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Chapter 2 Composition, Structure, Physical Data, and Chemical Reactions of Fats and Oils, Their Derivatives, and Their Associates
To understand the reactions of fats and oils and the technologies applied, as well as to be able to influence their characteristics and behavior during processing, it is important to know their properties and reactions. The number of main building blocks that make up common oils and fats is relatively small. Most of the different characteristics occur as a result of minor components and the immense number of possible combinations of these building blocks. Under natural conditions, the number of major reactions of oils and fats that lead to alteration is small.
Scheme 2.1 Fats are esters of fatty acids with the trihydric alcohol glycerol. Because of the symmetrical structure of the glycerol molecule, two identical outer positions (1-, 3-) and one central position (2-) exist, to which the fatty acids are esterified. The type of fatty acid, as well as the distribution across these positions, determines the characteristics of the triglyceride. Not only the composition by summation equation but also the structure of the triglyceride is therefore of great importance. The share of different fatty acids in the world production of common edible fats and oils is of interest (Table 2.1). The calculation of Boekenoogen (1941) is now more than fifty years old. Today’s distribution is different, mainly as a result of a shift of the main production areas from the tropical regions of the world to those with moderate climates and also because of new crop cultivation. The table shows the distribution of 1941, and 1950 onwards, each calculated for average fatty acid composition per species. TABLE 2.1 Fatty Acid Distribution of the Common Vegetable Fats and Oilsa,b C18:1 Oleic 34 30 29 30 31 31 C18:2 Linoleic 29 24 32 36 38 37 C16:0 Palmitic 11 14 14 13 14 16 C12:0 Lauric 7 8 7 4 4 3 C18:3 Linolenic 6 6 5 6 5 5 C14:0 Myristic 3 3 2 2 2 1 C22:1 Erucic 3 8 5 2 0 0 C18:0 Stearic 3 3 3 3 4 4 All others 4 4 3 4 2 3 aBecause fatty acid composition of vegetable oil is not uniform within the same kind of oil, these rounded figures are only indicative; however, they are based on thoroughly calculated figures from averages. bSource: Boekenoogen (1941) and own calculations. The fatty acid composition of the main seed and pulp oils does not differ substantially among the main types. Therefore, shifts of only a few percentage points in the table may be caused by immense shifts in cultivation. So little a share of erucic acid remains, for example, that it disappears by rounding off. In new rape varieties, it is almost completely substituted by oleic acid. Furthermore, the trend toward oilseeds with a high content of essential linoleic acid is clear; coconut oil has drastically decreased relative to the other oils, resulting in a parallel decrease in the appearance of lauric acid. The fatty acid compositions of the individual oils and fats are described in Chapter 4. Here, only the principles of composition and structure will be discussed. One unsaturated fatty acid is ubiquitous in fats and oils; two are almost ubiquitous. Oleic acid is present in all known fats and oils and also predominates in quantity (Table 2.1). Almost all fats and oils contain its next lower homologues, palmitoleic acid (although in very small quantities) and the double unsaturated acid with the same chain length as oleic acid, namely, linoleic acid, as well as other C18:2 acids. None of the saturated fatty acids is omnipresent. However, palmitic acid has almost the same distribution as oleic acid. The distribution of myristic acid and stearic acid is similar to that of linoleic and palmitoleic acid. To date, the fat and oil of ~5000 species (animals and plants) have been analyzed. No correlation between their composition and occurrence has yet been found. As early as 1935, Hilditch proposed a link between the fatty acid composition of fats and the evolutionary stage of the species in which they occur—beginning with microorganisms and marine monocellular organisms via marine plants and animals, to both terrestrial animals and plants. No real evidence could be found for this approach, although some indications for this connection may exist (Fig. 2.1).
Fig. 2.1 Proportion of certain fatty acids on the fat of sea and land animals. (after Hilditch and Williams 1964) It seems evident that the amount of saturated fatty acids and unsaturated C18-fatty acids (mainly oleic acid) increased during the course of evolution. Unsaturated long-chain fatty acids tended to disappear and palmitoleic acid (due to its occurrence also named zoomaric acid) diminished dramatically. Similar trends are also expected in vegetable fats, but an insufficient number of lower plants have yet been analyzed to prove this. However, some hints can be found in the change of fatty acid composition that occurs during the ripening of seeds following patterns similar to evolution (see also Chapter 4). In total, the spectrum of fatty acids that occur in main edible oils and fats is represented by no more than 10–12 compounds, representing >98 % of all fatty acids in food. Statistically, however, 10 different fatty acids allow the formation of more than 650 different triglycerides. Nature does not follow statistical distribution; therefore not all possible combinations can be found. The model of restricted statistical distribution best simulates reality (see Chapter 2.2). Only a part of the different physical and chemical characteristics of fats and oils is caused by the special characteristics of the fatty acids themselves, or by those fatty acids that represent the residual 2%. One of the main parameters influencing the characteristics of fats and oils is the degree of unsaturation of their fatty acids; this can be changed by hardening (see Chapter 6.5). The other concerns the distribution of the 10–12 main fatty acids over the three different positions of the glycerol molecule. Therefore, great efforts are undertaken to separate certain triglycerides (see Chapter 6.2) or to change their distribution in the glycerol molecule (see Chapter 6.4). Fats have a density d15 of 0.91–0.95 cm3/g, a very low vapor pressure and consequently, a very high boiling point. The melting point of oils usually lies below 0°C, the melting point of the highest melting fraction of fats at about 75°C. Solubility is very good in nonpolar solvents and chlorinated hydrocarbons. The main chemical reactions that occur naturally are saponification and oxidation. Fats usually are accompanied by lipids such as carotene, sterols and phosphatides. Many oilseeds contain other specific components, which usually have to be removed during processing. 2.1 Components of Fats and Oils
2.1.1 Glycerol
Glycerol (propane-1,2,3-triol) is the one and only alcohol to which fatty acids are esterified into triglycerides, i.e., oils and fats. Living organisms synthesize glycerol from hexose; in the body, this reaction is much faster than the synthesis of fatty acids. That is why glycerol is always available in sufficient quantities (Propjak 1953) for the synthesis of fats. The Swedish pharmacist and chemist, Scheele, discovered glycerol in 1783 when he experimented with olive oil. Chevreuil, the father of fat chemistry, gave the sweet-tasting substance its name, which is derived from the Greek word for sweet.
Scheme 2.2 Glycerol is a symmetrical triple alcohol (for physical data, see Table 2.2) and is important as the basic component of all triglycerides. However, as the one and only identical alcohol component that is present in all triglycerides, it becomes unimportant for the technology of fats and oils. The only importance is in its re-esterification with fatty acids to yield very special dietetic fats (see Chapter 8.7). TABLE 2.2 Physical Data of Glycerol...
