1. Kill the bacteria in the food and exclude others.
2. Subject the food to conditions which are unfavourable for bacterial growth.
In the first way, extreme heat is used to kill the bacteria in the food, and then while hot, the food is sealed to keep out other bacteria: Example, canning.
In the second way, conditions are made unfavourable to the bacteria in the food, as follows:
1. The bacteria are deprived of water; the food is dried.
2. The bacteria are deprived of sufficient heat to be active; cold storage is used.
3. Large quantities of certain substances which are detrimental to the growth of bacteria are put into the food, and the bacteria become inactive. Examples: salt, sugar, spices, vinegar, smoke, or certain chemicals.
When the lesson is finished, the class is ready to practise the principles it involves. The lessons on the special preservation of fruit may follow at once.
Utensils used in canning