1. Souring of the Milk



    Determine how the acidity of milk changes during storage.

    Research question

    The older the milk, the more acidic (lower pH) it becomes.


    • Beaker or glass jar with a lid, with a capacity of at least 200 ml.
    • Fresh Milk (not less than 200 g). Check on the milk carton or milk bottle for its suggested shelf life. Make sure the “last date” for its use is not less than a week from the day you start the experiment.
    • Digital pH–probe, laboratory pH-meter, pH test–strip or other pH–indicator.
    • Microscope with a magnification of at least 200X.
    • A standalone photo camera or camera built into a mobile device. The best, of course, would be using a special microscope attachment for making "microphotographs."

    Why use data from multiple participants?

    Working together, we will also be able to define more precisely how the acidity of milk changes over time.

    Investigation Protocol

    1. Pour 200 ml of liquid milk into a clean beaker or glass jar.
    2. Measure the pH-value of the milk. Measurements can be taken using a digital pH probe, laboratory pH-meter, litmus paper–strips or other pH-indicators.
      • If you are using a digital probe or laboratory pH-meter, then it is helpful to conduct a calibration procedure as described elsewhere. (see Getting on the same page project.)
      • Repeat the test three times, each time first washing a sensor thoroughly in distilled water, and then wiping it dry. Enter all three measurements in the Report Form. 
      • The program will automatically calculate the average pH value, summing these three measurements and dividing the sum by 3. The resulting value will be considered the average acidity of your milk. The findings also can be viewed in the "Findings" section in your profile after it is sent.
      • If you use litmus test-strip to measure milk’s pH, you should do it as follows: Dip a strip of indicator paper in the milk, and then put the test-strip on a white waterproof substrate and quickly compare it’s color with the reference pH color scale. Then select the closest color on the pH scale, and it will give you the pH value of the milk. Measurements made with a litmus test strip, as with a pH probe, should be repeated three times and all three values recorded in the Report Form.
    3. Extra: Microscopy
      • Take a small amount of milk or fermented milk product and filter it through cotton disc.
      • Put a drop of the filtrate (the clear or slightly cloudy liquid that passed through the filter) on a glass slide for study with the microscope.
      • Place the slide under the microscope. Adjust the microscope to 200X. (This is the minimum recommended resolution at which you might see bacteria or yeast that may be responsible for milk’s fermentation). Look by gently moving the glass under the microscope. Lactobacillus bacteria, the bacteria responsible for fermentation of sugars such as lactose into lactic acid, have shapes like sticks or balls, and are either single or in a group that looks like a chain. 
      • If you observe any microorganism (bacteria or yeast), take several photos of them, and then choose for the Report the one in which they will be seen most clearly. 
      • To take a picture using a smart phone, keep it close to the eyepiece lens. If your microscope has digital photo capability, the appropriate software option should be used. 
    4. After the measurement, cover the milk in the jar with the glass lid and put it back into refrigerator.
    5. Fill in the project Report Form.
    6. Repeat the measurements every day for one week, determining how the acidity of milk changes. Using a microscope, continue to check for the presence of microorganisms.
    7. During the project, participate in the discussion of the results obtained by other contributors.

    Safety tips

    In your study, do not use any milk products except those you purchased in a food store. Do not taste the milk products you measure. Always use safety goggles when conduct chemical experiments.

    Report Form Before filling in the Report Form, please read the Investigation Protocol