Compare the bacteria in plain yogurt with live/active bacteria and yogurt without live/active bacteria. Using a clean slide for each yogurt sample, thinly spread a bit of yogurt on the slide and add a drop of distilled water. Place the cover slip over the slide and put it under a light microscope on low power. Look for the bacteria in an area where the yogurt is thin. Compare the two types of yogurt. Change the power to 400X. Identify the arrangement and shape of the bacteria, and describe the differences you see between the samples.
Investigate how refrigeration affects yogurt bacteria. Put a tablespoon of plain yogurt with live/active cultures into two sterile containers with a covers. Place one container in a warm, dark environment, such as a cabinet in the kitchen. Place the other container in the refrigerator. After at least 24 hours, put a small bit of each sample on a clean slide, add a drop of distilled water and put on the cover slip. Place the slides under the compound light microscope and compare them. Identify the arrangement and shape of the bacteria you see.
Put a tiny bit of commercial plain yogurt with live/active cultures on a microscope slide and add a drop of water. Place the cover slip on top and put it under the microscope. Record what you see. Put several drops of the yogurt and several drops of water into a petri dish. Do not cover it and place it somewhere warm for 24 hours. Prepare a new slide with a sample from the petri dish. When you put it under the microscope, you will probably see more bacteria, and types that were not in the first slide. These bacteria were in the air and settled in the petri dish. Compare what you saw in the two slides.
Make your own yogurt and compare it under the microscope with commercial yogurt with live/active cultures. You'll need 3 cups of 2 percent milk, 3 tbsp. plain commercial yogurt with live/active cultures, 4 tbsp. nonfat dry milk, a cooking thermometer, a whisk, a glass or ceramic bowl and a saucepan. Warm the milk over very low heat to no more than 105 degrees. Remove the pan from the stove. Add the yogurt and dry milk and blend it with the whisk. Put the mixture into the bowl and let it sit four to six hours in a 100-degree oven. Refrigerate the mixture about three hours.