Lesson plan

Milk spoiling and milk for making cheese

Observe milk decomposition as it spoils, then make cheese by curdling milk with acid.
Science content
Biology: Classification of Living Things, Biodiversity (1, 3)
Biology: Food Webs, Ecosystems, Biomes (3, 4)
Chemistry: Physical Changes, Solutions, Mixtures and Separating (2, 4, 5, 6)
Activities in this lesson

Do the activity on decomposition, including milk in the foods that are observed. Note how the milk that is left out curdles. Bacteria growing in milk started the decomposition process. Although the milk was pasteurized to kill most bacteria, a few survived and grew and multiplied in the warmth of the classroom.
Some milk bacteria are bad (E.coli and Salmonella, whose spores sometimes survive pasteurization), which is why you should not drink spoiled milk. Many bacteria in milk are harmless (e.g. Lactobacilli, which are not killed by pasteurization). Lactobacillus also lives in our intestines.
Why did the lactobacillus clump the milk? As they grow they make an acid (lactic acid). The acid interacts with the protein in the milk and makes them clump together: called curds.

When cheese is made, bacteria like lactobacillus is often used to make the curds. The bacteria makes acid which clumps the milk proteins. Lactobacillus bacteria is used to make cottage cheese, swiss and emmenthal cheeses.
We can make a cheese by just adding acid directly to milk. Do the cheese activity, and sample other cheeses made with Lactobacillus.

Grades taught
Gr 4
Gr 5
Gr 6