1. Food Testing Carbohydrates
You are going to carry out a series of tests for finding
the presence, and type, of carbohydrate in food
products. There are a number of stages here so be
sure to read all instructions carefully and follow the
procedures as they are set out.
You are expected to answer the in-text questions as
you go along.
Preparing food stuffs
You need to prepare the materials to give optimum testing conditions. Any large items of
food (potato for example) should be chopped as small as possible or crushed in the pestle
and mortar. Dried food stuffs may need suspending in liquid.
Q1. What foods are you using and how have you had to prepare them?
Food Preparation Needed
Test one: The Benedictʼs test for Reducing Sugars
Safety
Eye protection – there is some risk
of hot liquids spitting when heating
test tubes Benedictʼs reagent:
LOW HAZARD Take care when
cutting food with a scalpel LET TRE
K N O W I F Y O U H AV E A N U T
ALLERGY. You should not carry out
tests with nuts if you are allergic to
Te s t t u b e s , t e s t t u b e Distilled water (in a
rack wash bottle)
Te s t t u b e h o l d e r B e n e d i c t ’s r e a g e n t i n a
dropper bottle
Thermostatic water
bath 1% glucose solution
Thermometer Freshly made 1%
sucrose solution
Scalpel (or a pestle and
mortar) Foods
Tile L a rg e p l a s t i c w a s t e
beaker
Spatula
250 cm3 beaker
Pasteur pipettes

2. Food Testing Carbohydrates
suspension suggests a low concentration. Confirmation of the absence of reducing sugars or a concentration below the sensitivity of
Confirmation of a reducing sugar: A colour change from a blue solution to a green/yellow/orange/brown or brick-red suspension. A green
Method
1. Set up the water bath to just below boiling point
2. Label some test tubes for the substances tested
3. Add 2 cm3 of each test solution or suspension to labelled test tubes (or 1
Possible results
cm depth of chopped food material and 2 cm3 of distilled water). One test
tube should contain the 1% glucose solution.
4. Add ten drops of Benedict’s reagent (about 0.5 cm3) to each test solution
the test for reducing sugars using Benedict’s reagent: when there is no colour change from the blue solution.
(without the dropper pipette touching the inside of the test tube).
5. Use the test tube holder to transfer the test tube with glucose into the water
bath. Observe the colour changes in the test tube over two minutes of heating
at boiling point. Return the test tube to the test tube rack. Make a note of
your observations in the spaces that are provided in Table 1.
6. Repeat the test for all of your samples.
Material tested Observations Deductions
1% glucose solution A change from a blue solution Confirms that glucose is
to a suspension a
Distilled water
(control)
1% sucrose solution
Table 1 A table of observations and deductions from the test for reducing
sugars using Benedict’s reagent

3. Food Testing Carbohydrates
Questions
1.
In the test for reducing sugars using Benedict’s reagent on glucose, (a)
Use pages 20 and 21 of the textbook to help you answer these questions
where does the colour change begin in the blue solution and (b) why does
it occur at this position first?
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2. When the test is carried out by heating the test tube directly, there can be
superheating of the test solution which can be ejected violently from the
test tube. Why is this unlikely to occur when heating the contents of the
test tube within a water bath?
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Discussion
1. Which of the materials tested (a) contained reducing sugars and (b) did not
contain reducing sugars?
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2.
Suggest one reason why there could be some reducing sugar in a test
solution that gives a negative result in the test for reducing sugars using
Benedict’s reagent.
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The Details
Examiners often expect you to know the following in relation to this test:
1. Why does the Benedict's reagent turn red in the
presence of reducing sugar?
2. Why do you have a glucose and water sample as
well as the food samples?
3. How precise is this investigation? How could you
colour in these test-tubes to reflect expected results

4. Food Testing Carbohydrates
Test two: Serial dilution: finding the minimum sensitivity of Benedictʼs
and Clinistix tests
Here we are going to learn how to make very Safety Wear eye
dilute solutions through a technique called serial protection – there
is some risk of hot
dilutions. We are going to dilute a 10% solution of
glucose through a series of dilutions until we have
a 0.001% glucose solution.
liquids spitting
You are then going to compare the sensitivity of when heating test
tubes Benedict’s
the Benedictʼs test with the clinical diagnostic
sticks, Clinistix.
This investigation is split into three parts. You will reagent: LOW
need to make observations and record results HAZARD Take
care with
from each section before you can move onto the
next. Keep your equipment set up until you have
completed the comparison between the two tests.
glassware
Equipment and materials
Eye protection White tile
Labelling equipment Waste beaker
Test tubes (10), bung (for test Paper towel
tubes) Water Bath
Test tube holder Distilled water wash bottle
10 cm3 pipette and pipette filler
1 cm3 pipette and pipette filler
Clinistix test strips
Step 1: Making serial tenfold dilutions of a 10% glucose
solution
✴ Label four test tubes, so that you can keep track of which dilution is which
✴ Add 9.0 cm3 of distilled water to each of the four labelled test tubes.
✴ Add 1.0 cm3 of the 10% glucose solution to the first test tube. Put a clean bung into
the neck of the test tube. Hold it in tightly and shake the test tube carefully ten times
through 30 cm in order to get an even mixture. This process achieves a tenfold
dilution.
✴ Using a clean pipette and a clean bung, repeat steps 2 to 3 for the three remaining
test tubes, on each occasion transferring 1.0 cm3 of your most recently diluted
glucose solution into the next test tube that contains 9.0 cm3 of distilled water.

5. Food Testing Carbohydrates
Step 2: Devising a standardised test For you to determine the minimum sensitivity of
the test for reducing sugars with Benedictʼs reagent, you need to conduct the test in precisely
controlled conditions, and you need a control.
Pointers towards devising a standardised test
In a trial, the mass of Benedict’s reagent that was dispensed by two
Work together to discover the answer to
methods was compared.
Method of dispensing the Mass of the dispensed Benedict’s reagent/g
Benedict’s reagent Individual values Mean
10 drops from a dropper pipette 0.34 0.37 0.39 0.35 0.39 0.37 (to 2
d.p.)
0.5 cm3 from a 1 cm3 glass pipette 0.58 0.59 0.58 0.59 0.59 0.59 (to 2
d.p.)
these questions
1.
From the data in the table, suggest the advantage in using measured
volumes from a 1 cm3 glass pipette rather than 10 drops
from a dropper pipette in a standardised Benedict’s test.
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2.
List other variables that you think should be kept constant for a
standardised test for reducing sugars using Benedict’s reagent.
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3.
Suggest a control for the experiment to find the minimum sensitivity
of the test for reducing sugars using Benedict’s reagent.
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Plan your standardised test here.....
get the method checked before going futher...

6. Food Testing Carbohydrates
Glucose Observations Deductions
concentration/%
10
1.0
0.1
0.01
0.001
Conclusion for the test for reducing sugars using Benedict’s reagent
What was the lowest glucose concentration where reducing sugars were shown
to be present (when compared with the control and against a white background)?
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Comparing the minimum sensitivity of the test using Clinistix and the test using
Benedict’s reagent
Now test a fresh sample of the least concentrated glucose solution that gave a
positive reaction with the test using Benedict’s reagent with the Clinistix test strip.
Clinistix contains glucose oxidase and peroxidase in the coloured pad and is used
to detect glucose in urine as a preliminary test for diabetes.
Discussion
1.
Was Clinistix able to detect the same low concentration of glucose as the test
using Benedict’s reagent? Could it detect lower values?
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7. Food Testing Carbohydrates
Food test 2: biochemical tests for non-reducing sugars and starch
Aims In this practical work you will carry out two Safety Wear eye
qualitative food tests to find out whether certain protection – there is
types of food molecules occur in various food some risk of hot
samples. For example, does starch occur in liquids spitting when
potatoes? You will test materials that should give heating test tubes
positive results and test controls that should give Benedict’s reagent:
negative results. LOW HAZARD Dilute
hydrochloric acid (0.1
Equipment and materials mol dm–3): LOW
As for the test for reducing sugars plus three items: HAZARD but may
 Dilute hydrochloric acid (0.1 mol dm–3) cause harm if it enters
 Sodium hydrogencarbonate powder the eye or a cut
Sodium
Method hydrogencarbonate:
1. Do this test only after a negative Benedict’s test. MINIMAL HAZARD IN
2. Label the tops of your test tubes. THIS TEST Take care
3. Use a Pasteur pipette to dispense 2 cm3 of a fresh when cutting food with
sample into a test tube (or finely chopped/ground food a scalpel LET TRE
to 1 cm depth and 2 cm3 of distilled water). KNOW IF YOU HAVE A
4. Use a dropper pipette to add 10 drops of dilute NUT ALLERGY. You
hydrochloric acid (HCl) to the sample.
should not carry out
5. Use a test tube holder to transfer the test solution to the tests with nuts if you
water bath. You can save time by having up to five test
tubes in the water bath. Heat to boiling point for two are allergic to them.
minutes. This is the stage when acid hydrolysis may
occur
Confirmation of a non-reducing sugar, for example,
sucrose, in the original solution: a brick-red/brown/
sugar (and the absence of reducing sugars) in the
6. Cool the test tube in cold water (in a beaker).
orange/yellow or green suspension at this stage
Confirmation of the absence of a non-reducing
7. Sodium hydrogencarbonate (NaHCO3) powder is needed to make the
solution alkaline. With a spatula add sodium hydrogencarbonate to
original solution: a blue solution remains.
the solution until the fizzing stops. The solution must be alkaline for
the Benedict’s reagent to work. You can confirm that the solution is
alkaline by using a Pasteur pipette to take a sample and adding it to
universal indicator paper on a tile; compare the colour with that in the
chart.
8. Add 10 drops of Benedict's reagent, about 0.5 cm3, to the test solution.
9. Heat the test solution to boiling point in a water bath and continue
heating for two minutes.
Results
10. Observe the colour changes in the test tubes over two minutes of
only.
heating at boiling point. Record your observation in Table 1.

8. Food Testing Carbohydrates
Material tested Observations (after acid and heat, Deductions
etc.)
1% sucrose solution
Distilled water (control)
1% starch suspension
The explanation of the test for non-reducing sugars using
Use Pages 22 and 23 in your textbook to help you
Benedictʼs reagent
Sucrose is a non-reducing sugar that occurs naturally in plants. Sucrose is a
disaccharide consisting of glucose and fructose units linked by a glycosidic
bond. It is not a reducing sugar because the reducing groups of glucose and
fructose are tied up in the glycosidic bond. When a sucrose solution is acidified
and heated; the hydrogen ions of the acid catalyse the hydrolysis of sucrose to
glucose and fructose, which are both reducing sugars, and cause the reduction
reaction.
Question
Which of the materials you tested: contained reducing sugars?
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contained non-reducing sugars?
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contained neither reducing sugars nor non-reducing sugars?
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Discussion
A student did not read the practical schedule carefully enough and found a
brick-red suspension for the test for reducing sugars and the test for non-
reducing sugars.
a) What can be concluded from these results?
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b) What cannot be concluded from these results?
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9. Food Testing Carbohydrates Safety Wear eye protection
Iodine solution – avoid contact
The iodine test for starch with skin and clothes Take care
when cutting food with a scalpel
Equipment and materials
 Eye protection  Spatula
 Labelling equipment  Distilled water
 Scalpel and tile  Iodine (in potassium iodide) solution
 Pasteur pipettes  1% starch suspension
 Dimple tile  Food materials, for example, potato
and onion
Method
Results Confirmation of
coloration Confirmation
1. Label individual dimples on the dimple tile with the foods to be tested.
starch: a yellow iodine
colour (or just no blue-
2. Use a clean Pasteur pipette to dispense a few drops of the test
starch: a blue-black
solution into a labelled dimple. For solid foods, use the scalpel to cut
of the absence of
the food sample into small pieces on a tile and use a clean spatula to
transfer some of them into a labelled dimple.
black colour)
3. Use the dropper pipette to add two drops of iodine solution (iodine in
potassium iodide) to the food samples.
Table 2 Observations and deductions from the iodine test for starch
Material tested Observations Deductions
1% starch suspension
The explanation of the iodine test for starch
Starch contains both amylose and amylopectin molecules. The coils of its amylose
molecules take up iodine molecules from the iodine solution and form the blue-black,
starch–iodine (polyiodide) complex.
Discussion
4. Which of the materials you tested
a) contained starch?
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b) did not contain starch?
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5. How could you show that a yellow liquid contains iodine?
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