This document provides instructions for making homemade yogurt. It explains that lactic acid bacteria ferment the lactose in milk into lactic acid, causing the milk to curdle into yogurt. To ensure safety, all materials and equipment must be thoroughly sterilized using bleach, ethanol, or heat to kill any harmful bacteria. The instructions then detail the steps to blend fruits, heat the milk, incubate the mixture with yogurt culture at 43-45°C for 6-8 hours to culture, then chill the yogurt in the refrigerator. One jar should be submitted to verify it was homemade.

1. Optional Experiment/Project: Making Homemade Yogurt
Biotechnology is the production of materials using biological organisms or their processes. In this
experiment, harmless lactic acid bacteria such as Lactobacillus bulgaricus use a process called
fermentation to convert lactose to lactic acid, which in turn curdles milk to give yogurt.
WARNING: It is important to follow the directions as closely as possible. Nobody wants to eat
yogurt contaminated with chemicals or harmful bacteria. To ensure this, all materials will be
provided by you.
Materials and Apparatus:
2L milk (whole or skim)
1/3 cup powdered milk (skim preferred)
¼ cup pure maple syrup or pure honey
Starter yogurt culture
Fruit(s) and flavoring(s)
Large pot
Stove
2 kitchen thermometers
8 250mL lidded jars
Thermochest/ice box
Candle
70% ethanol
Spray bottle
Spoon/ladle
Refrigerator
Cloth (optional)
Warm distilled/purified water (optional)
Procedure:
1. Sterilize all equipment (including thermochest, thermometers, containers of ingredients, etc.) by
soaking in a solution of bleach and water for 20 minutes. Rinse thoroughly with warm water to
get rid of the chlorine smell as much as possible.
2. Lay down a clean sterilized cloth on the work table. Do not lay any equipment on the table itself
as the table may be contaminated with bacteria. Alternatively, spray 70% ethanol on the table to
kill the bacteria. You can buy alcohol in the supermarket with “70% alcohol” label, and use a
clean spray bottle (not a spray bottle used for fertilizers, cleaning agents, etc.).
3. Light a candle. Remember to wash your hands afterwards as the candle may be contaminated
with chemicals. Heat kills bacteria, so a nearby candle minimizes the amount of bacteria in the
air that could contaminate the yogurt.
4. Blend your fruits in a blender, and line the bottom of eight clean sterilized 250mL transparent
jars with lid with the fruits. Add other flavorings you desire. Close the lid of the jar.
5. Place the lidded jars into a clean sterilized thermochest/icebox with the temperature maintained
at 43-45°C. You may pour warm distilled/purified water into the icebox to help maintain the
temperature. Make sure though that the jars are almost but not completely covered with water.
6. Pour milk into a clean sterilized large cooking pot and heat to 85°C using a clean sterilized
stove. The stove may be sterilized by spraying with 70% ethanol.
7. Allow the milk to cool down to 43-45°C. Cover the pot as much as possible while cooling down
the contents.
8. Add the remaining ingredients into the milk, and stir everything with a clean sterilized
spoon/ladle until dissolved. The starter yogurt culture is just a spoonful of original flavoured
yogurt available in any supermarket.
9. Take the jars out of the thermochest, open the lids, and pour the milk mixture into the jars.
10. Close the lid of the jars, return to the thermochest, and incubate for 6-8 hours at 43-45°C.
11. When the consistency of the contents of the jars is similar to the consistency of the yogurt you
bought, transfer to the refrigerator to chill.
12. Submit one jar to the instructor, and enjoy the other jars for yourself. Remember to use fruit and
flavouring combinations that are not available in the supermarket so the instructor knows you
did not just buy yogurt to submit.