A pulsed low resolution NMR study on crystallization and melting processes of cocoa butter
Pulsed low resolution nuclear magnetic resonance (NMR) was employed to measure the «melting» curves of different series of cocoa butter samples. The samples were prepared from completely liquid phase by cooling and tempering them at different temperatures Tc for varying time Δt. The «melting» curves were measured while keeping the sample at a fixed temperature Tm. The complex shape of each curve was interpreted in terms of cocoa butter polymorphism, and the results were compared with data obtained by other techniques available in the literature. Using just two tempering temperatures (+7 C and −18C), we were able to distinguish four solid phases and identify them with the phases II, III, IV, and V described in literature. Our data are in full agreement with literature. Several novel results have been also found. These include the kinetic constants of the melting processes of phases II and III, the rate constants of solidification of phase V, the conversion of phase III into phase IV before melting at temperatures ≥30 C, and the growth of phase V out of phase II at −18 C (including the rate constant of this process). We are convinced that NMR may serve as a principal tool in fat polymorphism investigations, especially if it is combined with other techniques such as differential scanning calorimetry. Its advantage, apart from rapidity of measurement, is the fact that the measurement itself does not interfere with the melting or solidification process studied. On the other hand, it does not distinguish in a direct way between different solid phases present in the sample; this can be done only in inference from the behavior upon melting.
Pulse NMR as a quick method for the determination of the solid fat content in partially crystallized fats
Pulse nuclear magnetic resonance is a good alternative to the slow, laborious dilatometer technique for determining the solid/ liquid ratio in partially crystallized fats and has some advantages over wide-line NMR. A processing unit is described which has the advantage that it displays directly the solid fat content on a digital voltmeter. Accepting a standard deviation of 0.6%, as is the case with the dilatometer technique, the solid fat content can be measured in 4 s.
Neutron Residual Stress Measurements on Rail Sections for Different Production Conditions
Rail sectioning with subsequent neutron diffraction experiments has been used to assess residual stresses in the rails. In this study we present the results of neutron stress measurements performed at the NIST Center for Neutron Research (NCNR) on rail sections from rails that were produced under various conditions. Specifically, these are air-cooled, air-cooled and roller-straightened, head-hardened and head-hardened and roller-straightened. More significantly, a head-hardened and roller-straightened rail was also studied after service to elucidate evolution of the service-induced residual stresses. In the latter case both a transverse-cut slice and the central region of a 0.53 m long piece were studied. Measurements on this piece are the first in which triaxial stresses have been determined for an intact rail. Neutron strain measurements with 3×3×3 mm3 spatial resolution were successfully employed for transversally cut slices to verify the difference in the stress state depending on the production process. Although examination of slices allows determination of only two-dimensional stresses in the plane of the slice, additional measurements on obliquely-cut slices, which were also carried out, and utilization of FEM gives the possibility of reconstructing the full triaxial stress distribution. Together, these approaches provide a better understanding of rail fabrication and the possibility of improving the durability and safety of rails in the future.Copyright © 2004 by ASME