to be able to stand cooling down to 130° C. for about 200 hours, and it is known from experiments in breweries that yeast cells can remain frozen in ice for months at a time without being killed (Prior).

Drying and the effect of temperature are related to

one another. Experiments by Hansen and others have shown that yeast cells die comparatively quickly when dried up, and that this occurs in nature, e.g., when the cells are on the uninjured surface of fruit. Under these circumstances the spores have a somewhat longer life than the vegetative cells, but the difference is not great. By a suitable treatment, however, life can be preserved in the dried cell for a long time. This will be mentioned under the heading following.

The injurious effect of violent shaking on yeast cells has already been touched upon (p. 217).

Experiments on the effect of light have been made from time to time by various investigators. Kny used as the source of light five flat gas flames, and carried out his experiments on pressed yeast (Sacch. cerevisiae) which was placed in an artificial culture solution in flat crystallising dishes and at the same temperature, part exposed to the action of light, and part set away in the dark. The heat from the source of light was removed by means of water. He arrived at the result that the budding of Sacch. cerevisia takes place in this moderate light with the same activity as in darkness. Marshall Ward remarked a destructive effect of light on the spores of Sacch. pyriformis.

Lohmann made

experiments with intense light (arc lamps and sunlight) and excluded the heat radiated from the sources of light by means of water cells. A distillery yeast, called Race II., was seeded in wort gelatine on glass slips and for eight hours at constant temperature part was subjected to light from the arc lamp, and another part was set away in the dark. For high temperatures (i.e., above 18°) it was found that budding was retarded in the cultures subjected to the light. The same yeast was also seeded on agar-agar plates in Petri dishes, a part exposed to sunlight and another part kept in darkness. Illumination for several hours in this case killed

the yeast. Diffused daylight also had a retarding influence on budding, but only after prolonged action. It resulted from experiments with Sacch. Pastorianus I. that this species has greater resisting power than the above one against the effect of light, both from the sun and from the arc lamp.

It has long been known in the brewing world that beer is sensitive to light in a high degree. Special experiments on this point were made by Ney, Beck and W. Schultze. The question put before one here is whether the disagreeable smell and taste, which beer gets when exposed to sunlight, originates in the effect of light on the yeast or on the beer itself. At the time when the first two named carried out their experiments (1878 and 1882), Sacch. exiguus was always looked upon as the cause of everything that went wrong, and it was found, in concordance with the above, that in those flasks least protected from light, and the contents of which had therefore acquired a very bad smell and taste, there was a more or less copious development of abnormal yeast cells," which they set down as the abovenamed species without further ado, and also of lactic acid bacteria. From this it would seem then that light was the cause of the good beer yeast, with which the beer was treated, being restrained and the supposed Sacch. exiguus being furthered in its development. But from the communications it appears rather to have been Sacch. Pastorianus I. or an allied species, and since the latter shows a greater power of resistance to the effect of light than Sacch. cerevisia, as we have seen from the above experiments of Lohmann, it is not impossible that this species. was responsible for the disagreeable smell and taste in the cases cited, since the action of the light in the experiments was very prolonged (three weeks). But it is not yet clear what the role played by the yeast cells in these undesirable transformations really is.