greatly reduces overcook of the food
that is adjacent to the container surfaces.
Unfortunately, many of our most
popular canned foods do not benefit
from aseptic processes or agitated
cooks because they are not sufficiently
fluid to permit convective heat transfer,
either in a heat exchanger or a can.
Tuna, ham, salmon, pork and beans,
corned beef and canned pumpkin, to
name but a few foods, heat within
the can largely or entirely by conduction.
Thermal processes for these
types of products have remained almost
unchanged for 50years or more,
although the containers and the retorts
may have changed greatly. Because
the heating rate into the product cannot
be substantially accelerated, many
have concluded that little potential for
improvement exists. Nonetheless, if a
process can improve the uniformity of
the heat process within the can, it may
be possible to improve product quality,
reduce energy consumption and
even reduce process time: VRT processing provides a
promising approach to this problem.
VRT for volume-average quality
Any retort process in which the environment temperature
within the retort is modulated during the process,
according to a predetermined temperature sequence, to
alter the heating profile within the product may be described
as a VRT process (Fig. 1). The terms ‘timevariable’
or ‘variable-temperature’ process have also been
used and are equivalent. It should be noted that although
the heating rate of the CRT can centre is greater early in
the process, the overall heating rate may be greater in
the VRT process, owing to a higher final retort temperature.
The VRT approach was not seriously considered in
the earlier days of canning research, in part no doubt
because the processes were cumbersome and unreliable
when retort operation was strictly manual. Also, as we
will see, VRT processes are difficult to study without
the aid of computer simulations of heat transfer.
Teixeira et ~1.‘~ first published a model of heat
transfer within a can, based on the numerical finite difference
technique; the model was used to examine CRT
processes. Even before the advent of fast personal computers,
computer simulations based on numerical calculation
methods had several very useful features for
thermal processing authorities engaged in the study of
canned foods. A properly prepared and verified model
can be used to carry out experiments when access to a
retort is restricted. Furthermore, such experiments are
quick, cheap and exactly reproducible. Also, the numerical
model allows calculation of the temperature history
of any spot or series of spots within the canned food.
Traditional CRT processes are based on measured temperature
histories of one spot within the food, usually