Anthocyanins in Fruit Juices

Learning Goals:

    1. To use UV-visible spectroscopy to monitor the properties of anthocyanins in fruit juices.
    2. To use a pH meter to monitor changes in pH in a juice sample.
    3. To understand the relationship between pH and absorbance in fruit juices. 

Abstract:

Anthocyanins are the pigments responsible for reds and blues in flowers and fruits. Anthocyanins are also responsible for the red color of cranberry juice, and many juice blends. They are also naturally occurring pH indicators. A typical anthocyanin is red in acid, violet in neutral solutions, and blue in basic solutions. A single type of anthocyanin can be responsible for different colors; for example a red zinnia, purple pansy, and blue morning glory may all contain the same anthocyanin in a different pH environment. Because of their indicator nature, anthocyanins can be used to demonstrate principles of acids and bases and light absorption. A pH meter is used to monitor changes in pH and can be used to take samples of various juices at different pH values. A UV-vis spectrometer can then be used to analyze these juices and to understand the effects of pH on absorbance intensity. The degree to which an anthocyanin acts as an indicator in several different juices can then be determined.

Lecture Connections

Prelab Assignment

In your lab notebook, prepare the following information:
    1. View the video clips on Using the UV-visible spectrometer and Using the pH Meter. You will need Quick Time video player to see them.
    2. A brief (2-3 sentence) introduction to the lab.
    3. A table of safety information that includes the chemicals used in the lab and any safety handling precautions. This information can be obtained from the MSDS safety sheets.
    4. Answer to the following question: Many young tree leaves contain large amounts of anthocyanins, making them appear red or purple. As the leaves mature, they turn green. Explain why the anthocyanin levels are high in young leaves. See Lecture Connections.

Give the information to your TA at the beginning of the lab. You will not be allowed to work in the lab without this information.

Procedure:
Chemicals
Equipment
Instruments
  • 2 50mL graduated cylinders
  • 150mL beaker
  • Magnetic stirrer & stirring bar
  • Cuvets
  • Glass stirrer
 

Part A: Cranberry Juice

  1. Measure out 25mL of cranberry Juice and 25mL of distilled water separately in 50mL graduated cylinders.
  2. Pour the juice and water into a 150mL beaker and mix thoroughly. Set up a pH meter, and calibrate the meter with the buffer solution provided.
  3. Place your beaker with the juice on a stirrer and add a stirring bar to the beaker. Obtain a dropper bottle with 1M Hydrochloric acid. Record the initial pH of your dilute juice sample.
  4. Adding the HCl one drop at a time, lower the pH of your juice sample to pH 1. Pour a sample of your pH 1 solution into a cuvet and place in the cuvet holder. Remember that you are handling very acidic substances.
  5. Obtain a dropper bottle with 1M NaOH. Adjust the pH of your juice solution to pH 2 by adding NaOH one drop at a time. Pour a sample of your solution into a cuvet, label, and place in the cuvet holder. Repeat the procedure for pH 3, 4, and 5. At each pH, pour a sample into a cuvet. 
  6. Take a spectrum of each sample using the UV-vis spectrometer. Remember to record the filename.

Part B: Cranapple Juice

  1. Repeat the above procedure for cranapple juice.

Part C: Assigned Unknown Juice

  1. Repeat the above procedure for the assigned unknown juice.
  2. Dispose of your samples and chemicals in their designated places as directed by your TA.

Post Lab Assignment:

(Include the following information in your lab report)
  1. For each juice sample, record the wavelength and maximum absorbance at each pH (1,2,3,4,5). Attach labeled copies of your spectra to your report.
  2. Make a graph of absorbance vs. pH for each juice samples. Note any similarities or differences between the three graphs. As the pH increases, what happens to the absorbance intensity?
  3. Does the assigned unknown juice sample contain anthocyanins?
  4. Is the anthocyanin as effective a pH indicator in the juice blend as in the cranberry juice? Explain.
  5. Anthocyanins in fruit juices act as pH indicators. Using Figure 16.4 in Brown, LeMay, and Bursten as a guide, what is the pH range and color change of the indicator in the cranberry juice? Is it the same in the juice blend?
  6. What is a possible function for anthocyanins in flowers?

Copyright (c) 2001 University of Maine, Chemistry Department.