3.4 Testing for Carbohydrates

Question 1

What’s the Benedict’s Reducing Sugars test and what’s the method

Answer 1

Benedict’s test for reducing sugars: (alkaline solution of copper sulfate II)
Reduction is a reaction involving the gain of electrons.
All monosaccharides and some disaccharides (maltose & lactose) are REDUCING SUGARS = can donate electrons, or reduce another molecule/chemical.

1. place sample in a boiling tube; if not liquid, grind up and then blend it in water.
2. add an equal volume of Benedict’s reagent.
3. heat mixture gently in a boiling water bath for 5 minutes.

Reducing sugars will react with the copper ions in Benedict’s solution. Results in addition to blue Cu2+ ions, reducing them to brick red Cu+ ions.

Brick red = pos. result

Test is qualitative because you can see the unchanged blue ions left compared to red ions changed, depending on conc. of reducing sugars present.

Question 2

What’s the Benedicts Non-reducing sugars test

Answer 2

Non-reducing sugars don’t react with Benedict’s and will remain blue even after the water bath, indicating a neg. result.

Sucrose is most common non-reducing sugar.

If sucrose is first boiled with dilute hydrochloric acid, it will give pos. result because sucrose has been hydrolysed by the acid to glucose and fructose, both reducing sugars.

Question 3

What’s the Iodine test for starch and it’s method

Answer 3

To detect starch:

1. drop few droplets of iodine dissolved in potassium iodide solution mixed with the sample.
2. if solution turns from yellow/brown to purple/black, starch is present.

If solution remains yellow/brown, it’s neg.

Reagent strips:
Manufactured reagent strips can be used for presence of reducing sugars, usually glucose.
Advantage= colour-coded chart, conc. of sugar can be determined.