Food Science and Estate Crop Technology

Definition of lipid is “a wide variety of natural products including fatty acids and their derivatives, steroids, terpenes, carotenoids and bile acids, which have in common a ready solubility in organic solvents such as diethyl ether, hexane, benzene, chloroform or methanol.” (Christie, 1982 in O’Keefe, 2002).

The terms fats and oils are used interchangeably, and the choice of terms is usually based on the physical state of the material at ambient temperature and tradition. Generally, fats appear solid at ambient temperatures and oils appear liquid. In the final analysis, it is the chemical composition that defines the characteristics of the individual fat or oil, which, in turn, determines the suitability of this ingredient in various processes and applications. Most of the oil-bearing tree fruits and kernels provide the highest oil yields. Oil seeds are obtained from annual plants that must be replanted each year, whereas fruit oils are harvested from trees with long life spans (O’Brien, 2009).

An important contributor to the availability of fats and oils is the fact that most fats and oils production is controlled by factors other than the demand. Weather is another factor that affects both availability and demand for fats and oils. For example, a hurricane in Malaysia can decrease the oil available on the world market (O’Brien, 2009).

Climate and availability certainly influenced the eating habits of our ancestors. Inhabitants of central and northern Europe obtained their edible fats from animals, whereas people in southern Europe, Asia, and Africa acquired their edible oils from vegetable sources. The food products developed in these different regions used the available fats and oils products. Consequently, the cuisine of central and northern European countries developed around the use of solid fats, such as butter, lard, and tallow for breads, pastries, and other baked and fried products. Similarly, the diets of inhabitants from warmer climates were developed around the use of liquid oils for their food products, which include many sauces and dressings. These trends appear to continue to be the preference of their descendants (Markey, 1960).

Fats and oils consumption has been categorized into visible and invisible sources. Visible fats and oils are those isolated from animal tissues, oilseeds, or oil fruits and used for food preparation as shortening, margarine, or liquid oil or for specialty uses. Invisible fats and oils are consumed as part of meats, poultry, eggs, dairy products, fish, fruits, or vegetables and account for approximately 60% of an individual’s fat consumption (O’Brien, 2009).

Fatty acids are almost entirely straight chain aliphatic carboxylic acids. The broadest definition includes all chain lengths, but most natural fatty acids are C4 to C22, with C18 most common. Naturally occurring fatty acids share a common biosynthesis. The chain is built from two carbon units, and cis double bonds are inserted by desaturase enzymes at specific positions relative to the carboxyl group. This results in even-chain-length fatty acids with a characteristic pattern of methylene interrupted cis double bonds. A large number of fatty acids varying in chain length and unsaturation result from this pathway (Scrimgeour, 2005).

The fatty acid components are distinguished in three ways: (1) chain length, (2) number and position of the double bonds, and (3) position of the fatty acids within the glyceride molecule. Variations in these characteristics are responsible for the chemical and physical differences experienced with edible fats and oils (O’Brien, 2009).

The fatty acid compositions of natural fats and oils vary significantly depending, not only on the plant or animal species, but also within the same species. Among the factors that affect the vegetable oil fatty acid compositions are climate conditions, soil type, growing season, plant maturity, plant health, microbiological conditions, seed location within the flower, and genetic variation of the plant (Sonntag, 1979 in O’Brien, 2009). Table 2.1. demonstrates the nomenclature, structure, and occurrence of fatty acid.

The melting point of a lipid is dependent on both the degree of unsaturation and the chain length (O’Brien, 2009). The melting point increases with chain length and decreases with increased unsaturation. Among saturated acids, odd chain acids are lower melting than adjacent even chain acids. The presence of cis-double bonds markedly lowers the melting point, the bent chains packing less well. Trans-acids have melting points much closer to those of the corresponding saturates. Polymorphism results in two or more solid phases with different melting points. Methyl esters are lower melting than fatty acids but follow similar trends (Scrimgeour, 2005). Vegetable oils’ saturated fatty acids are predominately even numbered carbon atoms ranging from 4 to 24 (O’Brien, 2009). Table 2.2. presents melting point of some fatty acid and methyl ester.

Oils and fats are now characterized mainly by their fatty acid composition determined by gas chromatography, replacing the titrimetric and gravimetric assays used previously. However, the saponification value (SV) or equivalent (SE) and iodine value (IV) are still used in specifications and to monitor processes. SE, expressed as grams of fat saponified by one mole of potassium hydroxide, is an indication of the average molecular weight and hence chain length, whereas the IV, expressed as the weight percent of iodine consumed by the fat in a reaction with iodine monochloride, is an index of unsaturation (Scrimgeour, 2005).

Table 2.1. Nomenclature, Structure, and Occurrence of Fatty Acid

*Abbreviations of the systematic names eicosapentaenoic acid and docosahexaenoic acid.

(Scrimgeour, 2005)