63 



and swelling of the starch granules (for future gelatinisation by heat), 

 the production of hydrated forms of gluten, itself a complex proteid, 

 biological changes during the growth of yeast, the occlusion of gas . 

 evolved during that gi'owth, the action of the yeast enzymes on the 

 carbo-hydrates, the working of the dough to secure suitable elasticity, 

 proteolytic enzyme action on the gluten, the hardening action of 

 salt on the gluten, gelatinisation and saturation of the starch- water 

 system on baking, and the gradual changes, physical and chemical, 

 during cooling and ageing which bring about eventual staleness. 

 This is but a broad outline of the problems to be attacked when the 

 barest necessities for the production of bread are used, and it is 

 evident that the addition of milk or fat must increase the complication 

 of the bread system. 



I. — Flour. 



The flour components of greatest importance are starch, gluten, 

 mineral salts and enzymes. 



(a) Starch. 



Some space has already been devoted in Report I. (p. 46 et seq.) 

 to the consideration of wheat starch and its behaviour in the presence 

 of water at a temperature to cause gelatinisation. It is not intended 

 to cover this ground again. 



Wheat starch in the presence of a sufficiency of water will commence 

 gelatinisation about 60° C, and every granule will be completelj^ 

 burst at about 65° C. Since the baking temperature of a bread oven 

 is usually between 200° and 240° C, and the amount of water present 

 in a normal dough is about 41 per cent., the baking lasting for one 

 hour, it might be imagined that, all other factors removed, a great 

 proportion of the starch granules to be found in bread-crumb would be 

 gelatinised. Yet, as a rule, the starch granules inside a loaf show 

 a mixture of those untouched by the damp heat and of those only 

 slightly swollen, whilst a few only have undergone complete gelatinisa- 

 tion. The reason for this divergence from theory may be found in 

 the fact that a temperature seldom higher than 95° C. is reached 

 withui the loaf, and then only for a very short period, during the 

 baking hour, though temperatures as high as 99-5° C. have been 

 recorded. It is probable that the time factor is of some importance 

 here. Further, the saturation of starch by water is reached when 

 a mixture shows 41 per cent, of the latter, hence, after allowing for 

 the absorption of water by gluten (which is not inconsiderable, being 

 as much as 200 per cent, of the dry gluten of a strong flour and slightly 

 less for weaker flour), there is an insufficiency of water present to 

 aUow complete gelatinisation of all the starch granules at the 

 temperature of baking. The complexity of the colloid systems in 

 a baked loaf is, therefore, considerable. 



Were there no component other than starch to consider, such 

 problems as the change in physical consistency from freshness to 

 staleness of a loaf would be comparatively easy of solution. But 

 there are so many underlying difficulties connected with dissolution 

 by enzymic action on the one hand, and coagulation by electrolytes 

 on the other, that the behaviour of the colloids in the loaf after 



