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- BIOLOGY AND CHEMISTRY (2.4)
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Summary of 3 pages for the course BIOLOGY AND CHEMISTRY at CAM (Explained in detail)
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B io Factsheetwww.curriculum-press.co.uk Number 173
How to identify foods: Food Tests and
Chromatography
Food tests and chromatography are techniques used for the Why do we need two methods for identifying foods?
recognition of biologically important chemical compounds (exam Food tests are less precise, they basically identify foods into the
questions on these techniques are common). The use of these category of proteins, lipids, starch, cellulose, reducing sugars or
techniques is not limited only to the food industry. Detectives can non-reducing sugars, whereas paper chromatography can identify
use chromatography to analyze fibres found at a crime scene and specific molecules, for example, finding out which amino acids or
when used in conjunction with sniffer dogs, gas chromatography monosaccharides are contained in a mixture.
is a common way to identify explosives in airports, it has also been
used in schools to perform drug screening.
Food Tests
It is worth learning the food tests as it can mean easy marks in an exam, it is pure recall. In an exam question always quote the reagent used,
method, starting colour and end colour for a positive result.
Table 1. Food tests
Biological Reagent/s used Method Starting Colour End Colour
Molecule to test (positive result)
Reducing Sugar Benedict’s solution Add 2cm³ Benedict’s solution an equal Blue Orange/Red precipitate.
(for example (10% CuSO4) amount of the test sample, heat over a
glucose, fructose water bath.
,maltose.
Non-reducing Benedict’s solution 1. Boil test sample with HCl (to break Blue Orange/Red precipitate
Sugar (sucrose) (10% CuSO4), HCl, the glycosidic bonds).
NaHCO3 2. Neutralise with NaHCO3/alkali
(otherwise the HCl would react with
the Benedict’s solution).
3. Add Benedict’s solution to the test
sample, heat over a water bath.
Starch Iodine solution (KI) Add a few drops of Iodine solution to Yellow/brown Blue-black
the test sample.
Lipids The Emulsion Test - 1. Shake some of the test sample with Clear Formation of a cloudy
ethanol (C2H5OH) about 4cm³ of ethanol. white emulsion.
and water 2. Decant the liquid into a test tube of
water, leaving any undissolved
substances behind.
Proteins Biuret reagent Add a few drops of Biuret reagent to Blue Lilac/purple precipitate
(alkaline CuSO4) the test sample
Cellulose Schultze’s solution Add a few drops of Schultze’s solution Yellow Purple
to the test sample
Three tips on food tests with sugars
1. To distinguish between reducing and non-reducing sugars, first test a sample for reducing sugars, to see if there are any present, if no
positive result is achieved test for non-reducing sugars.
2. When testing for a reducing sugar it is not possible to distinguish between glucose and fructose, paper chromatography would need
to be used.
3. When testing for a non-reducing sugar the solution will be neutralised when it stops fizzing.
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