THE PROCESS OF DIGESTION 301 



The Effect of Cooking on Foods. In general terms cooking may 

 be said to render food more easily digestible/ both directly and indirectly, 

 through increased palatability. Subjecting food to high degrees of heat also 

 serves to kill parasites, such as trichinae and the various tapeworms, which 

 may be present and alive in raw meats. In the case of meats various methods 

 of cooking are employed. In roasting, the meat in bulk is subjected to a 

 high temperature in an oven for a short time, followed by a somewhat lower 

 temperature until the cooking is completed, which causes a coagulation of 

 the outer layers of albumin so that the juices of the meat are retained. In 

 boiling, the meat is first immersed in boiling water for a time and then the 

 cooking continues at a lower temperature. If a broth is to be made, the ex- 

 tractives may be obtained by heating the meat in water for a long period at 

 a temperature below the coagulation point of albumin. Such a broth con- 

 tains the flavoring and the stimulating extracts of the meat, but is of only 

 slight nutritive value. For small pieces of meat, broiling practically serves 

 the same purpose as does roasting for larger pieces. Frying, as usually em- 

 ployed, is the least serviceable method of preparation, since the fat or other 

 oily material used so permeates the food as to render it difficult of penetration 

 by the digestive juices. 



Cooking produces upon vegetables the necessary effect of rendering them 

 softer, so that they can be more readily broken up in the mouth. It also 

 causes the starch grains to swell up and burst, and so aids the digestive fluids 

 in penetrating into their substance. The albuminous matters are coagulated, 

 and the gummy, saccharine, and saline matters are removed. The conversion 

 of flour into dough is effected by mixing it with water, and adding a little salt 

 and a. certain amount of yeast. Yeast consists of the cells of an organized 

 ferment (Torula cerevisia)', this plant in its growth changes by ferment action 

 the sugar produced from the starch of the flour, and a quantity of carbon 

 dioxide and alcohol is formed; the gas together with the action of heat 

 during baking causes the dough to rise, and the gluten being coagulated, the 

 bread sets as a perma nently vesiculated mass. 



THE PROCESS OF DIGESTION. 



The Enzymes. The digestive process involves both mechanical 

 and chemical changes. The former are secured by the crushing and grinding 

 in the mouth, together with the mixing and kneading that come from the 

 peristalses of the stomach and intestine. The chemical changes are the 

 most important factors of the digestive process. The various secretions that 

 are poured into the mouth, stomach, and intestines all contain substances 

 which react on the foods to render the latter more soluble. The special agency 

 in each secretion is the presence of representatives of the chemical groups 

 known as enzymes. These enzymes, or unorganized ferments, are the essen- 



