Mr. J. Lowe on the Development of the Yeast -plant. 305 



meuting liquor. The Unterhefe is the ferment of Bavarian beer, 

 which is allowed to ferment very slowly and at a low temperature. 

 The formation of lactic and acetic acids is thus avoided. The fol- 

 lowing is a brief account of the changes which I have observed yeast 

 to undergo in the process of fermentation at the distillery of Messrs. 

 Duncanson, and at the brewery of Messrs. Jeffrey, to whose kindness 

 I am much indebted. Before its application to the wort, yeast is 

 seen to consist of isolated cells of a spherical form, intermixed with 

 some which are oval or tubular. These latter are only formed on 

 the surface of the yeast where it has come in contact with the air. 

 They are the commencing mycelium, and should never be present in 

 any considerable quantity, as they materially affect the process of 

 fermentation. The spherical cells are seen to be of two kinds ; the 

 one having a thin, very transparent cell-wall, containing from two to 

 ten nuclei ; these are found in yeast which has become sour, and 

 they are usually met with at the bottom of the yeast-cask. They 

 appear to correspond with the Unterhefe of Mitscherlich. In speci- 

 mens of yeast kept in bottles, I have found that the cell- wall became 

 thinner and the nuclei more numerous in proportion as the fluid 

 became more acid. The other kind of cell has a thicker cell-wall, 

 and contains, instead of a number of nuclei, a large, globular, granular 

 mass or blastema, which, in older yeast, is converted into nuclei. 

 This is the most perfect form of yeast, and is the only kind which 

 should be used. Its activity I have found to be always proportionate 

 to the thickness of the cell-wall ; and this, a most important subject 

 to brewers, can easily be determined under the microscope, and thus 

 the value of any specimen of yeast made apparent. After being 

 added to the wort, yeast, which consists of the two varieties of cells 

 above mentioned, is observed to undergo two kinds of growth. The 

 nucleated cells, with the thin walls, burst and liberate the nuclei 

 {globulins seminiferes of Turpin), which then increase in size and 

 become like the second kind of cells. This is the form of propaga- 

 tion which Turpin observed in the rupture of the cells, although he 

 makes the cell-contents appear to be finely granular instead of nuclear. 

 1 am satisfied that it only takes place in old, acid yeast, and not, as 

 Turpin imagined, as a result of normal fermentation ; and this ex- 

 plains why others have failed to observe the process of bursting in 

 fermenting yeast, for it can only be seen on the first addition of the 

 yeast to the wort ; and, moreover, in new yeast these cells are often 

 altogether absent. The thick-walled cells and the enlarging nuclei, 

 after the period varying according to the temperature and the activity 

 of the yeast, are observed to put forth minute bud-like processes, 

 which soon separate and enlarge themselves, afterwards undergoing 

 the same process. This is the second mode of growth noticed by 

 Turpin, and is, in fact, the only result of true fermentation. My 

 own observations confirm those of Mitscherhch, who thinks the two 

 modes of propagation just mentioned are the only ones, and that the 

 conversion of 'ylobuline'' into cells is entirely erroneous. The budding 

 was observed by Turpin to begin after an hour, and the gemmations 

 were doubled in size in three hours ; in eight, they had attained the 

 Ann, ^ Mag. N. Hist. Ser. 2. Vol. xx. 20 



