[1.3] disaccharides and polysaccharides

Question 1

what is a reducing sugar?

Answer 1

they are sugars which lose their electrons and donate them to other chemicals, therefore reducing them (OILRIG)

Question 2

what happens when you add reducing sugars to benedict’s reagent?

Answer 2

• the reagent is reduced which causes it to change colour
• sugar donates electrons that reduce blue copper (II) sulfate to orange copper (I) oxide

Question 3

examples of reducing sugars

Answer 3

all monosaccharides:
- alpha glucose
- beta glucose
- fructose
- galactose

some disaccharides:
- maltose
- lactose

Question 4

examples of non reducing sugars

Answer 4

disaccharides:
- sucrose

polysaccharides:
- starch
- glycogen
- cellulose

Question 5

how can you turn a non reducing sugar into a reducing sugar?

Answer 5

• add HCl which hydrolyses the glycosidic bond
• this makes the polysaccharide or disaccharide into a monosaccharide

Question 6

why do you need to add dilute sodium hydrogencarbonate when doing the reducing sugars test?

Answer 6

• to neutralise the acid
• benedict’s reagent doesn’t work in acidic conditions

Question 7

in which order do you do the 2 sugar tests?

Answer 7

• reducing sugar test before the non reducing sugar test
• this is the only way to show that a reducing sugar is present

Question 8

what are the range of colours?

Answer 8

• blue - no reducing sugars
• green/yellow - traces of reducing sugar
• orange - moderate
• brick red - large amount of reducing sugar

Question 9

what is the method for food test 1: benedict’s test for reducing sugars?

Answer 9

1. set up a thermostatic water bath to 80°c
2. label some test tubes for the substances to be tested
3. add 2cm³ of each solution to a labelled test tube. one of the test tubes should contain the 1% glucose solution
4. add 10 drops of benedict’s reagent (about 0.5cm³) without the dropper pipette touching the inside of the test tube
5. transfer all the test tubes into the water bath
6. observe the colour changes in the test tube over 2 minutes and make note of your observations

Question 10

what is the method for food test 2: test for non reducing sugars using benedict’s reagent after acid hydrolysis

Answer 10

only do after negative test for reducing sugars test

1. set water bath to 80°c
2. add 2cm³ of the food sample being tested to 2cm³ of dilute hydrochloric acid in a test tube and place in the water bath for 5 minutes
   > dilute hydrochloric acid will hydrolyse any disaccharide present into its constituent monosaccharides
3. add sodium hydrogencarbonate solution to neutralise the hydrochloric acid (benedict’s reagent will not work in acidic conditions)
4. re-test the resulting solution by heating it with 2cm³ of benedict’s reagent in the water bath for 5 minutes
5. if a non-reducing sugar was present in the original sample, the benedict’s reagent will now turn orange-brown

Question 11

what is the test for starch?

Answer 11

1. place 2 drops of the sample being tested into a depression on a spotting tile
2. add 2 drops of iodine solution
3. the presence of starch is indicated by a blue-black coloration