Physical and Chemical Properties

Nonesterified fatty acids or free fatty acids have a polar (acidic) component and a neutral hydrocarbon component. The ratio of carbon to oxygen depends on the chain length and accounts for the solubility properties as well as the energy density of the lipid molecule. The hydrocarbon chain of the fatty acid is hydrophobic and the carboxyl end is hydrophilic, making the molecule amphipathic. This causes a dispersion of oil in water to form a mono molecular

Fatty acid models c-o-

Figure 3 Space-filling and conventional models of fatty acids: (A) stearic acid (18:0), space-filling; (B) stearic acid, conformational; (C) elaidic acid (18:1n-9t) trans, conformational; (D) a-linolenic acid, all-c/s, space-filling; (E) a-linolenic acid, conformational.

layer at the surface with the carboxyl end in contact with water and the hydrocarbon extending out of the water. The fatty acids may form a micelle (Figure 4) to separate the oil and water phases. These orientations of fatty acids and more complex lipids are a primary aspect of their participation in biological structures and functions. Furthermore, there is free rotation about the carbon-carbon bonds so the fatty acids and acylglycerols are capable of assuming a number of configurations.

Differences in the physical characteristics of fatty acids, particularly saturated compared with unsatu-rated, are extremely important in food and nutrition. SFAs with a chain length longer than 12 carbons are solid at usual ambient temperatures. As the chain lengthens, the melting point increases.

On the contrary, mono- and polyunsaturated fatty acids (PUFAs) are liquid at room temperatures. Uses of fats in food products are based on these properties. Salad oils, margarines, and shortenings are examples of such differences. Hydrogenation of oils containing PUFAs was introduced to provide food fats that are resistant to rancidity and of a desirable plasticity. The chemical hydrogenation process yields mixtures of cis and trans fatty acids (Figure 5). The physical conformation of trans fatty acids is important for their functions in foods and nutrition (Figure 3). The melting points are similar to those of SFAs of similar length and their shapes are linear rather than bent as forced by the cis configuration. These physical characteristics affect their space-filling functions and the mobility of the molecule.

Water

Figure 4 Micelle formed by oil dispersion in water.

Water

Figure 4 Micelle formed by oil dispersion in water.

CH3(CH2)n-CH=CH—C — CH=CH—C—CH=CH—(CH2)nCOOH

Polyunsaturated fatty acid

H2 (hydrogénation) Ni (catalyst)

Trans fatty acids

Figure 5 Hydrogenation of polyunsaturated fatty acids. trans fatty acids are produced when hydrogenation results in incomplete saturation of double bonds during chemical processing.

CH3(CH2)n-CH = CH-C-CH = CH-C-CH = CH-(CH2)nCOOH H H

Polyunsaturated fatty acid poty

antioxidant

, (autoxidation) -accelerated by heat

CH3(CH2)n-CH = CH — CH2_C_H —CH—_CH—_CH_—_CH —(CH2)nCOOH

(resonance) Free-radicals poty

O2 (peroxidation)

CH3CH2-CH = CH —CH2-CH —CH = CH —CH = CH —(CH2)nCOOH

(degradation)

short-chain compounds including aldehydes, ketones ^and short chain fatty acids

Figure 6 Autoxidation is caused by removal of a hydrogen from the methyl group between double bonds in polyunsaturated fatty acids. Resonating free radicals are produced and propagate peroxidation, degradation, and formation of polymers.

Polyunsaturated fatty acids with the methylene-interrupted double bond are also susceptible to oxidation (Figure 6). The hydrogen atoms on the methyl group between double bonds are susceptible to sequestration by oxidizing agents, such as iron or free radicals. This autoxidation results in a resonating free radical that is self-propagating and, with exposure to oxygen, yields peroxides. The peroxides may polymerize or degrade to smaller molecules. In foods, this process results in the condition of rancidity characterized by off flavors. In living systems, the products of peroxidation may cause reactions that damage proteins, membranes, and DNA resulting in pathological processes. Antioxidants are compounds that are capable of interrupting free radical propagation by reducing the peroxide to an alcohol without itself becoming a free radical. Tocopherols are a major antioxidant group in living systems and chemical antioxidants such as BHT (3,5-di-t-butyl-4-hydroxytoluene) are used in food products.

Another primary characteristic of naturally occurring PUFAs is that they cannot be synthesized by animals but are necessary for metabolism; therefore, they are an essential component of the diet. Animal organisms can introduce a double bond at the C-9 position but lack the enzymes to insert double bonds between the C-9 position and the methyl terminal carbon. The fatty acids are therefore considered to be in three families in relation to their biological functions: the mono-unsaturated (n-9 or omega-9) family and the polyunsaturated n-6 (omega-6) and n-3 (omega-3) families.

Phospholipids

The glycerol backbone is the central structure of phospholipids, as it is for acylglycerols. They are characterized by a phosphate group at the sn-3 position making phosphatidic acid (Figure 7).

CH3O xCH3 N+ I

CH2 CH2

CH2 O I II HC —O —C—R I O I II H2C—O —C — R

Phosphatidylcholine

OH OH

Phosphatidic acid

Figure 7 Structures of phospholipids.

OH OH

CH2 O I II HC-O-C —R I O I II H2C — O —C — R

Phosphatidyl-inositol

Phosphoglycerides have fatty acids esterified at the 1 and 2 positions. A number of compounds may be esterified to the phosphate moiety, including choline, ethanolamine, serine, and myo-inositol. The compounds are called phosphatidylcholine, etc. These molecules are obviously amphipathic, having very polar constituents at the sn-3 position and acyl chains at the 1 and 2 positions. This attribute is very important to their function in biological membranes.

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