INVISIBLE DOMESTIC SERVANTS 53 



yeast- cells have come to the end of their annual feast ; 

 they dry up, and are dispersed as dust. The great multi- 

 tude perish ; only one here and there, perhaps after many 

 vicissitudes, succeeds in getting lodged on an unripe grape 

 of the following season. 



When men began to press out the sweet juice of the 

 grape to make themselves a refreshing drink, yeast-cells 

 would often get from the grape-skins into the liquor, and 

 ferment it. Then the juice would froth with the gas 

 evolved, and it would soon be remarked that a little of 

 the fermenting juice would start fermentation in a second 

 vessel. It would be enough to keep a fermenting vat 

 unwashed till the following grape-harvest to start the 

 fermentation again with certainty. How yeast-cells came 

 to be employed in turning the sugar of malted barley to 

 alcohol nobody can tell ; but we know that the Egyptians 

 of old had found out the trick. 



It was a further step to employ these same yeast-cells 

 in making bread rise ; this time it was not the alcohol 

 but the gas (carbonic acid) which yeast gives off which 

 was useful. Flour contains starch and diastase, a ferment 

 which can under suitable conditions change starch into 

 sugar. The diastase is not a living thing, as yeast is ; but 

 it is formed within living plants and animals, and turns 

 starch into sugar for them. When the starch of a root 

 or a seed is to be circulated for the benefit of the growing 

 tissues, it is first turned into sugar by diastase. Fresh 

 green leaves contain diastase ; so do many seeds ; so do 

 the salivary glands of man and other animals. When we 

 mix flour with water, the diastase which the flour con- 

 tains soon gets to work, and much of the starch is changed 

 to sugar. If now a little yeast is introduced, and the 

 bread kept at the summer- temperature at which yeast is 

 most active, the yeast consumes the sugar, setting free 

 alcohol (which is allowed to escape in the baking) and 

 plenty of carbonic acid, which forms bubbles in the sticky 

 dough, and makes it spongy. 



