Lab 04: Determination of the content of reducing sugar using Benedict’s test
2007 Al-Bio Marking Scheme
Lab 3: What has happened to the banana?
Area B1 (max. 15 marks)
Problem: (1 mark)
What biochemical changes do you think have taken place in bananas during the period of storage?
Aim:
To find out what biochemical changes have taken place in the banana during storage.
Hypothesis:
Starch is converted to sugar during storage.
Or
Problem:
What biochemical changes do you think have taken place in bananas during the period of storage?
Aim:
To prove that the biochemical change in the banana during storage.is the conversion of starch into
reducing sugar
Or
Problem:
Why yellow bananas are sweeter than green bananas?
Hypothesis:
Yellow bananas are sweeter than green bananas because some starch is converted into reducing sugar
during ripening.
Aim:
To prove the hypothesis by comparing the amount of reducing sugar and starch in bananas of different
degree of ripeness.
Principle: (Experimental design and principles) (max. 7)
Extraction of chemicals from bananas
To grind up the tissue, so as to destroy the cell membrane and extract chemicals inside for chemical analyses
with Benedict’s solution and iodine solution.
Iodine test
a blue-black colouration is formed when adding iodine solution into starch
-
mechanism
iodine solution refers to iodine dissolved in an aqueous solution of potassium iodide. It is brown in colour. starch molecules
react with iodine to form a blue-black complex If the concentration of starch is relatively dilute, a dark brown colour is obtained
-
the higher the colour intensity, the higher concentration of starch is.
Amylase:
- Digest starch into reducing sugar (maltose)
 time for disappearance of blue black colour in iodine test
/ fold of dilution for appearance of a cross under test tube
or  time for no more ppt. formed
/ amount of extra ppt. formed in Benedict’s test in fixed digestion time period.
Page 1/4
Lab 04: Determination of the content of reducing sugar using Benedict’s test
2007 Al-Bio Marking Scheme
Benedict’s test
- condition / test
boiling of Benedict’s solution with reducing sugars / test with Benedict’s solution will give brick red
precipitate
-
mechanism
due to presence of blue solution Cu 2+ in Benedict’s solution changed into insoluble brick-red precipitate (Cu+)
-
observation and indicator for estimation


making the resulted mixture have a gradual change of colour with amount of reducing sugars inside the solution : from blue
to green, then yellow and finally brick red
the colour of boiled mixture / the amount of precipitate at bottom of test tube as indicator of comparing concentration of
reducing sugars.
the more the ppt.  more reducing sugar
Variables:
 independent variable: bananas of different degree of ripeness (yellow and green bananas / period of
storage in refrigerator)
 dependent variables: amount of reducing sugar (precipitate formed in Benedict’s test) and amount of
starch (intensity of dark blue colour in Iodine test)
Expected result
more reducing sugar and fewer starch in yellow banana than in green banana.
Others:
 pre-experimental trial:
to determine the ratio of banana extract and testing solutions to ensure complete reaction of reducing
sugar and starch during food tests. (dilution of extracts may be required)
 assumption:
1. the only reducing agent for precipitate formation in the experiment is reducing sugar in the solutions
2. all sweet taste of the yellow banana comes from reducing sugar (e.g. not from sucrose)
3. yellow banana and green banana are from the some plant (i.e. no genetic variation)
4. all chemical changes stopped during extraction and the whole experiment
(this assumption can be omitted if the bananas are boiled before extraction)
 Others
List of apparatus
Page 2/4
Lab 04: Determination of the content of reducing sugar using Benedict’s test
2007 Al-Bio Marking Scheme
Procedures (max. 7)
Steps
Labelling
Quantities and Units of control variables / observations
Extraction
volume of water, mass and size of sample, time for grinding
serial dilution (if required)
Volume of water, and extract of sample
resultant concentrations
Benedict’s test
- Benedict’s solution added
- Extract added
- Initial observations
- Boiled in water bath
- Final observations
place the boiled tube on test tube rack for:
 compare colour differences immediately; or
 wait for cooling and settlement of precipitate
and compare amount of ppt.
Iodine test
- Initial observation
- Iodine solution added
- Extract added
- Final observation
OR
Amylase digestion
- amylase solution added
- extract added
- iodine test
-- Initial observation
-- Iodine solution added
-- digested extract / extract added
-- Final observation
OR
Amylase digestion
- amylase solution added
- extract added
- Benedict’s test
- Benedict’s solution added
- Digested extract / extract added
- Initial observations
- Boiled in water bath
- Final observations
place the boiled tube on test tube rack for:
 compare colour differences immediately; or
 wait for cooling and settlement of precipitate
and compare amount of ppt.
- volume
- concentration (if diluted), volume
- colour, any ppt.
- time period, temp.
- colour, amount of ppt. (diameter / depth / ….)
-
Time waited
-
volume
concentration (if diluted), volume
observed relative intensity / appearance of a cross
-
volume,
concentration (if diluted), volume
- colour
- volume
- digestion time period, concentration (if diluted), volume
- colour / ….
-
volume,
concentration (if diluted), volume
-
volume
digestion time period / time waited, concentration (if
diluted), volume
- colour, any ppt.
- time period, temp.
- colour, amount of ppt. (diameter / depth / ….)
Time waited
Page 3/4
Lab 04: Determination of the content of reducing sugar using Benedict’s test
Precautions
Steps
- Pipette should be erect and be read at the same
level as eyes
- separate pipette for extracts, water (and amylase)
transfer; or wash pipette throroughly before
pipetting different fluids.
- use hot water for extraction of starch
excess Benedict’s solution used
heating in water bath with safety glass
sufficient heating duration
-
- others
Controls
- water boiled with Benedict’s solution
- Water boiled with extracts
-
Extracts without amylase in iodine test /
Benedict’s test
2007 Al-Bio Marking Scheme
Explaination
Prevent reading errors
Prevent contamination of solutions with each other
to increase the solubility of starch since starch is only slightly
soluble in water
For complete reaction of all reducing sugars
For even heating and protect the eyes
For complete reaction between reducing sugars and copper
(II) ion
To prove no contamination in water and reagents / apparatus.
To prove no oxidation of reducing sugar without Benedict’s
solution.
To prove no reduction of starch / no increase of reducing
sugar without amylase
Others (max. 1)
- technique descriptions, good table presentation, others
Area B2 (max. 7)
- Results recorded :
-- initial observations and measurements
-- final observation and measurements
-
Formula for calculation (if required)
-
Data table:
-- correct and proper title for all tables
-- proper heading with S.I. unit (if needed)
-- complete table lines
-
Graph:
-- correct and proper title for all and graphs
-- correct x-axis and y-axis with proper unit and scale
-- bar chart but not curves can be used if discontinuous independent variable is used
Area B3 (max. 8)
- data description and interpretation (describe the trend of changes in content of reducing sugar and starch
with different degree of ripeness and interpret into any biochemical changes occurred during ripening)
- other significant discussion (e.g. abnormalities, limitation, sources of error, wrong assumption, any new
assumption, improvement suggestion, suggestion of following experiment for further investigation…etc.)
- no explanation of principle or precaution except you have any new hypothesis suggested after the
experiment.
- conclusion (answer to your aim, no explanation in conclusion but in discussion )
* with reference to the above guidelines, reports were marked by overall impression.
Page 4/4