Safety spectacles should be worn when working with Benedict’s solution and sodium hydroxide.
Ethanol is flammable and must not be used near source of ignition.
Bunsen burner’s should always be kept on a safe flame when not in use.
Procedure 1.
These tests will be carried out on some common food substances.
- Starch
In a test tube put 2 – 3 drops of starch solution and then add a few drops of iodine solution with a dropper pipette.
- Vitamin C (Ascorbic Acid)
Place vitamin C solution into a test tube, and add DCPIP solution.
- Lipids (Fats)
Pour a small amount of fat solution into a test tube. Add 5ml of ethanol. The test tube needs to be shaken until the ethanol dissolves.
- Protein (Biuret Reaction)
Add 2 – 3 ml of protein suspension to 3ml of sodium hydroxide. Once the solution is shaken and mixed, add an equal amount of copper sulphate and shake again.
- Reducing Sugars (Simple Sugars)
Benedict’s Solution
NB/ In this reaction the simple sugar (monosaccharides) reduces the copper in sulphate into another form of copper, giving a red/brown colour in appearance.
- Complex Sugars (Disaccharides)
In a test tube pour 2 – 3ml of complex sugar and add an equal amount of hydrochloric acid. This is boiled in a beaker of water for a few minutes. The test tube needs to be cooled before adding sodium bicarbonate. The solution will fizz neutralizing the hydrochloric acid. Once the fizzing has stopped a test for simple sugar needs to be done.
NB/ The complex sugar has been converted into a simple sugar by the hydrochloric acid.
Results On Pure Food Substances
Table 1
Procedure 2.
Tests on actual food substances
Starch
2 gm of food substance to 5 drops of iodine. When starch and iodine solution is warmed up it turns colourless. This is due to the hydrogen bonds being broken. When the solution is cooled the colour reappears.
Vitamin C
20cm3 of solution to 1cm3 of DCPIP.
Fats
2 gm of food substance to 5cm3 ethanol and 5cm3 of water.
Protein
2gm protein suspension in 10cm3 of water. This is shaken to dissolve. Then 3cm3 of equal amounts of sodium hydroxide and copper sulphate is added.
Reducing Sugar
10cm3 of water solution and 1 gm of solid or liquid. Then add equal volume of benedict’s solution.
Complex Sugar
1 gm of food substance to 10cm3 of water. Add 10 drops of hydrochloric acid and an excess amount of sodium bicarbonate until it fizzes. Repeat the reducing sugars test.
The Results were recorded on a scale of 0 – 5, where 5 shows a high concentration of a chemical compound. No concentration would be 0.
Table 2
Results On Common Food Substances
Diagram below shows the protein test for cheese was positive. The initial colour is colourless, when the cheese was added to the sodium hydroxide and copper sulphate; once shaken the liquid turned violet.
Conclusion
After completing all the tests to different types of food substances it is concluded that the concentration of biochemical groups varies across the food range. This is in line with the aim of the experiment.
Evaluation
The methods and procedures that have been used have proved mainly successful. Using a colorimeter could have improved the results, this measures the colour density, the stronger the density the stronger the colour. Another aspect we could have considered if the resources were available would have been to access hotplates rather than Bunsen burners; this would have reduced one of the main safety aspects of the experiments taking place.
Theory
The biochemical nature of
- Starch test.
- Benedict’s test for reducing sugars.
Starch
Starch is a polysaccharide. Its molecules have 2 components amylose and amylopectin. Both are joined together by linking, a condensation reaction. Amylose is an unbranched polymer in which the glucose monomers are joined by alpha 1-4 glycosidic linkages. The molecule is wound into a helix.
Fig 1.
Glycosidic Bond
Fig 2.
Amylose
Fig 3.
Amylose Structure
Amylopectin
Amylopectin is a branched polymer, which also contains alpha glucose units joined largely by alpha 1-4 and alpha 1-6 glycosidic linkages.
Fig4.
Amylopectin
1-4 bond
Fig 5.
Amylopectin Branch Structure
Branches are 1-6 glycocidic bonds
The 2 components of starch fit together to form a three dimensional structure. The amylose helices are entangled in the branches of amylopectin molecules.
Fig 6.
Starch Test
I2 – Iodine molecule.
The amylose helix traps
Iodine molecules
Hydrogen bonds
It is this reaction that produces the blue coloured complex when starch is mixed with iodine.
Method
When the starch and iodine solution is warmed up the colour disappears. This is due to the hydrogen bonds being broken. On cooling the hydrogen bonds reform and the colour reappears.
Benedicts Test – Reducing Sugar
Reducing sugars are monosaccharides. Monosaccharides are the simplest of carbohydrates. They are sweet tasting and dissolve in water. All monosaccharides contain a carbonyl group, either as part of an aldehyde group or part of a ketone group. The carbonyl group readily donates electrons. It is this property, which makes monosaccharides reducing sugars. They have a chemical structure that allows them to reduce other chemicals while they are being oxidised. This is more commonly known as a redox reaction.
In glucose and galactose, carbon atoms are linked by an oxygen atom. In the presence of an oxidising agent this link is broken. The –CHO group is oxidised.
Fig 7.
Carbonyl Functional Group
CHO
H C OH
HO C OH
H C OH
H C OH
CH2OH
- In the benedicts test the CHO group is oxidised, that is, it loses electrons.
- The electrons are taken up by the copper in
Copper sulphate (blue) to form a
brick red precipitate of copper oxide.
2Cu(OH)2 + R-C=O Cu2 O + R.COOH + 2H2O
copper hydroxide ¦ copper oxide sugar acid
H
Ionic Reaction
Cu + e Cu
(ion) (ion)
The reductions of electron turn into a different kind of copper.
Complex sugars such as sucrose cannot reduce other chemicals because its chemical structure is slightly different. It is made up from glucose and fructose. The carbon 1 of the glucose becomes buried within the disaccharide and is not free to form the –CHO group. This is why it will not cause a colour change in the Benedicts test. It has to be split into its component monosaccharide first, by dilute acid.
Test For Complex Sugar.
In testing for complex sugar, firstly it most is hydrolysed to a reducing sugar.
Hcl/H2O
- Complex Sugar Reducing Sugar
Hydrolysis
Hydrolysis is one of the main reactions in digestion. Fats, proteins etc are broken down.
2.Carry out Benedicts test.
Bibliography
- Advanced Human Biology – J Simpkins, J Williams
