if the nutrient solution was charged from the beginning with a quantity of yeast exceeding the maximum amount (Prior).

Finally, it may be added that when aëration is so strong as to cause a violent commotion, its influence then becomes disadvantageous, and very markedly so with defective nutrimental conditions and yeast species of small fermenting power (Buchner and Rapp).

Energy of Fermentation and Fermenting Power.By activity or energy of fermentation is understood the intensity with which a yeast can decompose a sugar within a certain time. It of course varies in the different yeast species. Prior has determined it for some species by Meissl's method; the latter consists in noting the weight of carbonic acid which is liberated by 1 gram of yeast from a sugar solution of a certain composition in six hours at 30° C. According to Meissl, a normal yeast is one which liberates 175 gram of carbonic acid gas under the above conditions; the energy of fermentation of this is then put down as 100. Prior found the following values for the given species:

Permeability of the Cell Membrane. Since the transformation of the sugar into fermentable sugar takes place in the interior of the cells, the energy of fermentation is thus also a measure of the permeability of the cell wall (Prior). This varies according to the age and condition of the cells,

14.5 grams of a mixture of 400 grams of candy sugar, 25 grams of ammonium phosphate and 25 grams of potassium phosphate are dissolved in 50 c.c. of tap water.

and depends, besides, on the power of the latter to form fungous mucilage, since the permeability diminishes the more this substance is given off. The permeability varies, of course, with the different kinds of sugar. For instance, Prior obtained the following result for Carlsberg bottom yeast No. 2:

It may be seen from this that saccharose had the greatest diffusing power.

Products of Fermentation.-Besides ethyl alcohol and carbonic acid gas, the saccharomycetes form, during fermentation, other substances also, although in smaller amounts, viz., glycerine and succinic acid. Zopf discovered a species, Sacch. Hansenii, which forms oxalic acid. Raymann and Kruis have proved that, under certain conditions, the culture yeasts employed in the manufacture of spirits form amyl alcohol. In addition to these, volatile organic acids are formed, e.g., acetic acid and volatile, ester-like bouquet substances. The quantity of these substances varies according to the conditions under which the fermentation takes place, and their quality also varies in the different species and races, so that the product formed by the latter is. extremely variable. Having regard to this, the necessity urged by Hansen for the systematic selection of races in practice will be recognised. According to Prior, the wild yeast species (Sacch. Pastorianus I., II. and III., Sacch. ellipsoideus I. and II.) form larger amounts of volatile than of fixed acids, whereas with culture yeasts the opposite is usually the case. The ester-like substances produce a strong taste and smell in the finished product, even when present in small quantities, these substances, which the various species produce, being widely different from one another. Thus Sacch. anomalus brings out a strong taste

and smell of fruit ester, while some of the disease yeasts discovered by Hansen develop a very strong bitter taste and disagreeable smell.

There are formed in wine, by the different wine yeasts, those volatile compounds which Wortmann calls secondary bouquet substances (fermentation bouquet), and which are the substantial cause of the special taste of wines from particular places. These by-products are therefore of no little importance.

Many yeasts are capable of a reducing action, forming sulphuretted hydrogen when sulphur is present during fermentation. Other species are able to form sulphurous acid in must (B. Haas, W. Seifert) and also in wort (Schwackhöfer, Will). Many wine yeasts have an acidconsuming action (Schukow, Wortmann), gradually using up the organic acids present in the wines. (Cf. W. Seifert's researches mentioned later on.)

Auto-Fermentation.-When thick liquid yeast is set aside at a favourable temperature, it may be seen that, although no culture liquid is present, alcohol and carbonic acid gas. are formed. This phenomenon is called auto-fermentation. It takes place through the yeast transforming its selfcontained food stuffs. According to Lintner, the glycogen in the yeast cell is, in auto-fermentation, apparently turned first into sugar, and this then fermented into carbonic acid. and alcohol. During auto-fermentation yeast gives off a smell, more or less strong, of fruit ether, which probably arises from esters of the higher alcohols.

Yeast Types. In fermentation industries various yeast species occur which exhibit different actions during fermentation. We shall now consider yeast types from this point. of view. The intensity with which the sugars are attacked by the yeast species varies, as we have seen. At the very beginning, when Hansen introduced his pure culture system

into the brewery, he proved that there was a difference in this respect between the two brewery bottom yeasts named by him Carlsberg bottom yeast No. 1 and No. 2. Several such types were found later; the Berlin station has established the following three, viz., Saaz, Frohberg and Logos. These are thus characterised by Prior: The Saaz yeasts in a fermentation leave unfermented most achroodextrin III., and consequently also more maltose than those of the Frohberg type, which again leave more than the Logos yeast. Prior, however, does not recognise the Saaz and Frohberg types in the physiological sense of fermentation. According to this author the same degree of fermentation is finally reached with both, if the fermentation is conducted under the most favourable conditions (large yeast supply, high temperature, strong aëration).

Top fermentation is one in which the froth on the surface is often covered with a thick layer of yeast; in bottom fermentation this layer is never thick, and is sometimes entirely absent. In typical top and bottom fermentations this, the really only noticeable point of difference, is very prominent. Various investigators have attempted to find definite pronounced characteristics for each of these groups; but just as there are species which, with respect to the phenomenon of fermentation, stand between both categories, so is it also with those properties which have been classified as special characteristics. A. Bau considered he had found a distinctive property of the bottom yeasts in the fact that they completely ferment melitriose (raffinose), whereas top yeasts are unable to do this. The test applies generally, so far as his own experiments go, to forms included under Sacch. cerevisiae; but recent experiments by himself and by Schukow have shown that most of the typical bottom yeast forms among the wine yeasts cannot completely ferment melitriose, and the above char

acteristic has thus undergone a considerable limitation. The action is therefore as follows: The enzyme invertase present in yeast decomposes the melitriose into melibiose and fructose; the top yeasts especially are only capable of fermenting the latter of these (Bau). On the other hand, most bottom yeasts contain the enzyme melibiase which breaks up melibiose into dextrose and galactose, both of which are fermented by most bottom yeasts (E. Fischer).

Sometimes a bottom yeast may for a time exhibit feeble signs of top fermentation (Hansen, Kühle). In this respect, therefore, no absolute boundary can be drawn between top and bottom yeast. It is certain, however, that no one has hitherto been able to transform a typical top yeast into a permanent typical bottom yeast, and vice versa. It used to be a general belief that by the cultivation of a top yeast at a low temperature it could be transformed into a bottom yeast. But Hansen has cultivated such typical top yeasts as Sacch. cerevisiae I. and Sacch. Pastorianus III. for several years at a temperature of 5° to 7° C. without anything happening except that, as one might expect, the fermentation was feebler; but as soon as the cultivation was continued again at a high temperature, the signs of top fermentation became as prominent as before. Conversely, Hansen has cultivated typical bottom yeasts such as Sacch. Pastorianus I., Sacch. ellipsoideus I. and II., Carlsberg bottom yeast No. 1 and No. 2, and several others for years at ordinary room temperature, i.e., at a temperature considerably higher than that usually employed in bottom fermentation breweries, without any signs of top fermentation ever appearing. In earlier times the view was held that if a bottom yeast was allowed to form a film, it was then turned into a top yeast. Hansen has shown, by exact experiments, that this also is entirely false.

Culture yeasts is the name given to such yeast species