Test for Fructose, Pentose, Lactose

 

Fructose- Following a meal rich in fruits, honey, and jams, fructose may be present in the urine (fructosuria). If it is present in conjunction with glucose, it is indicative of diabetes. It is important to identify fructosuria so as not to wrongly classify it as glycosuria. Pentoses Pentosuria occurs under similar conditions as fructosuria. 

Test for Fructose-Seliwanoff's Test

Principle When fructose is boiled in the presence of hydrochloric acid, a derivative of furfuraldehyde is given off. This condenses with resorcinol to form a red coloured complex,

 

Reagent-

Resorcinol 0.05 g

Conc. HCI 33 ml

Make up to 100 ml with distilled water. This reagent is stable for about 6 to 8 weeks. 

Method- Add 0.5 ml urine to 5 ml of the reagent in a test tube. Mix well and boil. 

Result- Within 30 seconds fructose gives a red colour. The time factor is important as glucose also gives red colour, but never within 30 seconds.

Lactose:- Towards the end of pregnancy and during lactation lactose is sometimes found in urine (lactosuria). It is of no pathological significance. 

Test for Lactose:- Fearon's Methylamine Test

Principle:- The disaccharides lactose and maltose, when hydrolysed under alkaline conditions, form enediol due to rearrangement of either the ketone or the aldehyde group. This enediol reacts with methylamine hydrochloride to form a red complex. Since maltose is very rarely found in urine, this test can be considered to be specific for urinary lactose.

Reagents:- 0.2 % Methylamine hydrochloride; 10% Sodium hydroxide.

Method:- Add 1 ml methylamine hydrochloride solution and 0.2 ml sodium hydroxide solution to 5 mi urine. Mix well and place the tube in a water bath at 56°C for 30 minutes. Remove the tube from the water bath and allow it to stand at room temperature to cool. Prepare a blank consisting of the reagents with unheated urine and compare the colour of the test with the blank. 

Result:-Development of a red colour within 30 minutes indicates presence of lactose. Other sug. ars such as glucose, fructose, galactose, xylose and sucrose, when present in large amounts, give a yellow colour

Test for Pentose-

Name of method -Bial's test

Principle:- When a pentose is boiled in the presence of hydrochloric acid, furfuraldehydes are given off, which in the presence of orcinol condense to form a green coloured compound.

 

Reagent-

Orcinol (m-dihydroxytoluene)   300 mg

Conc. HCI                                         100 ml

Dissolve and add 5 drops of 10% ferric chloride.

The reagent is stable for 10 days

.

Method- Add 0.5 ml of urine to 5 ml of the reagent in a test tube. Mix well and place in boiling water for the solution to boil. 

Result- Pentoses give a green colour. If fructose is present, it gives a red colour. The boiling should not be prolonged, otherwise glucuronates will give similar greenish colour.