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MILK MICROBES

Test what affects the growth of microbes in milk

Suitable For

Grade 8

Difficulty

3

Time Required

 <1 Week

Supervision

Required

What’s it about?

Milk spoils or turns sour over time because microscopically small bacteria lives and grows in milk. When milk is not refrigerated, the conditions for these bacteria or ‘microbes’ is more favourable and they grow even faster, which causes the milk to go bad sooner when left outside at room temperature.

The ‘microbes’ inside the milk need oxygen to survive and they use up the oxygen over a period of time. In this science experiment we will use a ‘Methylene blue’ solution to test how much oxygen exist in various samples of milk:

Topics covered

Microbes, Methylene blue, Bacteria, Chemical indicator, Mycology

What will I need?

  • 4x PLASTIC CUPS
  • FULL CREAM MILK
  • MEASURING JUG
  • PLASTIC WRAP
  • MEDICINE DROPPER
  • METHYLENE BLUE (1% SOLUTION)
  • MAGIC MARKER

Procedure (Method)

Unfortunately, this section is only available in the e-book version of the project.

How does it work?

Unfortunately, this section is only available in the e-book version of the project.

This science experiment works because all food contains sugars, and if it is not sterile, it will contain microbes which break down the sugars into acid. In this experiment, milk, which has ‘lactose’ as the sugar, will be broken down into lactic acid by the bacteria present, which will lower the pH-level to make it acid. When the chemical indicator, methylene blue, is added to the milk, the milk is turned blue. The colour blue disappears over time as the milk is turned acidic by the growing and multiplying bacteria. The higher the concentration of microbes in the milk, the faster the blue colour disappears. The milk samples that have been out of the refrigerator for the longest will contain the most microbes and will loose its blue colour the quickest.

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