Carbohydrates

Monomers:

monosaccharides, glucose and amino acids

Test for reducing sugars:

reducing sugar - sugar that can donate electrons to another chemical.

benedict's reagent - alkaline solutin of copper sulfate

when a reducing sugar is heated with benedict's reagent it forms an insoluble red precipitate of copper (I) oxide.

• Add 2cm^3 of the food sample to be tested to a test tube. If the sample is not already in liquid form, first grind it up with water.
• Add an equal amount of Benedict's reagent.
• Heat the mixture in a gently boiling water bath for 5 minutes.

Test for non-reducing sugars:

In order to detect a non-reducing sugar it must first be hydrolysed into its monosaccharide components by hydrolysis.

• If the sample is not already in liquid form, it must first be ground up in water.
• Add 2cm^3 of the food sample being tested to 2cm^3 of Benedict's reagent in a test tube and filter.
• Place the test tube in a gently boiling water bath for 5 minutes. If the Benedict's reagent does not change colour, then a reducing sugar is not present.
• Add another 2cm^3 of the food sample to 2cm^3 of dilute hydrochloric acid in a test tube and place the test tube in a gently boiling water bath for 5 minutes. The dilute hydrochloric acid will hydrolyse any disaccharide present into its constituent monosaccharides.
• Slowly add some sodium hydrogen carbonate solution to the test tube in order to neutralise the hydrochloric acid. Test with pH paper to check that the solution is alkaline.
• Re-test the resulting solution by heating it with 2cm^3 of Benedict's reagent in a gently boiling water bath for 5 minutes.
• If a non-reducing sugar was present in the original sample, the Benedict's reagent will now turn orange-brown.

Iodine solution test:

Starch detected by the colour change of the iodine potassium solution from yellow to blue-black.

Test is carried out at room temperature

• Place 2cm^3 of the sample being tested into a test tube (or add two drops of the sample into a depression on a spotting tile)
• Add two drops of iodine solution and shake or stir
• The presence of starch is identfied by a blue-black colouration

Monosaccharides:

• Fructose
• Galactose
• Glucose - hexose (6 carbon) sugar, two isomers: alpha and beta

Disaccharides:

• Glucose + glucose -> maltose
• Glucose + fructose -> sucrose
• Glucose + galactose -> galactose

When monosaccharides join, a molecule of water…