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3The composition and nutritionalproperties of extra-virginolive oilManuela Mariotti1 and Claudio Peri21Department of Food, Environmental and Nutritional Sciences,University of Milan, Milan, Italy2University of Milan, Milan, Italy
Chapter 3 gives basic information about the composition and nutritional propertiesof extra-virgin olive oil. As triglycerides make up 97 to 99% of extra-virgin oliveoil, the main chemical-physical characteristics of the oil depend on the compositionof the triglyceride moiety. However, the minor components give an invaluable con-tribution to sensory and health-promoting properties. It is mainly the presence ofthese components that differentiates extra-virgin olive oil from all other edible oils.
3.1 Triglycerides and fatty acids
Extra-virgin olive oil essentially includes two groups of chemical compounds:
triglycerides: 9799% wt
minor components: 13% wt
Triglycerides mainly contain a monounsaturated fatty acid (oleic acid), a fairamount of polyunsaturated fatty acids (linoleic and -linolenic) and a slight amountof saturated fatty acids (palmitic and stearic).
The minor components are a complex mixture of polar, nonpolar and amphiphilicsubstances: hydrocarbons, tocopherols, phenolic compounds, sterols, chlorophyll,carotenoids, terpenic acids, monoglycerides and diglycerides, free fatty acids, esters
The Extra-Virgin Olive Oil Handbook, First Edition. Edited by Claudio Peri. 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.
22 CH03 THE COMPOSITION AND NUTRITIONAL PROPERTIES OF EXTRA-VIRGIN OLIVE OIL
and other volatiles. They contribute in a particular way to the sensory and health-promoting properties of extra-virgin olive oil.
Extra-virgin olive oil is separated from an aqueous medium so it still contains avery small, but essential amount of water. The water saturation threshold of extra-virgin olive oils is 300400 mg per kg of oil, but they often have higher amounts,ranging from 300 to 1200 mg per kg of oil. Water is present in micro-droplets,less than one-tenth of a m in diameter, impossible to separate by centrifugation.These microdroplets are associated with and stabilized by water-compatible, polaror amphiphilic substances of the minor components group.
Triglycerides belong to lipids, organic compounds that do not mix with water.They derive from the combination of three fatty acid molecules with one moleculeof glycerol. Glycerol is a short 3-carbon chain alcohol that serves as the frame towhich the three fatty acids can attach themselves with an ester link. Fatty acidsconsist of chains 4 to 30 carbons long, with an acidic group at one end: it is thisgroup that binds to glycerol to make a glyceride. Natural fatty acids usually have aneven number of carbon atoms, as their synthesis in vivo is based on the assembly ofa variable number of acetyl-CoA, a 2-carbon molecule (OKeefe 2008).
Figure 3.1 shows the structural formula of a fatty acid molecule (stearic acid). Itconsists of a long chain of 18 carbon atoms (C) with their four valence bonds. Twobonds create the basic connection of the chain, whereas the other two are saturatedby hydrogen atoms (H). At one end of the molecule there is a special group in whichthe carbon atom is linked to an oxygen atom (O) and to a hydroxyl group (OH). Theresulting COOH group is called the acidic group. At the other end, the carbon atomis linked to three hydrogen atoms forming a methyl group (CH3).
The position of carbon atoms is usually identified by a number in the sequencethat starts from the acidic group and ends at the methyl group. The acidic groupis also called (alpha), the first letter of the Greek alphabet. The methyl group iscalled (omega), the last letter of the Greek alphabet.
Figure 3.2 represents the same molecule of Figure 3.1 but with a different graphi-cal convention, which is called the skeletal formula. Carbon atoms are representedas black dots and hydrogen atoms, directly connected to the carbon atoms, are notindicated: each carbon atom is understood to be associated with enough hydrogenatoms to give the carbon atom four bonds. An even simpler representation is the
H H H H H H H H H H H H H H H H H
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
HO C C C C C C C C C C C C C C C C C H
H H H H H H H H H H H H H H H H
Figure 3.1 The structural formula of stearic acid.
3.1 TRIGLYCERIDES AND FATTY ACIDS 23
Figure 3.2 The skeletal formula of stearic acid.
skeletal formula without the black dots. In this type of representation, it is impliedthat the carbon atoms are located at the corners and ends of the line segments.
Figure 3.3 shows how a molecule of glycerol combines with three molecules ofthe fatty acid (in this case stearic acid), giving a triglyceride molecule (in this casetristearin) and three molecules of water.
In some fatty acids there are double bonds that derive from the elimination oftwo hydrogen atoms from two adjacent carbon atoms. Figure 3.4 shows a fatty acidderived from the stearic acid of Figure 3.2, by removing two hydrogen atoms fromcarbons 9 and 10, in the middle of the stearic molecule.
When double bonds are present, fatty acids are defined as unsaturated. Theunsaturated fatty acid represented in Figure 3.4 is the most important fatty acid ofolive oil and it is called oleic acid. It represents from 65 to 85% of all fatty acidsin olive oil.
Figure 3.3 The triglyceride of stearic acid (tristearin).
Figure 3.4 The skeletal formula of oleic acid.
24 CH03 THE COMPOSITION AND NUTRITIONAL PROPERTIES OF EXTRA-VIRGIN OLIVE OIL
A very particular structural change that takes place in the presence of doublebonds is the bending of the fatty acid molecule, as shown in Figure 3.4.
The same fatty acid molecule can have several double bonds, as illustrated inFigure 3.5, in which four fatty acids are shown, all with 18 carbon atoms but with adifferent number of double bonds:
stearic acid is a saturated fatty acid
oleic acid is a monounsaturated fatty acid (acronym: MUFA)
linoleic and -linolenic acids are polyunsaturated fatty acids (acronym: PUFA)with two and three double bonds, respectively.
Double bonds are the most reactive position in a fatty acid molecule, especiallyif multiple double bonds are conjugated, which means separated by a single CH2group, which is the case with both linoleic and -linolenic acid. Double bonds canreact with oxygen, thus spurring the oxidative spoilage of oil, or they can react withhydrogen, thus re-establishing a saturated condition.
If one double bond of a natural PUFA is saturated by chemical reaction with twohydrogen atoms, the resulting unsaturated fatty acid has a linear structure. Therefore,if a double bond of -linolenic acid is transformed into linoleic acid by saturationof a double bond, the resulting linoleic acid has a linear structure. Similarly, if anatural linoleic acid is transformed into oleic acid by saturation of a double bond,the resulting molecule of oleic acid has a linear structure (Figure 3.6). In order todistinguish these forms, all natural forms of unsaturated (and bended) fatty acidsare identified with the prefix cis-, while all trans-formed, artificial, unsaturated fattyacids are identified with the prefix trans-.
However having an equal number of carbon atoms and the same number of doublebonds, trans-isomers have physical, chemical and biological characteristics that aremore similar to saturated fatty acids than to unsaturated fatty acids. Thus, trans-oleicacid is more similar to stearic acid than to cis-oleic acid.
Trans oils increase the risk of coronary heart disease by raising the level of LDLcholesterol and lowering the level of good HDL cholesterol. The presence of trans-isomers in an extra-virgin olive oil is a clear sign of fraud and can be easily detectedthrough analytical methods that are common practice nowadays.
Due to the bending of the molecule, the cis structure makes it more difficult forthese fatty acids to solidify into compact crystals, so at a given temperature unsatu-rated fatty acids are softer than saturated fatty acids. In other words, unsaturated fattyacids have lower melting points in comparison to saturated fatty acids. Table 3.1shows the four C18 fatty acids that are present in olive oil. Despite the fact thatthey have very similar molecular formulas and molar masses, they have differentdegrees of unsaturation and very different melting points. It is especially worth not-ing that stearic acid, a saturated fatty acid, has a melting point that is much higherthan the human body temperature (37 C or 98.5 F) and therefore is solid in thebody, whereas all the others are unsaturated fatty acids and have a melting pointlower than the body temperature and therefore are liquid in the body.
3.1 TRIGLYCERIDES AND FATTY ACIDS 25
Figure 3.5 The molecules of stearic acid (a), cis-oleic acid (b), cis-linoleic acid (c) and cis--linolenic acid (d).
Lipid oxidation is influenced by many factors: the presence and concentrationof oxygen, temperature, light and metal catalysts, but, most of all by the degree ofunsaturation and the presence of conjugated double bonds.
The velocity of oxidative reactions is more than proportional to the number ofdouble bonds as is evident by comparing the data in Table 3.2.
26 CH03 THE COMPOSITION AND NUTRITIONAL PROPERTIES OF EXTRA-VIRGIN OLIVE OIL