Domain Sizes in Triglyceride Crystal Networks by 1H Spin-diffusion NMR
The use of spin-diffusion NMR for the measurement of domain sizes in multiphase materials has shown an increasing interest in the last decade, in particular for the study of polymers and biomaterials that exhibit structural micro-heterogeneity. The method is particularly attractive because it can be applied to materials lacking long-range order, in what case standard crystallographic methods do not perform. We present in this work the first attempt to quantify the mesostructure domains of triglyceride-based fat crystals by means of 1H spin-diffusion experiments. For this purpose the NMR spin-diffusion process was analyzed in fat blends consisting of different mixtures of triglycerides. A double-quantum (DQ) dipolar filter was employed to select the magnetization only from the crystalline phase of the fat network, corresponding to the strongest residual dipolar couplings. During the spin-diffusion time the magnetization is flowing towards equilibrium from the crystalline domains to the surrounding less rigid, non-crystalline phase. Data analysis was performed taking into account a lamellar morphology characteristic to β’ and β crystals. The crystalline domain sizes determined by spin-diffusion NMR were found in fair agreement with the long period determined by X-ray diffraction methods. Moreover, the size of the non-crystalline domains, not accessible by diffraction techniques, could also be estimated. These results demonstrate the ability of the NMR technique to characterize the mesostructure of fats. Since the micro- and macroscopic properties of fat-based products are greatly influenced by the growth of crystalline domains and their morphology, this topic has a high academic and industrial interest.