Practical
5.X
Comparison of the
food substances
in two types of
drinks
Objective: To compare the food
substances in milk and packaged fruit juice by using various food test.
Biological
principle and experiment design: (9 marks)
Different food tests are used to test for the
presence of particular food substances. The following table shows different
food tests and the corresponding food substances:
Food test
|
Food
substances
|
Using Glucose test paper (1)
|
Glucose
|
Benedict’s test
|
Reducing sugar (1)
|
Iodine test
(1)
|
Starch
|
Grease spot test
|
Lipid (1)
|
Using protein test paper (1)
|
Protein
|
Using DCPIP solution
|
Vitamin C
(1)
|
Milk and fruit juice contain different nutrients,
so we can use the above food tests to compare the food substances they contain.
To ensure that the positive result of the food test
is due to the presence of the food substance in the food
sample instead of the chemicals or contaminants in
the container, Control (1) should be set up by using the same
amounts of water to repeat all the food tests.
Expected results: (2)
Milk may contain … (1);
While the packaged fruit juice may contain … (1)
Results: (12
marks)
A Using
glucose test paper to test for glucose
Sample
|
Colour of the test end of the glucose test paper
|
|
Orginal
|
After the test
|
|
Milk
|
Pink
|
Pink
|
Fruit juice
|
Pink
|
Blue
|
Distilled
water
|
Pink
|
Pink
|
B Using
Benedict’s test to test for reducing sugars
Sample
|
Observation
|
Relative
amount of precipitate
(arbitrary unit*)
|
|
Before heating
|
After heating
|
|
|
Milk
|
Pale blue
|
Brown precipitate
|
+++
|
Fruit juice
|
Pale blue
|
Brick precipitate
|
++++
|
Distilled water
|
Pale blue
|
Pale blue
|
-
|
*Note: “-”
represents no brick-red precipitate
no.
of “+” represents the relative amounts of brick-red precipitate.
C Using
iodine test to test for starch
Sample
|
Colour of the iodine solution
|
|
Orginal
|
After the test
|
|
Milk
|
Brown
|
Brown
|
Fruit juice
|
Brown
|
Brown
|
Distilled
water
|
Brown
|
Brown
|
D Using
grease spot test to test for lipids
Sample
|
Appearance of translucent spot
|
|
Before adding the organic solvent
|
After adding the organic solvent
|
|
Milk
|
Yes
|
No
|
Fruit juice
|
No
|
No
|
Distilled
water
|
No
|
No
|
E Using
protein test paper to test for proteins
Sample
|
Colour of the test end of the protein test paper
|
|
Orginal
|
After the test
|
|
Milk
|
Yellow
|
Green
|
Fruit juice
|
Yellow
|
Yellow
|
Distilled
water
|
Yellow
|
Yellow
|
F Using
DCPIP solution to test for vitamin C
Sample
|
Colour of DCPIP solution
|
No. of
drops needed for decolorization
(drops)
|
|
Original
|
After the test
|
|
|
Milk
|
Blue
|
Milky violet
|
> 50
|
Fruit juice
|
Blue
|
Original colour of the fruit juice
|
e.g. 2 - 10
|
Distilled water
|
Blue
|
Pale blue
|
> 50
|
Discussion:
(5 marks)
1 Is it
necessary to measure the amounts of food samples accurately for comparison of
the food substances contained? Why? (2)
No, it is not necessary to measure the amounts of
food samples accurately (1).
It is because the objective of the experiment
is to compare the food substances in the two food
samples qualitatively only (1).
/ Yes (1). It is because we need to compare the
food substances in the food samples quantitatively (1).
2 Explain why the use of DCPIP solution in the
measurement of the amount of vitamin C of a food sample is not necessarily
accurate. [Challenging question] (3)
DCPIP may react with substances other
than vitamin C and decolourize (1). In fact,
substances other than vitamin C (e.g. anti-oxidant)
can also decolourize DCPIP (1), that may interfere
with the measurement of the amount of vitamin C (1)
Conclusion: (2 marks)
The
milk sample contains… (1) and the fruit juice sample contains … (1)
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