pubs.acs.org/crystal Published on Web 08/20/2009 r 2009 American Chemical Society DOI: 10.1021/cg900219b 2009, Vol. 9 4281–4288 Effects of Crystal Growth and Polymorphism of Triacylglycerols on NMR Relaxation Parameters. 2. Study of a Tricaprin-Tristearin Mixture Matthieu Adam-Berret, †,‡,§ Alain Riaublanc, ‡ and Franc -ois Mariette* ,†,§ † Cemagref, UR TERE, 17 avenue de Cucill  e, CS 64427, F-35044 Rennes, France, ‡ INRA-BIA, Rue de la G eraudi ere, BP 71627, 44316 Nantes cedex 3, France, and § Universit e europ eenne de Bretagne, France Received February 20, 2009; Revised Manuscript Received July 6, 2009 ABSTRACT: Being able to determine the physical properties of fats such as polymorphism and crystal size is very important for the food industry. After a relationship was observed between spin-lattice relaxation time and crystal size in a solid-liquid mixture of triacylglycerols, the effects of polymorphism and crystal size were investigated by low-field NMR relaxation and powder X-ray diffraction on three binary mixtures of tricaprin and tristearin in the solid state. Second moment (M 2 ) was proven to be only sensitive to polymorphism. Its measurements permitted the quantification of polymorphic forms in a binary mixture, with a model based on M 2 of the pure components. As for the spin-lattice relaxation time (T 1 ), it was proven to be only sensitive to crystal size and not to polymorphism. Quantification was not possible with T 1 measurements, but information on the pattern of the crystal thickness distribution was obtained using the maximization entropy method algorithm. The determination of polymorphism was thus possible because of the difference in size between the R and β triacylglycerol crystals. Finally, a phase diagram mainly based on NMR data was constructed for the tricaprin/tristearin system. Introduction The physical properties of fats depend on the polymorphic behavior and intersolubility of their major triacylglycerol components. The phase behavior of these mixtures is of paramount importance for the manufacture of fat-containing products such as chocolate and butter. 1 Indeed, fat structures formed by triacylglycerols determine the functional properties of these products such as their texture, plasticity, and mor- phology. 2,3 Various studies have been carried out on pure triacylglycerols, 4,5 but fats and lipids present in natural re- sources are mixtures of different types of triacylglycerols and the blends show a far more complicated polymorphic beha- vior. 6 Moreover, the complexity is increased by the intersolu- bility of the different triacylglycerols which can cocrystallize. 7 As an understanding of the physical properties of the triacyl- glycerol mixture system is essential, binary triacylglycerol mixtures have been extensively studied. 8,9 It has been demon- strated that the phase behavior of the binary systems can be classified into three situations, which are solid-solution, eu- tectic, and molecular compound. Differential scanning calori- metry (DSC) and X-ray diffraction (XRD) are the two techniques generally used to characterize triacylglycerol mix- tures. 10 Time-resolved synchrotron radiation XRD has pro- vided the most interesting results on the polymorphic behavior of pure triacylglycerols and in mixtures. 11-17 It was shown that the solid-solution phase was enhanced when the triacylglycerols were in the R and the β 0 form, and when the difference in chain length between the triacylglycerols was shorter than four carbons. However, crystal size is another important parameter for determination of the physical prop- erties of fats because it affects the rheological properties and consequently modifies taste, graininess, and texture. Maran- goni et al. 18 demonstrated that crystals with different sizes but the same polymorphism showed similar XRD patterns and had the same melting temperature. Low-field NMR relaxation is currently the reference method to evaluate solid fat content (SFC) in food with a measurement based on the intensity of the signals. 19,20 However, the latter technique has shown more potential with the measurement of relaxation parameters, especially in the case of determination of polymorphism. A recent study based on pure triacylglycerols showed that it was possible to assess polymorphism indepen- dently of temperature and chain length. 21 The distinction was proven to be possible via second moment (M 2 ) measurements, and greater sensitivity was found with spin-lattice relaxation time (T 1 ) measurements. Trezza et al. 22 developed a model based on T 2 measurements and semiempirical mathematical functions to quantify the R and β polymorphic forms as well as SFC through a single measurement. However, they were not able to distinguish between β and β 0 polymorphs. A recent study showed that spin-lattice relaxation time measurements could provide more information on the fat system. Indeed, evidence of a correlation between T 1 and crystal size, as well as the existence of cocrystals of tricaprin and tristearin after fast supercooling, was highlighted in this study. 23 T 1 measurements can thus provide interesting information on fat systems. After demonstrating the relationships between NMR re- laxation parameters and the design (i.e., their crystal size and polymorphism) for pure triacylglycerols, the aim of this study was to establish the ability of low-field NMR relaxation to obtain information on triacylglycerol mixtures. Second mo- ment and spin-lattice relaxation time measurements were combined for model mixtures of tricaprin and tristearin in the solid state at three different ratios. These two parameters were used to determine the behavior of the triacylglycerols in a solid mixture and to establish new methods for the quantification of blended triacylglycerol polymorphs. *To whom correspondence should be addressed. Address: Cemagref, Food Process Engineering Research Unit, CS 64426, 17 av de Cucill e, 35044 Rennes, France. Phone: (33) 2-23482178. Fax: (33) 2-23482115. E-mail: francois.mariette@cemagref.fr.