Triglycerides Are Esters of Glycerol and Fatty Acids

condensation

reaction

Structures linked by ester bonds (R-COOR') and water is released

Glycerol "backbone" is a water-soluble

alcohol

Fatty Acids are chains of carbon atoms with a methyl (-CH3) group at one end and a carboxylic acid (-COOH) group at

the other

Glycerol + 3 Fatty Acids

Triglyceride + 3 water molecules

What is the difference between fats and oils? •In the world of nutrition, they are the same. •They belong to a class known as triglycerides. They are composed of fatty acids and glycerol. •Sometimes, referring to the state at room temperature, the word "fat" is used for a solid triglyceride and "oil" is used for a liquid triglyceride. •Below, we will discuss saturated and unsaturated fats.•Unsaturated fats tend to be lower melting point than their saturated counterparts. For example, the fatty acid oleic acid (unsaturated, found in olive oil) melts at about 5.5°C, while its counterpart, the fatty acid stearic acid (saturated, found in tallow), melts at about 73°C. •Unsaturated fats (i.e. triglycerides made from unsaturated fatty acids) tend to be liquids at room temperature, while saturated fats (i.e. triglycerides made from saturated fatty acids) tend to be solid. •Unsaturated fats can be partially converted to saturated fats by a process called hydrogenation.

FATS AND OILS

Fats and oils are triglycerides with varying degrees of saturated and unsaturated fatty acids– Fats – generally solid at room

temperatures and high in saturated fatty acids

– Oils –generally liquid at room temperatures and low in saturated fatty acids

What is hydrogenation? • Many years ago, we discovered how to convert liquid fats into solid

fats by means of this process called hydrogenation. • Here, the liquid fat is put into a strong vessel, heated, and put under

high pressure with hydrogen in the presence of a catalyst. The catalyst, usually platinum, palladium or nickel, is necessary because the process would be so slow otherwise that it would, for all practical purposes, never happen.

• Under these catalytic conditions, however, the liquid fat becomes a solid fat. In this process, unsaturated fats are partially converted into saturated fats.

• Hydrogenation was employed by the food industry in an effort to convert liquid fats (unsaturated ones) to solids (saturated fats).

• There were two main topics that could be addressed by this approach: separation of oils (for example, in peanut butter), and shelf life problems.

• As a matter of fact, converting the liquid fats to solid fats did indeed solve both problems. Separation could be halted, and the off-tastes and odors produced over time by oxidation of the unsaturated fats (which is called rancidity) could be avoided, because the saturated fats do not oxidize (turn rancid).

Cardiovascular risk• But the price the consumer pays for the benefit of the

food industry is a cardiovascular risk, in the form of trans fats, which is why the government required the amount of trans fats to be listed on labels starting in January 2006.

• More recent studies have suggested that, even with the same caloric intake (including fat calories) in two groups, the group with the higher trans fats intake gained more weight.

• In addition, this weight tended to gather around the middle, giving the “apple” shape, which has been implicated in giving rise to an increased cardiovascular risk.

• The bad news about trans fats has been gathering for years, but commercial interests would rather bury the information (as well as their victims) than give up any profits.

Symbols

• C represents a carbon atom• H represents a hydrogen atom• or | (i.e. horizontal line or vertical line)

represents a single bond (that is, saturation)• = represents a double bond (that is,

unsaturation)

• R represents the rest of the molecule • COOH represents the acid portion of the

molecule• → means “goes to”

cis form

• Here we will discuss the hydrogenation process in terms of the structures of the fatty acids, which are the major components of the fats involved.

• The structure

     cis form • cis formis the usual molecular geometry, or

shape, of the unsaturated fatty acids. • This shape is called cis which means the two

H’s are on the same side of the molecule.• This is the geometry preferred by nature and

the human body.

H

C

H

CR COOH

 Primary reaction hydrogenation The hydrogenation process can be represented as follows:

unsaturated saturated

This is the desired result.

Unfortunately, this is not the whole picture. The problem of trans fats arises because the above reaction (Primary reaction, hydrogenation) is necessarily accompanied by the second reaction (Secondary reaction, isomerization), in which the molecular geometry, or shape, is re-arranged to convert the cis geometry to the trans geometry, like this

cis form

trans form

H

C C R

H

C

H

C R COOH

H

COOH

      Secondary reaction, isomerization

Notice that no hydrogen was taken up in this second reaction, and the double bond (or unsaturation) remains, but with a different geometry. Now the two H’s are on the opposite sides of the molecule.

Such a re-arrangement is called an isomerization, and the two shapes are referred to as isomers, which can be defined loosely as molecules which have the same composition, but with different arrangements.

Trans means “across” when describing molecular arrangements. Cis means “on the same side”.

Because total hydrogenation has not been found to be commercially viable, all hydrogenated fats in foods are partially hydrogenated fats, and trans fats are an inescapable result in their presence.

Unfortunately, then, we see that one type of by-product formed in the hydrogenation process is the group called the trans fats.

Trans fats are unsaturated fats, but their molecular geometry is what makes them so harmful. These are a very bad form of fats from a cardiovascular point of view.

Isomerization

Fatty Acids Vary in Chain Length and Saturation

saturatedstearic acidm.p. 73 oC

“cis”monounsaturated

oleic acidm.p. 5.5 oC

Longer Chain Higher Melting Point

Add Double Bonds Lower Melting Point

Poly-Unsaturation Confers Liquidity (And Reactivity Toward Oxygen)

“cis, cis”linoleic acidm.p. –13 oC

“cis, cis, cis”linolenic acidm.p. –24 oC

More Double Bonds Lower Oxidative and Thermal Stability

Hydrogenation Improves Stability BUT…

“sat”stearic acidm.p. 73 oC

“trans”elaidic acidm.p. 42 oC

“cis”oleic acid

m.p. 5.5 oC

Trans fats behave more like saturated fat

H2

Summary• Unsaturated fats tend to be liquid at room temperatures: the

double bond is, in the normal cis configuration, asymmetric and so forces a kink or bend into the carbon chain. As a result the unsaturated fatty acids are unable to pack so closely together, or crystallise so readily as straight-chain saturated fatty acids. This is why unsaturated oils are mostly liquid at room temperature, while more saturated fats (such as tallow) are hard.

• However a trans double bond does not form a sharp angle, as does a cis double bond. Instead the molecular chain forms a straight line similar to a saturated fat, except with a small kink at the double bond site. Consequently the trans isomer of a given fatty acid will always have a higher melting point than the cis isomer, but one lower than the corresponding saturated fatty acid.

• In a "monounsaturated" fatty acid, there is only one such double-bonded pair of carbon atoms. In a "polyunsaturated" fatty acid, there are two or more such pairs.

• When an unsaturated oil is hydrogenated, hydrogen is absorbed into the fatty acid molecules. The hydrogen atoms bind to the carbon chain in the vacant double-bonded sites, where only one hydrogen atom is attached to the carbon atoms. In so doing the extra hydrogen removes the double bond, straightens the chain, and makes the molecule more saturated.