Enzymatic Browning

FST 605 - Laboratory


Date Performed: January 29, 2002
Report Due: February 5, 2002

Polyphenoloxidase is an example of an enzyme that can lower the quality of a food product. This enzyme produces brown pigment and may change the texture and flavor of food products. The enzyme activity may be minimized by reducing agents, heat inactivation, lowering the pH of the food product and the presence of enzyme inhibitors.

Samples

Pear Apple Potato Banana Others


METHODS

A. Reducing Agents

Slice off about 1/4 of your sample and dice. Add to 100 ml water in a blender and puree. Strain through cheese cloth. Place 10 ml of liquid in each of 5 test tubes and treat as follows:


Immediately:

  • 1. Add 0.2% sodium bisulfite to decolor the sample . Use this as your end-point reference.

    2. Repeat 1 but this time record the volume required to decolor .

    3. Titrate with 0.2% ascorbic acid until decolored. Record volume.

    4. After 20 minutes titrate with . 2% ascorbic acid until decolored . Record volume.

    5. After 1 hour: Titrate with 0.2% ascorbic acid until decolored. Record volume.



    • B. Inactivation by Heat:

    1. Make six slices as nearly uniform in thickness as possible. Quickly place five slices in boiling water, leaving one for a control . Remove a slice after 0.1, 0.2, 0.3, 0.5, 1.0 and 1.5 min. Cool and compare the degree of darkening. Flood a portion of each slice with 0.02M catechol and observe. What are the blanching requirements of polyphenoloxidase.

    2 . Repeat the above section ( B1) with slices heated for the same time periods at full power in a microwave oven .


    C. Effects of pH on Browning:

    Puree as above and place 10 ml into 10 mls of each buffer (pH 2, 3, 4, 5, 6, 7, 8). As a control, place one in water. After 15 minutes add 1.0 ml of 0.02M catechol to each. Observe the color of the solution after 15 minutes. Rank the degree of browning. What appears to be the pH optimum of polyphenol oxidase. What is the natural pH of your sample?


    D. Effects of various agents on browning.

    Puree as above and place 10 ml into each of the following solutions:

  • 10 ml of water

    Ascorbic Acid ( 0.2% )

    Sodium Bisulfite ( 0 .2% )

    Citric Acid ( 0.2% )

    Sodium Bisulfite (0.1%) and Citric Acid (0.1%)

    NaCl ( 3%)


    • Add 0.2 ml of Catachol and wait 15 minutes. Rate each on a numerical scale from 0 (original color) to 10 (black). How much, if any, did each treatment affect the degree of browning compared to the control? What happens when an already brown tissue is treated with ascorbic acid? Discuss all results emphasizing the control of food products. Be sure to observe the results of the other groups with different food products.


    Questions

    1. How does each reagent utilized in part D function as an inhibitor of browning?

    2.Which of these agents are utilized in food products? What are their advantages and limitations?

    3. Why are the early steps of browning reversible but not the later?

    4. What is the dominant PPO substrate in your food product?