application of alternative food-preservation - Bentham Science

Non-Thermal Approaches Application of Alternative Food-Preservation Technologies 145
In order to assess the efficacy of this technology, some parameters should be taken into account:
1. Dp = Microwave power penetration depth
(eq. 1)
where Dp is in centimetres, f is in GHz and ε’ is the dielectric constant, whilst ε” is the dielectric loss factor (a
material property called the “dielectric property”, representing the material capacity to absorb microwaves).
Simply, the higher the frequency, the less is the depth of penetration; ε’ and ε” can be dependent on both the
frequency (f) and the temperature. This has practical consequences for batch processes [5].
1. Q = Rate of microwave heat generation per unit of volume at a particular location within the
foodstuff during the microwave irradiation process
(eq. 2)
where E is the strength of the electric field of the wave at the location, f is the frequency of the wave (generally
2450 MHz), ε0 is the permittivity of free space (a physical constant), and ε’’ is the dielectric loss factor [6]. It can
be seen from this formula that the temperature could be increased by choosing a higher frequency for the
microwave, a higher relative dielectric constant or through a larger loss factor; even if only certain frequencies
are permitted for microwave heating in order to prevent interference in radio traffic. This formula also shows
that air pockets in the foodstuff, which may be inevitable or are necessary for a good sensory quality of the
product, reduce the ability of food to be heated in the microwave field [2].
There is another dielectric property, called the dielectric constant, εr, which affects the strength of the electric
field inside food product; the Table 1 shows the relative dielectric constants of different materials. The dielectric
property depends on the composition of the food product, with moisture and salt being the most significant
determinants. The temperature increase in food depends on the duration of heating, the location of the food in the
reactor, the convective heat transfer at the surface and the extent of evaporation of the water inside the food and
at its surface [7].
Table 1: Relative dielectric constant of different foods-materials
Material Relative ε”
Water (0°C) 88
Water (20°C) 81
Ice (-20°C) 16
Ice (0°C) 3
Olive oil 3.01
Air 100.059
PASTEURIZING AND STERILIZING WITH THE MICROWAVE
Thermal pasteurization and sterilization are predominantly used in the food industry for their efficacy and
product safety record; however, excessive heat treatment may cause undesirable protein denaturation, nonenzymatic
browning and loss of vitamins and volatile flavour compounds. Advances in technology allowed
optimization of thermal processing for maximum efficacy against microbial contaminants and minimum
deterioration of food quality [1].
Another advantage to pasteurizing with microwave is due to the possibility of sterilizing packed foods, acting at
the same time on both foodstuffs and packaging. Nowadays, a new field in MW processing is the combination of
two microwave frequencies, used successfully to extend the shelf-life of packed sour milk products; in this batch
process, the whole product is heated with 10-30 MHz while the surface is treated with 2450 MHz.
To avoid hot and cold spots, which imply sensory and quality defects in foods, microwave long-term treatment
was tested with defined standing times. Even various foodstuff with different container shapes can be heated by
computerized processing units with a microwave hybrid system [2].