sealPurdue News

May 22, 2002

Post-packaging process halts deadly food contaminant

WEST LAFAYETTE, Ind. – Purdue University scientists have developed a post-packaging pasteurization process for ready-to-eat meats that eliminates contamination by the deadliest of the bacterial food contaminants.

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The scientists used sliced bologna tainted with Listeria monocytogenes, packaged it in vacuum-sealed plastic bags, and then submerged the packages in hot water, said lead researcher Tim Haley of Purdue's Center for Food Safety Engineering. Next, they immediately placed the bagged luncheon meat in cold water. This two-step procedure killed the microbes and also apparently extended the meat's shelf life.

The pasteurization method is called a high-temperature-short-time process. Although bologna and Listeria were used for this study, the process could be applied to similar deli meats and could eliminate other pathogens. Haley said the method possibly could be part of a national biosecurity system to protect against deliberate attempts to cause illness through food contamination. Haley will present his findings at the Institute of Food Technologists' annual national meeting June 15-19 in Anaheim, Calif.

"The problem with ready-to-eat meats, including luncheon meat, hot dogs and deli meat, is that prior to final packaging, Listeria still can contaminate it," said Haley, an assistant professor in the Department of Food Science and director of the Computer Integrated Food Manufacturing Center. "This can happen if the bacteria are present in the air, on the equipment or in the water in the processing plant. If the food handlers have been exposed to Listeria, they can spread it even if they are wearing gloves."

Researchers have focused on Listeria because as few as 10 of the bacteria cells can cause illness and, though it is a relatively rare biological contaminant, its fatality rate of 20 percent is the highest of the food pathogens, according to the Centers for Disease Control and Prevention.

Haley and his team began investigating pasteurization as a possible way to eliminate food pathogens from ready-to-eat meats because similar processes have been used for other foods, and they also wanted something food manufacturers could easily and quickly implement.

The research enabled the scientists to calculate the temperature and time needed to kill as many as 1 million of the organisms on one slice of bologna, Haley said. According to this model, the pasteurization process would require 85-degree Celsius (185 F) water for 20 seconds in such a scenario. If two slices were pasteurized, the length of hot water immersion would be increased to 60 seconds, and for four slices, 180 seconds. This would be followed by a 4-degree Celsius (39 F) water bath for the same amount of time as in the hot water.

However, the scientists report that it's not practical to do more than two slices in a package using the method. This is because the amount of time needed to pasteurize more than two slices would cause too much quality degradation.

But with just two slices in a package, the process didn't harm the quality of the bologna, Haley said. The researchers used both low-fat and regular bologna that can be purchased in retail food stores. His research team will study ways to modify the process to accommodate meats with higher fat content.

In addition, having one or two slices wrapped together would be a convenience to consumers, in much the same way as individually wrapped cheese slices, Haley said.

The process also appears to extend the product's shelf life. Some of the post-packaging pasteurized meat from the study has been refrigerated since December 2001 and, after five months, its quality is comparable to fresh packed bologna, Haley said.

"This approach is similar to the pasteurization method used for canned food and milk, so it should be acceptable to consumers as a safeguard against bacterial contamination," he said. "It will be easy for food processors to implement. This type of pasteurization also will allow pregnant women peace of mind."

Pregnant women and people with weakened immune systems are particularly vulnerable to Listeria-caused illness and death, according to the CDC. The death rate in immuno-compromised populations is between 34 percent and 70 percent. Compared to other healthy adults, pregnant women are 20 times more likely to develop listeriosis, the disease caused by the bacteria.

The bacteria are additionally dangerous because they can survive refrigeration and also can be found in raw meat, milk and cheese. The Clinton administration issued a "no tolerance" policy for Listeria , which means that if one organism is found on one piece of food, the whole batch must be discarded and/or recalled from stores, warehouses and consumers' shelves.

A 1999 multistate exposure to the bacteria caused 21 deaths, five miscarriages and the recall of more than 15 million pounds of frankfurters, according to the CDC. Last month, John Morrell & Co. of Cincinnati recalled 140,000 pounds of frankfurters and hot dogs that possibly were Listeria tainted, the U.S. Department of Agriculture Food Safety Service reported.

The other researchers involved in the study are Osvaldo Campanella, associate professor in the Department of Agricultural and Biological Engineering; Arun Bhunia, associate professor in the Department of Food Science; and David Gerrard, professor in the Department of Animal Sciences.

Purdue's Center for Food Safety Engineering includes nearly 90 university scientists studying ways to more quickly and exactly detect and protect against biological and chemical contaminants of food. They are collaborating with scientists from the USDA's Agricultural Research Service and other agencies.

The USDA Agricultural Research Service provided funding for this study.

Writer: Susan A. Steeves, (765) 496-7481,

Source: Tim Haley, (765) 494-9093,

Ag Communications: (765) 494-2722; Beth Forbes,;

Related Web sites:
USDA Food Safety and Inspection Service

Purdue researcher Tim Haley has developed a two-step pasteurization process that eliminates the food contaminating bacteria Listeria. Haley's process of dipping vacuum-sealed packets of bologna in separate baths of hot and cold water would make deli meats safer and possibly extend shelf life. (Agricultural Communication photo by Tom Campbell)

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