Mobility of solutes in frozen pork studied by electron spin resonance spectroscopy: evidence for two phase transition temperatures

The mobility of solutes in frozen pork (longissimus dorsi) was studied by X band electron spin resonance spectroscopy (ESR) using the three water soluble spin probes TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl), CAT 1 (4-trimethylammonium-2,2,6,6-tetramethylpiperidine-1-oxyl) and TEMPO choline (4-(N,N-dimethyl-N-(2-hydroxyethyl))ammonium-2,2,6,6-tetramethylpiperidine-1-oxyl). The mobility was quantified as the observed hyperfine coupling constant A′zz. The three spin probes, of which only the neutral TEMPOL may penetrate membranes, gave very similar values of A′zz as a function of the temperature. A′zz was constant at temperatures below −55 °C, whereas A′zz decreased as the temperature was increased above −55 °C. This behavior has been interpreted as an increase in the amplitude of the rotational oscillations of the spin probes (and other solutes) with temperature. Above −13 °C the spin probes gave isotropic ESR spectra indicating free rotational mobility in a molten liquid phase. These results are discussed in relation to glass transitions in frozen pork.