362 LIFE : OUTLINES OF GENERAL BIOLOGY 



and wine was made by 3500 B.C., if not much earlier, and wine- 

 making means familiarity with fermentation. The juice of the 

 gTapc. squeezed out in the winepress and left to itself, soon begins 

 to bubble and froth. It gives off carbonic acid gas and "boils up", 

 and this boiling up is what the word "ferment" refers to. It has 

 the same root as the words "fervent" or "perfervid". The outcome 

 of the boiling up is that the sugar of the grape juice is changed 

 into the alcohol of the wine; and we know what was hidden from 

 the ancients, that the agents in the change are yeast cells. These 

 are always to be found in the soil of the \aneyard, whence they 

 are carried by insects on to the fruit, which they enter through 

 minute wounds. There are always sufficient infected grapes to start 

 the fermentation in the vat, but nowadays the careful maker of 

 wine does not leave things to chance nor to stray "wild yeasts". 



There are other fermentations of very ancient origin, such as the 

 making of vinegar out of weak alcohol exposed to air. This is a 

 discovery that would readily be made by chance in connection 

 with wine-making, for dilute wine left exposed soon turns into 

 vinegar. In the course of a few days a slimy mass appears on the 

 surface, and we now know that this so-called "mother of vinegar" 

 consists of countless "acetic acid bacteria" entangled in a sort of 

 thin jelly which they produce. In various countries from very 

 ancient times drinks have been made from different kinds of milk 

 by means of "lactic acid bacteria", sometimes helped by yeasts. 

 Of course the ancients did not know about bacteria or yeasts, but 

 it is interesting to find that in certain cases they added "some- 

 thing" to what was to be fermented, just as they added "leaven" to 

 the dough that was to be baked. In making the milk drink called 

 "Kefir", which goes back to the time of Mahomet, what were called 

 "Kefir grains" were added to produce the fermentation. These 

 "Kefir grains" are now known to be little packets of "lactic acid 

 bacteria" and yeasts. 



In the case of bread-making there is a fermentation of the sugar 

 in the flour, and this produces carbon dioxide, which makes thr 

 bread pleasantly spongy, very different from the compact "un- 

 leavened bread". In old days what was introduced into the dough 

 was a little "leaven", that is to say, some dough reserved from a 

 previous baking and containing numerous yeast-plants and other 

 minute organisms. Later on this was improved on by the introduc- 

 tion of better and purer yeast bought from the brewer. Later still 

 there came the use of carefully selected yeast-plants in the form of 

 powder. But however the yeast be introduced it behaves in the same 

 way, bringing about a fermentation of the sugar into alcohol with 

 carbonic acid gas as a by-product. This makes the bread "rise". 



Hut we must now inquire more minutely into the living agents 

 that bring about certain kinds of fermentation. Towards the end 



