1. Searching for the Most Acidic Citrus

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    Compare the acidity of different citrus.

    Research question

    Are there are citrus fruits for which the acid taste and pH-values varies considerably?


    • Clean beaker or jar (100 ml).
    • Digital pH probe or laboratory pH-meter; litmus paper or universal pH–indicators.

    Why use data from multiple participants?

    Working together, we will explore a wider variety of citrus fruits and identify the "champions of acidity" among them. By studying the pH–value in a large variety of citrus fruits from different countries and regions, and grown in different climatic conditions, we can learn how variable their pH is, and whether variations in pH and acid taste are correspond with each other.

    Investigation Protocol

    In this project we will investigate fresh juice, that is, juice from the ripe fruit of various plants of the genus Citrus. The study cannot use ready-made juices, bought in a store, because they often are made from concentrate, and may contain preservatives, sugar, food additives and dyes. The presence of these substances can change the acidity of the freshly squeezed juice.

    1. Take one or two ripe fruit of any of the plants of the genus Citrus (Rutaceae family).
    2. Try to find out in what country they were grown.
    3. Take a picture of the fruit.
    4. Squeeze the juice of these fruits. This can be done in several ways, for example, by making an incision or by cutting the fruit in half, and then strongly squeezing the fruit by hand. It is important that you try not  to include juice from the rind of the fruit.
    5. Determine the acidity of the juice you obtained. Measurements can be made by a digital pH-sensor, a laboratory pH-meter, pH-litmus paper (strips) or other indicators of acidity.
      • If you are using a digital sensor or laboratory pH-meter, then it is desirable to calibrate it .(see the Project Getting on the Same Page
      • Repeat the test three times, each time washing the sensor or meter thoroughly first, and then wiping dry. Make all three dimensions in the questionnaire design. The program will automatically calculate the average value, summing these three values of acidity and dividing the total by 3. The resulting value will be considered and acidity. You will see the results at the time of sending in project Report Forms.
      • If you use a litmus test measurement of acidity, the test is conducted as follows: a strip of indicator paper must be dipped in the test solution, then put on a white waterproof substrate and quickly compared to a reference color scale. Selecting the closest color on the scale will give the value of the acidity of the liquid. Measurement using litmus or other indicator papers should, as in the case of sensors, be repeated three times and all three values averaged to make a profile.
    6. Fill in the project Report Form. If you decide to check out the fruits of the same plant, but grown in different countries, or different kinds of citrus, then when reporting the results of each experiment, it is necessary to fill in a Report form for each different kind.
    7. Follow the progress of the project, participating in the discussion of the results obtained by other contributors.

    Safety tips

    Working with an electrical appliance (pH meter), observe the safety precautions, and follow the instructions included with the device.

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