Testing for reducing and non- reducing sugars.

https://www.youtube.com/watch?v=LuAuCri2fKM

What is reducing sugar

Reducing sugars can donate an electron to another molecule, all monosaccharides are reducing sugars some disaccharides also such as maltose and lactose.

First step of food test

Grind food with distilled water in mortar and pestle, then filtering solid food particles.

How do we test for reducing sugars

Place 3cm³ of food solution into boiling tube, and add 3cm³ of benedict’s solution ( makes solution blue)

Place boiling tube into beaker of boiling water, and leave for 5 minutes

If solution remains blue, no reducing sugar present, if reducing sugar present, this adds an electron to the copper 2+ ion (found in benedicts which made solution blue)

This now forms copper 1+ ion and forms red precipitate.

What are the different colours formed in reducing sugars test.

If very small amount of reducing sugar, very small red precipitate, causes benedicts to appear green.

If more reducing sugar present, colour turns yellow#

A higher level produces orange

If theres alot, then we see brick red.

How is non reducing sugars tested for

We must break glycosidic bond between glucose and fructose ( to form sucrose) and release monosaccharides

How do we check for non reducing sugars

We see if solution also contains any reducing sugar ,

First we take small amount of our unknown solution and carry out the benedict’s test and note colour change

Then we take fresh boiling tube and add 3cm³ of our unknown solution, then 3cm³ of dilute HCl, and gently boil soltuion in waterbath for 5 mins.

If non reducing sugar present , then the HCl hydrolyses glycosidic bonds and releases monosaccharides

Then add 3cm³ of dilute alkali e.g sodium hydroxide, then add 3cm³ of benedicts, and heat in boiling water for 5 minutes, and note colour change

What is one possible outcome of NRS test

First benedicts test = negative ( blue) → Reducing sugar not present

Second benedicts test (orange) → Non reducing sugar present

Another possible outcome of NRS test

First benedicts = red → large amount of reducing sugar

Second test not possible , even if non reducing sugar present, we cant see beyond the red colour change

Another possible outcome of NRS test

First benedicts = green → Small amount of reducing sugar

Second benedicts = red → Non reducing present