Pathogenic microorganism hazards with reduced oxygen packaging of ground beef
Although most refrigerated, uncooked beef is still packaged in traditional air-permeable, moisture-impermeable film on a polyfoam tray, more ground beef is being packaged at a central facility with different proportions of atmospheric gases (nitrogen, carbon dioxide, oxygen) to inhibit the growth of spoilage and pathogenic (disease-causing) microorganisms.
Since spoilage bacteria usually grow faster than pathogens, consumers have been able to use the spoilage indicators of odor and discoloration to determine the relative safety of refrigerated foods. However, several studies have indicated that pathogenic microorganisms may outgrow the spoilage organisms in atmospheres with little or no oxygen. This hazard may be greater if elevated temperatures occur during storage or retail display.
For this experiment, ground beef patties were packaged in two reduced oxygen types to determine the relative growth rates and types of microorganisms during refrigerated storage and display. Patties weighing 1/4 pound were formed from ground chuck and assigned to two packaging treatments: vacuum or modified atmosphere (MAP) containing 50 percent nitrogen and 50 percent carbon dioxide.
After 15 days of storage at 30 degrees F, the gas atmospheres in the MAP packages were exchanged for 80 percent oxygen and 20 percent carbon dioxide. The patties were removed from vacuum packaging and were overwrapped in polyvinylchloride film. Packages were displayed three days at 45 degrees or 60 degrees F under simulated retail conditions.
Samples were taken at zero, 8, 15, 16, 17 and 18 days after initial packaging to determine color, microbial numbers and types of microorganisms.
There was no difference in microbial numbers on patties in MAP or vacuum packaging for patties during the first 8 days of the initial storage period. However, the microbiological counts on patties stored in vacuum increased greatly from days 8 to 15. This was unexpected since a storage temperature of 30 degrees F close to the freezing point (28 degrees F) of beef usually inhibits microorganism growth, as was observed with the patties in MAP.
Microbiological growth increased upon retail display with both temperatures. The 45 degrees F temperature is not uncommon for meat display cases, although 41 degrees F or below is recommended.
Retail display of patties at the abusive temperature of 60 degrees F caused rapid growth of microorganisms in both types of packaging compared with display at 45 degrees F. The numbers of pathogenic bacterial species increased from 11 percent to 70 percent in MAP and from 24 percent to 50 percent in overwrapped packages displayed at 60 degrees F during the three days of display.
Discoloration of the bright red beef pigments to brown is often used as an indication of the advancing spoilage of ground beef. Patties in MAP with high levels of oxygen were darker and redder than patties stored in vacuum. Patties in MAP were redder than patties in air-permeable packaging through the first two days of retail display because of the higher levels of oxygen in the packages. The lightness of patties in MAP remained fairly constant through three days of retail display. Display at 60 degrees F decreased lightness of patties in air-permeable packages and decreased redness of patties in MAP compared with display at 45 degrees F.
The implications from this research are that reduced oxygen atmospheres will alter the microbial populations, while exerting minimal influences on color during storage and retail display. The increased percentage of pathogenic microorganisms with retail display in oxygenated conditions was increased even more with abusive temperatures during display. Processors and retailers should be vigilant in maintaining low temperatures in display cases to safeguard their refrigerated meat products for consumers.
Influence of display gas mixture on shelf life of ground beef in modified atmosphere packaging
Modified atmosphere packaging (MAP) of fresh meat products can improve the shelf life, reduce economic loss and improve product quality. The use of vacuum to remove oxygen prevents oxidative changes, but also causes meat to become a purple color, which is unappealing to consumers. Inclusion of carbon dioxide in MAP will inhibit microorganism growth and increase shelf life.
Ground beef is the major fresh beef product, accounting for about half of the total beef consumed in the United States. To determine the levels of gases for optimal display shelf life of ground beef, different gas atmospheres during storage were exchanged with different gases before retail display.
Ground beef patties were manufactured from chuck rolls by grinding muscles through 1/2-inch and 1/8-inch plates before forming into 1/4-pound square shapes on a mechanical former. Patties were assigned to packaging in vacuum or one of three types of modified atmosphere packaging (MAP): 80 percent nitrogen and 20 percent carbon dioxide, 50 percent nitrogen and 50 percent carbon dioxide or 20 percent nitrogen and 80 percent carbon dioxide for storage in the dark at 30 degrees F.
After 15 days, the vacuum patties were removed, placed onto polyfoam trays and overwrapped with air-permeable, moisture- impermeable polyvinylchloride film. The atmospheres in the MAP packages were exchanged for 80 percent oxygen and 20 percent carbon dioxide; 50 percent oxygen, 20 percent carbon dioxide and 30 percent nitrogen; or 20 percent oxygen, 20 percent carbon dioxide and 60 percent nitrogen. All packages were displayed under simulated retail conditions of 45 degrees F.
The storage gas mixtures of nitrogen and carbon dioxide had a large influence on the growth of psychrotrophic microorganism plate counts on patties in MAP during storage and retail display. The display gases of oxygen and carbon dioxide had little influence on microorganism growth during display. The psychrotrophic bacteria are those able to tolerate and grow in refrigerated temperatures. Many of the pathogenic microorganisms that cause foodborne disease are psychrotrophs. Increased levels of carbon dioxide inhibited psychrotrophic microorganisms until the atmospheres were exchanged for oxygen and carbon dioxide on day 15. Then the carbon dioxide in the gas mixtures was unable to impede the growth stimulated by oxygen. The vacuum atmosphere did not inhibit bacterial growth during the storage period.
Color of fresh refrigerated meat is the main criteria used by consumers to make their purchase decisions. The values that indicate lightness of color were not different among patties from the different storage or display packaging treatments. The redness of patties during storage was similar between vacuum and MAP treatments. After gas exchange for oxygen mixtures, the color of beef patties in MAP became redder. The patties from vacuum storage treatment that were exposed to air became redder (bloomed) slightly, but less than the patties in oxygen. The red color faded after 2 days of display under lighted retail conditions and was not acceptable for consumer purchase after 4 days of retail display. Patties in higher levels of oxygen had higher yellow values than patties in lower levels of oxygen during display. Following re-packaging in air-permeable film, patties stored in vacuum were less blue during the dark storage and had yellow values similar to the patties in 50 percent oxygen, 20 percent carbon dioxide and 30 percent nitrogen,.
Oxygen is a concern with refrigerated packaged fresh meat because it causes oxidation of the lipids and rancidity to develop. In this study, lipid oxidation increased during storage in vacuum or in nitrogen/carbon dioxide gases. The lipid rancidity increased greatly during exposure to light during retail display and was higher with increased levels of oxygen in the display gas mixtures.
This study investigated the use of different gas mixtures on the shelf life of ground beef patties during storage and retail display. Increased levels of carbon dioxide inhibited the growth of psychrotrophic microorganisms while increased levels of oxygen increased red color and lipid oxidation. The most desirable gas combinations for MAP of ground beef in this study appeared to be 50 percent nitrogen and 50 percent carbon dioxide exchanged for 50 percent oxygen, 20 percent carbon dioxide and 30 percent nitrogen for retail display. This MAP gas system resulted in ground beef patties that had low microbial growth during storage and minimum lipid rancidity and desirable red bloomed color during display compared to other treatment combinations.
(This article was published in the spring 2000 issue of Louisiana Agriculture.)