As we are able to approximate the percentage enthalpy
residual in a food product during freezing with
the percentage of the food frozen, it is instructive to consider techniques developed to study the ‘‘freezing’’
(i.e., extent of crystallization) of lipid products. Dilatometry
is a well-established method to measure an index
of the solid fat content through the density change
on solidification. It is very precise, though laborious, for
fats, but would be unsuitable for aqueous systems as the
volume expansion on freezing would crack the glass.
Furthermore, dilatometric methods cannot be applied
on-line. Various nuclear magnetic resonance (NMR)
methods can measure lipid crystallization as the nuclei
in solid and liquid phases have different relaxation times
(McClements & Povey, 1988; Miles, Fursey, & Jones,
1985). Provided metal can be excluded and the product
held stationary for the several seconds required to make
a measurement, NMR can (in principle) be used as an
on-line solid fat content sensor. By analogy, it should be
possible to build an on-line freezer sensor based on
NMR and indeed such a device has been brought to
practice (McCarthy & Reid, 1997). Ultrasound has also
been used for solid fat content determination and may
be appropriate for the study of water freezing.