[OLD] Biological Molecules - Food Tests

Question 1

Food test for starch

Answer 1

Add a few drops of iodine

Positive result = iodine turns from reddy brown to deep blue/black

Question 2

Food test for protein

Answer 2

Add a few drops of biuret reagent

Positive result = biuret reagent turns from light blue to lilac

Question 3

Food test for lipids

Answer 3

Dissolve lipids in ethanol and shake the gentle pour into cold water
Positive result = white emulsion forms
Negative result = remains colourless

Question 4

Food test for a reducing sugar (all monosaccharides)

Answer 4

Add a few drops of Benedict’s solution and heat to 80C in a water bath
Positive result = Benedict’s solution turns from light blue to orange/red precipitate

Question 5

Food test for a non reducing sugar

Answer 5

Add a few drops of Benedict’s solution and heat to 80C in a water baths. There will be no change.
Boil the sugar with dilute acid, cool and neutralise with hydrogen carbonate solution, then repeat the Benedict’s test. The test will now be positive as the acid hydrolysed the polysaccharide into monosaccharides which are reducing sugars.

Question 6

Describe the quantitative Benedict’s test

Answer 6

1) prepare a range of sugar solutions of known concentrations
2) Do the Benedict’s test on each of these solutions
3) use a colorimeter with a red filter to measure % transmission of each solution
4) plot reading to prepare a calibration curve
5) perform a Benedict’s test on sample of unknown concentration and measure % transmission
6) read 5 transmission values off in the calibrates curve in order to obtain a sugar concentration value for your unknown sample