Crystallisation And Freezing

Decrease in mobility during freezing or crystallisation greatly attenuates MR signal amplitude, providing a means of following these related processes. Intensity of the MR signal depends on the relaxation rate of the interrogated nuclei; the very fast relaxation rates found in solids prevent observation by use of liquid MR techniques. This inability to observe nuclei in a solid matrix presents a method for observation of kinetics of the liquid-solid phase transition. Disappearance of signal from a volume containing liquid may signify crystallisation or amorphous glass formation. Observation of the movement of the intensity interface during solidification has provided kinetics information for crystallisation of fat/water emulsions and freezing of meats and vegetables.

2.1. Crystallisation

Magnetic resonance imaging has enabled the direct observation of crystallisation kinetics of fats in bulk or in fat/water emulsions containing one or more species of triglyceride. Initially, images and spatially-localized spectra acquired during crystallisation of warm (60-80° C) trilaurin:water and trimyristin:water (2:3) emulsions during cooling to 20° C validated the method (Simoneau, et al., 1991). Later, the same imaging techniques illustrated the slowing of ciystallisation kinetics resulting from "poisoning" of the emulsion with a mixture of fats. Calorimetry data permitted correlation of enthalpy with the crystallisation kinetics (Simoneau, et al., 1992). Images acquired during the crystallisation of bulk lipids and emulsions during cooling resemble those acquired during freezing; the decrease in mobility merely occurs in a different material at a lower temperature.

2.2. Freezing

Freezing, whether involving crystallisation or amorphous glass formation, decreases molecular mobility, thereby increasing relaxation rates and attenuating the MR signal. Movement of the freezing interface has been observed in food samples such as beef, chicken, potatoes, peas and corn (Figure 3). In meat and potato samples, imbedded thermocouples, visible in the images, permitted simultaneous measurement of temperature and freezing front progression (McCarthy, et al., 1993; Ozilgen, et al., 1993). Magnetic resonance observations of extent of freezing have also been correlated with enthalpy using calorimetry. These studies indicated that use of MRI for monitoring enthalpy of products leaving the freezer would allow adjustment of freezer temperature to quickly reduce enthalpy of frozen products to their storage enthalpy. The authors calculate that on-line monitoring of enthalpy to avoid under- or overcooling before storage could save 17% in total freezing energy expenditures (Ozilgen, et al., 1993). Lack of molecular mobility, leading to attenuation of the MR signal, confers an advantage in freezing applications.

Figure 3. Thermocouples (black dots) imbedded in a potato allow correlation of freezing interface position with temperature during freezing at -30° C within the magnet of an MRI spectrometer. Air velocity was 2.1 m/s. Calorimetry measurements accompanied identical freezing experiments without thermocouples to correlate freezing front progression with enthalpy.

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