To simplify the experiment a weak solution of malic acid may be used (seven grams per litre being about the average amount found in apples). Bring this to a boil in an aluminum cooker-pail and put it into a cooker for twelve hours. Transfer it to a porcelain vessel and add ammonia to precipitate the alumina. Filter and wash this, dry and weigh the aluminum oxide. It is probable that a smaller quantity of aluminum would be dissolved by foods of a mushy consistency than would be found in this clear solution.
8. The efficiency of home-made refrigerating boxes compared with other means of keeping foods cold.
Materials:
- One box fitted as for fireless cooking, with two or three covered crocks of at least one-half gallon capacity, packed as directed on [page 37], with either sawdust, hay, straw, excelsior or paper. Sawdust is specially recommended.
- Thermometer
- Ice
- Notebook and pencil
Fill the central crock with a weighed quantity of ice. Fill one or both of the other crocks with water at room temperature. Cover the crocks and close the box. Record the temperature of the water at the end of six, twelve, twenty-four, and forty-eight hours.
Make repeated observations of the temperatures found in ordinary household refrigerators, cellars, cold storage rooms, and any other places used for keeping foods cold. Compare these with the temperatures obtained with a home-made refrigerating box. Is there any economy in using these boxes?
Bacteriology of Insulating Boxes
9. Temperatures which kill disease and putrefactive germs, or check their growth.
It is taken for granted that the student of this subject will be more or less familiar with the nature of bacteria and the elements of bacteriology. It will be recalled that bacteria are a vegetable form of life; that, like all plants, they have, under certain conditions, the power of growth which is shown, largely, by their reproduction; and that under other conditions they are killed. When their growth is merely checked, they are in a dormant state, or perhaps form spores, in either of which cases they are ready to develop as soon as their environment permits. Temperature has much to do with the state of bacteria. If the temperature and other conditions are such that they are in an active or growing state, they will multiply with enormous rapidity. When in food stuffs they effect certain changes by reason of the products which they form as a result of their life processes, or of the alteration in the food materials, owing to their abstraction of some chemical elements or compounds used for their nutrition. When bacteria form unpleasant smelling or tasting substances we speak of them as “putrefactive bacteria.” Those which, if introduced into the bodies of humans or animals, will cause diseases, are called “disease bacteria.” Foods are liable to contain both kinds; and, therefore, it is, obviously, wise to do all that is possible to kill them or prevent their growth.
Most forms occurring in foods grow best at from 80 degrees to 98 degrees Fahrenheit. Few bacteria grow at above 100 degrees, and, if kept at 125 degrees, the weaker ones soon die. After subjection to a temperature of 150 degrees to 160 degrees Fahrenheit, for ten minutes, if water is present, almost all kinds are killed unless they are in the spore state. Prolonged boiling will often be resisted by spores. Dry heat is not as effective in killing bacteria as moist, and a higher temperature must, therefore, be reached to effect this end. Below 70 degrees Fahrenheit the growth of bacteria is more and more retarded, but not entirely checked until freezing point is reached. The popular idea that freezing may be relied upon to destroy bacteria is not true.