Establishing optimal parameters to mitigate the heating effects caused by CPMAS sequence in 13C solid-state NMR studies of cocoa butter and other fat samples

Abstract

The physical and chemical properties of solid triacylglycerols (fats) have been investigated through the use of ¹³C solid-state nuclear magnetic resonance (SS-NMR). However, the ¹³C SS-NMR experiments performed with cross-polarization (CP), magic angle sample spinning (MAS) and high power for ¹H decoupling (DEC), known as CPMAS, have the potential to increase sample temperature, which may disrupt the solid–liquid content, crystallization dynamics, and polymorphism of the fat samples. While the heating effects observed in CPMAS experiments have been well documented, they have not yet been studied in fat samples. Accordingly, the present study examines the influence of sample spinning frequencies (SF) and radio frequency irradiation (RFI) due to DEC, on sample heating using cocoa butter (CB), chocolate, and animal fats. The results of the variation in the ¹H chemical shift of the water peak in the butter sample indicate that the use of routine CPMAS parameter results in an increase in sample temperature of up to 15 °C. This temperature is sufficient to affect various physicochemical properties of fats, including partial or total melting when the experiments are performed with an air stream at ambient temperature. The results demonstrate that a SF of approximately 3 kHz and a recycle delay exceeding 10 s for a decoupler power of 70 W for 50 ms result in an increase of less than 1 °C in sample temperature in ¹³C CPMAS experiments. These experimental conditions were successfully employed to study the CB crystallization process, which exhibited the presence of the α form at the beginning of the process and the β form at its conclusion.