• Glasses or clear plastic cups -2
• Homogenized or 2% milk (not skim or 1%)
• White vinegar – 5 mL
• Strong paper towels or cheesecloth
• Lemon juice
• Tray or sink
• Spoon
• Baking soda

1. Pour some milk at room temperature into a glass so that it is ¼ full.
2. Add 5 mL of vinegar to the milk.
3. Wait about five minutes.
4. Swirl the milk around and look at the sides of the glass. What happened to the milk?
5. Use the paper towels or cheesecloth as a strainer to separate the curds and whey (the whey is the liquid part) in the tray or sink.
6. Squeeze most of the water out of the curds.

• Vinegar is a weak acid and so is lemon juice. What happens when you add lemon juice instead of vinegar?
• Add baking soda to form a sticky glue!

Milk is a mixture and contains water, sugar (lactose), protein, fats and minerals. The vinegar helps to curdle the milk — the solids in the milk (the proteins and fats) clump together and form a new substance called curds. You can separate the curds out of the milk by pouring the clumpy mess through a very strong paper towel or cheesecloth. The liquid that goes through is called whey... these are the same “curds and whey” that appear in the “Little Miss Muffett” nursery rhyme. Curdling can also happen if heat or salt are added.

Curdling is what happens when you keep milk after its expiration date or if it has been left out of the fridge too long: this is what it means when people say the milk has "gone bad". It will taste very different from milk that is still before it's expiry date! 

But curdled milk is still okay to cook with and you can also buy curds in the grocery store. In fact, cheese is actually curdled milk! This is really obvious when looking at cottage cheese.