APPENDIX. PASTEUR’S DISCOVERIES.
ONE talks glibly enough of fermentation, but the majority of us would be puzzled if asked to say what it is. For many years it has been known that minute particles of life are ever present in substances undergoing fermentation, but until recently proved beyond question by M. Pasteur, it was not known that the peculiar changes are wholly caused by these living atoms, which are so small that they can only be seen through the most powerful microscope. This discovery was the key to many problems. Pasteur soon traced the diseases of wine to the presence of various organisms which, fortunately, could be easily destroyed by heat. From this it followed that wine once heated to a certain temperature could be kept an indefinite length of time, provided, of course, no exposure to the air took place, for from the air germs of organisms similar to those killed by the application of heat might again enter the wine and multiply themselves.
The method of preserving the wine discovered by Pasteur is simple: In a suitable metal vessel the bottles of wine are placed, their corks firmly tied down. The vessel is then filled to such a depth that the water is level with the wires of the corks. One of the bottles, in which is placed the bulb of a thermometer, should be filled with water. The water in the vessel is then gradually heated until the thermometer shows that the water in the bottle has attained a temperature of 212 Fahr.
Pasteur proved by repeated experiments that the flavour of the wine is not in any way damaged by this operation. The discovery solved an important economic question, for wines heated in this manner can now be exported into all countries, or kept for an almost indefinite period without losing their flavour or perfume.
We have mentioned Pasteur’s labours for the wine-growers, for on them were based his studies on beer. {442}
At the close of the Franco-Prussian war, Pasteur, who was then recovering from an illness which lasted over two years, became eager to commence an investigation which would bring him again to the study of fermentation. Influenced, no doubt, by the patriotic wish of making for French beer a reputation equal to that of Germany, he attacked the diseases of malt liquors.
Beer is far more difficult to keep than wine, and it is said to be diseased when it has turned sharp, sour, ropy or putrid. To trace the causes of these undesirable conditions was Pasteur’s aim, and, as usual with any investigation undertaken by him, he met with success. In studying the fermentation of wine, he had discovered a new world peopled with minute beings of many different species, and in the fermentation of beer his discoveries were of much the same nature.
In wine the fermentation may be said to take place by itself, the organisms which cause it, finding their way into the liquid without the assistance of man; but in beer-brewing a small quantity of certain organisms (yeast) is put into the sweet wort by the brewer. These organisms multiply themselves, and, if of the right species, turn the sugar principally into alcohol and carbonic acid gas. The one remains in the beer, the other partly escapes and hangs about the surface of the liquid, as anyone who has put his nose into a fermenting tun has no doubt discovered. It is absolutely necessary for the production of drinkable beer that the right species of organism be set to work in the wort. If the wort was left to itself to ferment, as is wine, the results would be very unsatisfactory. Various kinds of organisms (from which wine is protected by its acidity) would enter into it from the air, divers ferments would take place, and more often than not an acid or putrid beer would be the result.
Every beer-disease Pasteur found to be caused by its own peculiar organisms, which enter into the liquid sometimes from the air and often in the yeast, and the causes of many mysterious occurrences in breweries at once become clear. Professor Tyndall said of this discovery: “Without knowing the cause, the brewer not unfrequently incurred heavy losses through the use of bad yeast. Five minutes’ examination with the microscope would have revealed to him the cause of the badness, and prevented him from using the yeast. He would have seen the true torula overpowered by foreign intruders. The microscope is, I believe now everywhere in use. At Burton-on-Trent its aid was very soon, invoked.”
The brewer has, therefore, to keep his wort free from foreign organisms {443} other than the yeast plant. When the wort is boiled, any harmful organisms it may contain are killed, and it can be indefinitely preserved even in a high temperature, provided the air with which it comes in contact is free from the germs of the lower microscopic organisms. Pasteur’s son-in-law, in the account he has written of the great savant’s life and labours, says that some brewers have constructed an apparatus which enables them to protect the wort while it cools from the organisms of the air and to ferment it with a leaven as pure as possible. At the Exhibition of Amsterdam were shown bottles only half full, containing a perfectly clear beer which had been tapped from the opening of the Exhibition.
As the causes of disease in beer are much the same as in wine, the same preservative—heating—may be applied. But beer differs from still wines in containing carbonic acid gas which heat displaces, and as beer which has lost its briskness is not pleasant drinking, it can only be advantageously so treated when contained in bottles. Both in Europe and America, says M. Pasteur’s son-in-law, the heating of beer is practised on a large scale. The process is called Pasteuration and the beer Pasteurised beer.
A very high temperature, as we have shown, kills the germs of disease in wine and beer. Extreme cold has the effect of indefinitely suspending the action of the ferment. A moderate temperature seems most favourable to the action of the organisms which work the wondrous changes in the wort. In England beer is usually fermented at a temperature of between 60° and 70° Fahr., and the process only lasts a day or two. In Germany the general practice is to ferment at about 40°, a temperature maintained by means of vessels containing ice, which are thrown into the fermenting tuns. This lower temperature checks the action of the ferment and the process lasts for fifteen or even twenty days.
The only other point to be noticed here in connection with fermentation is the peculiar fact discovered by Pasteur, that the organisms causing the ferment can live without air. When the wort is in the fermenting tun, the sides of the tun, together with the heavy carbonic acid gas hanging over the surface of the wort, exclude all air from the organisms, which can only obtain a small amount of oxygen from the liquid. There is, in fact, life without air. Pasteur made some interesting experiments which showed that there was a great difference in the action of the organisms according as they were placed in deep vats, cut off from the air by the carbonic acid gas, or in flat-bottomed {444} wooden troughs with sides a few inches high. In this latter situation the life of the ferment seemed enhanced, but the amount of sugar decomposed by the organisms was proportionately different from that decomposed in the vats. In the vats one ounce of ferment decomposed from seventy to a hundred and fifty ounces of sugar, while in the troughs the same quantity of ferment decomposed only five or six ounces of sugar. The experiment showed that the more the yeast was exposed to the air, the less was its power as a ferment, and that there is a remarkable relation between fermentation and life without air.
Dumas once said to Pasteur before the Academy of Sciences: “You have discovered a third kingdom—the kingdom to which those organisms belong which, with all the prerogatives of animal life, do not require air for their existence, and which find the heat that is necessary for them in the chemical decompositions which they set up around them.”
