The yeast-plant is a unicellular vegetable organism without chlorophyll, belonging to the genus Saccharomyces (Reess). Since they contain no chlorophyll, the yeasts, like other fungi, cannot obtain their carbon by decomposing carbon dioxide. On the contrary, they elaborate this gas and evolve it, absorbing oxygen at the same time.

Ordinary brewer's yeast seen under the microscope is found to consist of spherical or ovoid cells, either free or united in simple or branching rows. These rows are produced by the budding-off of daughter-cells from a single parent cell; and as this is the chief method of reproduction, the yeasts are classed among the "budding fungi"; but they are capable also of reproduction by the formation of spores. The genus Saccharomyces, in fact, is restricted to organisms which can form spores.

For technical purposes, the yeasts may be divided into two groups, the "cultivated" yeasts and the "wild" yeasts. The first includes those which have been used from time out of mind in the operations of brewing. They belong to one species only, namely, Saccharomyces cerevisice. There are, however, many races and varieties of this species, differing quite notably in certain of their properties, such as the degree to which they can carry on fermentation, the rapidity with which they can bring that process about, and the flavour of the product obtained thereby. Of these "cultivated " yeasts there are two main types, one known as "top fermentation" yeast, the other as ' bottom fermentation " yeast. These are so called because the former rises and collects on the surface of the fermenting liquid, whereas the latter falls to the bottom. "Top" yeast is the variety used in this country; whilst "bottom" yeast is employed on the Continent and elsewhere in the making of "lager" beer.

"Wild" yeasts are a numerous group. They occur wild in nature, and often on the surface of fruits - as, for instance, the wine yeasts which are present on the skins of grapes, and which set up fermentation of the "must" or grape-juice in the making of wine.1 As these wild yeasts occur in the air, they may find their way into the brewery, where they may cause much inconvenience, since some of them are obnoxious. In the distillery, they are less a source of trouble than are bacteria and moulds, since the relatively high temperature at which fermentation is carried out is not favourable to the development of the wild yeasts.

Classification

At one time the shape of the yeast-cells as seen under the microscope was taken as the basis of classification. Thus S. cerevisice included those yeasts of which the cells were approximately spherical, whilst those with elliptical cells were named S. ellipsoideus, and those the cells of which had the shape of a sausage were assigned to the species S. Pastorianus. It is true that the shapes mentioned are more or less characteristic of the respective species, but further study of the yeasts has shown that under certain conditions of cultivation the shapes of the cells may change, and one kind of yeast may temporarily assume the form of another. Hence the classification by shape alone was found to be misleading. More trustworthy methods were worked out by Hansen - to whom is also due the process by which pure yeast of any variety can be obtained by cultivation from a single cell of that variety (see p. 48).

Having by this process obtained pure yeasts of different kinds, Hansen found that each species could only develop spores between certain definite temperatures, and those temperatures were different for the various kinds of yeast. He recognised three distinct species of spore-forming Saccharomycetes, namely S. cerevisiæ, S. Pastorianus, and S. ellipsoideus, the first being the cultivated yeasts, the other two wild yeasts. These could be further subdivided into varieties, which were distinguished as 8. cerevisiæ I., S. cerevisiæ II., and so on. At the following temperatures, no spores are formed by the six varieties named, but they are produced at temperatures intermediate between the extremes given: - -

1 It should be mentioned, however, that certain of the wine yeasts are now regularly cultivated.

The yeast S. cerevisiæ I, it may be mentioned, was isolated from a "top" yeast used in an Edinburgh brewery.

Hansen also found that when these six varieties of yeasts were cultivated at temperatures at which they could all form spores, the time required for the commencement of spore-formation was different for the various organisms. This is illustrated for two temperatures in the table subjoined: -

Spore-formation occurs best when young and vigorous yeast is spread in thin layers on a moist surface - e.g., on slices of carrot or potato, or better, on a plaster of Paris block standing in distjUM water - and kept at a suitable temperature. To obtain the, young cells the yeast is cultivated for some time in beer wort, and then a few drops of this are mixed with some fresh wort and incubated at 25-26° for twenty-four hours.

If then it is required to ascertain whether a specimen of ordinary brewery " top " yeast, S. cerevisiæ I, is contaminated with any of the above five wild yeasts, the specimen may be tested by first obtaining a culture of young cells as just described, and then finding the time required for spore formation when a little of the culture is incubated at 11-12° on a plaster of Paris block.

The Mock used for this purpose has the shape of a truncated cone, and is placed in a glass vessel provided with a cover. To be trustworthy, the experiment must be made with sterilised apparatus. The covered glass vessel containing the dry block is therefore wrapped in paper and heated in an air-oven for 1 1/2 hours to a temperature of about 110°. After cooling, the paper is removed, and the cover raised a little to allow of a small quantity of the yeast culture being placed here and there on the top of the block.