Chapter 2 : Composition of dietary fat
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Dietary fat includes all of the lipids in plant and animal tissue which are eaten as food. The most common fats (solid) or oils (liquid) are a mixture of triacylglycerols (triglyceride) with minor amounts of other lipids. The fatty acids present in various lipid molecules are the moieties of great nutritional interest.
The most abundant fatty acids have straight-chains of an even number of carbon atoms. There is a wide spectrum of chain-lengths, ranging from a four-carbon fatty acid in milk to thirty-carbon fatty acids in some fish oils. Frequently, the fatty acids have eighteen carbons. Double bonds along the carbon chain or substituents on it are designated chemically by counting the carboxyl carbon as position 1. Thus, the double bonds in linoleic acid give it the chemical systematic name of 9, 12-octadecadienoic acid. A short-hand abbreviation for linoleic acid is 18:2 (eighteen carbon atoms: two double bonds). Its last double bond is six carbons from the methyl end, an important feature for some enzymes. This is considered a n-6 or w 6 fatty acid (Figure 2.1). The n-6 nomenclature is used in this report.
The common (trivial) name, the chemical systematic name and the short-hand abbreviation for some dietary fatty acids are provided in Table 2.1. The double bonds in the fatty acids are in the cisconfiguration. The first member of the n-6 series of fatty acids is linoleic acid and the first member of the n-3 series is a -linolenic acid (9, 12, 15octadecatrienoic acid). The n-6 and n-3 polyunsaturated fatty acids have cis double bonds that are interrupted by methylene groups. A double bond may change from a cis to a trans configuration (geometric isomerization) or move to another position along the carbon chain (positional isomerization) as illustrated in Figure 2.2. The shape of a trans fatty acid is similar to that of a saturated fatty acid. As a result, trans fatty acids have a higher melting point than their cis isomer. The trans isomer may be regarded as an intermediate between an original cis unsaturated fatty acid and a completely saturated fatty acid.
Acylglycerides. The type of fatty acid and the position in which it is esterified to glycerol determine the characteristics of acylglycerides. In addition to glycerides which have three esterified fatty acids, diacylglycerides (diglycerides) and monoacylglycerides (monoglycerides) occur in raw food or food ingredients (Figure 2.3).
There is some specificity in the position occupied by the fatty acids. Animal depot fats tend to have a saturated fatty acid in position 1 and an unsaturated fatty acid in position 2. Fatty acids in position 3 appear to be more randomly distributed, although polyunsaturated fatty acids often accumulate there.
Phospholipids. Phospholipids are components of membranes which occur in foods and extracted oils. The general structure of phosphoglycerides is shown in Figure 2.4.
A saturated fatty acid is usually esterified at position 1 and a polyunsaturated fatty acid at position 2. The polar groups which contain phosphorus and an organic base provide the lipid molecule with a hydrophilic region. In addition to phosphoglycerides, phospholipids include sphingomyelins and cerebroside which are based on sphingosine rather than glycerol. Although phospholipids constitute only a small fraction of total dietary fat, they can be an important source of essential fatty acids.
FIGURE 2.1 : Diagram of fatty acids
FIGURE 2.2 : Structure of cis and trans double bonds
Some dietary fatty acids
|Common name||Systematic name||Abbreviation||Fatty acid family|
The growing realization of the importance of the non-glyceride components, sometimes called "minor constituents", in fats and oils warranted inclusion of this topic in the expert consultation. Non-glyceride components are minor constituents only in terms of their
FIGURE 2.3 : Diagram of acylglycerides
FIGURE 2.4 : Diagram of phospholipids concentrations relative to triacylglycerols. New information about these constituents of fat stems from improvements in the ability to analyze them and studies of their properties.
Vitamin E. Vitamin E consists of a mixture of lipid-soluble phenols characterized by an aromatic chromanol head and a side chain of 16-carbon atoms. The tocopherols have a saturated hydrocarbon tail, whereas the tocotrienols are the farnesylated analogues having an unsaturated isoprenoid tail. The number and position of the methyl groups on the chromanol ring give rise to the different a -, b -, g - and b -tocopherol and tocotrienol isomers (Figure 2.5).
Vegetable oils and the products made from them usually contain large amounts of tocopherol, especially the a , b and g isomers. In addition, certain vegetable oils, particularly palm oil (Qureshi et al., 1991a) and rice bran oil (Rogers et al., 1993), are rich sources of tocotrienols which have weak vitamin E activity but act as antioxidants and provide stability against oxidation.
Carotenoids. Carotenoids are highly unsaturated polyisoprene hydrocarbons that are lipid soluble. Over 75 different carotenoids are known to occur in animal fats and vegetable oils. The most common are a, b and g carotene, Iycopene, lutein and xanthopylls (Figure 2.6). The carotenoids and their derivatives are generally responsible for the yellow to deep-red colour of fruits, vegetables, cereals and crude palm oil. Carotenoids are the precursors of vitamin A, with ß-carotene having the highest provitamin A activity.
Vitamins A and D. A traditional source of vitamin A is butterfat. Fish oils are a common source of vitamin D. Margarines, which are fortified with vitamins A and D by law in most countries, also make an important contribution to ensuring adequate intakes of these nutrients.
FIGURE 2.5 : Diagram of tocopherol and tocotrienol
FIGURE 2.6 : Diagram of or-carotene and b -carotene
Sterols. Cholesterol is the principal sterol of animal products. The major sterols of plants are b -sitosterol, campesterol and stigmasterol, although several others are known to exist (Formo et al., 1979); the side chains of cholesterol and several plant sterols are shown in Figure 2.7. They occur in the free form or are esterified to such compounds as fatty acids, glucosides or ferulic acid (oryzanol). The sterol content of dietary fats and oils ranges from 0.01 to 2 percent (Itoh, Tamura and Matsumoto, 1973a).
Methylsterols and triterpene alcohols. Sterols methylated at the 4-OH position occur in common vegetable oils at concentrations of 0.01 to 0.4 percent with rice bran oil and sesame oil having the highest levels (Itoh, Tamura and Matsumoto, 1973b). Corresponding concentrations of triterpene alcohols, comprised of five condensed cyclohexane rings, are 0.01 to 1.2 percent. Rice bran oil is the only one at the upper level.
Squalene. The predominant hydrocarbon in dietary fat is squalene. It is an intermediate in the synthesis of sterol from acetate, and it is found in particularly high quantities in some fish and olive oil. The concentration in most vegetable oils is below 30 mg/100 g (Formo et al., 1979).
Oryzanols. Oryzanols are compounds consisting of ferulic acid esterified to a variety of plant sterols and triterpene alcohols (Figure 2.8). Although large amounts are found in crude rice bran and linseed oils, oryzanols are not widely distributed in other oils (Ibid.).
FIGURE 2.7 : Cholesterol and several plant sterols
FIGURE 2.8 : Chemical structure for oryzanol (24-methylene-cycloartanol ester of ferulic acid)
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