Z. L. Carpenter

Attachment of Microorganisms to Pork Skin and Surfaces of Beef and Lamb Carcasses

A model system was developed to study attachment to and possibly detachment of bacteria from pork skin and thin-surface slices of beef and lamb carcasses. The technique involves embedding pork skin and beef and lamb surfaces in solidified wax with the skin surface exposed. After exposure of the skin or carcass surface to bacterial suspensions and subsequent manipulations, the sample is removed aseptically from the wax and subjected to agar plate counting methods. A direct relationship existed between bacterial counts of the skin or carcass surface and concentration of bacterial cells in the attachment medium. Much of the bacterial attachment occurred during the first minute of immersion in the attachment medium, although in some instances continued attachment occurred over a 30-min period. Gram-negative motile bacteria showed greater attachment than did gram-positive non-motile species. Temperature and pH of the attachment medium had little effect on the extent of bacterial attachment.

Microbiology of Beef Packaged in Various Gas Atmospheres

Boneless beef roasts ( longissimus muscles) were vacuum-packaged and then the bags were injected with one of six gas mixtures: 100% O2, 20% CO2 +80% N2, 50% CO2 + 50% O2, 20% CO2 +80% O2, 25% CO2 + 25% O2 +50% N2 or 51% CO2 + 30% O2 + 18% N2 + 1% CO. One group of roasts, vacuum-packaged without added gas served as controls. Roasts were stored for 0-35 days at 1-3 C. At five weekly intervals, steaks were removed from roasts in each treatment and examined after storage for 5 days under retail display conditions. Psychrotrophic plate counts of roasts stored in modified gas atmospheres were usually higher than those stored in conventional vacuum packages. Differences in lactobacillus counts between roasts stored in modified gas atmospheres and those stored in vacuum packages rarely were statistically significant. Counts of retail steaks prepared from roasts stored in various gas atmospheres were usually slightly higher than those prepared from comparable vacuum packaged roasts. In most instances these differences were not statistically significant. Initially, the microbial flora of vacuum-packaged beef roasts consisted primarily of Moraxella-Acinetobacter and Pseudomonas spp. Lactobacilli predominated on roasts at later storage intervals even on roasts stored in atmospheres initially containing 100% O2 or 20% CO2 + 80% O2. Pseudomonas spp. remained a substantial part of the microflora of roasts stored in high O2 containing atmospheres.

Appearance of Beef, Pork and Lamb Stored in Vacuum or Modified Gas Atmospheres

Beef, pork and lamb loins were vacuum-packaged as subprimal cuts, vacuum-packaged as reformed subprimal cuts, vacuum-packaged as retail cuts or packaged in modified gas atmospheres as retail cuts. Cuts were stored and evaluated, after 0, 7, 14 or 21 days of storage, for surface discoloration and overall appearance. Gas analysis of the heads pace of packages was also obtained. In all vacuum packaging treatments, the percentage of CO2, increased and the percentage of O2, decreased with increased storage. The gas composition in modified gas atmosphere packages changed little during the 21-day study. Using systems and gas combinations involved in the present study, retail cuts of beef, pork and lamb could not be successfully precut, retail-packaged and stored for 7 to 21 days in vacuum or modified gas atmospheres. Vacuum-packaged retail cuts and retail cuts stored in modified gas atmospheres sustained extensive surface discoloration and were of unsatisfactory appearance, even after only 7 days of storage and 1 day of retail display. Conversely, storage of retail cuts as vacuum-packaged reformed subprimal cuts for 7 to 21 days decreased (P < .05) surface discoloration and improved (P < .05) overall appearance in comparison to retail cuts from loins stored as subprimals for comparable times in vacuum packages.

Physical and Sensory Characteristics of Beef Packaged in Modified Gas Atmospheres

One hundred and ninety-six boneless beef roasts ( longissimus muscles) were vacuum packaged. Twenty-eight roasts remained vacuum packaged to serve as controls while the remaining packages were injected with one of six gas mixtures: (a) 100% O2, (b) 20% CO2 + 80% N2, (c) 50% CO2 + 50%O2, (d) 20% CO2 + 80% O2, (e) 25% CO2 + 25% O2 +50% N2 or(f) 51% CO2 + 30% O2 + 18% N2 + 1% CO. Five cuts from each packaging treatment were stored in a 1-3-C cooler for 7, 14, 21, 28 or 35 days and subsequently evaluated for relative percentages of various gases in the intact packages, off-odor, surface discoloration. overall appearance, metmyoglobin percentages and palatability. Packages initially injected with modified gas atmospheres containing O2 increased in relative percentages of CO2 with advancing storage times. Roasts stored in modified atmospheres initially containing high levels of O2 exhibited a greater incidence of off-odor, more surface discoloration, lower overall appearance ratings, shorter retail caselife and lower overall palatability ratings than vacuum-packaged roasts or roasts stored in an atmosphere initially containing 20% CO2 + 80% N2.

Effect of Heating, Freezing, and pH on Yersinia enterocolitica-like Organisms From Meat

No survivors of Yersinia enterocolitica were detected in beef roasts inoculated at levels as high as 3.1-3.8 × 106 viable cells per g when the final internal temperature in the center was 60 to 62 C. At 51 C some Y. enterocolitica survived. Extensive destruction of Y. enterocolitica occurred on beef during frozen storage. Counts of surviving organisms were greater on tryptic soy agar than on bismuth sulfite agar plates. Growth of Y. enterocolitica in brain heart infusion was better at pH 7 and 8 than at 6 or 9. Little or no growth occurred at pH 5.

Physical and Sensory Characteristics of Pork Packaged in Various Gas Atmospheres

One hundred and ninety-six boneless pork roasts were vacuum-packaged. Twenty-eight roasts remained vacuum-packaged to serve as controls. The remaining packages were divided into six groups of 28 packages each and injected with one of six gas mixtures: (a) 100% O2, (b) 20% CO2 + 80% N2, (c) 50% CO2 + 50% O2, (d) 20% CO2 + 80% O2, (e) 25% CO2 + 25% O2 + 50% N2 or (f) 51% CO2 + 30% O2 + 18% N2 + 1% CO. Five packages from each packaging treatment were removed from storage after 7, 14, 21, 28 or 35 days. Data collection included gas composition of the intact packages and off-odor, surface discoloration, overall appearance, retail caselife and palatability of cuts. The most prevalent gas in vacuum packages was CO2 with weight percentages of 62-88% during the 35-day storage period. Packages initially injected with an atmosphere containing 100% O2 showed a gradual decrease in O2 and an increase in CO2 with increased storage. Roasts stored in O2-containing atmospheres for 14 and/or 21 days had a higher incidence of off-odor and chops from such roasts had lower overall appearance ratings after 1 day of retail display and lower flavor and overall palatability ratings than comparable vacuum packaged meats. These differences were significant only for roasts stored in modified atmospheres containing high concentrations of O2 and only after extended periods of storage. Data suggest that a modified gas atmosphere of 20% CO2 + 80% N2 is a suitable alternative to vacuum-packaging.

Microbiology of Beef, Pork and Lamb Stored in Vacuum or Modified Gas Atmospheres

Beef, pork and lamb loins were vacuum-packaged as subprimal cuts (controls), reformed subprimal cuts and as retail cuts (steaks or chops). Subprimal cuts, reformed subprimal cuts and one group of retail cuts remained vacuum-packaged. Other groups of retail cuts were injected with atmospheres of either 20% CO2 + 80% N2 or 40% CO2 + 60% N2. Cuts were randomly assigned to storage periods of 0-21 days at 2 ± 1 C. At weekly intervals, steaks or chops were removed from each treatment and examined after storage for 5 days under retail display conditions. Psychrotrophic bacterial counts and lactobacilli counts of steaks and chops stored in CO2-N2 atmospheres usually were lower, though not often statistically significant, than those of comparable vacuum-packaged steaks, chops or loins. Psychrotrophic counts of steaks and pork chops, initially held in CO2,-N2 atmospheres and then subjected to retail display, usually were lower than those of comparable steaks and chops that had been vacuum-packaged (without added CO2-N2) or prepared from vacuum-packaged loins.

Effect of CO2-N2 Atmospheres on the Microbial Flora of Pork

Pork loins were vacuum-packaged; one group remained vacuum-packaged, other vacuum packages were injected with gas atmospheres of either 20% CO2 + 80% N2 or 40% CO2 + 60% N2. Loins were stored for 7, 14, 21 and 28 days at 1-3 C. After each storage interval, chops were prepared from loins and subjected to 5 days of retail display. Psychrotrophic bacterial counts of lean and fat surfaces of loins stored in 40% CO2 + 60% N2 were frequently significantly lower than counts of comparable sites of vacuum-packaged loins. Lactobacillus counts of the subcutaneous fat cover of loins stored in CO2 + N2 were in most instances (7 of 8 comparisons) significantly higher than those of vacuum-packaged loins; this was true in only 2 of 8 comparisons for the lean surface. Psychrotrophic bacterial counts of chops prepared from loins subjected to the various storage treatments were seldom significantly different. During storage of pork loins, lactobacilli became a predominant part of the microflora and comprised more than 90% of the microflora after 21 days for loins stored in CO2 + N2 and after 28 days for vacuum-packaged loins. Pseudomonas spp. continued to persist to a greater degree in the vacuum-packaged loins than in the loins stored in CO2 + N2.

Influence of Certain Processing Steps on Attachment of Microorganisms to Pork Skin1

Pseudomonas putrefaciens and lactobacillus counts of inoculated pork skin obtained with the maceration method usually were higher than were those obtained with the rinse method. Scalding and dehairing and shaving caused extensive destruction of the test organisms when placed on the skin before these slaughter-dressing steps. Smaller decreases in count occurred during singeing and washing and during evisceration and washing. Increases in S value (log count by maceration method minus log count by rinse method) for P. putrefaciens and a Lactobacillus sp. after freezing-thawing of inoculated pork skin may be related to a higher death rate of bacteria in the water film than for those entrapped in skin crevices. P. putrefaciens and the Lactobacillus sp. that were attached to pork skin exhibited greater heat resistance than did those bacteria which were not attached to skin. During storage of inoculated pork skin, S values of P. putrefaciens and the Lactobacillus sp. increased; this increase probably reflects increased strength of attachment of bacteria to the skin. Scanning electron microscopy of inoculated pork skin showed the formation of extracellular structures which may play a role in attachment of bacteria to skin or meat surfaces.

Physical and Sensory Characteristics of Pork Loins Stored in Vacuum or Modified Atmosphere Packages

Wholesale pork loins were either vacuum-packaged, packaged in an atmosphere of 20% CO2 + 80% N2 or packaged in an atmosphere of 40% CO2 + 60% N2. After storage, the primal cuts were evaluated visually for muscle color, fat cover appearance, off-odor, lean surface discoloration and overall appearance. Retail cuts fabricated from the wholesale loins were evaluated for surface discoloration and overall appearance during a 5-day retail display period or they were used for sensory panel evaluation. Few significant differences between packaging treatments were found during the storage period for traits evaluated on the primal cuts. Few significant differences between packaging treatments were observed for either retail display data or sensory panel evaluation.