Food Tests Flashcards
Describe how you would test whether a food sample contains protein [3 marks]
Biuret test confirms presence of peptide bonds,
1~ Add equal volume of NaOH to sample at room temp.
2~ Add drops of dilute copper (II) sulfate solution.
Swirl to mix.
Positive result: colour change - blue to purple.
Negative result: solution remains blue.
Steps 1 & 2 make Biuret reagent.
Describe how you would use the emulsion test to see whether a food sample contains lipids [3 marks]
1~ Place food sample in test tube.
Add 3cm³ of ethanol.
2~ Shake lidded tube thoroughly to dissolve any lipid in sample.
3~ Add 3cm³ of water and shake gently.
Positive test: solution turns cloudy (white emulsion forms)
Describe how you would use the Benedict’s test whether a food sample contains reducing sugars [3 marks]
1~ Add an equal volume of Benedict’s reagent to food sample.
2~ Heat mixture in water bath at 100°C for 5mins.
Positive result: colour change from blue to green, orange, brick red precipitate forms
Describe how you would test whether a food sample contains non-reducing sugars [3 marks]
Negative result: Benedict’s reagent remains blue.
1~ Add 1cm³ of dilute HCl and boil in water bath for 5mins.
2~ Neutralise mixture with sodium hydrogen carbonate solution.
3~ Proceed with Benedict’s test.
If positive: contains non-reducing sugars.
Describe how you would test whether a food sample contains starch [3 marks]
1~ Place food sample on spotting tile
2~ Add few drops of iodine solution - iodine dissolved in potassium iodide
Positive result: colour change from orange to blue-black.
How can colorimetry be used to give qualitative results for the presence of sugars and starch?
1~ Make solutions with known concentrations.
(Record absorbance or (% transmission values)
2~ Plot calibration curve: absorbance / % transmission (y-axis), concentration (x-axis).
3~ Record absorbance / % transmission values of unknown samples.
Use calibration curve to read off concentration.
Why do you add hydrochloric acid to the food sample to test for non-reducing sugars?
H+ ions in HCl hydrolyses the disaccharide glycosidic bonds to form monosaccharides that can be detected in a normal Benedict’s test.
Why do you add sodium hydrogen carbonate to the food sample to test for non-reducing sugars?
To neutralise the solution and make the conditions suitable for Benedict’s reagent to work.
What happens in a Benedict’s solution test?
Cu²⁺ (aq) ———-> Cu⁺ (g)
Cu²⁺ is blue.
Cu⁺ is red.
The Cu²⁺ ions in Benedict’s solution is reduced by the reducing sugars as they donate electrons.
Why do you shake the test tube during the emulsion test?
Shake lidded tube thoroughly to DISSOLVE any lipid in sample.
What would a test tube with a high concentration of sugars look like compared to one with a low concentration?
High concentration of sugars = Test tube is light blue.
Low concentration of sugars = Test tube is dark blue.
What would a test tube with a high concentration of sugars look like compared to one with a low concentration?
High concentration of sugars = Test tube is light blue.
Low concentration of sugars = Test tube is dark blue.
What are the trigger words when you see “test for proteins”?
NaOH
Copper (II) sulphate.
What are the trigger words when you see “test for non-reducing sugars”?
1cm³ HCl
Sodium hydrogen carbonate
How do you prepare a food sample?
1~ Grind the food sample with a pestle and mortar while adding distilled water to help dissolve.
2~ Filter the food sample and collect the food sample solution.
3~ Place liquid into a test tube.
