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.
Fully automated determination of solid fat content by pulsed NMR
Pulsed NMR has been developed into a quick, accurate fully automated method for the determination of the solid fat content in partially crystallized fats. Accepting a standard deviation of 0.3% solids, the percentage of solids is displayed on a digital voltmeter or printed out 6 sec after placing the sample into the sample holder. To allow for the dead time of the receiver, a correction factor has been introduced giving rise to only small errors (<1%) in the solid fat content. Due to the short measuring time, no temperature control of the sample holder is needed between 10–45 C. Pulsed NMR values can be converted into dilatations and vice versa.
Determination of the solid contents of fats by wide-line nuclear magnetic resonance: The signal of liquid oils
In order to ascertain the sources of error in the determination of the solid fat content by wide-line nuclear magnetic resonance (NMR) an investigation has been made of the factors governing the amplitude of NMR signals of liquid oils measured with a Newport Quantity Analyzer. The varying saturation properties of the oils concerned were found to be of major importance. The spin-lattice relaxation times (T1) of 16 oils were measured on a pulse spectrometer at 20, 40 and 60 C, and quantitatively related to the saturation phenomenon. The observed deviation from the theoretical linearity of the NMR signal with the inverse of temperature is explained by the strong dependence of T1 on temperature and viscosity, and new relationships are derived for these parameters. Since the improvement of the signal to noise ratio of the NMR signal, obtained by increasing the RF level (H1), increases the saturation as (H1)2, a compromise must be chosen between these two factors to obtain optimal measurement. The results are used to establish criteria for selecting two reference oils to be used in solid fat content determinations.