Sommers has also completed studies on the roles of food additives
in inhibiting growth of injured pathogens in food products during long-term
Food additives are commonly used to extend the shelf life of meat products.
Sommers says certain ones make Listeria more sensitive to radiation.
For instance, he determined that a mixture of salts of acetic acid (vinegar)
and lactic acid in bologna formulations decreased the radiation dose
needed to inactivate 99.999 percent of L. monocytogenes inoculated
onto the meat from 3.0 to 2.5 kiloGrays (kGy). The combined treatments
also prevented growth of spoilage microorganisms for 2 months. Ionizing
radiation, when combined with common food additives, has the potential
to significantly reduce the incidence of listeriosis associated with
consumption of RTE meats in the United States, Sommers says.
Fan and colleagues demonstrated that irradiation promoted production
of several volatile sulfur compounds associated with unpleasant odors
that can sometimes occur in irradiated or overcooked RTE turkey meat.
Adding antioxidants to RTE formulations had a very limited effect in
preventing production of the compounds. In additional work with Fan
and A.P. Handel from Drexel University in Philadelphia, Sommers found
that the combination of common additives and mild heating reduced levels
of sulfur compounds in RTE turkey meat by more than 50 percent compared
to irradiation alone.
Sommers collaborates with colleagues at ERRC in related research, including
irradiation of foodborne pathogens in fruits, vegetables, and juices.
Hes also examining ways to control other major pathogens, such
as E. coli, Salmonella, and Yersinia, in other
meat products. Yersinia infection most often occurs from eating
raw or undercooked pork products, such as chitterlings. Y. enterocolitica
is of great concern to researchers and the meat industry because, like
L. monocytogenes, it is capable of growing at refrigeration temperatures
and in high-salt environments.
To determine the radiation resistance of Y. enterocolitica in
different commercial practices, Sommers and co-researchers studied the
pathogen in ground pork at different temperatures. They were particularly
interested in the radiation dose needed to control Y. enterocolitica
in subfreezing temperatures. There are fewer undesirable effects on
some physical characteristics, such as texture and color, of meat products
at subfreezing temperatures than those irradiated at refrigeration temperatures
at the same dose. But it takes a larger dose to pasteurize frozen meat
than nonfrozen meat. They were able to determine the range of radiation
doses required to treat the same product at different temperaturesas
commercial processors do.
Research on Fruits and Vegetables Improves Safety and Quality
Brendan A. Niemira, a microbiologist and plant pathologist at ERRC,
focuses on methods to irradiate fruits, vegetables, juices, and meat
substitutes of vegetable origin, such as soy. He is interested in finding
the ideal dose of irradiation that increases safety but does not affect
a given products quality.
Since late 2002, government regulations have permitted use of irradiation
on imported fruits and vegetables to meet phytosanitary quarantine requirements,
which refers to controlling organisms that affect plant health. Higher
radiation doses could be used to inactivate foodborne pathogenic bacteria
as well. The National Food Processors Association has petitioned FDA
to allow the higher doses on fruits and vegetables. FDA is currently
evaluating the petition.
Grapes, for instance, have recently been suspected in Salmonella
outbreaks. Niemira and Fan tested red and white whole grapes and their
juices after purposely inoculating them with the pathogen. The textures
of both grapes were not significantly affected by irradiation doses
of up to 1 kGythe maximum dose approved for fresh fruits and vegetables.
Although the color of red grapes and red grape juice was influenced
by treatments, white grapes and white grape juice held up very well.
These results demonstrate that each product responds differently
to irradiation and has to be treated differently to ensure it remains
appealing to the consumer, Niemira says.
Niemira also recently studied the effectiveness of gamma irradiation
in inactivating Listeria and Salmonella placed on the
surface of lettuce. He studied four closely related but distinct types
of lettuce: Boston (butterhead), iceberg (crisphead), green leaf, and
red leaf (variants of looseleaf). The studies showed that the radiation
sensitivity of L. monocytogenes was similar on the four lettuce
types, but the sensitivity of Salmonella was slightly different.
Complexity of the leaf surface was the underlying factor. Although irradiation
was effective at reducing pathogen numbers, subtle differences between
lettuce types influenced the sensitivity of bacteria present on their
surfaces. Niemira says radiation doses would have to be tailored for
each type of lettuce to be effective.
The key to using irradiation is to eliminate foodborne pathogens without
affecting product quality. Fan wants to assess the extent of irradiations
effect on product quality and to develop novel ways to minimize any
adverse effects. He examines factors such as chemical composition, nutritional
qualities, aroma, flavor, and texture in a variety of foods.
Major quality changes in some vegetables appeared to be tissue browning
and decreased shelf life from loss of tissue integrity, which, in some
instances, gives a soggy appearance. Because fruits and vegetables contain
so much water, they are especially susceptible to the effects of irradiation
and other nonthermal treatments.
Fan found that irradiation and heat pasteurization of apple and orange
juice led to increased levels of several volatile sulfur compounds and
aldehydes. Studies have shown that compounds such as aldehydes might
contribute to off-odors in heated and irradiated foods. Fan believes
conducting the treatments at lower temperatures, adding antioxidants,
and combining irradiation with other treatments could help reduce undesirable
quality effects on juices.
Fan teamed up with collaborators to develop a method to treat sliced
apples used in fruit salads. They were interested in reducing tissue
browning, which happens when plant tissue is cut during processing.
They found that dipping apple slices into a calcium ascorbate (a form
of vitamin C) solution before exposure to a low-dose gamma radiation
treatment improved microbiological safety and inhibited tissue browning.
Fruits and vegetables are rich in antioxidants, which help protect
us against cancer and heart disease. These beneficial effects are partially
due to high amounts of phenolic compounds. Fresh and fresh-cut fruits
and vegetables are living organisms, capable of synthesizing novel antioxidants
in response to stresses even after they are picked and processed. Fan
showed that irradiated iceberg and romaine lettuce and endive developed
a higher antioxidant capacity than nonirradiated vegetables during a
postirradiation cold-storage period. The higher antioxidant capacity
was mostly due to the increased content of phenolic compounds. Vitamin
C content in the fresh-cut vegetables, however, was not significantly
affected by irradiation doses as high as 1 kGy.
In addition to the beneficial effect of foodborne pathogen inactivation,
irradiation of fresh-cut fruits and vegetables may result in a product
with enhanced antioxidant capacity, Fan says.By Jim
Core, Agricultural Research Service Information Staff.
This research is part of Food Safety (Animal and Plant Products),
an ARS National Program (#108) described on the World Wide Web at www.nps.ars.usda.gov.
Christopher H. Sommers,
Brendan A. Niemira,
and Xuetong Fan are with
the USDA-ARS Food Safety Intervention Technologies Research Unit, Eastern
Regional Research Center, 600 East Mermaid Lane, Wyndmoor, PA 19038;
phone (215) 836-3754 [Sommers], (215) 836-3784 [Niemera], (215) 836-3785
[Fan], fax (215) 233-6445.
"ARS Continues Advances in Irradiation of Ready-To-Eat Foods"
was published in the February
2005 issue of Agricultural Research magazine.