Biology SBA #1
KADEEN GRAHAM
Teacher: Ms. Hurlock
St. Mary High School
SBA #: 1
Date: October 05, 2017
Title: Food Test
Aim: To identify the biomolecules found in different food items.
Apparatus/Materials: Test tubes, Bunsen burner, tripod stand, test tube holder,
mortar and pestle, measuring cylinder, 500cm3 beaker, distilled water.
Reagents: Ethanol, hydrochloric acid, copper (II) sulphate, sodium hydroxide,
potassium hydroxide, Benedict’s solution, Iodine Solution.
Method:
Test for Starch
1. Solid samples were placed on a white tile while liquid samples were placed into test
tubes.
2. A pipette was used to apply iodine solution to the samples.
Test for Reducing Sugars
1. Solid samples were crushed and a solution made with water.
2. 1cm3 of the test solution was placed into test tubes and an equal volume of
Benedict’s solution added.
3. The test tubes were added to a boiling water bath and colour changes observed.
Test for Non-reducing sugars
1. 2cm3 of test solution was added to a test tube.
2. A few drops of hydrochloric acid were added and the test tube was placed in a
boiling water bath.
3. The test tube was cooled and dilute sodium hydroxide solution was added.
Benedict’s solution was added afterwards.
Test for Protein
1. Solid samples were crushed and a solution made with water
2. 1cm3 of the test solution was placed in a test tube
3. An equal volume of biuret solution was added and the test tube and the contents
mixed by shaking.
Test for lipids
1. Solid samples were crushed and added to ethanol while ethanol was added directly
to liquid samples in a test tube.
2. The mixture was poured into a test tube of water.
All observations were recorded.
OBSERVATIONS:
Observation Inference
Test For: Sample
Starch Bread Yellow to blue-black colour Starch is present
change
Cane Juice No colour change observed Starch is absent
Oil No colour change observed Starch is absent
Cheese No colour change observed Starch is absent
Fried Chicken No colour change observed on Starch is only
flesh, blue-black colour change present in the skin
on skin
Protein Bread No colour change Observed Protein is absent
Cane Juice No colour change Observed Protein is absent
Oil No colour change Observed Protein is absent
Cheese Purple colour observed Protein is present
Fried Chicken Purple colour observed Protein is present
Reducing Bread Blue to green colour change Reducing sugars are
Sugars seen. Later changed to orange present
Cane Juice Green to yellow to orange Reducing sugars are
colour changes observed present
Oil No colour change Observed No reducing sugars
present
Cheese Different layers seen. Green at Reducing sugars are
top, yellow in middle then present
orange at bottom.
Fried Chicken No colour change Observed No reducing sugars
present
Non-reducing Bread No colour change Observed Non-reducing
Sugars sugars are absent
Cane juice No colour change Observed Non-reducing
sugars are absent
Oil No colour change Observed Non-reducing
sugars are absent
Cheese No colour change Observed Non-reducing
sugars are absent
Fried Chicken No colour change Observed Non-reducing
sugars are absent
Lipids Bread No change Observed Lipids are absent
Cane Juice No change Observed Lipids are absent
Oil Dissolved in ethanol. Cloudy Lipids are present
emulsion observed
Cheese Cloudy emulsion observed Lipids are present
Fried Chicken Cloudy emulsion observed Lipids are present
TABLE SHOWING OBSERVATIONS AND INFERENCES FROM FOOD TESTS DONE IN
EXPERIMENT
DISCUSSION:
Living organisms need energy and nutrients in order to performs tasks which allows
them to survive. These include reproduction, growth and repair, and defence. The
energy needed is stored as chemical energy in food and is made available to our cells
by respiration, which involves a number of processes that break the bonds of the
molecules in food, producing the energy. These molecules are referred to as
biochemicals or biomolecules and include carbohydrates, lipids and proteins.
Carbohydrates are macromolecules containing carbon, hydrogen and oxygen. They
consist of sub-unit molecules of glucose which is a reducing sugar. Lipids are also
made of carbon, hydrogen and oxygen but the ratios of carbon to oxygen and
hydrogen to oxygen are higher. They contain fatty acids. Proteins contain carbon,
hydrogen, oxygen, nitrogen and sometimes sulphur or phosphorus. Proteins are held
together by polypeptide bonds and are the most diverse group of biomolecules as
they perform the greatest variety of cellular functions.
In the experiment, tests were done on several food samples in order to identify the
biomolecules present in them.
STARCH
Starch was found to be present in the bread and fried chicken skin. This was made
obvious by the blue-black colour change. Starch is a mixture of amylose, a poly-(1-4)
glucose, and amylopectin, a poly-(1-4) glucose with (1-6) branches. Amylose forms a
floppy chain and has a tendency to coil into a helix shape. When in contact with
amylose, iodine ions can slip into and become bounded to the centre of amylose
chain leading to a starch-iodine complex, resulting in the blue-black colour change.
No colour change was observed on the flesh of the fried chicken itself, but since fried
chicken is coated in flour (which contains starch), the skin tested positive for starch.
PROTEIN
Protein was present in the cheese and fried chicken. A biuret test consists of alkaline
copper (II) sulphate. When exposed to polypeptide bonds, the copper (II) ions will
form a violet-coloured complex in an alkaline solution (hence, why potassium
hydroxide was added). Therefore, it can be said that polypeptide bonds are present
within the cheese and the chicken.
LIPIDS
Oil, cheese, and fried chicken tested positive for lipids. Lipids are organic compounds
that are known to be soluble in organic solvents like ethanol but insoluble in
inorganic ones such as water. Ethanol was added to dissolve the lipids. When the
mixtures were added to water, a white emulsion was observed because lipids cannot
dissolve in water and so the lipids become dispersed throughout the water in tiny
particles.
REDUCING SUGARS
Cheese, cane juice and bread all contain reducing sugars. Reducing sugars are sugars
that can act as reducing agents due to their ketone or aldehyde groups. Benedict’s
solution contains copper (II) ions. Reducing sugars will reduce copper (ii) ions to
copper (I) ions, forming a precipitate of copper (I) oxide. All monosaccharides (like
glucose) are reducing sugars. Some disaccharides and polysaccharides are also
reducing sugars.
NON-REDUCING SUGARS
No food samples were found to contain non-reducing sugars, the most popular being
sucrose. Non-reducing sugars have no free ketone or aldehyde groups which makes
them unreactive so they wont reduce copper (II) ions.
CONCLUSION:
It can be concluded that lipids are found in oil, cheese and fried chicken, reducing
sugars in cheese, cane juice and bread, protein in cheese and chicken and starch in
bread while none of the samples tested contain non-reducing sugars.
LIMITATIONS:
1. There was contamination with samples leading to positive/negative results for
tests.
2. Benedict’s solution was added before the mixture was neutralised in the test
for non-reducing sugars or enough acid was not added.
