90 DISPERSE SYSTEMS 



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 

 exposed directly to heat. The proteins in the outer layers are 

 immediately coagulated, thus forming a more or less impermeable 

 covering which prevents the escape of the meat juices, leaving the 

 centre portion of the flesh only slightly altered chemically, but with 

 all sols converted into hydrogels. On the other hand, if the meat is 

 immersed in cold water and boiled much of the protein-sol and 

 practically all the salts and extractives are dissolved out and form 

 soup. In this soup the protein-sol is coagulated as the tempera- 

 ture rises, and on cooling it is adsorbed to the surface and often 

 is removed with the fats as a scum. The remaining meat under- 

 goes coagulation, but is flavourless. ' Stewing is a modification of 

 boiling, but the extractives, salts and soluble proteins, are served 

 as gravy. 



