Principal Components Of Food

Principal components of food

Principal components

 Proteins  Carbohydrates  Fats

and oils

Proteins  Function

– maintains the structure and proper functioning of all living organisms.  Structure – a polymer formed from about 20 amino acids by peptide linkage.

Some amino acids COOHCOOH COOH H 2N C H C H N C H H 2N 2 H CH3 CH2COOH

Aminoethanoic 2-Aminopropanoic Aminobutanoic acid acid acid (Aspartic acid) (Glycine) (Alanine)

Peptide linkage H R

H R’

H N CH C OH H N CH C OH O

O -H2O

H R

H R’

N CH C N CH C O Peptide linkage

O

Hydrolysis of proteins  Breaks

the peptide linkages in a protein molecule  The composition of the protein molecule may be deduced by using paper chromatography

Hydrolysis of proteins H R

H R’

N CH C N CH C O

O

acid H R

H R’

N CH C OH O

H N CH C O

Past paper questions

1. 2001 6c 2. 2001 6d 3. 2000 7b

Carbohydrates  Function

– provide energy  Formula – Cx H2yOy  Monosaccharides C6H12O6 (glucose, fructose)  Disaccharide C12H22O11 (sucrose, maltose)  Polysaccharides (C6H10O5)n (starch, cellulose)

Stereoisomers of C6H12O6

H

H

H

H

H

H

H

C

C*

C*

C*

C*

C

OH OH OH OH OH 4 chiral carbons 24 = 16 stereoisomers

O

Glucose

Glucose  Open

chain (acyclic

form)  Two Ring forms (cyclic form) – M.p. 146oC , 150oC – Optical rotations • +112o , +19o → +52.7o

Fructose

Reducing sugars

Reducing sugars

Disaccharides

Maltose

Maltose

Sucrose

Polysaccharides  Carbohydrate

polymers  Storage polysaccharides – Energy storage – starch and glycogen  Structural

polysaccharides

– Use to provide protective walls to cells cellulose

Starch

Cellulose

Hydrolysis of sucrose  Sucrose,

like all disaccharides, is hydrolysed by dilute mineral acids to two monosaccharides, glucose and fructose

 C12H22O11  The

H+

+ H2O → C6H12O6 + C6H12O6 glucose

fructose

reaction can be effected by enzyme

Hydrolysis of starch 

 

A solution of starch can be hydrolysed in the presence of an enzyme to a disaccharide, maltose. 2(C6H10O5)n +nH2O → nC12H22O11(maltose) It starch is boiled with dilute sulphuric acid, it is hydrolyzed to a monosaccharide, glucose. (C6H10O5)n +nH2O → nC6H12O6

Past paper questions

1. 2001 7c 2. 2000 6c 3. 1998 7b

Fats and oils

Triglyceride

Fatty acids

Fatty acid

Fatty acid

Hardening of vegetable oil  Unsaturated

oils usually have a lower melting point and exist as a liquid.  Hydrogenation of some of the C=C bonds converts to solid fats.  E.g. Margarine

Hydrolysis of fats and oils  



Fats and oils are hydrolysed into carboxylic acid and glycerol in human body. The substances are then used as fuel, or used in building cell membranes and fatty tissues. In laboratory, hydrolysis can be carried out in an alkaline medium (i.e. saponification)

Iodine value  Unsaturated

fat is considered desirable

in our diet.  Iodine value is defined as the number of grams of iodine that reacts with 100 grams of fats/oils.

Fats / Oils Animal fats Butter Dripping Lard Vegetable Coconut oil oils Cotton seed oil Ground-nut oil Olive oil Almond oil Corn oil

Iodine values

25-30 35-65 45-65 8-10 80-140 85-105 80-90 90-110 115-130

Rancidity  Rancidity

is caused by reactions of fats / oils which release foul smelling aldehydes and fatty acids.  Two types: – Hydrolytic rancidity – Oxidative rancidity

Hydrolytic rancidity  Occurs

as a result of hydrolysis of glyceride molecules to glyerol and free carboxylic acids by the presence of moisture in the oils.  E.g. Frying of chips in oil at high temp.  The reaction is speeded up in the presence of certain micro-organisms or enzymes.

Oxidative rancidity  Occurs

when fats/oils are exposed to air and undergo oxidation.  Fats/oils have a high degree of unsaturation are more susceptible to oxidation.  The oxidation is a free radical mechanism and is accelerated by trace metals, light and free radical initiators.

Past paper questions 1. 2000 6a 2. 1999 7d