Thermal Food Processing

498 Thermal Food Processing: New Technologies and Quality Issues
O
H H
λ = 2.73 µm
(υ = 3657 cm –1 )
FIGURE 16.4 Normal vibration of water.
H H
When radiant electromagnetic energy impinges upon a food surface, it may
induce changes in the electronic, vibrational, and rotational states of atoms and
molecules. The kinds of mechanisms for energy absorption are determined by
the wavelength range of the incident radiative energy: 2 changes in the electronic
state correspond to wavelengths in the range of 0.2 to 0.7 µm (ultraviolet and
visible light), changes in the vibrational state correspond to wavelengths in the
range of 2.5 to 100 µm (FIR), and changes in the rotational state correspond to
wavelengths above 100 µm (microwaves).
Generally, food has been composed of water and organic compounds such as
carbohydrates, proteins, and fat, and the infrared absorption characteristic of those
materials is important. The normal vibration of water is represented as follows: 2 (1)
the symmetrical stretching vibration, (2) the antisymmetric stretching vibration, and
(3) the symmetrical deformation vibration, as shown in Figure 16.4. These vibrational
frequencies, namely, the wave number, are shown in the figure. The IR energy
in proportion to these frequencies is efficiently absorbed in a body. Therefore, food
absorbs the IR energy at wavelengths greater than 2.5 µm most efficiently through
the mechanism of changes in molecular vibration state, which can lead to heating.
16.3 ATTENUATION FACTOR AND PERMEABILITY
The penetration of infrared energy into the food is an important factor to discuss
with regard to infrared heating.
The incident energy would be partly reflected and transmitted at the air–food
boundary. The energy transmitted into the food is attenuated exponentially with
penetration distance. The attenuation factor determines the energy absorption
within the food as a function of depth from the surface of the food, as described
by Lambert’s law:
(16.7)
where I l is the energy flux at the wavelength of l and a l is the spectral attenuation
factor. The attenuation factor of water, which is shown in Figure 16.5, changes
remarkably with wavelength. For example,a l =0.355, 10800, and 2138 cm –1
when l =1, 3, and 6 µm, 5 respectively.
O
λ = 6.27 µm
(υ = 1595 cm –1 )
O
H H
λ = 2.66 µm
(υ = 3756 cm –1 )
(a) (b) (c)
I I x α
= exp( − )
λ λ0 λ