It is better to provide each pupil with a sample of the food to be studied, but where conditions make this difficult, the one used by the teacher will suffice.
STUDY OF MILK
Lesson I
COMPOSITION
Milk is the best food to examine first, because it contains all the food elements except starch and because these can be easily found.
The pupils may each be asked to bring a half cup of milk from home. It may be allowed to stand in glasses while other work is taken.
When ready for the lesson, ask the pupils to look at the contents of the glass, and they will observe a difference of colour where the cream has risen. Nature itself has divided the milk into two parts. Pour off the top part and feel it. It feels greasy. Butter is made from this part. We have found fat—a carbonaceous food.
Move the milk around in the glass and let the pupils see that it is a liquid. Tell them that all liquid in a natural food is mostly water. We have, therefore, another food substance—water, a builder and regulator.
Let the pupils compare a glass of water with a glass of skimmed milk, and they see that something is dissolved in the water of the milk, giving it the white colour. Show them a glass of sour milk, where the white substance is separate from the water. Get the names curd and whey. Tell them how the cheesemaker separates sweet milk into curd and whey. If advisable, let them do it, but in any case show them some sweet milk separated by rennet. Examine the sweet whey. It tastes sweet, denoting the presence of sugar—another carbonaceous food.
Notice the greenish-yellow colour. Recall this same colour in water in which potatoes, cabbage, or other vegetables have been cooked. Tell the pupils that this colour is given by mineral matter being dissolved in the water.