Lecture 10

Physical constants
i. Specific gravity
 Since different oils have different specific gravity, any variation from normal value
shows mixture of oils.
ii. Refractive index
 Fats have definite angles of refraction.
 Variation from the normal value indicates adulteration of fats or oils.

iii. Solidification point or setting point

 Solidification point is the temperature at which the fat after being melted, sets
back to solid or just solidifies.
 Each fat has a specific solidification point.

Chemical constants
i. Saponification number
 It is defined as milligrams of KOH required to saponify 1 gm of fat or oil.
 Saponification number is high for fat or oil containing low molecular weight
or short chain fatty acids and vice versa.
 It gives a clue about the molecular weight and size of the fatty acid in the fat or
oil.
ii. Iodine Number
 It is defined as the number of grams of iodine taken up by 100 grams of fat
or oil.
 Iodine number is a measure of the degree of unsaturation of the fatty acid.
 Since the quantity of the iodine absorbed by the fat or oil can be measured
accurately, it is possible to calculate the relative unsaturation of fats or oil.
iii. Reichert-Meisel number (R.M.number)
 This is a measure of the volatile soluble fatty acids.
 It is confined to butter and coconut oil.
 It is defined as the number of millilitres of 0.1 N alkali required to neutralise
the soluble volatile fatty aicds contained in 5 gm of fat.
 The determination of Reichert-Meisel number is important to the food chemist
because it helps to detect the adulteration in butter and ghee.
  Reichert-Meisel value is reduced when animal fat is used as adulterant in butter
or ghee.
iv. Polanski number
 Ghee may be adulterated by the addition of insoluble, non-volatile fatty acids
(by addition of animal fat).
 This can be tested by finding out the Polanski number.
 It is defined as the number of millilitres of 0.1 N potassium hydroxide
solution required to neutralise the insoluble fatty acids (not volatile with
steam distillation) obtained from 5 gm of fat.
v. Acetyl number
 It is defined as the amount in millilitres of potassium hydroxide solution
required to neutralise the acetic acid obtained by saponification of 1 gm of
fat or oil after acetylation.
 Some fatty acids contain hydroxyl groups. In order to determine the
proportion of these, they are acetylated by means of acetic anhydride.
 This results in the introduction of acetyl groups in the place of free hydroxyl
groups.
 The acetic acid in combination with fat can be determined by titration of the
liberated acetic acid from acetylated fat or oil with standard alkali.
 Acetyl number is thus a measure of the number of hydroxyl groups present
in fat or oil.
vi. Acid number
 It is defined as the milligram of potassium hydroxide required to neutralise
the free fatty acids present in one gram of fat or oil.
 Acid number indicates the amount of free fatty acids present in fat or oil.
 The free fatty acid content increases with age of the fat or oil.
Molecular aggregation of phospholipids
 Glycerophospholipids are virtually insoluble in water.
 Depending on the precise conditions and the nature of lipids used, three types
of lipid aggregates can form when amphipathic lipids are mixed with water.
Micelles
 Free fatty acids, lysophospholipids and sodium dodecyl sulphate (SDS) form
micelle.
  Micelles are relatively small spherical structures involving a few dozen to few
thousand molecules arranged so that their hydrophobic regions aggregate in
the interior excluding water and their hydrophilic head groups are at the
surface in contact with water.
 This molecular arrangement eliminates unfavourable contacts between water
and the hydrophobic tails
Bilayer
 A second type of lipid aggregate in water is the bilayer in which two lipid
monolayers combine to form a two dimensional sheet.
 The hydrophobic portions in each monolayer interact excluding water.
 The hydrophilic head groups interct with water at the two surfaces of the bilayer
lipid bilayers form the structural basis of biological membranes

Liposomes
 The third type of lipid aggregate is formed when a lipid bilayer folds back on
itself to form a hollow sphere called a liposome or vesicle.
 These bilayer vesicles enclose water creating a separate aqueous compartment
Biological membranes
 Proteins and polar lipids account for mass of biological membranes.
 The relative proportions of protein and lipid differ in different membranes,
reflecting the diversity of biological roles.
 Amphipathic molecules form a lipid bilayer with the non polar region of lipids
facing outward.
 In this lipid bilayer, globular proteins are embedded at regular intervals held by
hydrophobic interactions.
 Some proteins protrude from one or other face of the membrane (peripheral
proteins); some span its entire width (integral proteins).
 The individual lipid and protein subunits in a membrane form a fluid mosaic
 The membrane is fluid because the interactions among lipids, between lipids and
proteins are non covalent, leaving individual lipid and protein molecules free to
move laterally.
 One of the key functions of a membrane is to control the passage of
substances across it.
 They are said to be selectively permeable. The different membranes of the cell
have different selective permeabilities.