PROPERTIES OF LIPIDS BIOCHEMISTRY

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.

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-

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.