188 SCIENCE PROGRESS 



carbon dioxide. When the dough so prepared emerges from 

 the kneading machine, it expands because of the gas within it. 

 The whole process is a rapid one, and the difficulty of retaining 

 the gas within the dough is minimised as compared with 

 " baking-powder " methods. But " aerated bread," good though 

 it is, lacks the characteristic and pleasant flavour of first-class 

 yeast-fermented bread. The chemistry of flour and bread is 

 particularly abstruse, and as a layman I am content to say that 

 the complicated actions and reactions resulting from yeast 

 fermentation do cause, when the best of flours and the best 

 methods are used, a particularly pleasant flavour, which aerated 

 bread does not possess. If the alcohol produced has any direct 

 effect, it must be evanescent, for if any at all be left in the bread 

 as it leaves the oven, the proportion is so exceedingly small that 

 it may be regarded as non-existent. Any carbon dioxide that 

 may be left rapidly disappears by diffusion. 



For practical purposes yeast holds the field. One important 

 modern development of bread-making is the comparative ease 

 with which high-class yeasts can now be obtained. It is not 

 essential to obtain pure cultures, for the other micro-organisms 

 which exist in commercial yeast can only develop very slightly 

 in dough ; but however that may be, modern yeast is a high- 

 class article, produced generally under favourable conditions 

 by specialists and distributed rapidly by our modern wonderful 

 facilities for transport. There can be no doubt that these 

 modern developments are wholly good. In days gone by, some 

 of the fermentations were of the very highest class, but the 

 results were often irregular and poor. A far higher percentage 

 of regularly good results is now obtained. In old literature 

 the term " leaven " is frequently met with. That consists of a 

 portion of dough held over from a previous baking. The fresh 

 fermentation is started by the mixing of the old dough with fresh 

 flour and water, and after an interval the admixture of a little 

 beer yeast and more flour and water. The first batch produced 

 by this method in any day is relatively sour and dark. The 

 advantages of starting an entirely fresh fermentation for each 

 batch of bread are so great that the older method can be said 

 to have been discontinued in England. Formerly bakers used 

 to make their own yeasts from flour, malt, hops, potatoes, sugar, 

 and water as the principal ingredients, but the comparative 

 disuse of the term " barm " indicates that that way of making yeast 



