Food tests

What are the two types of sugars?

* Reducing (donate/lose electrons)
* Non-reducing (gain electrons)

What is the Benedict’s test?

* Benedict’s reagent is a blue solution
* Contains copper sulfate ions
* Test for reducing sugars
* In the presence of reducing sugars it forms a precipitate
* It forms copper oxide which is insoluble

What is the test for reducing sugars?

Benedict’s test

What is the method for the Benedict’s test?

1. Add Benedict’s reagent to a sample
2. Heat the mixture
3. If a reducing sugar is present, a coloured precipitate will form
4. Positive test is colour change from blue to any other colour (green, yellow, brick red)

What is a positive result for the Benedict’s test?

Colour change from ^^blue^^ to ==brick-red== (red means high concentration of reducing sugar, yellow or orange means there is a lower concentration)

What are the reducing sugars?

* Glucose
* Galactose
* Fructose
* Maltose

How can reagent strips be used?

Test for reducing sugars e.g. glucose

What is the non-reducing sugar?

Sucrose

What is the test for non-reducing sugars?

* If the solution tests negative for reducing sugars, it is necessary to test for non-reducing sugars as well
* Adding dilute HCl acid breaks down bonds so that Benedict’s solution can have an impact

What is the method for the test for non-reducing sugars?

1. Add dilute HCl to the sample and heat
2. Neutralise the solution
3. Carry out Benedict’s test as normal

What is the iodine test for?

Starch

What is the test for starch?

The iodine test

What is the method and colour change of the iodine test?

* Add a few drops of brown iodine to the sample
* If starch is present, the iodide ions interact with the starch molecules causing a change to blue-black

What is the test for lipids?

The emulsion test

What is the emulsion test?

The test for lipids

What is the method of the test for lipids?

* Add ethanol to the sample
* Shake
* Add the mixture to a test tube of water

What is the positive result for the lipids test?

* If lipids are present, a milky emulsion will form
* The more lipid present, the more obvious the milky colour of the solution
* If no lipid is present the solution remains clear

What is the test for proteins?

The Biuret test

What is the Biuret test?

The test for proteins

What is the method of the Biuret test?

1. Add sodium hydroxide to the sample (makes it alkaline)
2. Add copper sulfate
3. Repeat steps 1 and 2 using the control solution
4. Compare the colours of the control solution and the food sample

What is the positive result for the Biuret test?

* If the colour changes from blue to lilac, the sample contains protein
* If no colour change is observed, no protein is present