Production Practices and Quality Assessment of Food Crops. Vol. 1

248 S. A. Ordoudi and M. Z. Tsimidou
fumigants like propylene oxide and methyl bromide are used, too (Sjöberg et al.,
1991). However, fumigation with microbicidal gases has several disadvantages
such as toxic residues, changes in the organoleptic properties of spices and health
hazards for the workers. On the other hand, heating is restricted to low temperatures
otherwise the content of the heat sensitive aroma compounds will be
dramatically reduced. An alternative to these methods of decontamination, widely
disputed for several years, is the treatment of spices with ionising energy. The process
of irradiation produces ions, free radicals and excited molecules in the food that
cause the desired effects. Conversely, in a microwave oven foods are exposed to
microwaves, a type of non-ionising radiation that generates heat by increasing the
molecular motion of the water molecules in moist foods. Therefore, when spices
are subjected to irradiation the decrease in their volatile content is rather low,
while none chemical residue is left. Legnani et al. (2001) have treated various
herbs and spices with microwaves and irradiation in their attempt to examine the
microbiological quality of these products. Comparing the two methods they found
that only irradiation (5–10 kGy) could eliminate the faecal indicators completely.
Irradiation is also believed to have an after-effect in dry spices as the number of
the viable cells is reduced even during post irradiation storage depending on the irradiation
dose (Sjöberg et al., 1991).
The process involves exposing food to a source of radiation such as a tightly
sealed metal container of radioactive elements – cobalt 60 or caesium 137 – that
produce gamma rays. The rays are directed onto the food being irradiated, with food
itself never being touched by the cobalt of caesium. Another type of radiation source
is an apparatus that produces X rays and high-energy electrons. Neither of these
sources has enough energy to make the irradiated foods radioactive.
Radiation decontamination of dried spices with doses of 8–10 kGy is regarded
as the most appropriate for killing bacteria and moulds. Irradiation at doses of
10 kGy or lower has been found to be capable of killing the most common foodborne
parasites but is unlikely to kill all bacterial spores unless the initial level of
contamination is low. Irradiation doses of up to 20 kGy may be required to achieve
‘sterility’, that is, a reduction of the total viable cell count to less than 10 per
gram in natural spices. Microorganisms that survive the low and medium dose
radiation treatment have lower resistance to environmental stresses or subsequent
processing treatments than the microflora of untreated spices (Sjöberg et al., 1991;
Farkas, 1998). The irradiation for decontaminating herbs and spices in the United
States is permitted by FDA since 1983. In other countries the situation varies. A
major compromise between producers and consumers concerns the packaging and
labelling status of the irradiated product and their exhibition on separate selves.
As regards to the legal status of food irradiation in the European Union two
Directives no2 and 3/1999 became applicable on 20 September 2000. The first
Directive set the framework for the introduction of this technology to food sector.
It covers general and technical aspects for carrying out the process, labelling and
conditions for authorising food irradiation. Then, in the second Directive, the first
category in the positive list of foods and food ingredients became the dried aromatic
herbs, spices and vegetable seasonings. The maximum overall average absorbed radiation
dose permitted is 10 kGy.