Abstract
ESR has been used to study multi-phase food systems, including ice cream and chocolate, using both spin probes and spin labels. Chemical and physical methodologies were developed and applied to study the behaviour of 1,1,3,3-tetramethylisoindolin-2-yloxyl (TMIO), the 5-pentyl derivative (PTMIO) and the water-soluble 5-sulphonate (NaTMIOS) spin probes in hydrophilic and hydrophobic phases over a range of temperatures. Linewidths and hyperfine coupling constants (aN) were derived using fitting and simulating computer software, from which rotational correlation times (tauc) and enthalpies of activation of molecular rotation were calculated. These give an indication of the local environment of the probe. In the hydrophilic phase of ice cream there was a sharp reduction in probe mobility at -18°C, which did not occur in the hydrophobic phase. In chocolate, an essentially hydrophobic system, no sudden changes in mobility were detected although at around 60°C there was a change in the enthalpy of activation. Spectral deconvolution techniques enabled the simultaneous study of NaTMIOS and TMIO in the hydrophilic and hydrophobic phases of ice cream and enabled the study of TMIO spectra in the presence of the underlying spectrum of the radical present in chocolate. Guar gum and xanthan gum are used as stabilisers in ice cream and were spin-labelled using 5-carboxy-TMIO (CTMIO) and 5-amino-TEMPO respectively. The ESR spectra showed that the mobilities of the nitroxide moieties were comparable with the free spin probes. The addition of the gums to a sucrose solution, a model hydrophilic system, had no appreciable effect on the mobility of the NaTMIOS spin probe.