Download radiation resistance

Radiation Protection of Foods



Radiation may be defined as the
 “emission and propagation of energy through space or
through a material medium”.
Because they destroy microorganisms without appreciably
raising the temperature, the process is termed “cold
sterilization”.
APPLICATION OF RADIATION
 The two most widely used techniques of irradiating
foods are:
 Gamma radiation (from either 60Co or 137Cs)
 use of electron beams from linear accelerators.






Gamma Radiation
These are electromagnetic radiations emitted from the
excited nucleus of elements such as 60Co and 137Cs.
This is the cheapest form of radiation for food preservation,
because the source elements are either byproducts of
atomic fission or atomic waste products.
Gamma rays have excellent penetration power.
60Co has a half-life of about 5 years; and the half-life for
137Cs is about 30 years.
Electron Beams/Accelerated Electrons
 The use of electron accelerators offers certain
advantages over radioactive elements that
make this form of radiation somewhat more
attractive to potential commercial users.


X-ray is expensive and not usually used.




Some other sources of radiation:
Microwaves
When electrically neutral foods are placed in an
electromagnetic field, the charged asymmetric molecules
are driven first one-way and then another.
As the molecules oscillate about their axes while
attempting to go to the proper positive and negative poles,
intermolecular friction is created and manifested as a
heating effect.






Ultraviolet Light
Ultraviolet (UV) light is a powerful bactericidal agent, with
the most effective wavelength being about 2,600 A˚ .
The mechanism of UV death in the bacterial cell is the
production of lethal mutations as a result of action on cell
nucleic acids.
The poor penetrative capacities of UV light limit its food use
only to surface applications.
Also, it may catalyze oxidative changes that lead to
rancidity and discolorations.
Small quantities of ozone may also be produced when UV
light is used for the surface treatment of certain foods.
PRINCIPLES UNDERLYING THE
MICROORGANISMS BY IRRADIATION
1-Types of Organisms
 Resistance to radiation (generally):




DESTRUCTION
OF
“Radio-resistance of microorganism generally parallels
heat- resistance among bacteria”.
Most sensitive to radiations are:
 Pseudomonads
 Flavobacters
.


Among spore formers, Paenibacillus larvae seem to
possess a higher degree of resistance than most
other aerobic spore formers.
Spores of Clostridium botulinum type A appear to
be the most resistant of all clostridial spores
The most radiation resistant spore-formers
noted are:
 Geobacillus stearothermophilus
 Clostridium sporogenes


The most resistant non-sporeformers are:
 Enterococcus faecium
 Alcaligenes spp.,
2- Numbers of Organisms
 The numbers of organisms have the same effect on the
efficacy of radiations as is the case for heat, chemical
disinfection, and certain other phenomena:
“The larger the number of cells, the less effective is a given
dose”


3- Composition of Suspending solvent (Food)
Microorganisms in general are more sensitive to radiation
when suspended in buffer solutions than in proteincontaining media.
The presence of nitrites tends to make bacterial
endospores more sensitive to radiation.

4- Presence or Absence of Oxygen
The radiation resistance of microorganisms is greater in
the absence of oxygen than in its presence.


5- Physical State of Food
The radiation resistance of dried cells is, in general,
considerably higher than that for moist cells.
This is most likely a direct consequence of the radiolysis of
water by ionizing radiations.


6- Age of Organisms
Bacteria tend to be most resistant to radiation in the lag
phase just prior to active cell division.
The cells become more radiation sensitive as they enter
and progress through the log phase and reach their
minimum at the end of this phase.





PROCESSING OF FOODS FOR IRRADIATION
Blanching or Heat Treatment
Sterilizing doses of radiation are insufficient to destroy the
natural enzymes of foods.
In order to avoid undesirable postirradiation changes, it is
necessary to destroy these enzymes.
The best method is a heat treatment—that is, the blanching
of vegetables and mild heat treatment of meats prior to
irradiation.








RADAPPERTIZATION, RADICIDATION, AND RADURIZATION
OF FOODS
1- Radappertization:
Radappertization is equivalent to radiation sterilization or
“commercial sterility,” as it is –understood in the canning
industry.
Typical levels of irradiation are 30–40 kGy.
The term was coined to honor N. Appert.
The effect of this treatment on endospores and exotoxins
of C. botulinum (Type A, B, E) is of obvious interest.
Type E spores are the most radiation sensitive of these
three types.
Type A spores are the most radiation resistant of these
three types.
“Resistance increases at the colder temperatures
and decreases at warmer temperatures”
The use of a radiation 12D process for C.
botulinum in meat products would result in
the survival of virus particles unless
previously destroyed by other methods such
as heating.
 Enzymes are also highly resistant to
radiation, and a dose of 20–60 kGy has
been found to destroy only up to 75% of the
proteolytic activity of ground beef.






2-Radicidation:
Radicidation is equivalent to pasteurization—of milk, for
example.
Specifically, it refers to the reduction of the number of
viable specific non-spore-forming pathogens, other than
viruses, so that none is detectable by any standard
method.
Typical levels to achieve this process are 2.5–10 kGy.
A radiation dosage up to 7 kGy has been approved by the
World Health Organization (WHO) as being “unconditionally
safe for human consumption”





3- Radurization:
Radurization may be considered equivalent to
pasteurization.
It refers to the enhancement of the keeping quality of a
food by causing substantial reduction in the numbers of
viable specific spoilage microbes by radiation.
Common dose levels are 0.75–2.5 kGy for fresh meats,
poultry, seafood, fruits, vegetables, and cereal grains.
The ultimate spoilage of radurized, low-temperature-stored
foods is invariably caused by one or more of the
Acinetobacter-Moraxella or lactic acid types noted above.


In general, shelf-life extension is not as great for radurized
fruits as for meats and seafood because molds are
generally more resistant to irradiation than the Gramnegative bacteria that cause spoilage of the latter
products.
Insect eggs and larvae can be destroyed by 1kGy.



LEGAL STATUS OF FOOD IRRADIATION
In 1983, the FDA permitted spices and vegetable
seasonings to be irradiated up to 10 kGy In 1981, a joint
Food and Agriculture Organization (FAO)/International
Atomic Energy Agency (IAEA)/WHO Expert Committee on
food irradiation found that foods given an overall average
of up to 10.0 kGy were unconditionally safe.
It is considered an additive rather than a process, which it
is. This means that irradiated foods must be labeled as
such.


Another area of concern is the fate of C. botulinum
spores, and yet another is the concern that
nonpathogens may become pathogens or that the
virulence of pathogens may be increased after
exposure to subradappertization doses.
However, There is no evidence that the latter occurs.



The U.S. Centers for Disease Control and Prevention has
estimated that if only one-half of the ground beef, pork,
poultry, and processed luncheon meats in the U.S. were
irradiated, there would be over 880,000 fewer cases of
foodborne illness (Food Protection Trends, July 2003).
When low-acid foods are irradiated at doses that do not
effect the destruction of C. botulinum spores, legitimate
questions about the safety of such foods are raised,
especially when they are held under conditions that allow
for growth and toxin production.
Because these organisms would be destroyed by
radappertization, only products subjected to radicidation
and radurization are of concern here.




EFFECT OF IRRADIATION ON FOOD QUALITY
The undesirable changes that occur in certain irradiated
foods may be caused directly by irradiation or indirectly as
a result of post irradiation reactions.
1- Water :
Water undergoes radiolysis when irradiated in the following
manner:




In addition, free radicals are formed along the path of the
primary electron and react with each other as diffusion
occurs.
By irradiating under anaerobic conditions, off-flavors and
off-odors are minimized due to the lack of oxygen to form
peroxides.
One of the best ways to minimize off-flavors is to irradiate
at subfreezing temperatures.
The effect of subfreezing temperatures is to reduce
radiolysis and its consequent reactants.



2- proteins and other nitrogenous compounds :
Other than water, proteins and other nitrogenous
compounds appear to be the most sensitive to irradiation
effects in foods.
Among the most sensitive to irradiation are:
 Methionine
 Cysteine
 Histidine
 Arginine
 Tyrosine



3- Lipids:
Irradiation of lipids and fats results in the production of
oxidation products (such as carbonyls and peroxides),
especially if irradiation and/or subsequent storage take(s)
place in the presence of oxygen.
The most noticeable organoleptic effect of lipid irradiation
in air is the development of rancidity.


Fruits and vegetables:
One of the most serious detrimental effects is the
softening of these products caused by the irradiation–
degradation of pectin and cellulose, the structural
polysaccharide of plants.


STORAGE STABILITY OF IRRADIATED FOODS
Foods subjected to doses of ionizing radiation may be
expected to be as shelf stable as commercially heatsterilized foods.
 There are, however, two differences between foods
processed by radappertization and heatappertization
that affect storage stability:
1) Radappertization does not destroy enzymes
2) Some postirradiation changes may be expected to
occur.






NATURE OF RADIATION RESISTANCE OF MICROORGANISMS
The most resistant of all known non–sporeforming bacteria
consist of:
four species of the genus Deinococcus (isolated from
meat, the hides of animals, water and feces)
one specie of Deinobacter (isolated from animal feces and
freshwater fish)
one specie of Rubrobacter (isolated from a radioactive hot
spring in Japan)
one specie Acinetobacter (isolated from cotton and soils)







Apparent Mechanisms of Resistance
1-The extreme resistance of deinococci to desiccation has been
observed and presumed to be related in some way to
radioresistance.
2- All are highly pigmented and contain various carotenoids, a
fact that suggests some relationship to radiation resistance.
3-DNA repair process and abundance of DNA.
4-The deinococci do not contain phosphatidylgylcerol or
diphosphatidylglycerol in their phospholipids but contain,
instead, phosphoglycolipids as the major component.
5- possession of an outer membrane (deinococci), unlike other
Gram-positive bacteria.
6-Among other unusual features of deinococci is their
possession of palmitoleate (16:1), which makes up about 60%
of the fatty acids in their envelope and about 25% of the total