Dr. Noha Soliman

Introduction Carbohydrates, proteins,

and fats are all essential nutrients.

We cannot manufacture these nutrients so we must obtain them from our environment.

Introduction

In this lab, with the use of indicators as chemical detection tools, you will analyze a variety of foods for the presence of nutrients.

Detection is based upon observing a chemical change that takes place most often a change in color.

Objective

Identify the presence of major nutrients such as simple carbohydrates (glucose), complex carbohydrates (starch), protein and fat in common foods.

What is an indicator?

Indicators are chemical compounds used to detect the presence of other compounds.

Background InformationINDICATOR MACRO-

MOLECULENEGATIVE

TESTPOSITIVE

TEST

Benedict’s solution

simple carbohydrate

blue yellow/green

/orange/Brick red precipitate

IKI solution complex carbohydrate

dark red Blue-black

Biuret solution protein blue purple

Sudan IV lipid dark red reddish- orange

Carbohydrates

Carbohydrates are the most abundant organic molecules in nature.

Carbohydrates formula : (CH2O)n

Types of Carbohydrates

Monosaccharides: Are the simplest carbohydrates-Examples: glucose, fructose, galactose , ribose.

Disaccharides: Are two monosaccharides bonded together. - Examples: sucrose , lactose and maltose.

Polysaccharides :Are polymers of monosaccharidesCan be split into many monosaccharides with acid or enzymes. - Examples: starch, cellulose, glycogen.

1. MONOSACCHARIDE - These are crystalline compounds, soluble in water, sweet to taste, and needs digestion in order to be absorbed into the blood stream. 2. DISACCHARIDES -These are crystalline, water-soluble, sweet to the taste, and must be digested to monosaccharides before they can be absorbed and used for energy.

3. POLYSACCHARIDES -These are not water soluble and are not crystalline. They form colloidal suspensions instead of solutions. They are not sweet and must be digested before being absorbed.

The physical properties of carbohydrates

The most important chemical property Of

carbohydrates is The Reducing power.

The chemical properties of carbohydrates

Reducing and Non-Reducing Sugars

• Reduction is the chemist’s term for electron gain.• A molecule that gains an electron is thus……

– “Reduced”• A molecule that donates electrons is called a……

– “Reducing agent”• A sugar that donates electrons is called a……

– “Reducing sugar”• The electron is donated by the carbonyl group.• Benedict’s reagent changes colour when exposed

to a reducing agent.

The standard of the reducing power of carbohydrate is the power to reduce copper (II) ion in alkaline solution into copper (I) ion in copper oxide brick-red precipitate.

If a sugar can do this,It is said to be a reducing sugar.

If it cannot,It is said to be a non-reducing

sugar.

A  sugar that serves as a reducing agent due to  its free  aldehyde or ketone  functional groups  in its molecular structure.

REDUCING SUGAR

The carbonyl group - monosaccharides

• The carbonyl group is “free” in the straight chain form.

• But not free in the ring form.

• BUT remember – the ring form and the straight chain form are interchangeable.

• So all monosaccharides are reducing sugars.

• All monosaccharides reduce Benedict’s reagent

The carbonyl group – disaccharides - sucrose

• In some disaccharides e.g. sucrose both of the carbonyl groups are involved in the glycosidic bond.

• So there are no free carbonyl groups.

• Such sugars are called non-reducing sugars.

• They do NOT reduce Benedict’s reagent.

The carbonyl group – disaccharides - sucrose

• The subunits of sucrose (glucose and fructose) are reducing sugars.

• If sucrose is hydrolysed the subunit can then act as reducing sugars.

• This is done in the lab by

acid hydrolysis.

• After acid hydrolysis sucrose

will reduce Benedict’s reagent.

Benedict’s solution (Benedict's reagent)

A solution of sodium citrate, sodium carbonate, and copper (II) sulfate that changes from blue to yellow or red in the presence of

reducing sugars.

Monosaccharaides, such as glucose, fructose and , such as glucose, fructose and galactose, and galactose, and disaccharides, such as maltose and , such as maltose and lactose, are known as lactose, are known as reducing sugars.

Reducing sugars

Benedict’s Test For Reducing Sugars

Method:-Add a few drops Benedict's reagent to the solution to be tested. -Heat for 5 minutes in a water bath.

Observations:Will turn from blue solution to an yellow/green /orange/Brick red precipitate if a reducing sugar is present.Original Pale Blue = no reducing sugar.

Basis of test:Benedict’s solution contains copper sulphate. Reducing sugars reduce soluble blue copper sulphate, containing copper (II) ions to insoluble red- brown copper oxide containing copper (I) which is seen as a precipitate.

Benedict's solution is a chemical indicator for simple sugars. Aqua blue: negative test. yellow/green/ brick red : positive test.

Non-Reducing sugars do not reduce copper sulphate.

However, if it is first hydrolyzed to its constituent monosaccharaides, it will then give a positive Benedict’s Test.

Sucrose is a non-reducing sugar but it will give a positive result with Benedict's if it is first boiled with dilute acid to hydrolyze (split) it into its monosaccharaides.

Benedict’s Test For Non-Reducing Sugars

Sucrose

Benedict’s Test For Non-Reducing Sugars

Method:1-Add 2cm of sucrose solution to a test-tube. 2-Add 1cm dilute hydrochloric acid. 3-Boil for one minute. 4- Neutralize, by adding small amounts of solid sodium hydrogen carbonate until it stops fizzing.5- Perform the Benedict’s testAdd a few drops Benedict's reagent to the solution to be tested. Heat for 5 minutes in a water bath.

Observations:Will turn from blue solution to an orange/Brick red precipitate if a reducing sugar is present.

Benedict’s Test For Non-Reducing Sugars

Basis of test•A disaccharide can be hydrolysed to its monosaccharide constituents by boiling with dilute hydrochloric acid.

•Sucrose is hydrolysed to glucose and fructose, both of which are reducing sugars and give the reducing sugar results with the Benedict’s test.

Test for starchIodine test

Method:1- Add 2cm 1% starch solution to a test-tube. 2- Add a few drops of I2/ IkI solution.

Observations:A blue-black colouration.

Basis of test:A polyiodide complex is formed with starch.