Grow a bacteria culture
Grow a harmless bacteria culture by making yogurt using store-bought yogurt as a starter and warm milk, then observe and record growth safely.
About this activity
Grow a harmless bacteria culture by making yogurt using store-bought yogurt as a starter and warm milk, then observe and record growth safely.
What you need
Step by step
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Step 1
Wash your hands with soap and water for 20 seconds.
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Step 2
With an adult, wash the jar and spoon in hot soapy water and rinse well.
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Step 3
Measure 4 cups 1 quart of milk into the saucepan.
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Step 4
With an adult, heat the milk to 180°F 82°C while stirring occasionally and check the temperature with the thermometer.
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Step 5
Remove the saucepan from heat and let the milk cool to 110°F 43°C using the thermometer to check.
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Step 6
Pour about 1/2 cup of the cooled milk into the small bowl and stir in 2 tablespoons of the plain yogurt until smooth.
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Step 7
Pour the starter mixture back into the cooled milk and stir gently for 30 seconds.
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Step 8
Pour the milk mixture into the clean jar and close the lid loosely.
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Step 9
Use the labeling tape to write today’s date and stick it on the jar.
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Step 10
Wrap the jar in the towel and place it in a warm spot around 100–110°F 38–43°C to incubate for 4 to 8 hours.
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Step 11
Put the jar in the refrigerator for at least 2 hours to let the yogurt thicken.
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Step 12
Ask an adult to inspect the yogurt for any mold or off smells and then write your observations about texture color and smell in your notebook.
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Step 13
Share your finished yogurt creation and observations on DIY.org.
Fun facts
How do you grow a harmless bacteria culture by making yogurt with store-bought yogurt as a starter?
This activity is a simple, supervised kitchen experiment: use pasteurized milk and a small amount of store-bought yogurt that lists “live cultures” as the starter. Follow the package directions on the yogurt container or a trusted recipe rather than step-by-step instructions here. With an adult, combine the starter and milk, keep the container in a stable warm spot until it changes consistency, then observe and record changes over time in a notebook or with photos.
What materials do I need to make yogurt as a child-friendly bacteria culture activity?
Gather household items: pasteurized milk, a small container of store-bought yogurt labeled “live cultures,” a clean heatproof jar or container with a lid, a spoon, labels or masking tape, a notebook and pen for observations, and a camera or phone for photos. Also have clean towels and a waste bag for disposal. Ensure all utensils and containers are washed well and that an adult oversees the setup and cleanup.
What ages is this yogurt-making bacteria activity suitable for?
This activity suits different ages with adult supervision: preschoolers (3–5) can watch and help stir, early elementary children (6–9) can record simple observations and take photos, and older kids (10–14+) can learn more about cultures and keep detailed notes. Always have a responsible adult handle any heating or hot liquids and explain hygiene and safety throughout the activity.
What safety tips should parents follow when doing a yogurt culture activity with children?
Prioritize safety: use pasteurized milk, read and follow manufacturer instructions, and keep adults in charge of any heating. Practice good handwashing and clean all utensils and surfaces. Label the jar with date and starter used. Do not taste or use the yogurt if it smells off, shows mold, or looks unusual — discard it. If anyone feels ill after handling or tasting, seek medical advice. Supervise children at every step.
Watch & Learn
Need help?
What can we substitute if we don't have a kitchen thermometer, a clean jar, or plain yogurt starter?
An adult can use a candy or instant-read meat thermometer to check 180°F and 110°F, any clean heatproof glass jar or mason jar in place of the labeled jar, and store-bought plain yogurt with live cultures as the starter.
My yogurt didn't thicken or smells off—what might have gone wrong and how can we fix it?
Check that the milk was heated to 180°F and then cooled to about 110°F before stirring in 2 tablespoons of plain yogurt, ensure the jar stayed wrapped in a warm 100–110°F spot for the full 4–8 hours, and discard any batch with visible mold or an off smell.
How can we adapt the steps for different age groups so it's safe and fun?
For younger kids have an adult do the hot steps while the child helps wash the jar, stir the starter into the 1/2 cup cooled milk, stick on the labeling tape, and draw observations in the notebook, whereas older kids can measure 4 cups of milk, monitor temperatures, and post the finished yogurt on DIY.org.
How can we extend or personalize the yogurt activity once it's set and refrigerated?
Try small experiments by labeling multiple jars with tape and different plain yogurt starters or milk types to compare texture, color, and smell after refrigeration, then flavor or decorate your finished yogurt before sharing observations on DIY.org.