E about 25°, or alternatively at the ordinary temperature. In the course of two or three days, or more, according to the temperature, active fermentation will have set in, and after a week or so enough yeast will have been formed to allow of a larger quantity of wort being "pitched." This in its turn produces a still larger amount of yeast. Hence, given that precautions are taken to prevent contamination during the operations, any desired quantity of pure yeast can be obtained from the original single cell.

Special propagating apparatus, designed for the preparation of a regular supply of pure yeast on the foregoing principles, is installed in many breweries and distilleries.

Single-cell yeasts are employed with good results in distilleries and in numerous Continental and American breweries, but their application to English " top-fermentation ' brewing has been less satisfactory, on account of the difficulty of obtaining a good secondary fermentation in casks when such yeast is used.

Yeasts are sometimes referred to as being of the "Saaz" type or the "Frohberg" type. The former term indicates yeasts which have a relatively small " attenuating" power - that is, which produce a low percentage of alcohol, about 8 per cent, by volume, or 14 per cent, of proof spirit; whereas Frohberg-type yeasts have a greater attenuating power, producing about 11 per cent, of alcohol, or 19 of proof spirit. More of the malto-dextrins are fermented by the "Frohberg" than by the "Saaz" types of yeast.

Two well-known yeasts largely used abroad for distillery fermentations are known as "Race II" and "Race XII." They were developed in Germany, and have given very good results in distillery practice by reason of their ability to ferment rapidly worts of high strength. Race II is a top yeast of the Frohberg type, distinguished chiefly by its large cells; it is well adapted for the fermentation of strong mashes, and has a good power of resistance to the inhibiting effect of the high proportion of alcohol produced. The use of a quick-fermenting yeast is advantageous, because on account of its rapid growth it suppresses bacteria and wild yeasts. Also a dilute wort is relatively more costly for distillation than one yielding more alcohol in the same volume; hence the advantage of a yeast which will rapidly ferment the more concentrated worts.

Mention may also be made here of the "Annam yeast," S. anamensis, which has been employed in distilleries using the amylo-process (p. 79) for fermenting mashes saccharified by moulds. This yeast is especially useful because it ferments sugars at the same temperature as the mucors (moulds) require which are employed to saccharify the mash, namely at 35-38°. It can therefore be used together with the moulds, so that saccharification and fermentation can go on simultaneously. The organism was isolated from a mixture of wild yeasts found on the sugar-cane in Cochin China. Its cells are normally oval, though some small, spherical ones may be present. The optimum temperature for spore-formation is 33°, at which temperature spores form in nine hours; the lower limit of temperature is 12°, with fifty hours as the time of spore-formation, and the upper limit 35°, when twelve hours are required for the production of spores. S. anamensis ferments and assimilates dextrose, lævulose, galactose, sucrose, maltose, and raffinose. Lactose is assimilated, but only slightly fermented.1

To produce alcohol in quantity, yeast must not only live, but grow. ' Alcoholic fermentation never occurs without simultaneous organisation, development, and multiplication of cells, or the continued life of cells already formed " (Pasteur). Suitable nutriment must therefore be supplied, so that the cells may increase and multiply. What is suitable nutriment for yeast ? A knowledge of the substances which go to form the tissues and contents of yeast-cells will partly answer the question, and this brings us to a consideration of the chemical constituents of yeast.