Thermal Food Processing

Infrared Heating 521
and thawing time when the lowest temperature of the sample reached –4°C. The
ambient temperature was maintained at 7°C during thawing. In Figure 16.25 the
results are also shown for natural thawing, in which heat was supplied by natural
convection. As can be seen in Figure 16.25, a 17.5-mm-thick sample was thawed
within 30 min using FIR thawing, four times faster than natural thawing. Therefore,
it is effective to use an auxiliary heater to heat from the bottom of the food, when
the thicker food is thawed. The simulated correlation between food thickness and
thawing time using an auxiliary bottom heater is shown in Figure 16.25 as a solid
line. In this case, the heat flux of the auxiliary heater was assumed to be almost
equal to that of the FIR heater. As can be seen, the use of the auxiliary heater increases
the thawing speed and enables 30-mm-thick tuna to be thawed within 30 min.
Thawing equipment has been developed for frozen foods 56,57 and for frozen
small packs of prepared meals. 58 The equipment can maintain the temperature at
60 or 100°C after thawing and is expected to be used in office kitchens and fastfood
restaurants. Ohashi 59 reported on a refrigerator with a partial defrosting system,
which consists of heating by an FIR heater and blowing with cold air. This system
also keeps raw foods such as tuna at a partial freezing temperature (approximately
–3°C) after thawing. The thawing time of beef (400 g) or tuna (200 g) in the device
is less than 30 min, three times faster than thawing at room temperature.
16.5 CONCLUSIONS
Infrared heating applications are expected to grow as a result of the increasing
demand for safe food products of high nutritional value and organoleptic quality.
There are many examples that IR heating is superior to conventional heating.
However, there are few theoretical explanations of the IR effects; therefore, there
is a need for the basic research on mechanisms of energy transfer and its effect
on changes in taste and nutritional components in the food. Furthermore, in order
to provide an improved basis for infrared heating process design, the practical
development of quantitative models is needed to predict temperature and product
quality as a function of temperature. Finally, innovations of infrared heating
equipment combined with microwave or ohmic heating will lead to greater energy
efficiency and higher-quality products in the food industry.
NOMENCLATURE
c Velocity of light (m/sec)
E Emissive power (W/m2 )
F 21
Radiation shape factor
h Plank’s constant (m·K)
I l
Energy flux (W/m2 )
k Boltzmann’s constant (J/K)
Q Interheat generation (J/sec)
T Absolute temperature (K)