Making yogurt is a great experiment to do with your little ones. Although this experiment requires adult supervision and an adult doing most of the “heavy lifting,” this experiment will introduce some basic science concepts to your child: contamination, sterilization, microbes, growth, incubation, and boiling.
The first step in making yogurt is to assemble your materials. You will need milk and a starter culture of yogurt. Pick any yogurt from the grocery store, but make sure it contains live cultures. Other materials include two rinsed quart-size spaghetti jars with lids, a large stock pot, a metal spoon with a long handle, tongs, thermometer and a measuring spoon.
Step 1: Pour milk into the quart size jars until you reach the top, leaving about an inch of space – do not place your lids on the jars. Place the jars into the stock pot and fill the pot with water up to, and slightly above, the level of milk in your quart jars. You are making a double boiler.
Step 2: Heat the water up to boiling. When the water boils, the temperature of your milk will be around 170F. Keep boiling until you reach about 185-190F. Dip the lids into the boiling water using tongs and then place on the jars. Turn off the heat, but leave the jars of milk in the water bath for about 10 minutes.
Step 3: Carefully remove the jars from the water bath and set on the counter for about 2 hours. You will need to cool the milk to 115F.
Step 4: Once the milk reaches 115F, start boiling your water again. When it reaches a boil, using your tongs, dip the measuring spoon in the water. Using that spoon, add 1 Tablespoon of the yogurt starter to each jar of milk. Sterilize your long-metal spoon by dipping in the boiling water and then vigorously mix the milk. Redip your lids in the boiling water and recap the jars of milk. Its good to sterilize your equipment this way, but if you have to put down the spoons to let them cool before you touch them, that is okay. Just place them on a clean towel.
Step 5: Incubate. You will have to set up some sort of incubator in your house. I use a cooler (see picture to the right). Just fill it up with half boiling water and half cold tap water. You want it at 115F, no higher. Place the jars in the cooler so that the water line reaches the height of the milk in the jars. Close the lid and incubate for about 5-7 hours.
When you are done you will notice that the milk has now become think and creamy. You have yogurt! Stay tuned to future blog posts for explanations of the basic scientific concepts behind this experiment.