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Physical , chemical properties

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1 Physical , chemical properties
And identification tests of Carbohydrates Dr. Noha Soliman

2 Introduction Carbohydrates, proteins, and fats are all essential nutrients. We cannot manufacture these nutrients so we must obtain them from our environment.

3 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.

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

5 What is an indicator? Indicators are chemical compounds used to detect the presence of other compounds.

6 Background Information
INDICATOR MACRO-MOLECULE NEGATIVE TEST POSITIVE TEST Benedict’s solution simple carbohydrate blue yellow/green /orange/Brick red precipitate IKI solution complex carbohydrate dark red Blue-black Biuret solution protein purple Sudan IV lipid reddish- orange

7 Carbohydrates are the most abundant organic molecules in nature.
Carbohydrates formula : (CH2O)n

8 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 monosaccharides Can be split into many monosaccharides with acid or enzymes. - Examples: starch, cellulose, glycogen.

9 The physical properties of carbohydrates
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.

10 The chemical properties of carbohydrates
The most important chemical property Of carbohydrates is The Reducing power.

11 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.

12 It is said to be a reducing sugar.
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.

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

14 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

15 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.

16 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.

17 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.

18 Reducing sugars Monosaccharaides, such as glucose, fructose and galactose, and disaccharides, such as maltose and lactose, are known as reducing sugars. Sucrose (table sugar) contains two sugars (fructose and glucose) joined by their glycosidic bond in such a way as to prevent the glucose isomerizing to aldehyde, or the fructose to alpha-hydroxy-ketone form. Benedict's reagent can be used to test for the presence of glucose in urine. Glucose found to be present in urine is an indication of diabetes mellitus. Once a reducing sugar is detected in urine, further tests have to be undergone in order to ascertain which sugar is present. Only glucose is indicative of diabetes.

19 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.

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

21 Benedict’s Test For Non-Reducing Sugars
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. Sucrose


23 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 test 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 orange/Brick red precipitate if a reducing sugar is present.

24 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. 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 hydrolyse (split) it into its monosaccharides.

25 Test for starch Iodine 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.


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