There is still the curd of milk to examine. The use of the senses does not allow us to definitely decide what food substance the curd is. Tell the pupils it is protein, or find the name by a process of reasoning, thus: Recall the fact that babies live for several months on milk alone and during that time build all tissues of the body. Milk, therefore, must contain all tissue-building substances. Review the food substances which are necessary to build all body tissues—mineral matter, protein, and water. We have found the mineral matter and water in milk, but not the protein. Since curd is the only remaining part of milk, it must be largely protein.
Tell the pupils that the scum which comes on the top of milk, when it is boiled, is another kind of protein of which there is a small amount in solution in milk.
Lead the pupils to see that if starch were present, it would be in a raw form, and in this form is indigestible.
Lesson II
FOOD VALUE
The analysis of milk gives a key to the food value of milk and each of its by-products (cream, butter, butter-milk, sour milk, skim milk, curd, whey, cheese, junket). These may now be briefly discussed as to composition, food value, and cost.
CARE
Milk readily absorbs odours, bacteria, etc., and should be kept in covered, sterilized dishes in a pure, cool atmosphere.
EFFECT OF HEAT
Experiments should be made to show the effect of simmering and boiling temperatures. To save time, a different experiment may be given to each pupil, and the results reported.