94- BACTERIOLOGY 



The sugar is fermented by the yeast with the production of the 

 gas and a small amount of alcohol. The gluten of the wheat 

 is rendered soft and digestible. During the heating, the yeast 

 is killed off, starch granules are ruptured, the albuminoid con- 

 stituents are coagulated, and some of the starch is converted 

 into dextrin, especially in the outer portion or crust. 



Various physical and chemical methods have from time to 

 time been introduced to replace the yeast-fermentation. 

 Aeration by the mechanical forcing of carbonic dioxide gas 

 into the dough, or by the freeing of the gas from various 

 baking powders on heating the bread, are not infrequently 

 used, but in none of them is the resulting bread so well 

 flavoured and digestible as that made by the older methods 

 with yeast or leaven. 



Alcoholic Fermentation. In 1837, three investigators, 

 Kiitzing, La tour, and Theodor Schwann, independently dis- 

 covered that alcoholic fermentation was due to a living organism 

 in the " Yeast." Their discovery was met with ridicule at the 

 hands of the great chemists and other scientists of their time ; 

 and it was not till Pasteur announced, in 1857, that he was 

 " of opinion that alcoholic fermentation never occurs without 

 simultaneous organisation, development, multiplication of cells, 

 or the continued life of cells already formed," and subsequent re- 

 search by others confirmed his statement, that its truth was 

 recognised and accepted. The chemistry of the process is one 

 of great complexity. In a recent important monograph on 

 " Alcoholic Fermentation " by Dr. Arthur Harden, it is stated 

 that "the sugar which has diffused into the (yeast) cell 

 unites with the fermenting complex and undergoes the char- 

 acteristic reaction with phosphate, already present in the cell, 

 yielding carbon dioxide, alcohol, and hexose-phosphate." 



Beer, in all its numerous forms ale, mead, porter, and the 

 like is the result of two chemical processes : (1) the conversion 

 of the starch in the germinating seeds of various cereals such 

 as barley, maize, oats, wheat, and rice (the first-mentioned 

 barley being the most important) into sugar under the action 

 of the ferment diastase. Germinating barley contains the 

 necessary starch and diastase ferment in the fullest amount, 

 and barley-malt, as the resulting product is called, is most 

 used for brewing in this country. The germinating barley is 

 killed by heat, dried, crushed between rollers, and treated 

 with hot water in the mast-tun to form a wort (that is, the 

 clear, sugar-containing liquid which contains the products of 

 the alteration or conversion of the starch), which is then boiled 

 with hops, filtered, cooled, and : (2) fermented by the addition 



