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Understanding the Impact of Fruit Enzymes on Oxidative Browning

and its Degradation of Gelatin Gels

Megan Hovey
NTR 502L Fall 2016
Fruit Enzymes Lab

Cut surfaces on fruits are likely to undergo enzymatic oxidative

browning. In this browning process, the oxidation of colorless polyphenolic
compounds present in fruits and vegetables is catalyzed by enzymes called
polyphenolases (Walter & Beathard, 2015, p.105). Lowering pH can inhibit
enzymatic oxidative browning to fruit. The process in which pH can inhibit
enzymatic oxidative browning is by the addition of an antioxidant, or
immersion in sodium chloride solution or sugar syrup, or lastly by blanching
the fruit. Length of cooking time and pH are two factors that determine the
color of fruits. Chlorophyll, carotenoids, and flavonoids are the three classes
of pigments in fruit, each of which have different chemical structures and
properties (Walter & Beathard, 2015, p.105).
Different procedures, such as adding an acidic medium to fruit, effect
the preservation of the fruit. Objectives of fruit and vegetable cooking
include minimizing vitamin and mineral losses, maximizing the development
of desirable textures, and maximizing the retention of characteristic
desirable flavor compounds (Walter & Beathard, 2015, p.105). Through
sucroses hydroscopic properties it too is able to preserve fruit. Cooking also
changes the texture of the fruit as the structural tissues soften and pectic
substances are hydrolyzed.
Certain fruits can degrade a gelatin dessert. Raw pineapple contains
the enzyme bromelain, which breaks up gelatin into its amino acid building
blocks. Fresh kiwi contains the enzyme actinidin that has the same nongelatin binding abilities. Pineapple and kiwi contain proteases that cut into

the gelatin and result in a non-solid gelatin. However, cooking or heating

certain fruits can inactivate proteases resulting in solid gelatin (McWilliams,
2017, p.218).
The purpose of this lab is to observe the impact of fruit enzymes.
Throughout this lab enzymatic oxidative browning will be examined, the
results on different acidic mediums will be viewed, and gelatin-forming
abilities (with raw versus fresh fruit) will be studied. Based on the
understanding of enzymatic oxidative browning, it is to be expected that
there will be changes in coloration, taste, and texture in some of the fruit
procedures that will be tested. Knowing acidic-based solutions and high
sucrose solutions can aid in the preservation process of fruits, it is likely a
few of the fruit samples will remain preserved. Lastly, based on active and
inactive enzymes and their abilities to form a solid gelatin or not, there will
be notable changes to the different mixtures of Jell-O.

A: Enzymatic Oxidative Browning
Apples and bananas were the two fruit samples that were tested in
various procedures to observe enzymatic browning, taste and texture.
Apples and bananas were cut into thirteen pieces to ensure each student
could taste each sample. The peel remained on for all samples. Procedures
were conducted as indicated in Section A: Enzymatic Oxidative Browning of
the Lab Manual (Walter & Beathard, 2015, p.107) with the exception of

completing only six of the procedures listed. The six different procedures
that were performed were: exposed to air, blanched, pineapple juice, lemon
juice, cream of tarter solution and sucrose dry.
All procedures conducted were left to incubate for thirty minutes.
Afterwards, all observations were recorded in Table 1.

B: Jell-O Experiment
In this procedure four Jell-O mixtures were prepared: control Jell-O, Jell-O
with canned pineapple, Jell-O with fresh pineapple, and Jell-O with fresh kiwi.
The four Jell-O mixtures were prepared using the instructions on the back of
the Jell-O box. The first sample was prepared per instructions on the box
with no modifications. The second Jell-O was prepared with 1 cup canned
pineapple, the third Jell-O was prepared with 1 cup raw pineapple, and the
fourth Jell-O was prepared with 1 cup fresh kiwi. The four Jell-O mixtures
were set aside to incubate for thirty minutes. Afterwards, observations were
recorded in Table 2.

Observations for enzymatic oxidative browning, taste, texture and
appearance are listed in Table 1. The apples and bananas that were exposed
to the air underwent the highest amount of enzymatic oxidative browning,
resulting in brown apples and bananas. The taste of the apples and bananas
in both the exposed to air solution and the blanched solution were

undesirable as they became soft and discolored. Blanched apples turned the
white pigment to a pinkish hue. Lemon juice, with the lowest pH of all
procedures, kept the fruit the lightest in color (without any sign of enzymatic
oxidative browning). The cream of tarter solution also kept the apples and
bananas light but there was a slight acidic aftertaste when sampled. Sucrose
solution kept the fruit preserved very well, both the apple and banana
remained light with no effect of enzymatic oxidative browning.

Table 1 Appearance, Taste and Texture of Various Apple and Banana


Treatment Type:
Exposed to air

Pineapple Juice
pH= 4

Lemon Juice
pH= 2

-Oxidized within a few
-Highest oxidative
browning observed in
this lab
-Tasted fairly normal,
wasnt extra mushy
-Turned brown
-Was very soft and
-Banana sample kept
very well, didnt brown
-A bit mushy, felt like it
fell apart in my mouth
when chewing it

-Banana tasted a bit

sour and little mushy,
not as bad as blanched
and exposed to air

-Oxidized within a few
-Turned brown
-Tasted a bit crunchy,
but mostly mushy
-Very mushy, tasted like
-Color was pink
-Apple sample kept very
well, didnt brown
-Tasted sweet but was
not overwhelming
-Apple still had a crunch
to it
-Looked crisp, in this
solution it was
preserved nicely
-Apple still tasted crisp
and sour
-Didnt undergo
enzymatic browning

Cream of tarter

Sucrose dry

-Didnt undergo
enzymatic browning
-Fruit the lightest
-Middle of banana was
started to brown;
appeared this acid
solution didnt work as
well (preservation wise)
as the pineapple and
-Can taste a bit of acidic
solution (stronger
aftertaste than the
-Banana was a little
-Banana was extra
sweet with a great
sugar crunch when
chewing it
-Banana did not
undergo enzymatic

-Fruit the lightest

- Little browning
occurred in the apples
-Did not taste the acid
solution (as compared
to the banana)
-Apple remained crisp
with a strong apple

-Apple was extra sweet,

almost tasted like apple
-Apple didnt undergo
oxidative browning

Observations for all mixtures of Jell-O are listed in Table 2. The results
were obtained at the end of class through observation. The control Jell-O
remained firm, whereas the fresh kiwi and fresh pineapple mixtures
degraded the top of the Jell-O mixture. The fresh fruit did not form a strong
gelatin gel mixture with the Jell-O. The canned pineapple mixture did form a
gelatin gel with the mixture.

Table 2 Appearance of Gelatin with Added Fruit

Jell-O Mixture Type:

Control Jell-O
Jell-O with Raw Pineapple

Observations of Mixtures:
Jell-O was firm, strong gelatin
The raw/fresh pineapple was not firm

in the Jell-O, degraded Jell-O on top,

didnt allow to form gelatin structure
The canned/cooked pineapple
remained gel, stronger gelatin
structure compared to fresh fruits
The fresh kiwi was not firm in the JellO, degraded Jell-O on top, didnt
allow to form gelatin structure

Jell-O with Canned Pineapple

Jell-O with Kiwi

The results were consistent with the expectations from this
experiment. Referring to the data as listed in Table 1, enzymatic oxidative
browning was observed in various procedures. The exposed to air procedure
resulted in the highest amount of enzymatic oxidative browning. Certain
solutions, such as the sucrose and acidic-based solutions, preserved the fruit
allowing the fruit to remain light in color. Referring to the data listed in Table
2, the fresh fruit did not hold a strong gelatin gel formation with the mixture
of Jell-O. The canned pineapple mixture did allow a strong gelatin gel
formation with no sign of degradation.
The apple developed a pinkish hue in the blanching procedure.
Apparently this color change is caused by the conversion of the
proanthcyanin to a pigmented and closely related compound, cyanidin
(McWilliams, 2017, p.152). During the blanching process this occurs because
the apple was overheated. The anthoxanthins often undergo browning when
cut or exposed to air for long periods of time. Polyphenoloxidases are a
group of enzymes that attribute to the darkening of the fruits that underwent
oxidative browning in this lab (McWilliams, 2017, p.152).

As bananas change color though enzymatic oxidative browning they

first form a reddish compound called dihydroindole-quinone. The banana
eventually turns a grayish black color of melanin (McWilliams, 2017, p.153).
The exposed air solution sample allowed the banana and apple to
immediately have access to oxygen, which accelerated the enzymatic
oxidative browning process. Phenolases are naturally present in some fruits
that can catalyze oxidative reactions resulting in less desirable colors
(McWilliams, 2017, p.150). The acidic solutions such as cream of tarter,
lemon juice and pineapple juice stopped the browning from occurring in the
fruits because it eliminated the fruits access to oxygen.
Adding an acid solution is often used to inhibit enzyme action. It was
evident that the acidic-based solutions kept the fruit preserved and (for the
most part) eliminated enzymatic oxidative browning. The apples remained
very light and crisp in the cream of tarter, pineapple juice and lemon juice
solutions. As far as the cream of tarter solution, there was an acidic
aftertaste, making this solution a less desirable option for preserving fruit.
The sucrose solution in this lab exposed the principle of hygroscopicity,
which is the ability to attract and hold water. Hygroscopicity is a
characteristic of sugars to fluctuating degrees (McWilliams, 2017, p.74). The
appearance of the apple and banana in the sucrose solution remained the
same without any sign of enzymatic oxidative browning. The sucrose
procedure preserved the apple and banana and gave it an extra sweet

crunch. The lemon juice procedure, versus the sucrose procedure, would be
a much better option for preservation as far as calorie and sugar content.
Fresh pineapple and kiwi have active enzymes, bromelain and actinidin
respectively, and do not form strong gelatin gels with Jell-O. Bromelain is
inactivated between 77 and 82 degrees Celsius, which therefore allows
canned pineapple to form a gelatin structure with Jell-O (McWilliams, 2017,
p.272). Raw pineapple and kiwi have pectic acids that are composed of
shorter polymers of galacturonic acid. Protopectin is a non-methylated
polymer of galacturonic acid incapable of forming a fruit. This type of pectic
substance has lost the gel-forming ability characteristics of the longer methyl
esters of galacturonic acid polymers (McWilliams, 2017, p.134). The canned
pineapple lacks the pectic enzyme, polygalacturonase, which promotes the
degradation of pectic substances.
Overall the results were consistent with the expectations. Eliminating
oxygen or using acidic solution can eliminate oxidative enzymatic browning
of fruit and inhibit enzyme action. Lemon juice would be the most desirable
way to preserve fruit because of the minimal aftertaste and high
preservation properties due to its low pH. Canned pineapple can be added to
Jell-O because their active enzymes are inactivated during the heating
process. Raw fruit, such as kiwi and pineapple, have active enzymes and will
not form a strong gelatin structure.
Walter, J. & Beathard, K. (2015.) Understanding Food Principles and
Preparation Lab Manual (5th ed.). Stamford, CT: Cengage Learning.

McWilliams, M. (2017) Foods: Experimental Perspectives (8th ed.). Upper

Saddle River, NJ: Prentice-Hall Inc.