ISOLATION OF STARCH FROM POTATO EXPLAINED EXPERIMENT WITH SCIENTIFIC REASONS

STARCH EXTRACTION FROM POTATO PRACTICAL / EXPERIMENT 

Starch is found in a lot of naturally occuring food items mostly those which are rich in carbohydrates like Potato, rice etc. Like potato, starch can also be extracted from rice. But here we will discuss the extraction of starch from potato including the chemistry and reasons behind each step. 

 REQUIRED EQUIPMENT :

APPARATUS:                                                                    SUBSTANCES AND REAGENTS

1- Beaker                                                                            1-Potato

2-Water Bath                                                                      2-Ethyl Alcohol  

3-Funnels                                                                           3-NaCl solution

4-Filter papers                                                                   4- 0.01M NaOH 

5-Test Tubes                                                                      5- Conc. H2SO4

6-Muslin cloth                                                                   6-Iodine        

                                                                                          7- Alpha-Naphthol

                                                                                          8-Lugol's Iodine reagent

PROCEDURE:

STARCH EXTRACTION METHOD 

a. Blend 0.5Kg peeled potato with 400ml 1% NaCl solution.

b. Filter through fine muslin cloth.

c. Re-extract residue using 150ml 1% NaCl solution and re-filter.

d. Mix up both filtrates.

e. Allow to stand combined filtrate until the granules of starch are obtained.

f. Decant and discard the supernatant.

g. Wash the wet starch with 1% NaCl (thrice).

h. Wash the starch with 0.01M NaOH (once).

i. Cool this solution when pure d-glucose crystallizes out.

j.  Dry and weigh. Calculate its % yield.

Q : How to test the presence of starch in potato?

ANALYSIS / CHARACTERIZATION OF STARCH

MOLISCH TEST: (dehydration of carbohydrates to give aldehyde using H2SO4)

The molisch test is a general test for the presence of carbohydrates. Molisch reagent is a solution of alpha-naphthol in 95%ethanol. This test is useful for identifying any compound that can be dehydrated to furfural or hydroxymethylfurfural in the presence of H2SO4. Furfural is derived from the dehydration of the pentoses and pentosans, while   hydroxymethylfurfural is produced from hexoses and hexosans. Oligosaccharides and polysaccharides are hydrolyzed to yield their repeating monomers by the acid. The alpha-naphthol reacts with the cyclic aldehydes to form purple colored condensation products. Although this test will detect compounds other than carbohydrates (i.e glycoproteins), a negative result indicates the ABSENCE of carbohydrates.

PROCEDURE

a. Add 2 drops of Molisch reagent to 2ml of sugar solution and mix thoroughly.

b. Incline the tube, and gently pour 5ml of concentrated H2SO4 down side of the test tube.

c. A purple colour at the interface of the sugar and acid indicates a positive test.

d. Disregard a green colour if it appears.

IODINE TEST (ONLY FOR STARCH AND GLYCOGEN)

The use of Lugol's iodine reagent (IKI) is useful to distinguish starch and glycogen from other polysaccharides. Lugol's iodine yields a blue-black color in the presence of starch. Glycogen reacts with Lugol's reagent to give a brown-blue colour. Other polysaccharides and monosaccharides yeild no colour change ; the test solution remains the characteristic brown-yellow of the reagent. It is thought that starch and glycogen form helical coils . Iodine atoms can then fit into the helices to form a starch-iodine or glycogen-iodine complex. Starch in the from of amylose and amylopectin has less branches than glycogen. This means that the helices of starch are longer than glycogen, therefore binding more iodine atoms. The result is that the colour produced by a starch-iodine complex is more intense than that obtained with a glycogen-iodine complex.

PROCEDURE 

a. Add 2-3 drops of Lugol's iodine solution to 5ml of solution to be tested.

b. Starch gives a blue-black colour.

c. A negative is the brown-yellow colour of the test reagent.

REASONS AND THEIR CHEMISTRY:

Q: How to remove starch from potatoes fast?

Ans: Soaking and Rinsing from cold water can remove the starch from potato fast. Moreover, if you increase the surface area of potato, then rate of starch removal will increase. e.g; starch can remove fast from a sliced/cut potato rather than only a peeled potato.

Q: Why NaCl solution was added in the first step while blending?

Ans: We had a mixture of potato starch and water. So to free starch from water we added salty water, so due to hypertonic solution osmosis occurs and there is high water potential inside potato starch. So, water moves from starch to the salty water.

Q: How starch is identified using Iodine test?

Ans: By adding iodine to starch, iodine bind to the centre of the starch helix, which forms a Starch-Iodine complex giving a blue-black colour.

Q: Why the colour produced by starch-iodine complex is more complex than glycogen-iodine complex?

Ans: Helices of starch are longer than that of glycogen because starch in the form of amylose and amylopectin has less branches than glycogen therefore, starch can bind more iodine atoms to it giving intense blue-black colour.

Q: Why KI is added to make iodine reagent?

Ans: KI (potassium iodide) helps to make iodine reagent beacuase iodine is usually not much soluble in water. This makes a linear tri-iodide ion which is water soluble. That tri-iodie ion slips into the coil of starch causing intense blue-black colour.

Q: Why NaOH was added ?

Ans: NaOH was added to remove acidic impurities present in the starch.

Q: Why the unwanted green colour appears during Molisch test?

Ans: Sometimes green colour appears due to the presence of gum arbinose.

Q: Why glucose and fructose do not give positive molisch test?

Ans: Because the hydrolyzed glucose and fructose are monosaccharides and they can not split further.