Test for Starch and Reducing Sugar Present in Apple and Pear

Test for Starch and Reducing Sugar Present in Apple and Pear

Title: Quantitative test for starch and reducing sugar present in

apple and pear

Aim:

To find and compare the amount of starch and reducing sugars present

in apple and pear.

Principle:

In testing the amount of reducing sugars, the fruits had to be

grounded into juice. Since reducing sugars are soluble, the sugar

present in the fruit would dissolve into the juice. We can dilute and

control the volume of fruit juice when doing the test, hence it is

easier to compare the quantity of reducing sugar in the same amount of

apple and pear. To test for reducing sugars, Benedict’s solution is

used. Benedict’s solution contains blue copper (II) ions that reduce

to red copper (I) oxide precipitates when added to reducing sugars. By

comparing the amount of precipitate present at the end of the test, we

can find out which fruit contains more reducing sugars.

The independent variable in this test is apple juice and pear juice,

the dependent variable is the amount of red precipitate present at the

end of the test. The controlled variables are the volume and

concentration of fruit juices, the temperature of the water bath, the

time of boiling and the volume of Benedict’s solutions added.

Since starch is insoluble, the above method is not suitable to test

for the amount of starch present in the fruits. The fruits had to be

cut into same sizes. Iodine solution is used to find the amount of

starch. Iodine solution turns from reddish-brown to dark blue when it

meets starch.

Procedure:

Test for reducing sugar:

1. apple and pear were grounded...

... middle of paper ...

... the pear rusted in the

air, also the juices might be contaminated while squeezing with the

same filter paper (teabag). The concentration of reducing sugar then

might not be true for the whole fruit. Also, the degree of rusting in

apple and pear was not the same. Thus this might alter the result.

In the iodine test, the size of drops of iodine solution might be

different, thus the measurement might be incorrect.

Conclusion:

The apple was determined to contain a larger amount of reducing sugars

than the pear did.

Possible improvement:

Use a pipette for the transferal of solutions and juices to ensure

equal volume in both independent variables. Cut away the rusted

surface of the fruits before grounding them into juices. Use the same

dropper to add iodine droplets to the fruit, so as to minimize the

error.