activity 12: Make your own yoghurt Normally you don't want bacteria growing all over your food - they spoil it. However, in this experiment we are going to carefully grow some types of harmless bacteria. They grow so much that any harmful ones don't stand a chance. You will need: 100 g skimmed milk powder 600 ml hot water Tablespoon of plain live yoghurt - (containing either lactobacillus delbrueckii subspecies bulgaricus or streptococcus thermophilus) 2 forks 2 mixing bowls 2 clean glass jars with lids What to do: Label the bowls A and B and put 50g of skimmed milk powder in each bowl. Slowly stir 300ml of the hot water into a bowl mixing well with a fork. Repeat step 2 for bowl B. Add the tablespoon of yoghurt to bowl A. Rinse both jars with boiling water and label them A and B - be careful not to scald your hands. Pour the contents of bowl A into jar A and close lid tightly. Pour the contents of bowl B into jar B and close lid tightly- this is your control. Leave the jars in a very warm place overnight - an incubator or an airing cupboard would be ideal - if not place in a warm spot for four to five days. Compare the contents of jar A and B What is happening? Providing you didn't use pasteurised yoghurt in step 4 then the yoghurt you added to the bowl of skimmed milk would have contained live bacteria. These bacteria feed on the lactose sugar present in the milk. The lactose is converted into lactic acid, which curdles the milk and gives it the firmer texture and distinct taste that is yoghurt. The acid also stops many nasty micro-organisms from growing.
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