6 hours

Figure 4. Sodium MRI illustrates diffusion of sodium into a milk protein gel as a model of brining in cheese manufacture. A proton image (left) shows the form of the gel, which lies in an overturned beaker, and the 1% (w/v) sodium chloride/water standard, in a 1 cm cuvette. In this experiment, water saturated with sodium chloride was poured over the gel in the upright beaker. After durations of 2 and 6 hours, the salt water was decanted and the standard was replaced for sodium imaging. The lower MR-sensitivity and abundance of sodium results in much smaller signal-to-noise ratios compared to the proton image.


where d is the droplet diameter, g is acceleration due to gravity, T| is the viscosity of the continuous phase, and Ap the difference in phase densities. Creaming rates of milkfat emulsions proved much slower than predicted on the basis of particle sizes determined by a laser diffraction particle-size analyser.

Another method for non-invasive measurement of oil/water ratios in emulsions exploits the chemical shift phenomenon. Deliberate non-refocusing of dephased signals during a spin-echo imaging experiment permits spatial mapping of chemical shift (Majors et al., 1990). Incrementing the second, refocusing period by a small duration (another method of phase encoding) produces a dataset composed of two superimposed frequencies resolvable by Fourier transformation. An emulsion of hexadecane in water thus appears as two bands of intensity, proportional to the content of each component; chemical shift on one axis, positional information on the other (Figure 5).

In addition to volume fraction determination, MRI diffusion techniques allow calculation of particle size distribution, important in resulting texture and emulsion stability. The same principles and techniques apply to discerning emulsion particle size as to cheese oil droplet size, discussed in the diffusion section.

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