Food tests +questions

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FOOD TESTS
EXAMINER'S TIPS
In answering recall questions about the biochemical tests for biological molecules, you must:
State that heat to 80 degrees Celsius is needed to be applied in Benedict's tests.
State the colour change you would see in a positive result for any of the tests.
REDUCING SUGARS
Used for monosaccharaides and some disaccharides.
Add Benedict regant and heat (>80 degrees Celsius).
Blue brick red (due to the formation of a red precipitate).
NON REDUCING SUGARS
Disaccharides, e.g. sucrose.
Hydrolyse with heat and acid.
This breaks glycosidic bonds.
Carry out normal Benedict's test (add Benedict regant and heat).
If a non-reducing sugar is present then there should be more of a colour change than without hydrolysis.
HOW MUCH GLUCOSE?
Use a colorimeter which measures the absorbance/transmission of light. This can detect the strength of colour.
Make a series of tubes with known glucose concentrations.
Ensure they are the same volume to start with.
Add the same of Benedict's to each.
MEASURING THE STANDARDS
Filter/centrifuge tubes to remove the precipitate.
Add filtrate to glass tube.
Insert in colorimeter.
Plot transmission for each tube against initial concentration.
FINALLY
Repeat the procedure with your unknown sample.
Use the percentage transmission from the unknown sample to read the glucose concentration off the graph.
PRINCIPLE
The more the transmission, the more of the original precipitate. The more precipitate , the more glucose in the original
sample.
STARCH
Add iodine in POTASSIUM IODINE SOLUTION.
Colour changes from orange- brown to blue-black.
LIPIDS
Emulsion test

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Lipids will dissolve in ethanol but not water.
Add lipids and shake with ethanol.
Add shaken ethanol to water.
If the solution goes cloudy, lipids are present.
Quantity measured with a colorimeter.
PROTEINS
Add biuret reagent.
This turns the solution from blue to purple if there is protein present.
Quantity measured by a colorimeter.
QUESTIONS
1. Describe how you would test a solution for starch. What would you expect if:
a) Starch was present?
b) Starch wasn't present?
2.…read more

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