food tests Flashcards
what type of sugars are known as reducing sugars?
all monosaccharides and many disaccharide sugars are known as reducing sugars.
what is a reducing sugar?
a reducing sugar is a molecule that can react with other molecules by giving electrons to them (reducing agent)
what is the name of the test for reducing sugars?
Benedict’s test
what is the method for a Benedict’s test?
- place about 1cm3 of a solution in a test tube
- add an equal volume about 1cm3 of Benedict’s solution (usually blue)
- boil or heat to 80oC in a water bath for 2 mins
what will a positive result show for the Benedict’s test?
the Benedict’s reagent will gradually turn red through a series of colours
BLUE –> GREEN –> YELLOW –> ORANGE –> RED
what will a negative result show for the Benedict’s test?
no change to the blue colour
what factor affects the extent of the colour change of the Benedict’s reagent?
The higher the concentration of reducing sugar, the further the colour change goes
why is there a test for non-reducing sugars?
there is a test for non-reducing sugars because some sugars do not react with Benedict’s solution at all, so a reducing sugar test would show up as negative - no colour change
name a common non-reducing sugar?
Sucrose is the only common non reducing sugar
How is sucrose formed?
sucrose is a disaccharide and is formed by a condensation reaction between a glucose molecule and a fructose molecule
what is the method to be followed if a Benedict’s test proves to be negative?
- Add several drops of dilute hydrochloric acid –> 2cm3 of the test solution in a test tube and boil in a water bath for several minutes.
why is hydrochloric acid added to 2cm3 of test solution and left to boil in a water bath?
hydrochloric acid breaks down sucrose into its monosaccharides
what should be done after adding the acid?
- cool the solution and neutralise it by adding some sodium hydrogencarbonate solution
- carry out the Benedcict’s test again
why is sodium hydrogencarbonate added to the cooled solution?
the alkali neutralises the acid, which is important as the acid inhibits Benedict’s reagent
what will a positive result of the non-reducing sugar test look like?
The benedict’s reagent will now give a positive result because the monosaccharide’s glucose and fructose are present
what will a negative result of the non-reducing sugar test look like?
no change - which indicates no reducing or non-reducing sugars present
what is the test for lipids known as?
the test for lipids is known as the Ethanol emulsion test
what is the method for a lipid emulsion test?
- mix and crush the material to be tested with a glass rod in some ethanol in a test tube - shake for 1 minute to help it mix
- then filter, or carefully decant, the ethanol into a second test tube containing cold water (do not mix)
- discard the solid residue - avoid any solid getting in the water
what does a positive result for the ethanol emulsion test look like?
a milky emulsion forming in the water indicates lipid in the ethanol
what does a negative ethanol emulsion test look like?
no milky emulsion
what is the test for polypeptides known as?
the Biuret test
what is the method for the Biuret test?
- Place about 1cm3 of a solution or suspension of the test substance in a test tube
- Add an equal volume (about 1cm3) of biuret solution into the test tube
- mix the biuret solution and test solution by swirling
What is biuret solution?
Biuret solution is a blue solution which contains copper sulphate and sodium hydroxide
what is a positive result for the biuret test?
VIOLET/PURPLE or LILAC colour
What is a negative result for the biuret test?
No change to the blue colour of copper sulphate
what is the test for carbohydrates and polysaccharides known as?
the iodine test
what is the method for the iodine test?
- Test substance may be solid or liquid
- place a small amount on a white tile/in a test tube
- add iodine solution
what is iodine solution?
iodine solution is iodine dissolved in potassium iodide solution - It is yellow orange
what does a positive iodine test result look like?
blue black colour
what does a negative iodine test result look like?
a yellow orange colour - does not change
