Just What Is It ?
Astronauts prefer foods sterilized by ionizing radiation
to all other types of preserved foods because they look
and taste like the original fresh food. This process
preserves foods without need for refrigeration or freezing
since it destroys all microorganisms that can cause
spoilage. If astronauts can relish such foods made
especially for them, why can't other people have them too?!!
What is ionizing radiation? An atom is conceived as a
nucleus with a positive charge. Negatively charged electrons
revolve around the nucleus like satellites. The positive
charge of the nucleus balances the negative charge of the
electrons so that the atom is electrically neutral. If an
atom is bombarded by radiation having sufficient energy
to knock the electrons from orbit, the neutral atom is
converted to a positively charged particle called an "ion",
thus giving rise to the term "ionizing radiation".
Radiation preservation of foods is performed in two ways:
pasteurization - accomplished with low dosages, and
sterilization - with higher dosages. Food can be irradiated
by bombarding it with beta particles, developed from high
voltage electron generators or linear accelerators, or with
gamma rays from Cobalt-60 or Cesium-137 radioactive isotopes.
These isotopes are produced in nuclear reactors by neutron
bombardment.
Consumer activists who usually play the role of devil's
advocate do not approve of this method of food preservation
in spite of its many safeguards. Their criticisms must be
heard, however, in fairness to all concerned.
In order to understand their argument one must examine what
happens in irradiation. Radiation speeding through a food
ionizes some of the atoms in its path. Simultaneously, it
causes an alteration of some vital molecules in the
microorganisms resulting in their destruction. Incidentally,
the food does NOT become radioactive, and it suffers no
apparent harmful effects.
The same radiation that kills the harmful bacteria also
penetrates the product, breaking some of the atomic bonds
within. Electrons broken out of orbit create molecular
fragments which often combine with other fragments to create
so-called radiolytic particles. These are new chemical
compounds previously not present in the original product.
Most of these new particles are identifiable as known
non-injurious substances. Others are unique and unidentified.
It follows that these unidentified compounds, according to
the activists, may have unidentified effects on health. In
order to minimize the risk associated with these compounds,
guidelines have been established by the responsible scientific
and government agencies.
The radiation dose used in the processing of foods is expressed
in terms of the "rad" (the acronym for radiation absorbed dose). It
is defined as that quantity of ionizing radiation resulting in the
absorption of a measured amount of energy. The kilorad (1,000 rads
or Krad) and the megarad (1,000,000 rads or Mrad) are the practical
operating levels of the dose.
(Please click on image for detail. 25K)
The amount of radiation to be delivered depends upon
the food itself and the result desired. If the goal is
to prolong shelf life, the pasteurization dose is
measured in Krads. If for sterilization for long term
storage without refrigeration, the dose is in Mrads.
At doses lower than required for pasteurization, the
radiation performs other effective chores. It prevents
sprouting of onions and potatoes, disinfests insects in
grains and cereals, and inactivates the destructive
Mediterranean Fruit Fly in fruits.
Interest in radiation processing of food is worldwide.
Up to a third of the world food harvest is lost because
of spoilage by microorganisms and animal pests. This
interest goes back to 1966 when 200 scientists from 25
countries attended an International Symposium in Karlsruhe,
Germany. The scientists discussed radiation sources, the
wholesomeness of irradiated food, chemical and physical
effects of radiation, status of various commodities such
as meat, poultry, seafood, grain, fruits and vegetables.
Also discussed were programs in operation, economics, and
national legislation for clearance of irradiated items.
The mechanism to deliver radiation is called a food
irradiator, quite obviously. It is essentially a deep tank
filled with water to shield operating personnel from
radiation. Th radiation source is placed at the bottom
between two plates. A control system lowers the food in
watertight containers to the source. Irradiation is done
automatically by presetting the length of time the food
is exposed.
Low pasteurizing doses of radiation lengthens the refrigerated
life of fresh fish and chicken for several days. Strawberries
treated this way, now being done in Florida, can last a month.
Hermetically sealed foods sterilized at Mrad doses can remain
on store or home shelves for years, just like canned foods.
Irradiation has lately been allowed on shipments of Hawaiian
papayas coming to the U.S. mainland. It is estimated that
Hawaii loses several million dollars a year in produce sales
because some products are barred from entry due to the
Mediterranean Fruit Fly infestation. For import quarantine
purposes, early in 1995 the Animal and Plant Inspection
Service approved irradiation of untreated papayas shipped in
sealed air cargo containers to Chicago and treated upon
arrival there. This opens the door for greater use of
irradiation as quarantine treatment for other producing nations.
Editor's Note: Paparella is a retired University Extension
Professor in Food Science and Technology, and a
practicing consultant.
Copyright © 1996 Michael W. Paparella
All Rights Reserved.
mpaparel@shore.intercom.net