372 Our Surroundings 



affects vegetable tissue. It causes the fibers to loosen, the starch 

 grains confined in the cells to expand, and the covering of the 

 cells, called cellulose, to burst, thus liberating their contents. Since 

 cellulose is insoluble, it must be broken up or the contents of 

 the cells cannot come into contact with the digestive fluids. In 

 green vegetables, germs of a harmful nature are destroyed by 

 cooking. 



Chemical Reactions or Changes. The only chemical change 

 occurring in cooking that needs to be considered is the production 

 of carbon dioxide in the making of bread. This has the effect 

 of making bread light. It is usually brought about by the use of 

 yeast which, in fermentation, yields carbon dioxide. 



Economy in the Purchase of Foods. Economy in the pur- 

 chase of food means the spending of money, whether the amount 

 is large or small, for the kinds of food that will provide the 

 greatest amount of nourishment with the least expenditure of 

 money. Food purchased at small cost is cheap only when it is 

 capable of keeping the body in a high state of health. No food 

 can be called cheap if its use causes illness, or lessens in any 

 degree the efficiency of the body. It leads to doctors' bills and 

 decreases one's ability to earn. It will always be found more 

 economical to purchase food of high grade. This does not neces- 

 sarily mean that the most expensive brands of food should be 

 bought, but simply that whatever food is used should be of 

 good quality. 



It should always be borne in mind that the human body 

 requires food which will produce energy, and also food which 

 will replace tissue that is worn out. The best energy-building 

 foods are sugars, starches, and fats, which are found in sufficient 

 quantity in cereals, cereal products, potatoes, beans, peas, butter, 

 fat meat, and oils. The chief material for the repair and building 

 of living tissue is food containing protein. Protein comes from 

 lean meat and the white part of eggs, and from grains and vege- 

 tables, all of which will also produce energy when properly oxi- 

 dized in the body. Other materials, like lime, iron, and phos- 

 phorus, are also needed, but they are usually present in sufficient 

 quantity in ordinary food substances. 



