Food Tests for sugars, starch, proteins & lipids - methods and observations
Doc Brown's biology revision notes Some of these biology revision notes on food tests are suitable for UK KS3 Science Biology (~US grades 6-8)
as well as GCSE level students
This page will help you answer questions
such as ... How do you carry out the test for a
reducing sugar in food? How do you carry out the test for
starch in food? How do you carry out the test for
protein in food? How do you carry out the test for
lipids in food?
Sub-index for this page on food tests
1.
Introduction - preparing the starting material
2.
Benedict's test for reducing
sugars
3.
Testing for
the carbohydrate starch with iodine solution
4.
The Biuret test
for proteins
5.
Two tests for lipids
(animal fats/vegetable oils)
6.
The DCPIP test for vitamin C
See also
Respiration page for a simple investigation of food calorimetry
The food tests described below are
qualitative only, they area NOT quantitative analysis, you simple get
a 'YES or NO' result for the presence of particular type of food molecule in
your prepared sample.
ALL my Biology Notes
Find your GCSE
science course for more help links to revision notes
Use your
mobile phone or ipad etc. in 'landscape' mode
This is a BIG
website, you need to take time to explore it [Website Search
Box]
email doc
brown
1.
Introduction - preparing the starting material
(need picture of pestle and mortar)
Examples of biological
molecules mentioned in conjunction with food tests
Carbohydrates like sugars and starch are a combination of
the elements carbon, hydrogen and oxygen (C, H and O).
They are synthesised in plants from the sugar molecules made in
photosynthesis.
Lipids (vegetable oils/animal fats) are mainly a
combination of the elements carbon, hydrogen and oxygen (C, H
and O), sometimes phosphorus (P) too.
These are synthesised in plants and animals from fatty acids,
glycerol and other molecules.
Proteins are mainly a combination of the elements carbon,
hydrogen, nitrogen and oxygen (C, H, N and O),
sometimes sulfur (S) too, enzymes proteins might contain metal ions
like Zinc Zn2+.
However, in these tests, you are testing directly for a
type of molecular compounds, not their constituent chemical elements.
These are synthesised in plants and animals from amino acids and
other molecules.
Take the food sample and mash it up using a pestle and mortar.
Mix it with some pure water (distilled/deionised) and mix it all up.
Try to have a good 25 cm3 of food
mixture solution from various types of food, i.e. enough to carry
out a variety of tests on each food sample.
AND, remember that negative results are just as important as
positive results!
AND, make sure you were safety goggles
throughout the experiments.
With a filter funnel and paper, filter the mixture to remove the
'waste' bits of solid food.
The filtrate can now be tested for specific types of food with
various chemical reagents.
In chemistry, the word 'reagent' just means a
chemical you use in carrying out a test.
e.g. like silver nitrate or sodium hydroxide
in
chemical tests for ions
in your GCSE chemistry
These four tests described, are qualitative,
you don't get numerical data, so no graph work to do.
All you get is a positive or negative
result.
TOP OF PAGE and
sub-index
2. Benedict's test for reducing
sugars
(a) A positive direct test for a reducing sugar
Sugars are an important ingredient in biscuits,
bread, breakfast cereals, cakes and sweets.
There are two types of sugar - reducing and
non-reducing sugars.
Glucose and fructose are reducing sugars. Sucrose
is a non-reducing sugar.
You can use Benedict's reagent to test for
reducing sugars.
Put a few cm3 of your previously
prepared filtered food sample mixture into a test tube.
Set up a water bath and set the thermostat to
~75oC and wait for it to warm up.
This is safer than directly heating the
test tube, but still wear safety glasses.
With a teat pipette, add at least half a dozen
drops of Benedict's solution (blue) to the food sample solution and
carefully shake the mixture gently.
(Benedict's solution is based on a complex
copper salt mixture, hence the blue colour)
Place the test tube in the water bath using a
test tube holder and leave it for 5 minutes - make sure the test
tube points away from you in case of any accident.
If the food sample contains a reducing sugar,
the solution in the test tube will change from the initial blue
colour through a series of colours from a green solution ==>
yellow solution ==> orange ==> brick-red precipitate.
The more reducing sugar in the food, the
more likely you are to see the full colour sequence changes to
give the final brick-red precipitate - the best confirming
positive result that a reducing sugar was in the food.
If no reducing sugars are present, you should
not see any significant colour change.
However, this does not mean that no sugars are
present - there might be non-reducing sugars in the food ... read on
in section (b) below
(b) Using Benedict's solution to test for a
non-reducing sugar
BUT, follow the logic carefully, its an indirect
test!
Put a few cm3 of your previously
prepared filtered food sample mixture into a test tube and add a few
cm3 of dilute hydrochloric acid and gently and carefully
shake the mixture.
Set up a water bath and set the thermostat to
~75oC and wait for it to warm up.
Place the test tube in the water bath using a
test tube holder and leave it for at least 10 minutes - make sure
the test tube points away from you in case of any accident.
At this point you need to appreciate a bit
of chemistry!
The hydrochloric acid catalyses and
hydrolyses non-reducing sugars like sucrose into reducing sugars
like glucose or fructose.
e.g. sucrose ====> glucose +
fructose
C12H22O11
+ H2O ===> C6H12O6
+ C6H12O6
Therefore, you can now test the solution for
reducing sugars derived from non-reducing sugars!
Remove the test tube from the water bath and
place it in a test tube rack.
You then add sodium carbonate to neutralise
the acid.
Then test the neutral solution with Benedict's
reagent.
If reducing sugars have been formed, the
solution in the test tube will change from the initial blue
colour through a series of colours from a green solution ==>
yellow solution ==> orange ==> brick-red precipitate.
If the test with Benedict's solution is
negative for both tests (a) and (b), the food probably didn't contain
any sugar at all.
TOP OF PAGE and
sub-index
3. Testing for
the carbohydrate starch with iodine solution
The carbohydrate starch is present in many foods
such as beans, bread, cereal grains, corn, pasta, peas, potatoes and
rice.
Put a few cm3 of your previously
prepared filtered food sample mixture into a test tube.
Add a few drops of a dilute iodine solution
(pale orange-brown) and carefully shake the mixture gently.
If the food sample contains starch the mixture
will turn from pale orange to a dark blue-black colour.
(If the solutions are very dilute, you might
see a nice blue-purple colour.)
If no starch is present, you should not see any
significant colour change - the solution remains a pale orange-brown.
(In chemistry you might test for iodine using
starch solution - its the same test and result!)
TOP OF PAGE and
sub-index
4. The Biuret test
for proteins
Foods such as meat, fish and cheese are rich in
protein.
Put a few cm3 of your previously
prepared filtered food sample mixture into a test tube.
Add a few cm3 of potassium hydroxide
followed by a few cm3 of copper sulfate solution to the food sample and carefully gently
shake the mixture.
If protein is present in the food sample the
solution should turn from blue to pink or purple.
Biuret reagent solution can be made from copper
sulfate, sodium/potassium hydroxide and sodium tartrate.
Using the 'proper' Biuret reagent
is better in my opinion.
If there is no protein in the food sample the
mixture just stays the characteristic blue of a copper salt solution.
TOP OF PAGE and
sub-index
5. Two tests for lipids
(vegetable oils/animal fats)
Foods such as margarine, milk, vegetable oils like
olive oil or sunflower oil contain lipids (chemically they are often
fatty acid esters).
(a) The emulsion test for lipids
For this test you do not use the mixture
prepared for the other tests.
You mash up a sample of food, as before, with
a pestle and mortar and mix with a few cm3 of ethanol
(NOT water) and
mix up in a test tube for a few minutes (there is no need to filter it).
Let the mixture settle out.
The ethanol dissolves any lipid (fatty/oily)
molecules present - lipids are NOT soluble in water.
You then carefully pour the contents of the 1st test tube
into a 2nd test tube containing a few cm3 of pure water
and carefully shake the mixture (try to avoid food particles getting
into the test tube).
Since lipids are not soluble in water, any
lipids dissolved in the ethanol are precipitated out as fine
globules of oil, so you see a milky emulsion.
The more lipids in the food sample, the more
milky the mixture becomes, but this is not a quantitative test!
If no fatty/oily lipid molecules are present,
you don't see a milky emulsion.
(I wonder if you could add the Sudan III stain
at this point to get the red colour described below?)
(b) The Sudan III test for lipids
For this test you do not have to use the
mixture prepared for the other tests.
You mash up a sample of food, as before, with
a pestle and mortar and mix with a few cm3 of water and
there is no need to filter it.
With a teat pipette add three drops of
Sudan III stain solution and carefully and gently shake the
mixture. Sudan III stain is red-brown in colour
If the food sample contains lipids, the
mixture will separate into two layers.
The 'thinner' top layer contains the lipids
(insoluble oil) which becomes stained a bright red by the
Sudan III stain if lipids are present.
If there are no lipids in the food sample, you
will not get an upper separate red layer forming.
Keywords for gcse biology revision notes on food
tests for sugars, proteins & lipids-fats: GCSE 9-1 biology biological science IGCSE revision
notes food tests for sugars, proteins & lipids-fats KS4 biology Science notes on
food tests for sugars, proteins & lipids-fats GCSE biology guide
notes on food tests for sugars, proteins & lipids-fats for schools colleges academies science course tutors images
pictures diagrams for food tests for sugars, proteins & lipids-fats science revision notes on
food tests for sugars, proteins & lipids-fats for revising biology modules biology topics notes to help on understanding of
food tests for sugars, proteins & lipids-fats university courses in biological science
careers in science biology jobs in the pharmaceutical industry
biological laboratory assistant
apprenticeships technical internships in biology USA US grade 8 grade 9 grade10 AQA
GCSE 9-1 biology science notes on food tests for sugars,
proteins & lipids-fats GCSE
notes on food tests for sugars, proteins & lipids-fats Edexcel GCSE 9-1
biology science notes on food tests for sugars, proteins &
lipids-fats for OCR GCSE 9-1 21st century biology science
notes on food tests for sugars, proteins & lipids-fats OCR GCSE
9-1 Gateway
biology science
notes on food tests for sugars, proteins & lipids-fats WJEC gcse
science CCEA/CEA gcse science gcse biology revision notes on food tests
for sugars, proteins & lipids-fats
Website content © Dr
Phil Brown 2000+. All copyrights reserved on Doc Brown's biology revision notes
on tests for food substance - method, reagents, observations,
deductions, images,
quizzes, worksheets etc. Copying of website material is NOT
permitted. Exam revision references to science course specifications
are unofficial.
|