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FOOD TESTS

You are also expected to learn know how to carry out these food tests in future.


CAUTION: YOU ARE EXPECTED TO WORK CAREFULLY AND SAFELY.

MAKE SURE YOUR WORKING SPACE IS CLEAR, INCLUDING FLOOR AREA.

WORK FROM YOUR NORMAL POSITIONS BEHIND THE BENCH.

DO NOT MOVE AROUND THE LAB UNNECESSARILY.

WEAR A LAB COAT TO PROTECT YOUR CLOTHES AGAINST THESE CHEMICALS, AND SAFETY GLASSES TO PROTECT EYES, UNLESS YOU ARE A SPECTACLE WEARER.

KNOW WHEREABOUTS IN THE LAB APPARATUS AND CHEMICALS ARE TO BE FOUND, AND RETURN THEM AFTERWARDS

(EXCEPT WET GLASSWARE, AND EMPTY CHEMICAL BOTTLES!).

BE ESPECIALLY CAREFUL WITH HOT LIQUIDS AND WATERBATHS.

KEEP CHEMICAL BOTTLES OFF THE BENCH, AND DO NOT CONTAMINATE PIPETTES.

WIDE MOUTHED TEST-TUBES (BOILING TUBES) ARE USED FOR EASE OF CLEANING, BUT CHEMICALS MUST BE USED ECONOMICALLY.

RE-USE TUBES, ETC BY TIPPING OUT CONTENTS, RINSING WITH COLD WATER.

AT THE END OF THE LESSON, PUT YOUR WASTE INTO THE CONTAINER PROVIDED, THEN RINSE AND SORT GLASSWARE AND OTHER APPARATUS AS REQUIRED FOR WASHING UP.

Activity: INVESTIGATION OF DIETARY COMPONENTS OF VARIOUS FOODS

Test for the presence of simple sugars and starch in the food substances provided, using the information below. Later on, the other tests (overleaf) will be demonstrated. Record those results too.

Tests for carbohydrates

There is no standard test for carbohydrates, but 2 simple laboratory tests for the main carbohydrates.

Click for a photo

Benedict's test - for simple (reducing) sugars :

(does not work with ordinary cane/beet sugar: sucrose!)
- Make a solution of the substance to be tested in a boiling tube, and add Benedict's solution (blue).
- Alternatively, add Benedict's solution to a small chunk (of solid fruit, etc) in a tube.
- Heat the tube gently for about 2 minutes in the waterbath.
A colour change from blue, through murky green to yellow/orange/red shows the presence of a simple (reducing) sugar.
Actually an orange precipitate is formed so the (hopefully) initially clear solution not only changes colour but also becomes opaque.

Click for a photo

Iodine test - for starch:

- Solid foods: Add a couple of drops of iodine solution (brown) directly to the substance to be tested, in a plastic dish.

- Liquids (starch does not dissolve, but forms a cloudy liquid called a sol):

- Add 1-5 drops to test substance in a boiling tube or welled tile.

- THERE IS NO NEED TO HEAT.

The result is almost instantaneous, but the liquid needs to soak in to dry solids.

A colour change from orange to blue-black shows the presence of starch.

You will be using these tests on a variety of food substances.


Tests to be demonstrated

Click for a photo

Biuret test - for Proteins

- Add Biuret solution (sodium hydroxide + small amount of copper sulphate) - blue, like Benedict's - to the test substance.

- THERE IS NO NEED TO HEAT, but the result is not quite instantaneous.

A colour change from blue to mauve shows the presence of a protein.



Click for a photo

Emulsion test - for Fats and Oils

Add ethanol (alcohol, industrial methylated spirit) to a very small amount of the test substance.
Shake or crush (and possibly heat gently using a water-bath - CAREFULLY - DO NOT USE A BURNER! - ethanol is flammable!), in order to dissolve.

Filter or dilute if necessary to obtain fairly clear liquid (which is of course a solution of fat in ethanol).

Take another tube containing (tap)water, and pour the ethanolic solution (prepared above) into top.

A white (milk-like) emulsion indicates the presence of fats or oils.


Results (final colours, or ++, +, - , etc) from the following tests
Food substance tested Benedict's test (Simple sugars) Iodine test (Starch) Others to be demonstrated Comments and conclusions - food substance contains:
Biuret test (proteins) Emulsion test (fats)
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