Properties of lipids- rancidity- hydrogenation- emulsification- hydrolysis- oxygenation

1. PROPERTIES OF LIPIDS
BIOCHEMISTRY

2. Lipids, together with carbohydrates, proteins and nucleic acids, are one of the four major classes
of biologically essential organic molecules found in all living organisms; their amounts and
quality in diet are able to influence cell, tissue and body physiology.
The term lipid was first used by the German biochemist Bloor in 1943.
DEFINITION OF LIPID:
Chemically lipids are defined as esters of glycerol and fatty acids or triglycerides of fatty acids.
General formula of lipid = Glycerol + Fatty acid = Triglycerides
FUNCTIONS OF LIPIDS
 They are stored in adipose tissue (triglycerides) and are one of the major energy sources, as
they provide the major part of calories. (contribute up to 30% of the total daily energy intake)
 Some lipids are essential nutrients like fat-soluble vitamins A, and D, present in some fats
and oils of animal origin, vitamin E, present in vegetable oils, and vitamin K present in green
leaves etc.
 During growth they are utilized as “bricks” for construction of biological membranes, so
contributing to construction of that barrier that separates intracellular environment from
extracellular one.
 Many hormones are lipids (steroid hormones, like estrogens, androgens and cortisol).
 They can act as receptors, antigens and membrane anchors for proteins.
 Many lipids act as regulators of intracellular processes.
 A subcutaneous layer insulates the body reducing the loss of body heat.
 On epidermis they are involved in maintaining water barrier.
 They are electrical insulator of axon of neurons. (lipoprotein coating called myelin sheath)
 On digestive tract they facilitate the digestive process depressing gastric secretion, slowing
gastric emptying and stimulating biliary and pancreatic flow.
 Bile salts are natural detergents synthesized in the liver and secreted into bile. They solubilize
phospholipids and cholesterol in the bile. Bile salts also aid in the digestion and absorption of
fat and soluble-fat vitamins in gut.
 Some lipids are secreted as pheromones that attract or repel other organisms.
 They affect the texture and flavor of food.
PROPERTIES OF LIPIDS:
 HYDROLYSIS OF LIPIDS
Hydrolysis is the breakdown of a substance by the addition of water. Fats and oils are hydrolyzed
by moisture to yield glycerol and 3 fatty acids. 3 molecules of water are needed to add -H and -
OH groups to break the ester bonds, an action which is catalyzed by the enzyme lipase.

3.  SAPONIFICATION OF LIPIDS
A process by which triglycerides are reacted with NaOH/KOH to produce glycerol and a fatty
acid salt, called 'soap'. When sodium hydroxide is used, a hard soap is produced. Using
potassium hydroxide results in a soft soap. Lipids that contain fatty acid ester linkages can
undergo hydrolysis. This reaction is catalyzed by a strong acid or base. Saponification is the
alkaline hydrolysis of the fatty acid esters.
Triglyceride + sodium hydroxide (or potassium hydroxide) → glycerol + 3 soap molecules
 EMULSIFICATION OF LIPIDS
Emulsification is the dispersion of lipids into tiny droplets, by reducing the surface tension,
which provides a larger surface area on which the enzyme pancreatic lipase can act to digest the
fats into fatty acids and glycerol. Emulsification is assisted by the action of the bile salts. When
soaps have more unsaturated fatty acids they act as germicides or the emulsifying or detoxifying
agents for Tetanus or Diphtheria.
 HYDROGENATION OF LIPIDS
Generally, hydrogenation can be represented as:
Hydrogenation – to treat with hydrogen – is a chemical reaction between molecular hydrogen
(H2) and another compound or element, in the presence of a catalyst such
as nickel, palladium or platinum. The process is commonly employed to reduce or saturate
organic compounds. Catalysts are required for the reaction to be usable; non-catalytic
hydrogenation takes place only at very high temperatures. Normally, partial hydrogenation is
carried out as complete hydrogenation solidifies the product making it hard to use. For example;
butter.
 RANCIDITY OF LIPIDS
Rancidity is the process which causes a substance to become rancid, i.e.; having an unpleasant
smell or taste. Specifically, it is the hydrolysis or autoxidation of fats into short-

4. chain aldehydes and ketones which are objectionable in taste and odor. When these processes
occur in food, undesirable odors and flavors can result. In some cases, however, the flavors can
be desirable. In processed meats, these flavors are collectively known as warmed-over flavor.
Rancidity can also detract from the nutritional value of food, and some vitamins are highly
sensitive to degradation.
FACTORS:
Rancidity can be caused by temperature, time, air, catalysts, light, and water.
RANCIDITY PATHWAYS
Three pathways for rancidity are recognized:
 Hydrolytic rancidity
Hydrolytic rancidity refers to when triglycerides are hydrolyzed and free fatty acids are released.
This reaction of lipid with water sometimes requires a catalyst. In particular, short-chain fatty
acids, such as common butter fats, are odorous.
 Oxidative rancidity
It is associated with the degradation by oxygen in the air. Via a free radical process, the double
bonds of an unsaturated fatty acid can undergo cleavage, releasing
volatile aldehydes and ketones. For example, even though meat is held under refrigeration or in a
frozen state, the poly-unsaturated fat will continue to oxidize and slowly become rancid. The fat
oxidation process, potentially resulting in rancidity, begins immediately after the animal is
slaughtered and the muscle, intra-muscular, inter-muscular and surface fat becomes exposed to
oxygen of the air. This chemical process continues during frozen storage, though more slowly at
lower temperature. The process can be suppressed by the exclusion of oxygen or by the addition
of antioxidants. Thus, airtight packaging will slow rancidity development.
 Microbial rancidity
Microbial rancidity refers to a process in which microorganisms, such as bacteria or molds, use
their enzymes such as lipases to break down fat. This pathway can be prevented by sterilization.