1.3 Carbohydrates - disaccharides and polysaccharides

Question 1

Alpha Glucose + Alpha Glucose = ?

Answer 1

Maltose (disaccharide), joined by a condensation reaction and forms a glyosidic bond

Question 2

Alpha Glucose + Fructose = ?

Answer 2

Sucrose (disaccharide), joined by a condensation reaction and forms a glyosidic bond

Question 3

Alpha Glucose + Galactose = ?

Answer 3

Lactose (disaccharide), joined by a condensation reaction and forms a glyosidic bond

Question 4

What is the test for non-reducing sugars?

Answer 4

1) Same as testing for reducing sugar but after water bath if there is no change in colour then a reducing sugar is NOT present
2) Add 2cm^3 of food sample to 2cm^3 of dilute HCl into a water bath for 5 minutes. Dilute HCl will hydrolyse any disaccharide into a monosaccharide.
3) Add sodium hydrogencarbonate to test tube to neutralise it. Test with pH paper to see if its alkaline.
4) Re-Test the solution by adding 2cm^3 of Benedict’s reagent for 5 minutes
5) If a non-reducing sugar is present than will turn from blue to orange-brown/brick red.

Question 5

What is the test for starch?

Answer 5

1) Add 2cm^3 of sample into test tube
2) Add 2 or 3 drops of iodine to it, shake and/or stir
3) Presence of starch is indicated by a blue-black colouration