Polymorphism of tristearin

Polymorphism means many bodies. Having determined the melting point of tristearin in... 

A slow heating scan at 0.1°C/min from -10 to 40°C using coupled DSC and XRDT confirmed the existence of five of the six forms already observed (4). It was also shown that the second solid phase displays at least two polymorphic varieties, the LS and SS of which correspond more or less to the a and p forms of tristearin (4,8). 

TABLE 15.3 Some Properties of the Three Main Polymorphic Modifications of Tristearin... 

The behaviour was different for the three mixtures. There was one peak broadened at the bottom for the 25-75 (w/w) mixture, there were two peaks for the 50-50 (w/w) mixture and three peaks for the 75-25 (w/w) mixture. The first peak was found for all three mixtures around 150 ms and was attributed to T of tristearin in the a form. This peak confirmed the presence of small crystals of tristearin in the systems. This peak shifted a little to a higher value when the tricaprin concentration increased. A second peak was characterized for the three mixtures, and attributed to the p polymorph of tricaprin. It was... 

C and 80 C is of special interest melting enthalpies are between 100 and 200 J/g. In the context of quality control, such calorimetric curves can be used as fingerprints for the considered fats. As examples. Figs. 2 and 3 present the thermal polymorphism of two glycerides, distearin and tristearin, as displayed by a power compensation DSC instrument. 

FIG. 3 Calorimetric curves of tristearin showing, during the first heating (soUd curve), the melting of the stable polymorphic form around 75°C and, during the second heating, (-------), the melting of two less stable forms around 55°C and 70°C, with partial crystal-... 

Figure 6.5 Cryo-TEM micrograph of tristearin nanoparticles (scale bar = 100 nm) showing a characteristic plate-like morphology in the 5 polymorphic form. (Reproduced with permission from Langmuir, Bunjes eto/., 2007, copyright 2007, the American Chemical Society.)...

Emulsifiers sensitively modify the rates of crystal growth and polymorphic transition of fats through the preferred adsorption at or inclusion in fat crystals [55,56]. The retardation or acceleration of the polymorphic transformation is influenced by the hydrophobic moiety structure. Figure 20 shows the effect of emulsifiers on the a-to-p transformation of tristearin during aging at room temperature... 

Figure 32. The effect of heating rate on the DTA curve of tristearin (6 mg). The assignment of the peaks is as follows a, melting of a-polymorph b, crystallisation of p -polymorph c, melting of P -polymorph d, crystallisation of P-polymorph e, melting of P-polymorph. Reprinted from reference [112] with permission from Elsevier Science and The Royal Society of Chemistry.

Monotectic mixtures arise when the individual components have similar melting points, molecular volumes and polymorphic forms. Figure 17.12(a) represents a possible phase diagram for monotectic mixtures. A typical monotectic solution occurs when SSS is mixed with SOS. Lutton (1955) determined that the a form was present and associated with limited solid solution formation, and contrasted with the a forms of other glyceride mixtures that formed continuous solid solutions (Rossell 1967). It was found, for this system, that tristearin incorporates about 50% of the SOS into a solid solution on the other hand, SOS incorporates very little SSS into a solid solution. 

The existence of a substance in two or more forms, which are significantly different in physical or chemical properties, is known as polymorphism. The difference between the forms arises firom different modes of molecular packing in the crystal structure of certain triglycerides. Certain pure or mixed fatty acid triglycerides may show as many as five different melting points. Each crystal system has a characteristic melting point, x-ray diffraction pattern, and infrared spectrum. For example, tristearin can exist in three polymorphic forms with melting points of 54.7, 63.2, and 73.5°C. 

Fats and oils often show multiple melting points. As an example, tristearin has three melting points at 52°C, 64°C and 70°C, because fats and oils solidify in more than one crystal form this property is known as polymorphism. Crystallization of fats and oils occurrs in two stages of nucleation and growth. 

Tricaprin/Tristearin Mixtures. The tricaprin/tristearin mixtures were studied at 10°C. According to melting temperatures, tricaprin is in the P form and tiistearin in the a form at 10°C. However, the polymorphic behaviour depends on the thermal history of the sample. It was confirmed by XRD measurements (data not shown) that in our system, tricaprin was in the p form and tristearin in the a form. We thus had a solid mixture of two different polymorphs, and it was interesting to obtain information on the crystal size distribution profile. The Ti distributions for the three different ratios are presented in Figure 3. 

Modified triacylglycerols can occur in different polymorphic forms, and these influence the melting properties of the fat, the softening point, the solids content, etc. Analysis of such properties is commonly performed using differential scanning calorimetry (DSC). DSC has been used to determine the physical properties of sTAG formed from interesterification products of triolein and tristearin (Seriburi and Akoh, 1998) and from acidolysis reactions between stearic acid and coconut oil (Rao et al., 2001). 

Figure 7 Interfacial shear viscosity as affected by the interaction of sodium caseinate and different polymorphic form of fat crystals located at the interface. (O), P-polymorph hydrogenated pahn oil mid fraction (HPMF) crystallized in hexane ( ), P-polymorph tristearin crystallized in oil ( ), p -polymorph HPMF crystallized in hexane ( ), P -polymorph HPMF crystallized in oU. (Results from Ref. 36.)...

Figure 9.17 DSC thermograms (endotherms up) showing isothermal crystallisation (left) and melting (right) of tripalmitin-tristearin blends (labelled as wt % tristearin) at (a) 62.5°C, (b) 58.5°C, (c) 54.5°C and (d) 40.5°C. The y-axis scale divisions are 1 and 10 W/g for crystallisation and remelting thermograms, respectively. This demonstrates the use of DSC in identifying the polymorphic forms of fats. (Taken from [64], Copyright (2006), with kind permission of Springer Science and Business Media.)...

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