The addition of a large quantity of water to the yeast dissolves the network. Nevertheless, as Hansen has shown, it forms anew, unless the treatment be repeated sufficiently often and prolonged. According to H. WILL (VII.), however, it is permanently removed when the washing is repeated ten times in twenty-four hours. Now, since the percentage of the constituents forming the network in beer yeast (and also pressed yeast) influences the compressibility of the mass, it follows that this property may under certain circumstances be impaired by the method pursued for washing the yeast (§ 255). On the other hand, it becomes indispensable to remove these mucinous constituents from the yeast when the latter has to be subjected to a quantitative macrochemical analysis for the purpose of ascertaining the composition of the corpus of the cell. This purification might be effected by the washing process, were it

FIG. 158.-Fragments of Gelatinous Network.

From a beer yeast. Unstained. Magn. 1000. (After Hansen's original drawing.) not that the treatment so far modifies the cells themselves as to make the results of the analysis, under certain circumstances, not worth the trouble expended thereon.

In this case also, practical use may be made of the general property of the vegetable gums for thickening to a jelly in presence of borax solution, and thus separating from the liquid. On the basis of an observation by DUMAS (III.), H. WILL (I.) found that a thick barm of beer yeast, when treated with a 5 per cent. solution of borax, immediately agglomerates into pure white lumps, which quickly subside and unite into large solid masses that fairly crackle when spread out on plates of gypsum. This behaviour is advantageously utilised in the production of pure-culture yeast on a practical scale. The thick barm of yeast from the fermenting cylinder of the pure yeast apparatus is treated with a similar solution of borax, when destined to be sent out in a pressed state. This treatment not only accelerates the deposition of the yeast and considerably facilitates pressing, but also greatly ameliorates the colour of the yeast, and therefore improves its appearace, without impairing the fermentative power to any appreciable extent. It would therefore seem that

the precipitated mucinous matter carries down the yeast cells, but not the brown hop resins, &c.

The foregoing explanations may also prove useful in practical analytical chemistry, namely in cases where the percentage of sugar in wine and similar liquids is to be determined with the aid of Fehling's solution. In such event it should be remembered that this reagent also precipitates the gummy substances present, the resulting precipitate of cuprous oxide also including the precipitated copper compound of the gum; consequently too large an amount of copper is weighed, and the quantity of sugar calculated therefrom will be far in excess of the truth. This applies more especially to the so-called yeast wine, which is an inferior wine obtained by pressing the sediment (the so-called wine yeast) from the wine casks. This sediment is very rich in gum, and therefore (as was shown by K. AMTHOR (III.)), should not be examined for its sugar content, by the aid of Fehling's solution, without bearing this in mind. With the said yeast wine, in this respect, must be classed the fruit wines (mostly very rich in mucinous matter) and such grape wines as have been racked from the yeast very late, and have therefore absorbed large quantities of gum from the latter during the protracted period of contact therewith. It may also be mentioned that certain gummy substances find their way from the grapes into the wine must, and that experiments have been made by G. NIVIERE and A. HUBERT (I.) with regard to the gum recoverable from wine. Gummy substances are also found in malt, having been isolated therefrom by C. J. LINTNER (IV.). It may occasionally happen that such a gum, when not eliminated during mashing and boiling, renders the beer dichroic, instances of which kind have been reported by H. WILL (XI.).

$ 255.-Albuminoid Mucinous Substances. "Head" and Frothy Fermentation.

The network enveloping the yeast cells is not invariably composed of the carbohydrate gums described in the preceding paragraphs. Careful investigations performed by H. WILL (VII.) have shown that, in the case of a considerable number of stocks of beer yeast, the enveloping, agglomerating network is constructed of a material that gives all the reactions of albumin. This albuminous matter is more abundant in the upper layer of the sedimental yeast found in the fermenting tun at the close of primary fermentation, but is less plentiful in the lower or core yeast; and it differs from the other albuminous admixtures present therein, not only in its lack of definite form, but also in point of origin. From the fact that it is also encountered in cultures grown in nutrient solutions free from albumin,

one may conclude that it is elaborated by the yeast cells themselves. Of course the further question must be left undecided whether, in nutrient solutions that already contain albumin, the amount is increased by some portions of this latter substance.

Beer yeast (sedimental yeast) when well shaken up with ether (1:1), takes up a certain quantity of the same, the amount depending on the percentage of this albumin and the aforesaid mucinous substances present. Yeast from the upper layer takes up two to three parts by volume of ether, but core yeast only about one part, the mucinous substances forming bubbles charged with ether. In these laboratory experiments the ether plays the same part as carbon dioxide does in practical fermentation in the vat; and as the mucinous substances do not remain in the sedimental yeast, but are in part ceded to the supernatant liquid, they are able, in the above-described manner, to effect a firmer retention of the carbon dioxide than can be done by mere physical solution. Consequently, under certain circumstances, they may considerably augment the so-called permanent absorptive capacity for carbon dioxide on the part of the beer.

The fact that a soluble albuminoid takes part in the formation of the "head" or scum thrown up by fermenting wort was first suspected by Habich, and then proved, by isolation, by C. LINTNER, sen., and REISCHAUER in 1876. This froth glutin (Krauesenglutin) is said to originate in the wort or the malt. By means of comparative fermentation experiments with different beer yeasts, it was then established by ALB. REICHARD (III.) that, in addition to this malt albuminoid, the presence of certain mucinous excretions (especially those of an albuminoid character) from the yeast is essential to the formation of a normal "head" composed of fine bubbles. The condition of these formative materials in the head at different stages of fermentation has not yet been more closely investigated.

In the case of a liquid absolutely devoid of viscosity and free from mucinous constituents, the bubbles of carbon dioxide liberated in the interior would immediately unite to large ones and make their escape without delay as soon as they arrived at the surface. By means of these mucinous substances, however, each bubble of gas is surrounded by a tough envelope at the instant of liberation, and thereby prevented from becoming merged into others. These bubbles collect on the surface of the liquid, stick together, and thus form by degrees the fine, permanent "head" so desired by the brewer and the pressed-yeast maker. The ascending bubbles also carry to the surface other constituents of the fermenting wort, and incorporate them with the structure of the head, chief among these being yeast cells (see below), then fragments of tissue from the

mashed materials, and finally (in the case of hopped wort), glutin substances and globules of hop resin, to which latter the disagreeable bitter flavour of the head is due. A permanent head of the kind is peculiar to beer, and is not produced in the fermentation of unhopped wort or on wine must; and one must therefore conclude that the presence of hop constituents is indispensable to the structure of a normal head on beer. The accuracy of this conclusion has been rendered more probable by the results furnished by the experiments of E. EHRICH (I.).

The thickness of the head thrown up by the activity of yeast in wort, mash and wine must, is dependent-other conditions being equal-on the available supply of the materials, by means of which the initially naked gas bubbles are converted into mucinous bubbles charged with gas. However, the amount of these substances produced by different stocks of yeast, stands in no definite relation to their fermentative activity, i.e. the amount of carbon dioxide liberated in unit time. Hence it will occasionally happen, in comparative fermentation experiments, that a relatively high loss in weight (i.e. liberation of carbon dioxide) is noticed in specimens which, from their appearance during fermentation, would seem to have fallen behind considerably. Thus, for instance, in a set of experiments conducted by MUELLER-THURGAU (IV.) with 25 stocks of fruit and wine yeasts, the one (a Wædensweil cider yeast) that liberated the largest amount of gas fermented its nutrient substratum without the slightest formation of head. So far, no close attention has been devoted to the capacity of the wine yeasts for depositing albuminoid mucinous substances, though, in addition to the above named, other observations have been recorded indicating the existence of this capacity. Thus, for example, the well-known loss of colour in red wines when treated with white-wine yeasts, seems attributable to the action of similar albuminoids. In contrast to the red-wine yeasts, which are already saturated with colouring matter, the albuminoid mucinous constituents of white-wine yeasts are still in a position to take up tinctorial substances. The case is analogous to that of browned wines when treated with yeast, this procedure, according to NESSLER (III.), affording a remedy for the malady in question. The combining and precipitating power of these albuminoid excretions of yeast is probably the cause of the phenomenon-well known in practice, and recommended for utilisation by H. MUELLER-THURGAU (III).—that a wine rendered turbid by bacteria can be cured by pitching with a small quantity of must in active fermentation. This must is very rich in yeast cells, MUELLER-THURGAU (V.) having detected about 3 milliards per litre. P. LINDNER (XVI.) has reported a similar instance of the cure of a sarcina-ropy white beer by adding yeast and rousing.

The chief practical difference between top fermentation and bottom fermentation is due to the aforesaid relation between the yeast cells and the mucinous substances, either produced in the nutrient medium or excreted by the cells themselves. A typical example of top-fermentation yeast in its fullest development is afforded by the stocks forming the chief component of unadulterated pressed yeast. The main features in the preparation of this article by the old or Viennese method are as follow: About three to four hours after the wort has been pitched with yeast a so-called "artificial" yeast (§ 148) of suitable quality—the mash begins to work. The scum of husks and grains that has accumulated on the surface in the meantime, is now penetrated by an ascending white head, the development and growth of which, during the next twelve hours, presents a picture of ebullient motion of considerable briskness. Under ordinary circumstances the head at the end of this time will have attained a thickness of twelve to fifteen inches, whilst the gravity of the wort will have decreased to about half its initial value. This head forms the vehicle containing the major part of the yeast crop reproduced from the pitching yeast. The bubbles are still transparent. Whereas, during the next three to four hours, the head does not increase in thickness to any great extent, the young cells therein begin to grow; this stage is termed the ripening of the head. The bubbles begin to turn cloudy, and finally reveal the presence of large yeast colonies which, to the unassisted eye, appear as white spots. The reproduction of the cells is now ended; the vat is ripe, to use a practical term; and the gravity of the wort has fallen to about one-third the original strength. The head is next skimmed off by means of suitable skimmers, and is transferred to a sifter in order to separate the coarser adherent particles of the grains, the elimination of the finer undesirable admixtures being effected by the subsequent washing, to which reference will be made in the paragraphs dealing with the autodigestion of yeast. For the present we will merely utter a warning against a one-sided judgment and imperfect appreciation of the importance of the mucinous excretions in connection with facilitating the ascent of the yeast in the fermentation vats during the manufacture of pressed yeast, by calling attention to the influence exercised by the viscosity and chemical constitution of the mash on the working of the yeast, and therefore on the yeast crop. More detailed information on this matter will be found in the volume on pressed-yeast manufacture, issued by O. DURST (I.).

The so-called riotous or bladdery fermentation of beer yeast may be left out of consideration here. The structural materials for the large, tough bladders forming the head in this case are not supplied by the yeast but from the sludge-as was shown