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Molecules 2020, 25, 5572 16 of 26 Several studies have been conducted on the mechanisms driving the thermal degradation of triglycerides and the conditions under which this occurs. Upon investigating the thermal degradation behaviour of olive oil, which is rich in triglycerides, Vecchio et al. [52] hypothesize that the decomposition of the saturated and unsaturated carboxylic chains occurs approximately in the range 160–370 ◦C, whereas in oxidative conditions above 370 ◦C a fast combustion of the evolved volatile components takes place, followed by oxidation of the carbonaceous residues. This is confirmed by Lee et al. [53], who observed a second peak in the thermogravimetric (TGA) curve of coconut oil around 363 ◦C corresponding to the combustion of volatile residues. Sari et al. [40] have also noted a two-step degradation mechanism when evaluating the thermal stability of trimyristin, tripalmitin and tristearin. However, in contrast to reports by Morad et al. [47], they observed the onset of degradation at 150, 170 and 180 ◦C for trimyristin, tripalmitin and tristearin, respectively. According to Nawar et al. [54], under oxidative conditions triglycerides may also undergo a 6-atom-ring-closure, which was confirmed when a series of lactones were shown to form by heating milk fat. Crossley et al. [55] observed this under non-oxidative conditions as well when studying the effects of heat on tricaprin in the absence of moisture at 300 ◦C. After 4 h of heating under a constant nitrogen flow, both acrolein and di-n-nonyl ketone and, to a lesser extent, the symmetrical ketones di-n-butyl, di-n-amyl, and di-n-hexyl were produced in addition to the major product, decanoic acid. Noble et al. [56] also investigated the hydrolysis of corn oil, cottonseed oil, and lard heated at 200 ◦C in the presence of moisture, and noted a preference for the hydrolysis of the shorter chain and the unsaturated acids. In addition, they observed an increase in the rate of hydrolysis with the degree of unsaturation. Therefore, unsaturated triglycerides undergo earlier onset thermal degradation. Other thermally induced reactions which may occur under non-oxidative conditions include dehydration, decarboxylation, hydrolysis of the ester bond, double bond conjugation, polymerization, dehydrocyclization, aromatization, dehydrogenation, and degradation by carbon-carbon cleavage [54]. 4. Modelling and Prediction of Thermal Properties Due to the importance of the thermal properties of triglycerides in defining the final characteristics of foods rich in fat, researchers have been trying to develop models able to accurately predict such properties. In fact, being able to predict the thermophysical properties of a given chemical or to assign a certain chemical structure to the required thermal property would considerably reduce the time required by the formulation of specific foods and would greatly benefit to researchers operating in this field. Several models have been developed over the years to identify the thermal properties of a molecule starting from its chemical structure alone. As mentioned by Moorthy et al. [57], despite the great advances made in developing models capable of predicting the thermal properties of triglycerides, research for the prediction of unsaturated triglycerides is far less developed than it is for saturated triglycerides due to the lack of data collected on their thermal properties. In 1936, King and Garner [58] formulated a simple equation to predict the enthalpies of fusion of fatty methyl or ethyl esters with chain lengths exceeding carbon number equal to 8. Their model parameters were then corrected and implemented to improve predictions for specific triglycerides groups, such as saturated and cis unsaturated compounds, and some of their polymorphs. Hagemann and Rothfus [18] contributed to this with investigations on the packing geometries and the molecular rotation freedom. However, up to this point the majority of the models for predicting either the enthalpy of fusion or the melting point of the different polymorphic forms relied only on the total number of carbon atoms in the three alkyl groups as input variable [28]. Therefore, often when calculating the melting point of different polymorphic forms having the same carbon number the models failed. In 1990 Wesdorp [27] introduced a new model where the position and the chain length of the three alkyl groups in the triglyceride are also taken into account. Based on the work conducted by Wesdorp, Constantinou et al. [59] introduced the group-contribution (GC) method, where aside from the positionPDF Image | Triglycerides as Novel Phase-Change Materials
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