Practical (Food Tests)

Question 1

Test for reducing sugar?

Answer 1

BENEDICT’S TEST

For: all monosaccharides (glucose, fructose, galactose) and some disaccharides

Method:
1. To 2cm3 of sample solution, add an equal volume of Benedict’s reagent
2. Shake well and heat in a boiling water bath at 100 °C for 1-2min (if no results by 3min, confirm negative test)

Positive test: brick red precipitate is observed
Green —> yellow —> orange —> brick-red
(Least to most)
Negative test: solution remained blue

Explanation:
Benedict’s reagent: contains copper (II) ions
Reducing sugars reduce the soluble copper (II) ions to insoluble red-brown copper (I), which is the precipitate

Question 2

Test for non-reducing sugar?

Answer 2

BENEDICT’S TEST but acidified (hydrolysed by acid to the constituent monosaccharides that will give positive results for Benedict’s test)

For: non-reducing sugars (sucrose)

Done when Benedict’s test for reducing sugars is negative

Method:
1. To 2cm3 of (fresh) sample solution, add an equal volume of dilute HCL (for hydrolysis)
2. Shake thoroughly to mix and heat the mixture in a boiling water bath at 100 °C for 1-2 min
3. Neutralize by adding excess sodium carbonate or dilute sodium hydroxide. Transfer 2cm3 of this solution into a clean, dry test tube
4. Proceed with Benedict’s Test (add an equal volume of Benedict’s reagent, shake well and heat in a boiling water bath at 100 °C for 1-2min, if no result by 3min means negative test)

Explanation:
Negative test prior to acid hydrolysis indicates absence of reducing sugars at the start
positive result subsequent to acid hydrolysis indicates the presence of reducing sugar(s) formed by hydrolysis of non-reducing sugar(s)

Question 3

Test for starch?

Answer 3

STARCH IODINE TEST

For: starch (polysaccharide)

Method: Add 3 drops of iodine in dilute I2/KI to the sample /tissue

Positive: sample turned blue-black in colour
Negative: sample remained yellow/brown in colour

Explanation: a poly iodide complex is formed with starch, whereby the iodine takes up the position in the Centre of the starch helix, giving rise to the intense blue-black colouration

IMPORTANT NOTES
1. ensure that the starch-Amylase mixture is dropped onto a white tile BEFORE adding I2/KI solution
2. Do NOT add I2/KI solution to the reacting test tube as the poly iodide complex formed interferes with the digestion of starch by amylase

Question 4

Test for lipids?

Answer 4

EMULSION TEST

Method:
1. Add 2cm3 of ethanol to the sample solution (one drop is enough) and shake vigorously. Any lipids present will dissolve in the ethanol (to form a clear solution)
2. Decant the ethanol mixture into another test-tube
3. Add an equal volume of water to the ethanol mixture and shake

Positive: a cloudy white emulsion is formed
Negative: a clear solution is observed

Explanation: lipids are insoluble in water, but soluble in ethanol (alcohol), with which water is miscible
Lipid-water emulsion is formed when water is added to a solution of lipid in alcohol
Tiny droplets dispersed in the water reflect light and give a white, cloudy appearance
Intensity of whiteness of the emulsion is proportional to the concentration of fats/oil in the test sample

Question 5

Test for proteins?

Answer 5

BIURET TEST

Method:
1. To 2cm3 of sample test solution, add an equal volume of 5% sodium hydroxide solution
2. Add 1-2 drops of copper (II) sulphate solution and shake to mix

Positive: a violet/purple colouration is observed
Negative: the solution remained light blue in colour
Explanation: test for peptide bonds
In the presence of dilute copper (II) sulphate in alkaline solution, nitrogen atoms in the peptide chain form a purple complex with copper (II) ions

Question 6

How to rinse a syringe?

Answer 6

Rinse twice with deionised water
Rinse with new reagent
NEVER ASSUME A SYRINGE IS CLEAN