1 - reducing/non reducing sugars

What are reducing sugars?

Sugars that can reduce Benedict's reagent, including all monosaccharides and some disaccharides.

Which monosaccharides are reducing sugars?

All monosaccharides are reducing sugars.

Which disaccharides are reducing sugars?

Some disaccharides, including maltose and lactose.

What is Benedict's reagent used to test for?

Reducing sugars.

What colour is Benedict's reagent before the test?

Blue.

What is a positive Benedict's test?

The solution changes from blue to a coloured precipitate, ranging from green to brick-red depending on the concentration of reducing sugar.

What is a negative Benedict's test?

The solution remains blue, showing that no reducing sugar is detected.

What does a green Benedict's test indicate?

A low concentration of reducing sugar.

What does an orange Benedict's test indicate?

A medium concentration of reducing sugar.

What does a brick-red Benedict's test indicate?

A high concentration of reducing sugar.

What does the colour change in Benedict's test indicate?

The presence and approximate concentration of reducing sugar.

What is a precipitate?

An insoluble solid that forms in a solution during a chemical reaction.

What colour indicates the highest concentration of reducing sugar in the Benedict's test?

Brick-red.

What are the steps for testing for reducing sugars?

Add food sample to a test tube, add Benedict's reagent, heat in a boiling water bath for 5 minutes and observe the colour change.

How much food sample is used in the reducing sugar test?

2 cm³.

How much Benedict's reagent is added?

An equal volume to the food sample, so 2 cm³.

Why is the mixture heated in a boiling water bath?

To provide the heat required for Benedict's reaction safely and consistently.

How long is the reducing sugar test heated for?

5 minutes.

Why is a boiling water bath used instead of directly heating the test tube?

It provides controlled, even heating and reduces the risk associated with direct heating.

What happens if reducing sugar is present?

Benedict's reagent changes from blue to a coloured precipitate.

What happens if no reducing sugar is present?

The solution remains blue.

What are non-reducing sugars?

Sugars that do not give a positive result with Benedict's reagent unless they are first hydrolysed into reducing monosaccharides.

Give an example of a non-reducing disaccharide.

Sucrose.

Why does sucrose give a negative Benedict's test?

It is a non-reducing sugar and therefore does not reduce Benedict's reagent.

How can you test for a non-reducing sugar?

First carry out the reducing sugar test; if negative, hydrolyse the sugar with dilute hydrochloric acid, neutralise it, then repeat the Benedict's test.

What does hydrolysis do to a disaccharide?

It breaks the glycosidic bond, producing monosaccharides.

What is used to hydrolyse a non-reducing sugar?

Dilute hydrochloric acid.

Why must the acid be neutralised before repeating the Benedict's test?

Benedict's test requires suitable conditions and the acid would interfere with the test.

What is used to neutralise the hydrochloric acid?

Sodium hydrogencarbonate solution.

How long is the sample heated with dilute hydrochloric acid?

5 minutes in a gently boiling water bath.

What happens after hydrolysis of a non-reducing sugar?

The disaccharide is broken down into its monosaccharide components, which can reduce Benedict's reagent.

What result indicates a non-reducing sugar was originally present?

The first Benedict's test is negative, but after hydrolysis, neutralisation and repeating the test, a brick-red precipitate forms.

What is the first step when testing for a non-reducing sugar?

Carry out the Benedict's test for reducing sugars.

What should you do if the initial Benedict's test is positive?

There is a reducing sugar present, so the non-reducing sugar test is not required.

What should you do if the initial Benedict's test is negative?

Continue with the hydrolysis and neutralisation steps to test for non-reducing sugar.

What is added after the initial negative Benedict's test?

2 cm³ of dilute hydrochloric acid.

What happens when the sample and hydrochloric acid are heated?

The acid hydrolyses any non-reducing disaccharides into monosaccharides.

What happens after heating with hydrochloric acid?

The mixture is neutralised using sodium hydrogencarbonate solution.

What test is carried out after neutralisation?

The Benedict's test is repeated.

What indicates that a non-reducing sugar was present?

The mixture changes from blue to a brick-red precipitate after hydrolysis and neutralisation.

What is the difference between reducing and non-reducing sugars?

Reducing sugars react directly with Benedict's reagent, whereas non-reducing sugars must first be hydrolysed into reducing monosaccharides.

What happens to sucrose during hydrolysis?

It is broken down into glucose and fructose.

Why does hydrolysed sucrose give a positive Benedict's test?

Hydrolysis produces glucose and fructose, which are reducing sugars.

Why can't Benedict's reagent directly detect sucrose?

Sucrose is a non-reducing sugar and does not reduce Benedict's reagent.