210 



KNOWLEDGE ♦ 



[Aug. 25, 1882. 



lived towards the end of the last century, that some light 

 began to dawu upon the sulyect. About 200 years ago, 

 tlie famous Dutch uiicroscopist, Leeuwenhoek, discovered 

 tliat yeast consistetl of a collection of minute cells or grains, 

 as he called them, rtoating about in a thin liquid. No 

 further investigations apjx\ir to have been made into the 

 nature of this subst^ince for another hundred years, when 

 M. Appert, at the beginning of the present century, turned 

 his attention to the subject of decay, and its prevention. 

 He found that meat and other organic matter might be 

 j.res.'rved from putrefaction for almost any length of time 

 by cutting oil" all access of air. Yeast was one of the sub- 

 stances which came under his examination, and he found 

 that air was essential to its formation, and tliat, if the 

 wort of beer or the must of wine is boiled and kept in a 

 <:losely-stoppered bottle, no fermentation will ever be set 

 up. This was a verj- important discovery, because it re- 

 vealed to us tlie e.Kistence of minute germs or sporules in 

 the atmosphere, which have the power of setting up those 

 chemical changes in sugar which accompany the process of 

 fermentation. Further experiments showed that if we 

 tilt«r the air in its passage into a bottle of fermentable 

 matter by a plug of cotton wool, so as to prevent the 

 entrance of the sporules, no fermentation will ensue, not- 

 withstanding the fact that the air has free access through 

 the plug. If the vitality of the sporules be destroyed by 

 pa.'ising the air through a red-hot platinum tube, all power of 

 originating fermentation will, in like manner, be lost. It 

 had before been noticed by Cagniard de la Tour that 

 Leeuwenhoek's little grains grew from one another ^ that, 

 when a cell became fully developed, it gave off from its 

 side a minute bud or embryo cell, which, in its turn, en- 

 larged, and ultimately sent forth another bud. This propa- 

 gation Ijy gemmation or l)udding is e.xactly analogous to 

 the mode of growth in cellular plants, as mildew and other 

 fungi Yeast is, therefore, neither more nor less than a 

 plant of a low order, and has received the name of tonda 

 (Mycoderma cerevisi;e). Fabroni soon afterwards ex- 

 amined these yea.st-cells chemically, and found that they 

 consisted of a kind of bag, composed of cellulose, or woody 

 fibre, containing a liquid whose composition was protein- 

 aceous or nitrogenous. As the cells which compose the 

 fle-sh of anim.ils contain an all)uminous liiiuid of similar 

 composition, Fabroni called yeast a vegeto-animal. So far 

 the yeast-plant \\ax\ come within the range of experimental 

 demonstration ; but when chemi.sts began to theorise upon 

 fermentation and the inodua oppraruhnu, they entered the 

 arena of speculative uncertainty, within which they are 

 still groping. That yeast is the agent which causes the 

 decomposition of sugar into alcohol and carbonic acid is 

 indi.sputable, and that it appropriates some of this sugar for 

 the purposes of its ownnutrition is equally certain, but ill what 

 way its power is exert*;d is a question upon wliich eminent 

 fhemLsts are still at issue. M. Thenard and M. Pasteur 

 l<elieve that the torula attacks the sugar, and by assimil- 

 ating a jKjrtion for its own growth, causes the decomposition 

 of the remainder. Liebig, however, denies this, and as- 

 .•rilK?8 the decomposition to the action of the vital forces 

 having their origin within the plant itself. The doctrine 

 of spontaneous generation, which some have held in refer- 

 ence to the yeast-genns, is altogether inadmi.s^?ible in the 

 face of modem evidence. The protoplasm, or liquid matter 

 of the yfawtsacs, contains ammoniacal salts and alkaline 

 I.liosphat<.-.s, so that the plant lives not only at the expense 

 of the sugar, but of the gluten which is associated with it 

 in the must of wine and the wort of beer. The cells are 

 ••xc**ding!y minuU;, the largejit being only the two- 

 thousandth of an inch in diamet<T, while the smallest are 

 not more than the 8»;ven-thousandth of an inch in diameter. 



Now let us see what takes place when sugar undergoes 

 fermentation. AVhen glucose or grape sugar is exposed to 

 the air at a temperature of 2.")° C, or 30° 6, the sporules of 

 the torula tiud in it the conditions of their existence, and 

 chemical changes are at once set up. There is an appear- 

 ance of frothing from the escape of gas through the syrupy 

 mass ; and when the action is complete, all sweetness will 

 have vanished, and a spirituous taste become apparent. 

 In fact, alcohol has been produced, and carbonic acid gas 

 given off. 



The formula for glucose is C„IT,jO„ which in plain 

 language means, that it is composed of 6 equivalents of 

 carbon, 12 of hydrogen, and G of oxygen. This may be 

 shown to be equal to alcohol plus carbonic acid by the 

 following equation : — 



C„H,30, = 2 (C,II„0)-|-2 (COJ 

 The formula for alcohol being C^H^O, and that of 

 carbonic acid being CO, the equation represents that from 

 one equivalent of grape sugar we obtain 2 equivah^nts of 

 alcohol and 2 of carbonic acid. Lavoiseur was the lirst to 

 examine iiuantitatively the products of the decomposition, 

 and came to the conclusion that the weight of the carbonic 

 acid and alcohol together was exactly equal to the weight 

 of the sugar employed. For many years this was accepted 

 as true ; but subsequent experiments proved that this was 

 only true of about 94 per cent, of the sugar, and that the 

 remaining G per cent, was split up into other compounds, 

 as glycerine 3 -.5 per cent., succinic acid OG per cent., 

 cellulose and fatty matter, forming the sac or bladder of the 

 yeast-cells, 1-7 per cent Fernunitation is, therefore, a 

 much more complicated affair than was at one time sup- 

 posed ; in fact, a fractional part of the percentage is not 

 yet satisfactorily accounted for. 



There are other kinds of fermentation besides the 

 alcoholic ; thus, we have the acetous and the mucous fer- 

 mentations, which are likewise duo to vegetable sporules, of 

 different species; while the butyric fermentation is caused 

 by the ova of an animal, which can exist only in an atmo- 

 sphere of hydrog(!u. The alcohol we are now dealing with 

 is called ethi/l alcohol, to distinguish it from other alcohols 

 obtained from other sources. Thu.s, an alcohol can be 

 obtained by the dry distillation of wood, which is called 

 methi/l alcohol, methylated spirit, or wood spirit, which is 

 commonly used in the arts, as a cheap substitut<! for spirits 

 of wine or ethyl alcohol. Again, the potato yields an 

 alcohol called anii/l alcohol, potato spirit, or fusel oil. All 

 these varieties contain the same constituents, but in 

 different proportions, and are therefore iaomerir. They 

 differ in specific gravity, solubility, volatility, and in their 

 action upon the human system. They boil at different 

 temperatures. Rectifi<!d ethyl alcohol boils at 172° Fah. 

 (77 C). Methyl alcohol boils at a still lower temperature 

 — 110 F. (GO C) and when placed in an exhau.sted tube, as 

 the " pulse " glas.s, boils with the heat of the hand. Amyl 

 alcohol is very heavy, and does not boil below 270 F., or 

 .')H'' above the boiling-point of water. The heaviest of 

 these alcohols is metliyl alcohol, or wood .spirit Water 

 being taken as 1000, the specific gravity of this spirit is 

 HM, while that of ethyl alcohol is only 792. Fusel oil is 

 nearly as heavy as wood spirit, having a density of 811. 

 As our glass of wine contains, or ought to contain, only 

 ethylic alcohol, we shall leave the other varieties out of 

 consid<!ration. Wo have seen tliat the composition of 

 common alcohol is Cjl{„0. This is regarded as a radical 

 C'jH, combined with the elements of water, HO, one 

 equivalent of H in HjO being replaced. To this radical 

 the name of ethyl is given, hence the ti^-ni ethyl alcohol 

 in the same way. Methylated spirit is suyii)OS<;d to consist 

 of a radical methyl combined with the elements of water. 



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