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Fruit Enzymes: Control of Enzymatic Browning and Proteolysis of Gelatin
Mary Johnson
INTRODUCTION
Enzymatic browning is caused when the colorless polyphenolic compounds in
vegetables and fruits are oxidized by enzymes called polyphenolases (Walter & Beathard,
2011, p. 101). This oxidative browning can occur on the bruised or cut surfaces of most
fruits and vegetables (Walter & Beathard, 2011, p. 101). Various methods exist to
prevent this browning, and these include lowering the pH, adding an antioxidant,
blanching the vegetable or fruit or immersing it in a sugar syrup or a dilute sodium
chloride solution (Walter & Beathard, 2011, p. 101).
In this experiment, apples and bananas were evaluated to determine the
effectiveness of different treatments on the development of enzymatic browning. The
pigments found in apples and bananas are the anthoxanthins which can be colorless,
white or slightly yellow (McWilliams, 2012, p. 218). Anthoxanthins will undergo
oxidative browning if exposed to air for a long period of time, bruised or cut
(McWilliams, 2012, p. 220). Typically oxidative browning is an undesirable outcome,
with one exception. This exception is the production of black or oolong tea
(McWilliams, 2012, p. 220). The green teas leaves are purposely bruised and exposed to
air and the tea pigments are oxidized to form the final pigmentation, thearubigens which
is responsible for the pleasing coloring of tea (McWilliams, 2012, p. 221).
Jello was also evaluated in this experiment to determine the effect fresh fruit
versus canned fruit had on gel formation. Some proteolytic enzymes have the ability to
cleave long gelatin molecules and form shorter polypeptides (McWilliams, 2012, 357).
Once the molecular length has changes, the gelatin no longer has the ability to form a
satisfactory gel (McWilliams, 2012, 357). Some foods that contain these enzymes are
“papaya, which contains papain; pineapple, which contains bromelain; figs, which
contains ficin, and kiwi fruit, which contains actinidin (McWilliams, 2012, 356). If these
enzymes are denatured by heating, then they use their catalytic ability and can be used in
gelatin without disrupting the gel formation (McWilliams, 2012, 356).
The purpose of this lab experiment is to examine the effects various treatments
have on the enzymatic oxidative browning. The secondary purpose of this lab
experiment is to examine the effect fresh fruit versus canned fruit has on the formation of
gels.
Methods
Enzymatic Oxidative Browning
Follow Procedure A. Basic Procedures to Evaluate Enzymatic Oxidative
Browning using apple and banana slices and following steps 1-6, 9, 11 and 13 in Walter &
Beathard, 2011, p. 103. Evaluate and record the appearance, flavor and texture in a table.
Evaluate and record the pH for the pineapple juice, lemon juice, cream of tartar solution
and the dry sucrose in a table.
Fresh and Canned Fruits in Jello
Prepare four packages of instant jello. Refrigerate the jello until it has set. Cut up
¾ cup of fresh pineapple and ¾ cup of fresh kiwi. The first jello is the control. Gently
stir in ¾ cup canned pineapple to the second bowl of jello, ¾ cup fresh pineapple to the
third bowl of jello and ¾ cup fresh kiwi to the fourth bowl. Evaluate and record the
appearance, texture and flavor in a table.
RESULTS
As shown in the table, the apple slices that were blanched, covered with pineapple
juice or covered with lemon juice, showed the least amount of oxidative browning.
Cream of tartar and the dry sucrose turned the apple slice a white-ish color. The Fruit
Fresh powder apple slice showed slight browning and the apple slice exposed to air was
very brown. Also shown in the table, the banana slices that were blanched showed no
browning, while the banana sliced covered in pineapple juice, lemon juice, cream of
tartar and dry sucrose were all slightly brown. The Fruit Fresh powder banana slice
showed very little browning.
Enzymatic Oxidative Browning
Apple
Appearance
Flavor
Fruit
Slightly
Fresh
brown
powder
Exposed to Brown
air
Blanched
Sweet,
bitter
aftertaste
Sweet with
sour
aftertaste
No browning Not a lot
of taste
Banana
Texture
Appearance
Flavor
Crisp
Very little
browning
Mushy
Crispy
Brown in
center
Mushy
Texture
Sour, offtaste
Banana
taste, light
sweetness
Extremely Falls apart
mushy
in mouth,
watery
Firm
Strong
pineapple
taste
Watery
Undesirable,
and mushy mushy look,
no browning
Pineapple No browning Strong
crisp on
Slight
juice
pineapple outside,
browning
pH = 4
taste
mushy
center
Lemon
No browning Strong
Crispy on Slight
Mushy
Very strong
juice
lemon
outside,
browning
lemon taste
pH = 2
juice taste, mushy
sour
center
Cream of Very white,
Bitter
crisp
Slight
Mushy
Sour taste
tartar
whitest
browning
solution
sample
pH = 3
Sucrose – White
Sweet
Crisp
Slight
Mushy,
Sweet
dry
browning, wet gooey on
pH = 5
surface
surface
As shown in the table, the control jello and the jello with the canned pineapple
had smooth, firmly set surfaces with no noticeable air bubbles. The jello with the fresh
pineapple had noticeable air bubbles around the pineapple chunks and visible liquid. The
jello with the fresh kiwi was no completely set, with visible liquid on the surface of the
jello and around the kiwi slices.
Jello Control
Jello with canned
pineapple
Jello with fresh
pineapple
Jello with fresh
kiwi
Fresh and Canned Fruits in Jello
Appearance
Texture
Smooth gel surface Firm, fully set
No liquid, smooth
Firm, no liquid
gel surface with
around pineapple
chunks of pineapple chunks
Noticeable air
Jiggly, not as set as
bubbles around
the jello with canned
pineapple chunks,
pineapple
visible liquid around
pineapple chunks
Not a complete gel, Jiggly, with surface
visible liquid around liquid
kiwi pieces
Flavor
sweet, orange flavor
Sweet, orange and
pineapple
Sweet, orange,
pineapple flavor, tart
aftertaste
Sweet, kiwi and
orange flavor, slight
bitter aftertaste
DISCUSSION
The fruits used to examine oxidative enzymatic browning in this experiment were
bananas and apples. The apple slices with the least browning were the blanched slices or
those covered in lemon juice or pineapple juice. Blanching denatures the enzymes that
are necessary for oxidative enzymatic browning to occur. The lemon and pineapple juice
both are acidic (a pH or 2 and 4, respectively) and a lowered pH also denatures the
enzymes, preventing browning. While the least amount of browning occurred, these
slices were also the least palatable and would not be the best choice to prevent apple
slices from browning. The dry sucrose imparted a sweetened flavor to the apple, while
also preventing browning and could be the best option in terms of preventing browning as
well as maintaining texture, appearance and flavor.
The banana slices that demonstrated the least browning were the Fruit Fresh,
pineapple juice, lemon juice, cream of tartar and dry sucrose. These treatments prevented
enzymatic browning because the pineapple juice, lemon juice and cream of tartar are all
acidic and therefore lower the pH and denature most of the enzymes. The cream of tartar
acts as a whitener when combined with anthoxanthin pigments, like those in bananas and
the outcome is a whiter product. Finally the dry sucrose effectively covered the surface
of the banana, preventing the exposure of oxygen and less browning occurred.
The second part of the experiment looked at the effect fresh fruit versus canned
fruit had on gel formation. The results were as expected. The canned fruit allowed the
jello to form a complete gel while the fresh pineapple and kiwi prevented gel formation.
Fresh pineapple contains the enzyme, bromelain and fresh kiwi contains the enzyme
actinidin (McWilliams, 2012, 356). Both of these enzymes are proteolytic and are able to
cleave the gelatin molecules preventing the formation of a true gel (McWilliams, 2012,
356). The proteolytic activity was visible in the jello with the fresh pineapple and fresh
kiwi. Air bubbles were noticeable, showing the enzyme activity around the fresh fruit.
Fresh fruit needs to be heated to denature the proteolytic enzymes, prior to putting it in
jello (McWilliams, 2012, 356).
While there are various ways of preventing enzymatic browning, not all produce a
palatable food. In this lab experiment, the Fruit Fresh powder and the dry sucrose
prevented browning in both the apple and banana without significantly altering flavor,
appearance and texture. These sensory characteristics must be all taken into account
when evaluating the effects of the different treatments. In deciding which treatment is the
best to prevent enzymatic browning, it is important to remember that for enzymatic
browning to occur, oxygen and the enzyme must be present, the sample must be in a
normal pH range and the pigment has to be exposed. Changing one or more of these
factors will lessen the amount of enzymatic browning that occurs.
REFERENCES
Walter, J. & Beathard, K.. (2011). Understanding Food Principles and Preparation Lab
Manual (4th edition). Belmont, CA: Cengage Learning.
McWilliams, M. (2012). Foods: Experimental Perspectives (7th edition). Upper Saddle
River, NJ: Prentice Hall.