Food Testing Carbohydrates

1. 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
Food Testing Carbohydrates
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
KNOW IF YOU HAVE A NUT
ALLERGY. You should not carry out
tests with nuts if you are allergic to
Test tubes, test tube
rack
Test tube holder
Thermostatic water
bath
Thermometer
Scalpel (or a pestle and
mortar)
Tile
Spatula
Pasteur pipettes
Distilled water (in a
wash bottle)
Benedict’s reagent in a
dropper bottle
1% glucose solution
Freshly made 1%
sucrose solution
Foods
Large plastic waste
beaker
250 cm
3
beaker

2. Food Testing Carbohydrates
Method
1. Set up the water bath to just below boiling point
2. Label some test tubes for the substances tested
3. Add 2 cm
3
of each test solution or suspension to labelled test tubes (or 1
cm depth of chopped food material and 2 cm
3
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
(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.
Possible results
Confirmation of a reducing sugar: A colour change from a blue solution to a green/yellow/orange/brown or brick-red suspension. A green
suspension suggests a low concentration. Confirmation of the absence of reducing sugars or a concentration below the sensitivity of
the test for reducing sugars using Benedict’s reagent: when there is no colour change from the blue solution.
Material tested Observations Deductions
1% glucose solutionA change from a blue solution
to a suspension
Confirms that glucose is
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. The Details
Examiners often expect you to know the following in relation to this test:
Food Testing Carbohydrates
Questions
1.In the test for reducing sugars using Benedict’s reagent on glucose, (a)
where does the colour change begin in the blue solution and (b) why does
it occur at this position first?
...............................................................................................................................
..............................................................................................................................
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?
..............................................................................................................................
..............................................................................................................................
Discussion
1. Which of the materials tested (a) contained reducing sugars and (b) did not
contain reducing sugars?
..............................................................................................................................
..............................................................................................................................
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.
..............................................................................................................................
..............................................................................................................................
Use pages 20 and 21 of the textbook to help you answer these questions
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. Test two: Serial dilution: finding the minimum sensitivity of Benedictʼs
and Clinistix tests
Here we are going to learn how to make very
dilute solutions through a technique called serial
dilutions. We are going to dilute a 10% solution of
glucose through a series of dilutions until we have
a 0.001% glucose solution.
You are then going to compare the sensitivity of
the Benedictʼs test with the clinical diagnostic
sticks, Clinistix.
This investigation is split into three parts. You will
need to make observations and record results
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.
Equipment and materials
Eye protection
Labelling equipment
Test tubes (10), bung (for test
tubes)
Test tube holder
10 cm
3
pipette and pipette filler
1 cm
3
pipette and pipette filler
Clinistix test strips
White tile
Waste beaker
Paper towel
Water Bath
Distilled water wash bottle
Food Testing Carbohydrates
Safety Wear eye
protection – there
is some risk of hot
liquids spitting
when heating test
tubes Benedict’s
reagent: LOW
HAZARD Take
care with
glassware
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 cm
3
of distilled water to each of the four labelled test tubes.
✴ Add 1.0 cm
3
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 cm
3
of your most recently diluted
glucose solution into the next test tube that contains 9.0 cm
3
of distilled water.

5. Plan your standardised test here.....
get the method checked before going futher...
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
methods was compared.
Method of dispensing the
Benedict’s reagent
Mass of the dispensed Benedict’s reagent/g
Individual values Mean
10 drops from a dropper pipette0.340.370.390.350.390.37 (to 2
d.p.)
0.5 cm
3
from a 1 cm
3
glass pipette0.580.590.580.590.590.59 (to 2
d.p.)
1. From the data in the table, suggest the advantage in using measured
volumes from a 1 cm
3
glass pipette rather than 10 drops
from a dropper pipette in a standardised Benedict’s test.
......................................................................................................................
......................................................................................................................
2.List other variables that you think should be kept constant for a
standardised test for reducing sugars using Benedict’s reagent.
..................................................................................................................................
..................................................................................................................................
3.Suggest a control for the experiment to find the minimum sensitivity
of the test for reducing sugars using Benedict’s reagent.
......................................................................................................................
Work together to discover the answer to
these questions

6. Glucose
concentration/%
Observations Deductions
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)?
...................................................................................................................................
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?
..................................................................................................................................
...................................................................................................................................
Food Testing Carbohydrates

7. Food test 2: biochemical tests for non-reducing sugars and starch
Aims In this practical work you will carry out two
qualitative food tests to find out whether certain
types of food molecules occur in various food
samples. For example, does starch occur in
potatoes? You will test materials that should give
positive results and test controls that should give
negative results.
Equipment and materials
As for the test for reducing sugars plus three items:
Dilute hydrochloric acid (0.1 mol dm
–3
)
Sodium hydrogencarbonate powder
Food Testing Carbohydrates
Method
1.Do this test only after a negative Benedict’s test.
2.Label the tops of your test tubes.
3.Use a Pasteur pipette to dispense 2 cm
3
of a fresh
sample into a test tube (or finely chopped/ground food
to 1 cm depth and 2 cm
3
of distilled water).
4.Use a dropper pipette to add 10 drops of dilute
hydrochloric acid (HCl) to the sample.
5.Use a test tube holder to transfer the test solution to the
water bath. You can save time by having up to five test
tubes in the water bath. Heat to boiling point for two
minutes. This is the stage when acid hydrolysis may
occur
6.Cool the test tube in cold water (in a beaker).
7.Sodium hydrogencarbonate (NaHCO3) powder is needed to make the
solution alkaline. With a spatula add sodium hydrogencarbonate to
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 cm
3
, to the test solution.
9.Heat the test solution to boiling point in a water bath and continue
heating for two minutes.
10.Observe the colour changes in the test tubes over two minutes of
heating at boiling point. Record your observation in Table 1.
Safety Wear eye
protection – there is
some risk of hot
liquids spitting when
heating test tubes
Benedict’s reagent:
LOW HAZARD Dilute
hydrochloric acid (0.1
mol dm
–3
): LOW
HAZARD but may
cause harm if it enters
the eye or a cut
Sodium
hydrogencarbonate:
MINIMAL HAZARD IN
THIS TEST Take care
when cutting food with
a scalpel LET TRE
KNOW IF YOU HAVE A
NUT ALLERGY. You
should not carry out
tests with nuts if you
are allergic to them.
Results
Confirmation of a non-reducing sugar, for example,
sucrose, in the original solution: a brick-red/brown/
orange/yellow or green suspension at this stage
only.
Confirmation of the absence of a non-reducing
sugar (and the absence of reducing sugars) in the
original solution: a blue solution remains.

8. Material tested Observations (after acid and heat,
etc.)
Deductions
1% sucrose solution
Distilled water (control)
1% starch suspension
Food Testing Carbohydrates
The explanation of the test for non-reducing sugars using
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?
……………………………………………………………...……………………………
contained non-reducing sugars?
……………………………………………………………………………………………
contained neither reducing sugars nor non-reducing sugars?
…………………………..………………………………………………………………
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?
...............................................................................................................................
...............................................................................................................................
b) What cannot be concluded from these results?
...............................................................................................................................
...............................................................................................................................
Use Pages 22 and 23 in your textbook to help you

9. The iodine test for starch
Equipment and materials
Eye protection
Labelling equipment
Scalpel and tile
Pasteur pipettes
Dimple tile
Spatula
Distilled water
Iodine (in potassium iodide) solution
1% starch suspension
Food materials, for example, potato
and onion
Method
1.Label individual dimples on the dimple tile with the foods to be tested.
2.Use a clean Pasteur pipette to dispense a few drops of the test
solution into a labelled dimple. For solid foods, use the scalpel to cut
the food sample into small pieces on a tile and use a clean spatula to
transfer some of them into a labelled dimple.
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?
.......................................................................................................................................
b) did not contain starch?
.......................................................................................................................................
5.How could you show that a yellow liquid contains iodine?
.......................................................................................................................................
Food Testing Carbohydrates
Results Confirmation of
starch: a blue-black
coloration Confirmation
of the absence of
starch: a yellow iodine
colour (or just no blue-
black colour)
Safety Wear eye protection
Iodine solution – avoid contact
with skin and clothes Take care
when cutting food with a scalpel