Melting points of fatty acids

Table 10.2 lists the structure and melting point of four fatty acids containing 18 carbon atoms. Stearic acid is one of the two most common saturated fatty acids, and oleic and linoleic acids are the most common unsaturated ones. The data show the effect of Z double bonds on the melting point of fatty acids. 

The Effect of Double Bonds on the Melting Point of Fatty Acids... 

The amount of polyunsaturated fatty acids (PUFA) affects both melting and flashpoints of vegetable oils. Melting temperature of oil is directly related to the melting point of fatty acids, which decreases with unsaturation (7). The flash point of flaxseed oil is relatively low compared with other vegetable oils this can be attributed to a high contribution of PUFA. 

Low-temperature crystallization is based on the fact that the melting point of fatty acids changes considerably with the type and degree of unsaturation (73). At low temperatures, long-chain saturated fatty acids that have higher melting... 

Fractionation by crystallization using the differences of melting point of fatty acids or triacylglycerols is done in two ways. Dry fractionation, also known as... 

III. Double bonds in the structure decreases the melting points of fatty acids. 

Table 2.16 Melting point of fatty acids and triacylglycerols of soybean oil and its partially hydrogenated product...

The melting points of fatty acids. Melting points of both saturated and unsaturated fatty acids increase as the number of carbon atoms in the chain increases. The melting points of unsaturated fatty acids are lower than those of the corresponding saturated fatty acid with the same number of carbon atoms. Also, as the number of double bonds in the chain increases, the melting points decrease. 

Figure 3.35 Relationship between the melting point of fatty acids and their chain length.

Problem 19.7. Why do cis double bonds lower the melting points of fatty acids Trans double bonds do not have the same effect as cis double bonds. Why Hint Use appropriate drawings to show the effect of double bonds on molecular shape and indicate how molecular shape affects melting point. 

The transition temperature, i.e., the melting point, of fatty acids when suspended in water is very close to their anhydrogenous melting point (38). Fatty acids do not interact with water and thus behave as di- and triglycerides, i.e., as insoluble amphiphiles irrespective of experimental temperature. 

Fig. 9. Phase equilibria for the bile salt (bile acid)-fatty acid-water system at constant water concentration in relation to temperature (see Fig. 5). Six mixtures varying in molar ratios of bile salt (bile acid) and palmitic acid with total concentration of micellar bile acid plus palmitic acid equal to 40 mM were examined. Fatty acid has a finite solubility in the micellar bile acid solution, the excess being crystalline at body temperature. At 50-60 C, there is a marked increase in micellar solubility, and the fatty acid melts. At higher fatty acid/bile acid ratios, the micellar solubility is exceeded, and an immiscible oil phase occurs. The melting point of fatty acid in the presence of water is nearly identical to that in the anhydrous state (38), in contrast to the behavior of monoglyceride (Table I). As shown in Fig. 3, the size of the micellar area decreases with increasing chain length. Unsaturated fatty acids (not shown) behave similarly to saturated fatty acids, but their micellar solubility is greater, and at most experimental temperatures a crystalline phase will not occur.

The melting points of fatty acids are highly dependent on the length of the carE)on chain and the degree of unsaturation of the molecule. Short fatty acids tend to be more volatile. In fact, acetic, propionic, and butyric acid are collectively called volatile fatty acids (VFA). There is a steady rise in melting point eis the chain lengths increase. However, eis the numlser of double bonds increases, the melting point decreeises. 

Knothe, G., Dunn, R.O., 2009. A comprehensive evaluation of the melting points of fatty acids and esters determined by differential scanning calorimetry. J. Am. Oil Chem. Soc. 86, 843-856. 

The influence of fatty acid structure on its melting point has already been mentioned with branch chains and cis double bonds lowering the melting points of equivalent saturated chains. In addition, the melting point of fatty acids depends on whether the chain is even- or odd-numbered (Table 3.5). 

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