COLLOIDAL NATURE OF FOODS 113 



the contents ean be utilised. Far and away the most important 

 of our foodstuffs are derived from cereals. From .*J() to 50 per 

 cent, of the energy of an ordinary- diet comes from them. They 

 are generally used as flour, baked into bread, or as meal made into 

 porridge. Wheat flour is a complex gel powder consisting of 

 about 10 per cent, protein, about 75 per cent, carbohydrate 

 (starch and cellulose), and about 2 per cent, fat in the colloidal 

 state. The individual particles contain molecidarly dispersed 

 salts, sugar, water, and adsorbed gases such as air and carbon 

 dioxide. Of the 10 per cent, of protein, gliadin forms a]:)out 

 4 per cent, and glutelin about 4 per cent. There is less than 

 1 per cent, of globulin (0-6 per cent.) and albumin (0-3) present. 

 The mixture of glutelin and gliadin is known as gluten. Gluten 

 is insoluble in water or in dilute salt solutions, and therefore readily 

 forms a disperse system with water called dough. Dough is a 

 polydispersoid composed of the glutelin (and other proteins) 

 carbohydrates and crystalloids mentioned above, bound together 

 by colloidal gliadin. It is a viscous semi-liquid mass which, 

 however, may be cut like a solid, and when torn exhibits a fil)rous 

 surface. The elastic properties of dough depend upon the pro- 

 portion of electrolytes present, especially on the phosphates. 

 When it is dried it changes into a gel and later becomes brittle 

 like glue. There is doubtless a close connection between the 

 viscosity of flour- water mixtures, and the stickiness, rising property, 

 power of absorbing COg of the dough, hydration of the starch and 

 the porosity and volume of the resultant loaf. 



The viscosity is found to increase with the concentration of 

 the flour and also to become greater for some time after mixing. 

 This is doubtless due to the slow swelling of the starch and albumin. 

 If concentrated solutions are suddenly diluted the viscosity is 

 too great at first, but gradually approaches a normal value. This 

 is probably caused by a slow increase in the dispersion, because 

 when the larger particles are removed by means of filter paper 

 normal results are obtained. 



Cooking. While many reactions occur in cooking, the changes 

 that are of paramount importance are of a colloidal nature. 

 Dough, for instance, undergoes a marked alteration in its physical 

 characters during the baking process. The proteins are coagu- 

 lated (gel formation) and the degree of dispersion of the starch 

 is increased. Adsorbed gases are set free and the bread " rises," 

 Further alterations take place in the loaf after it is removed from 

 the oven. 



The physical nature of flesh is profoundly altered by subjection 

 to cooking. In roasting, grilling, boiling, or frying, the meat is 



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