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Benedict's Test for Non-reducing Sugars

The Benedict's Test for Non-reducing Sugars determines the presence of non-reducing

sugars- sugars which do not have an aldehyde functional group.

Many students confuse the Benedict's Test for Non-Reducing Sugars with the Benedict'sTest for Reducing Sugars, perhaps ecause some of the steps in the procedure are similar.

Benedict's solution is the principle reagent in oth tests. The test for non-reducing sugars isoften conducted on a food sample which tested negative for reducing sugar. !f reducingsugars have een shown to e present, a heavier precipitate is often oserved when the testfor non-reducing sugar is conducted.

The "ehling's Test for Non-reducing sugar is an alternative to the Benedict's Test. #owever itis less popular as it less sensitive and re$uires tha the reagents - "ehling's solutions % and B -e &ept separate until the eperiment is carried out.

(hat is a Non-Reducing Sugar)

Sugars can e classified as either reducing or non-reducing ased on their aility to reduce

copper*!!+ ions to copper *!+ ions during the Benedict's Test.

Non-reducing sugars do not contain an aldehyde group - the reducing species.

Reducing sugars are simple, disaccharide sugars. Sucroseis the most common disaccharide

non-reducing sugar.

Procedure

.% li$uid food sample does not need prior preparation ecept dilution if viscous orconcentrated.

"or a solid sample prepare a test solution y crushing the food and adding a moderateamount of distilled water. ecant the suspension to remove large particles. se thedecanted li$uid as the test solution.

/.%dd / cm0 of the sample solution to a test tue.

0.%dd cm0 of dilute hydrochloric acidand boilfor one minute.

1.%llow the tue to cool and then neutrali2e the acid with sodium hydrogen

carbonate. 3ercise caution due to 'fi22ing' or effervescence.

Check with blue litmus paper or pH paper. Do not worry if the resulting solution is

slightly alkaline; Benedict's solution is also alkaline since an alkaline medium inneeded for oxidation of the copper (! ions by the sugars

Then carry out Benedict's Test as for reducing sugars:

4.%dd an / cm0 of Benedict's solutionto the test tue and swirl or vorte themiture.

5.6eave the test tue in a boiling water bathfor aout 4 minutes, or until the colourof the miture does not change.

7.8serve the colour changes during that time as well as the final colour.

9.To prepare a control, repeat steps /-5 using / cm0 of distilled water instead ofsample solution.

8/10/2019 Benedict's Test for Non-reducing Sugars

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8servations and !nterpretation

OBSERVATION

(FINAL COLOUR CHANGE) INTERPRETATION

No colour change *miture remains lue+ No non-reducing sugar present

:reen Trace amounts of non-reducing sugar present

;ellow 6ow amounts of non-reducing sugar present

8range Moderate amounts of non-reducing sugar present

Bric& Red 6arge amounts of non-reducing sugar present

(hilst oiling the colours of the miture transition in this order green--> yellow--> orange--> brick red

The final colour may e any of the colours aove, depending on the $uantity of non-reducingsugar present. !f you do no oserve the ric&-red colour, it does not mean that youreperiment has not een successful.

!t is important to note that the Benedict's Test for Non-reducing Sugars is not specific to anyone type of non-reducing sugar, and that the colour corresponds to the totalreducing andnon-reducing sugarpresent.

Princile of the Benedict's Test for Non-reducing Sugar

isaccharides are hydroly!edto their constituent monosaccharides when oiled in dilute

hydrochloric acid. The monosaccharides are reducing sugars are they contain the aldehydegroup which is the reducing species.

Neutrali!ationof the miture y sodium hydrogen caronate is necessary as the reductionof the copper*!!+ ions will not ta&e place in acidic conditions - of ecess acid is present.

Benedict's Solution contains copper*!!+ sulphate, sodium caronate and sodium citrate.

The blue coer"##$ ionsfrom copper*!!+ sulphate are reduced to red coer"#$ ionsythe aldehyde groups in the reducing sugars. This accounts for the colour changes oserved.The red copper*!+ oide formed is insolule in water and is precipitated out of solution. Thisaccounts for the precipitate formed. %s the concentration of reducing sugar increases, thenearer the final colour is to ric&-red and the greater the precipitate formed.

Sodium carbonateprovides the al&aline conditions which are re$uired for the redo reactionaove.

Sodium citratecomplees with the copper *!!+ ions so that they do not deteriorate to

copper*!+ ions during storage.