THE power of fungi to decompose this or that organic substance, sugar, starch, cellulose, fat, alcohol, protein, etc., depends on their ability to produce those agents of specific chemical change which are known as enzymes. Enzymes are not peculiar to fungi. All living things possess them but fungi often provide magnificent large-scale examples of enzyme activity in the processes known as fermentation. The fungus yeast converts sugar into alcohol. The change is brought about by enzymes. Another fungus changes alcohol to acetic acid : a fungus-like organism produces the enzyme that does it. An enzyme is a substance which, though it takes no permanent chemical part in the change it produces, is nevertheless a potent agent of that change. Minute quantities of an enzyme can so speed up the rate of a given chemical change that large quantities of the end product of that change are produced and hence the importance of enzymes in industry : brewing and the like. The yeasts may be taken as examples of fungi which do spectacular work by means of their enzymes.
Yeasts are single-celled plants that do not usually form a thread-like spawn of the kind produced by the mushroom, though they can be induced to do so under special conditions. They possess the faculty of converting sugars into alcohol and carbon-dioxide. The yeasts are of many kinds. Some act on only a few kinds of sugar and others on many kinds.
The beer yeast consists of a number of races that are termed bottom and top yeasts according to whether they live within the liquid at the bottom or at the surface. When they live at the surface, they cause a brisk fermentation with a large amount of froth. The ‘head ‘of Munich lager-beer is a product of top-yeast activity. The yeast of beer is found only in cultivation, but the wine yeast is a wild species living normally on the outside of grape skins. The flavour of particular wines is due to the particular variety of yeast used to
ferment the grape juice. Every wine district seems to have its own particular form of yeast with a different kind of growth. Some yeasts, like the Burgundy and Champagne forms, settle quickly and leave a clear liquid while others remain suspended for a long time and take a long time to settle. Sometimes unwanted fermentation occurs and turbid diseases of beer result from the presence of unwanted yeasts.
THE LITTLE BACILLI THAT MAKE THE BREAD RISE THE cultivated yeasts are probably among the oldest of all cultivated plants, as even in biblical times there was a distinction drawn between unleavened bread and leavened bread that had been ‘raised ‘by yeast. The leaven was a lump of dough kept from one baking to the next and contained the active yeasts, while unleavened bread was simply the tough mass produced by mixing flour and water and
then baking the mixture. One of the old methods of obtaining yeast for baking was that known as ‘salt-raising,’ by which salt was added to milk so that the bacteria of the sour milk were delayed in development until the yeasts from the air could develop. Modern investigation of ‘salt-raising,’ however, declares the action to be produced by bacteria.
To-day the baker does not have to rely on brewers’ yeast, nor the housewife on chance fermentation by wild forms, because ‘pressed-yeasts ‘are produced commercially. Grain of either rye, wheat or barley is malted as in brewing, the process being stopped by heat at the right point when all the starch in the grain has been converted to sugar. The mash is then soured with acid and filtered and the resultant liquid, called a ‘wort,’ is inoculated with yeast. After growth the yeast is filtered and pressed into cakes.
Yeasts are not the only fungi concerned in fermentation; in fact in recent years many fungi have tended to assume importance in’ many industrial processes of which the time-honoured production of alcohol is only one. Citric acid is produced by the fermentation of sugars with one of the common ‘mildews ‘called Aspergillus, and for a long time alcohols and vinegar have been produced on an industrial scale by means of the fermentative properties of bacteria. The use of bacteria in industry is, however, so specialised that it is a subject in itself. To show how wide are its possibilities it is only necessary to mention that the whole of the rubber industry depends on the fact that the milky-juice of the rubber tree is coagulated into rubber by a special bacterial flora.