Components of Food

Chemistry Project 2005-2006

Carbohydrates

Chemistry and Food

What is Carbohydrates ?

Compounds composed of carbon and water General formula CxH2yOy

H:O = 2 : 1 Divided into 3 types ---Monosaccharides ---Disaccharides ---Polysaccharides

Monosaccharides

Eg.---Glucose Eg.---Fructose Basic unit of carbohydrates Sweet in taste, soluble in

water General formula C6H12O6

Disaccharides Formed from 2 monosaccharides 2C6H12O6 -----C12H22O11 + H2O Condensation

SucroseMaltose

Polysaccharides Condensation polymer of

monosaccharides nC6H12O6--(C6H10O5)n + n(H2O) Condensation Example: ---Starch ---Cellulose

Open Chain and Ring Structures of Glucose and Fructose

Glycosidic Linkage in Carbohydrates Bond formed between 2

monosaccharides Condensation---2 OH group Removal of H2O

Hydrolysis of Sucrose

Add water 2 simple sugars Dilute mineral acids

Hydrolysis of starch (1) With enzymes To maltose

Hydrolysis of Starch (2) Boiled with dilute H2SO4

To glucose (C6H10O5)n + nH2O-- nC6H12O6

Reducing and Non-reducing Sugars Reduces basic solution of Cu2+(aq) or Ag+

(aq)

Sugar converted to acid Aldehyde group Keto group

Fehling’s test

Reducing sugar and Non-reducing sugar

Reducing sugar and Non-reducing sugar

Reducing sugar and Non-reducing sugar

Proteins

Physical properties of proteins

Large Molecular mass ,typically several thousands. eg. hemoglobin :64 500 viral proteins :40 000 000

Not truly soluble on water,but form colloidal suspension

Constituent element: C,H,O,N may contain S and P

Limitless type of protein e.g. E.coli :800 types human:10 000 types

Vary form species to species

Therefore it determines the characteristics of a species

Rarely stored in organisms except in eggs or seeds.

Structure of amino acidsStructure of amino acids A group of over a hundred members

The commonest are the 20 essential ones,which cannot be synthesized by our bodies.While the rest are non-essential,and can be synthesized from the essential ones.

Zwitterions formationZwitterions formationBasic amino group : -NH2

Acidic carboxyl group : -COOH

Neutral Amino Acid:

no of amino group = no of carboxyl group

Basic Amino Acid:

no of amino group > no of carboxyl group

Acidic Amino Acid:

no of amino group < no of carboxyl group

Dipolar : with both positive and negative pole

Form zwitterions

Soluble in water but not in organic solvent

Non-volatile, crystalline organic compound with high melting point

Amphoteric : with both acidic and alkaline properties

Buffer

Biological significance : Constant Ph for enzymatic reaction

Polypeptide FormationPolypeptide FormationAmino acid (condensation)dipeptidepolypeptide

Structure of polypeptideStructure of polypeptide

Three dimension

Four types of bonding:

a) disulphide bond

b) hydrogen bond

c) hydrophobic interaction

d) ionic bond (broken by alternation in pH)

Level of proteinsLevel of proteins

DenaturationDenaturation

Change in shape but not the sequence

Factors:

- Heat

- Acid

- Alkali

- high electropositive eg.Ag+ Hg +

- high electronegative eg. CN-

- organic solvent

- Mechanical force

Function of proteinsFunction of proteins

1.cytoskeleton : cytoplasm consists of a network of fibrous

proteins

2. Membrane protein

3. Raw material for growth

4. Formation of enzymes, hormones, antibodies

5. Fibrous proteins for support and protection

6. Osmotic balance and buffering

7. Energy source

Source of proteinsSource of proteins

Egg

Milk

Daily products

Soya bean

meat

fish

etc…..

Site for protein digestionSite for protein digestion

Stomach & duodenum

Ileum

peptide

protein

Protease in intestinal juice

Protease in gastric juice and pancreatic juice

Amino Acid

Absorption of amino acidsAbsorption of amino acids

Amino acids

Capillaries in villi of small intestine

Liver

DeaminationDeamination

carbohydratesH2N-C-COOH

H

H

NH2

Urea Kidney for excretion

Deficient diseaseDeficient disease

Kwashiorkor

Symptoms of Kwashiorkor:

a) Inflammation of skin

b) Anaemia

c) Swelling of abdomen

Test for ProteinsTest for Proteins

1) Protein Turns Yellow Albustix paper green

2) Biuret test:

Protein + NaOH + CuSO4 purple colouration

(blue)

IdentificationIdentification

Paper chomatography

2 dimensioned 3 dimenstioned

Fats and Oils!!!

Foods containing Fat and Oils:

What are fats and oils?

Fats and Oils are different lipids.

Lipids are rather diverse class of organic

compounds of organic compounds that include

triglycerides, phospholipids, steriods, etc.

insoluble in water, soluble in organic solvents.

They are mainly composed of C, H, O but with a

very low proportion of oxygen in the molecules.

Structure of fats and oils:

Most natural fats and oils are mixed glycerides.

Glycerides are esters formed from propane-1,2,3-triol

(glycerol) and a mixture of different long chain

carboxylic acids.

The carboxylic acids(fatty acids) making up fats and

oils are usually unbranched, having 14 to 18 carbons.

There are three ester groups per glycerol and the three

R groups are usually different, fats and oils are often

called triglycerides.

Glycerol

A fatty acid

Triglycerides

Synthesis of Triglycerides

Microscopic views of fat cells

A more colourful one

Animal fats and Vegetable oils:

Fats and Oils are found in animals and plants.

Animal fats, such as lard and butter, are composed of glycerides rich in heavy chain, saturated fatty acids, Therefore they are solids at room temperatures.

Vegetable oils are liquids because of their high content of glycerides composed of light chain unsaturated fatty acids.

Hydrolytic and Oxidative Rancidity

Fats and Oils develop an unpleasant smell if they

are kept for too long.

They are liable to spoilage that produces an ‘off’

odour and a flavour described as rancidity.

Rancidity :

->(1) Hydrolytic (2) Oxidative

->Both of which release foul smelling aldehydes

and carboxylic acids.

Hydrolytic rancidity Presence of moisture in oils, which hydrolyzes the glyceride

molecules into propane-1,2,3-triol and free carboxylic acids.

This reaction is speeded up in the presence of certain

micro-organisms or in the presence of some enzymes.

Over a period of time, more molecules of carboxylic

acids are liberated which may be volatile and have

extremely unpleasant odours and flavours.

At room temperature, hydrolysis proceeds rapidly so that butter soon turns

rancid.

So, to duel with, butter is usually covered and refrigerated.

Oxidative Rancidity Oxidative spoilage occurs when fats/oils are exposed to

air and undergo oxidation.

It results in the production of flavours such as ‘tallowy’,

A taste of fatty according to “yahoo dictionary”.

Fats and oils with a high degree of unsaturation are

more susceptible to oxidation.

The oxidation has a free radical mechanism and is

accelerated by trace metals, light and free radical

initiators.

Autoxidation

C

H

H

C

H

C

H

C

H

H

O2 in the air

AutoxidationC

H

C

H

C

O

C

H

H

H

OH

segment of carboxylic acid in fat/oil hydroperoxide

Hydroperoxide

It is flavourless and odourless

It easily decomposes to form highly reactive

hydroperoxide free radicals

HydroperoxideHydroperoxide

free radical

cleavage of

double bonds and carboxylic acidsAldehydes,ketones

Autoxidation

Can be contolled, But not be eliminated

Can be slow down by antioxidants. Examples :

butylated hydroxyanisole (BHA)

butylated hydroxytoluene (BHT)

carotene (Pro-Vitamin A)

Vitamin E

Hydrolysis of Fats and oils

Hydrolysis is a chemical process in which a molecule is

cleaved into two parts by the addition of a molecule of

water.

Fats can be hydrolysed into carboxylic acids and

glycerol in an alkaline medium (NaOH).

It is a reversible reaction.

Saponification

Hardening of unsaturated fat An unsaturated fat is a fat in which there is one or

more double bond between carbon atoms of the

fatty acid chain.

Such fat molecules are monounsaturated if each

contains one double bond, and polyunsaturated if

each contain more than one.

Unsaturated fat cannot pack together closely,

because of their bent structure. As a result,

unsaturated oils exist as a liquid at room

temperature.

Saturation of fatty acid

Hydrogenation Hydrogenation is a chemical reaction which can convert an

oil to a semisolid fat by adding hydrogen to some of the

carboxylic acid C=C double bond , thus decreasing the

degree of unsaturation.

As a result , they can pack together closer and has a

higher melting point .

It is an important reaction to produce margarine.

Soft spread margarine are prepared by the catalytic partial

hydrogenation of vegetable oil .

Catalytic HydrogenationMargarine: An unsaturated fats

Solid forms of vegetable oil

Widely used as a substitute for butter

It is healthier than butter. (Why?)Use your brain to think…

Examples:

Corn oil

Soy bean oil

Cottonseed oil

Iodine value: Iodine value is used to measure the degree of

unsaturation in fats and oils.

It is determined by reacting fats or oils with excess

iodine which adds on across the double bonds in the

carboxylic acid side chains .

The degree of unsaturation is defined as the number of

grams of iodine needed to react with 100 grams of

fats/oils.

The greater the value is , the greater the degree of

unsaturation in the fat or oil.

Energy Source The energy yield of lipids is more than twice those of

carbohydrates and proteins, as shown in the below table.

On average, around 20-30% of the daily energy requirement of the human body comes from oxidation of lipids

Approximate amount of energy released on complete oxidation(kg g-1)

Carbohydrates 17Proteins 17Lipids 38

Energy Reserve

They provide much more energy per

gramme than carbohydrates and

proteins.

They are insoluble in water so that they

do not diffuse out of the cells and do not

upset the osmotic balance of the cells.

They can be stored in the animal body

in almost unlimited amount.

Triglycerides are common energy reserve in the adipose tissue of animals. They are an excellent storage form of energy because of the followings:

Component of cell membrane

The cell membrane is formed by two layers (bilayers) of

phospholipids, with the lipophilic hydrocarbon ends facing each

other and the hydrophilic phosphate ends pointing outward to

the aqueous environment.

Cholesterol in the cell membrane helps to limit the leakage of

small molecules, and hold the hydrocarbon chains of the

phospholipids together but not changing them into a solid form.

Regulatory components

Cholesterol is also the precursor for the synthesis of

steroid hormones. Some of them are sex hormones

that stimulate the development and maintenance of

secondary sexual characteristics.

Vitamin D, which regulates the absorption of calcium

inside the intestine, is derived from cholesterol.

Component of digestive juice

Bile salts are made in the liver with cholesterol as a raw

materials. They emulsify dietary lipids into small oil

droplets which increase the surface area for the

enzymes to work.

Heat insulation

Being a poor heat conductor, fats effectively reduce heat

loss from the bodies of many animals, such as human

beings, polar bears and penguins.

Protection Fats, being soft, light and

shock-absorbent, protect many

internal organs such as the

kidneys and the eyeball from

the mechanical injury by

cushioning them.

Cell Membrance

Credits

Ngo Yu Hin Chung Man Chuen Fung Ho On Yim Pui Kin Yeung Sheung Yai Chan Kai Hung