Food Tests.

This note looks at Food Tests as required by Specific Objective 2.6 of Section B of the latest edition CSEC Biology Syllabus, which asks us to “perform tests to distinguish among food substances”.

Kindly note that a subsequent note will look at the chemical and physical properties of proteins, lipids and carbohydrates, as also suggested by this Specific Objective of the syllabus.

Firstly, there are a few key concepts to understand:

Hydrolysis Reaction- the chemical breaking down of a substance as it reacts with H2O.

Condensation Reaction- the chemical combination of one molecule, from two atoms or molecules, producing H2O as a byproduct.

Dehydration synthesis is simply another term for a condensation reaction.

Test for Proteins (Biuret Test)

Steps:

1.      Obtain a crushed food sample.

2.      Place 2cm3 of this crushed food sample into a test tube.

3.      Add 2cm3 of Biuret Solution to the test tube.

Positive Result- Colour change from blue to purple, meaning that proteins are present.

Negative Result- No colour change observed (blue solution remains), meaning that proteins are not present.

Fats- Method #1: The Grease-Spot Test

1.      Obtain a crushed food sample.

2.      Rub some of the crushed food sample onto brown paper or filter paper.

3.      Allow time for this to dry.

Positive Result- The spot turns translucent, indicating the presence of fats.

Negative Result- No translucency observed, indicating that fats are not present.

Fats- Method #2: The Ethanol Emulsion Test

1.      Add a crushed food sample to a test tube.

2.      Add a small amount of ethanol, equivalent to the amount of crushed food sample present.

3.      Shake the test tube vigorously (important for proper dissolving to occur).

4.      Add an equal volume of water to the mixture.

Positive Result- a cloudy white solution is observed, indicating that fats are present.

Negative Result- No cloudy white solution observed, indicating that fats are not present.

Reducing Sugars:

1.      Place crushed food sample in a boiling tube and add about 2cm3 Benedict’s Solution.

2.      Heat in a water bath at about 80°C for 5 minutes.

3.      Observe the change in colour.

Negative Result- no change in colour observed as the solution remains blue.

Positive Result- Colours are based on the amount of reducing sugar present. (This test may be referred to as semi-quantitative, as it partly accounts for the quantity of reducing sugar present).

·        Green- low level of reducing sugar present.

·        Yellow- moderate level of reducing sugar present.

·        Orange- high level of reducing sugar present

·        Brick-Red- very high level of reducing sugar present.

Non-Reducing Sugars

This experiment is a combination of two parts- turning the non-reducing sugar components into reducing sugar components, and then if a non-reducing sugar was present, the reducing sugar components now obtained can be used in a previously described reducing sugar test. The overall experiment can be seen below:

1.      Place crushed food sample in a boiling tube.

2.      Add 2cm3 dilute HCl for hydrolysis of non-reducing sugar components into reducing sugar components (monosaccharides).

3.      Speed up the hydrolysis reaction by heating up the mixture in a water bath.

4.      Add a small amount of sodium hydroxide, NaOH, to neutralise the solution.

5.      Now add about 2cm3 Benedict’s Solution.

6.      Heat in a water bath at about 80°C for 5 minutes.

7.      Observe the change in colour, the same as in the reducing sugar test:

·        Green- low level of reducing sugar present.

·        Yellow- moderate level of reducing sugar present.

·        Orange- high level of reducing sugar present

·        Brick-Red- very high level of reducing sugar present.

The importance of Step 2 (hydrolysis):

Hydrolysis (the breaking down of a substance as it reacts with H2O) is important so that a non-reducing sugar can have its components separated into reducing sugar components, which is imperative for this test to work.

The Importance of Step 4 (neutralisation):

Neutralisation of the acidic solution (into a neutral one) is important, as Benedict’s solution cannot work in acidic conditions. Note that pH paper can be used to ensure the solution is neutral.

Starch

1.      Add crushed food sample to the test tube.

2.      Add a few drops of iodine solution (also called Lugol’s Iodine).

Positive Result- colour change from brownish colour iodine solution to blue-black solution.

Negative Result- no colour change, i.e. brownish colour remains.