32 BIOLOGICAL PROBLEMS 



fermentation and baking. It would seem, therefore, 

 that differences in strength in wheats are likely to 

 depend upon variations in the amount of gluten con- 

 tained in the grain. This is not the case. Owing to 

 the interesting investigations of my colleague at Cam- 

 bridge, Professor T. B. Wood, we are now clear as 

 to the real factors at work. The yeast plant directly 

 ferments sugar, and if, during the rising of the dough, 

 it has to depend mainly upon the small amount of 

 sugar present as such in the flour, the amount of gas 

 given off from the fermentation Avill be small, and the 

 dough will rise to a correspondingly small degree. 

 Strong wheats, however, contain an enzyme capable 

 of rapidly converting into sugar some of the starch of 

 the flour made from them as soon as it is mixed with 

 water, so that the yeast is stimulated, fermentation of 

 the dough proceeds more rapidly, more gas is evolved, 

 and there is a greater expansion of the dough. Weak 

 wheats contain less of this enzyme. In this way dif- 

 ferences in the ultimate size of the loaf are accounted 

 for. The shape and texture of the loaf depend upon 

 a different factor. Because it is a substance which 

 when in solution displays colloidal properties, the 

 physical state of the gluten in the dough is profoundly 

 affected by the salts which are also present. Phos- 

 phates make gluten tough and elastic, exaggerating, 

 therefore, its power of retaining the gas bubbles from 

 yeast fermentation. Chlorides and sulphates, on the 

 other hand, tend to make it hard and brittle, and so 

 reduce that power. Now strong wheats are found 

 to contain more phosphates and less chlorides and 

 sulphates than weak wheats. It is for this reason that 

 the former yield a porous, well-shapen loaf and the 

 latter a loaf which is flat and heavy. 



Home-grown wheats are usually of the weak type, 

 and in order to make flour which would bake such 



