Enzymatic Browning Experiment

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					Enzymatic Browning Experiment
Appropriate for grade levels: 3-8
                                                        Cutting fruit⇒ Browning occurs: Pigments (e.g.
Adaptation by Beth Calder and Susan Brawley
                                                        catechols and tannins) form.
                                                        (Exposing enzymes in the fruit to air, causes them to
Potter, N.N. and Hotchkiss, J.H. 1995. Food Sci-
ence, 5th edition. Chapman and Hall, New York, NY.
                                                        There are two ways to prevent enzymatic browning:
                                                           1) Inactivate the enzymes (stop their action) or
                                                           2) Prevent oxygen from coming in contact with
Materials: (Note: These materials will be ad-
                                                               the enzyme
equate for one group of students or for one
“hands-on” demonstration for a classroom of
                                                        Methods to decrease browning:
1 Banana
                                                            1) Heat Blanching: Fruits and vegetables are
1 Apple
                                                               often boiled for 1-5 minutes (depending on
1 Vitamin C tablet
                                                               size of produce) before freezing. Blanching
1 cup of warm water
                                                               inactivates enzymes by causing these
1 cup of white granulated sugar
                                                               proteins to lose their normal state (causing
                                                               proteins to denature).
Paring knife
                                                            2) Vitamin C Dip: Vitamin C is a great antioxi-
Small plastic dishes or bowls
                                                               dant and oxidizes instead of the catechol/
Clear plastic cups and spoons
                                                               tannin pigments.
½ cup of lemon juice
                                                            3) Sulfur Dioxide Dip: Sulfur dioxide is able to
                                                               stabilize the color of fresh and also pro-
                                                               cessed fruits and vegetables. Sulfur dioxide
Enzymatic browning is caused by the oxidation1 of
                                                               stops the activity of oxidizing enzymes by
pigments in fruits and vegetables, which then
                                                               removing the oxygen from the enzyme
activates specific enzymes in plant tissues. En-
                                                               before pigments are formed. Sulfur dioxide
zymes are proteins that function as catalysts.
                                                               also has antioxidant properties. It is often
Catalysts speed up chemical reactions, but are not
                                                               used in salad bars at restaurants to prevent
changed by the reaction. They often function by
                                                               unattractive browning.
lowering the activation energy of reactions. By
                                                            4) Sugar Syrup Dip: This method is one of the
bringing the reactants closer together, the reactions
                                                               oldest to prevent browning. The sugar syrup
will occur at a faster rate when the catalyst is
                                                               coats the fruit and prevents the fruit surface
present. Enzymatic browning is why lighter colored
                                                               from contacting oxygen. Sugar dips also add
fruits and vegetables (e.g. bananas, apples,
                                                               a nice sweet taste to tart fruit.
peaches and potatoes) turn brown when they are cut
open and exposed to air. This type of browning is
considered unacceptable because of the resulting
                                                        This activity can be done by students working
dark color and changes in flavor.
                                                        together in groups or as a hands-on demonstration.
        1                                               Have students prepare dipping solutions. At least
       Oxidation = reaction of oxygen with a
                                                        two replicate samples (duplicates) of each dipping
                                                        solution should be prepared to demonstrate that
experiments must have replicates. Place one vitamin       observation will be helpful to show students why
C tablet into ½ cup of warm water and mix until           replicates are needed during experiments to provide
dissolved in a clear plastic cup; label cup as “Vita-     an estimate of experimental error.
min C”. Place ¾ cup of sugar into ½ cup of warm           A discussion with the students should follow to allow
water and mix solution in a separate clear plastic        them to discuss which methods were best to prevent
cup; label as “Sugar Dip”. Pour lemon juice into a        browning.
third plastic cup; label as “Lemon Juice”. Have
students peel off the apple skin(s) with a paring knife   Questions:
and cut the banana(s) and apple(s) into ¼ - ½ inch
slices. Place a few pieces of apple and banana on a           1) Which method did you think would be the
plate and label as “Control”. Have students immerse              best to stop browning and why?
pieces of fruit in each of the dipping solutions for 3
minutes (hold the fruit into the solution; do not allow       2) Which method(s) was the best and which
it to float to the surface). After three minutes, the            one(s) seemed to work less well?
students should take the fruit out of the solution and
place the fruit onto a plate with label to identify the       3) Can you think of reasons why some of these
dipping solution used to treat the fruit. While stu-             methods are better than others to decrease
dents are waiting for the fruit in the dipping solu-             browning?
tions, they can then add some of the cut fruit to ¼
cup of dry granulated sugar in a small dish. Have             4) What is the ingredient in Fruit-Fresh® that
students roll the fruit in sugar until totally covered           decreases browning? Was it an effective
and then place the fruit onto a plate and label it as            treatment in preventing browning?
“Dry Sugar”. Students can then roll cut fruit in a dish
of Fruit-Fresh® until covered; place on a third plate
and label as “Fruit-Fresh”. There should be a total of
12 plates for each group of students to observe:
labeled as “Control” replicates A and B (fruit not
treated and exposed to air), “Sugar Dip” replicates A
and B (soaked for 1 minute in sugar solution),
“Lemon Juice” replicates A and B (soaked for 1
minute in lemon juice), “Vitamin C” replicates A and
B (soaked for 1 minute in Vitamin C solution), “Dry
Sugar” replicates A and B (rolled in dry sugar until
covered) and Fruit-Fresh replicates A and B (rolled
in Fruit-Fresh® until covered). Set aside enzymatic
browning experiment for at least 1 hour so that the
degrees of browning can be best observed.
Have the students observe the differences in the
degree of browning across treatments and compare
them to the controls. The students can also compare
replicates to each other. If replicates are not consis-
tent, have students brainstorm with their group to
think of reasons why the replicates are different.
Were all the steps followed exactly for preparing
each replicate? Did a group(s) miss a step? This

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