Title: Investigation of the effect of vitamin C of different concentrations on the
prevention of apple oxidation (Fuji apple)
Aim: To find out the effect of different concentrations of vitamin C on the prevention
of apple oxidation (Fuji)
Hypothesis:
Higher Vitamin C concentration is more effective in preventing the apple from
oxidation.
Background / Introduction:
Vitamin C is very effective in preventing the oxidation of the apple surface. However,
high dosage of vitamin C is harmful, thus the minimum amount of vitamin C should
be added. The formation of the brown residue causes the apple surface to appear
brown in color and greatly the appetite.
Fuji apple was used in this experiment as it is a common variety of apple used in salad
and cold dishes in which heating is not preferred as a kind of preservation method.
Principle of experimental design:
The browning of apple is due to the oxidation of phenolic compounds into quinones
by the action of an enzyme, "polyphenol oxidase" (PPO) (which is also known as
tyrosinase). The quinones will then rapidly polymerize into a brown residue.
When the apple is cut open, the apple cells along the surface have their cell walls
damaged and the contents in them are exposed to the oxygen in air and trigger the
oxidation.
Besides, bruising of the apple enables the contents of those cells to flow freely inside
the apple, where they react with air that is also inside the apple (apples are 80%
air--that's why they float), thus healthy apples with no damage should be used.
Vitamin C of different concentrations will probably prevent the apple from oxidation
for different period of time. Thus by measuring the time before there is change in
color of the apple, the efficiency of different vitamin C concentration can be
estimated.
Materials and methods:
1.
A tablet of Vitamin C with 500mg vitamin C content was crushed and dissolved
in 50ml of water to make up a Vitamin C solution of concentration of 10g per
litre. (brand of Vitamin C tablet used: Jamieson)
2.
The vitamin C solution was diluted to different concentration step by step.

About 25 ml of the Vitamin C solution was taken out and mixed with 25 ml
of water, thus Vitamin C solution of half the original concentration was
prepared.

The process was repeated with the diluted solution again and again for 3
more times.

At the end, vitamin C solutions of 5 different concentrations were obtained.
(0.675 g L-1, 1.25 g L-1, 2.5 g L-1, 5 g L-1, 10 g L-1)
3.
Another cup of tap water (25ml) was also used as a control.
4.
12 apple slices of equal thickness were prepared using knife and egg slicer (A
Fuji apple was cut in to four pieces with its skin peeled off. Two of the four
pieces were then deseeded and sliced using the egg slicer and 12 slices of
roughly the same size were chosen.)
5.
2 apple slices were dipped into and coated with each of the solution prepared.
6.
After half a minute, the apple slices were taken out and placed on a piece of
transparency separately (2 in a group) and each group of apple slices was then
covered with a plastic cup. (The cup is used to prevent the drying up of the apple
slices and the attraction of insects as it was left overnight and we usually cover
the apple if we want to preserve it in our daily life. The cup does not completely
isolate the apple slice from the surrounding as there is a gap between the cup and
the transparency.)
7.
The color of the apple slices was noted in each 5 minute interval for the 1st 45
min. and each 15 min. interval for the next 155min. and 4 1/4 hours & 3 1/2
hours later. The color was also noted at the end of the experiment (after 24
hours.)
8.
The texture of the apple slices was also recorded at the beginning and at the end
of the experiment.
9.
The temperature at which the experiment was performed was noted.
Note: The plastic cups were regularly ‘open’ for a very small period of time to let
in oxygen so that the ceasing of color change is not only due to the limit of
oxygen available inside the cups.
Assumption:

The time lag in which the apple slices was dipped into the solution and that in
which they were taken out between different solutions were negligible.

All the vitamin C in the tablet has been dissolved in the solution.

There is insignificant change in the environmental conditions.

The decrease in oxygen concentration inside the plastic cup cause insignificant
effect on the oxidation rate. (The size of the container is much larger in volume
compared with the apple slices to ensure the decrease in oxygen concentration is
slow enough to cause insignificant effect.)

The difference in oxidation rate due to the difference in the surface area of each
groups of apple slices is negligible.
Precaution:

The apple chosen should be healthy as bruising will speed up the browning of it.

The time after the apple surface has exposed to the air but before dipping in the
solution should be minimized so that it has negligible contact with the air which
speed up the oxidation.

The selections of the 2 of 4 the apple portions and of the 12 slices of apple from
the apple should be random processes.

Only the color of the large surface is noted as the side surface has much longer
time of contact with the air and has become slightly brown before dipping into
the solutions. (The skin is peeled before the apple is sliced.)

The observation should be taken under the same lighting device so that there is
fair comparison of the color.
Photo showing cups of solutions of 6 different vitamin C concentrations with a piece of transparency on top
Photo showing 6 groups of apple coated with different vitamin C concentrations covered with plastic cups on a
piece of transparency.
Photos showing the vitamin C tablet used (the container and the tablet)
Prediction:
The apple slices coated in higher Vitamin C concentration will take longer time to
turn brown.
Result:
Temperature at which the experiment is performed. = 18 - 19 ℃
Table of result showing the color of the apple slices coated with vitamin C solution of
different concentrations with respect to time
Time
elapsed
/min.
0 g L-1
1.25 g L-1
0.675 g
L-1
2.5 g L-1
0
1
5
1
10
2
1
15
3
1
20
3
1
25
3
5 g L-1
10 g L-1
2
30
3
1
35
3
1
40
3
2
45
4
3
60
4
3
1
1
2
1
75
4
2
1
90
4
2
1
105
4
3
1
120
4
3
1
135
4
1
150
4
1
165
4
1
180
4
1
195
4
1
210
4
1
225
4
1
240
4
1
300
4
1
555
4
1
765
4
1
144(24hrs)
4
1
Code of brownness*
1: white (no change in color, actually, a bit green in color)
2: pale yellow
3: pale yellowish brown
4: light brown
The order of the oxidation of the apple: 1 < 2 < 3 < 4
*: the color is comparatively speaking only.
Time elapsed
before there is
observable
oxidation of the
apple slice (min.)
0 g L-1
0.675 g
L-1
1.25 g
L-1
2.5 g L-1
10
25
40
60
5 g L-1
10 g L-1
Table of result showing the initial and final texture and appearance of the apple slices
Texture
and
appearance
Initial
Final
0 g L-1
0.675 g
L-1
1.25 g L-1
2.5 g L-1
5 g L-1
10 g L-1
fleshy
Light brown surface (browner round the core
region), dull, dry.
Look freshly cut, firm
but a little bit spongy.
Discussion:
The whole mechanism of how vitamin C prevents the apple oxidation is still not very
clear. It is suggested there are two main ways that vitamin C can inhibit oxidation on
apple surface:
1.
Vitamin C tablets come in the form of ascorbic acid, which slows the oxidation
of the apple slices by preventing the PPO (polyphenol oxidase).
2.
Vitamin C prevents oxidation because of its strong reducing power. Thus the
phenolics are kept reduced, avoiding the formation of quinones and thus no
brown residues can be formed. (The apple is prevented from ‘rusting’.)
Explanation of the result:

The time period before the turning brown of the apple is longer for higher
vitamin C concentration.


When the apple slice is dipped into the solution of higher vitamin C
concentration, more vitamin C can be attached to the apple slice. When the
apple slice is exposed to oxygen, vitamin C is oxidized instead. Besides,
with more vitamin C attached to apple slices, more PPO is inhibited. Thus
the rate of oxidation of phenolics is slowed down.
For concentration  5 g L-1, the apple slices has no color change after a long
period of time.

At these concentrations, the inhibition of oxidation by vitamin C may be
mainly due to the inhibition of the PPO as the reducing property of vitamin
C would probably be lost after the exposed to the air for a long period of
time. (Apple slices coated in half the vitamin C concentration has turn
brown with in an hour. ) Most of the PPO has been prevented by the vitamin
C for concentration  5 g L-1, thus no brown residue is formed. (However,
this explanation is based on the assumption that the oxidized form of
vitamin C can still inhibit PPO but no evidence was found on this.)
In concentrations below this, there is not enough vitamin C to inhibit all the
PPO and some active PPO can still oxidize phenols to quinones, forming
the brown compound.

The oxidation of the apple usually develops from the core.

This may be due to the more damaged cells there because of the more
irregular cutting using the knife or more phenolics / PPO around the core.
Figure showing that the oxidation of the apple slice develops from the tip of it
Source of error:

The measuring cup used is not very accurate in measuring the volume (compared
with measuring cylinders) and the concentrations vitamin C solutions may not be
very accurate. But this is not very significant to the result as the concentrations
used differs quite greatly (successive concentrations differs by half).
Limitation of the experiment and suggestion for improvement:

The color described in the result does not really represent the color. They
are only used to compare the color of the apple slices. For example, even at
the end of the experiment, the color of the apple is said to be light brown in
color, it only means the surface color is browner than the beginning.

The vitamin C tablet has a coating which is difficult to dissolve. Thus the
total amount of Vitamin C dissolved may be fewer than 500mg.

The choice of apple and the ripeness of it may greatly affect the result.

There is difficulty in judging the color of the apple slices. Photos may be
taken regularly so that the color can be compared all together at the end of
the experiment.

Besides, the color change is a gradual process that cannot be shown by the
result. Smaller time intervals with photos taken may show the gradual
change in color better. (However, only the first change in color is used, so
this does not affect the result much.)

The table does not represent the color change directly as the changing in
color is a gradual process but color was noted at particular intervals only.

The oxygen concentration inside the cup is actually decreasing each time
after the covering of the cup. Cups with many pores may be used to imitate
the open air environment.
Conclusion:
There is observable oxidation of the apple after longer period of time for higher
Vitamin C concentration.
Higher vitamin C concentration is more effective in preventing oxidation.
In this experiment, 5 g L-1 is the critical concentration, there is no significant
oxidation observed even after a long time (24 hours) for concentration equal to above
5 g L-1.