The aerobic growth of Aeromonas hydrophila and Listeria monocytogenes in broths and on pork.

Flasks of tryptic soy broth (TSB), unacidified (pH 7.2) or acidified with HCl or lactic acid to pH 6.3 or 5.5, and samples of sterile pork fat or muscle tissue, were inoculated with logarithmic phase cultures of a strain of Aeromonas hydrophila or a strain of Listeria monocytogenes. The broth cultures were incubated at temperatures between 0 and 25 degrees C, and growth rates were determined from optical density increases. The tissue samples were incubated at temperatures between -2.4 and 25.2 degrees C, and growth rates were determined from viable count increases. Both organisms grew without lag in all broths at temperatures greater than 10 degrees C. A hydrophila did not grow at 5 degrees C in TSB acidified with lactic acid to pH 5.5, and grew in other broths at that temperature after a lag of about 10 h. The organism did not grow in either broth of pH 5.5 at 2 degrees C, but grew in other broths at that temperature after a lag of about 40 h. A hydrophila did not grow in any broth at 0 degrees C. L. monocytogenes grew in all broths at 5 degrees C only after a lag of about 60 h, and did not grow in any broth at 2 degrees C. For both organisms, the rates of growth, at any temperature, were lower in broths of pH 6.3, and lower again in broths of pH 5.5, than in the unacidified broth. Growth rates in the broths of pH 6.3 were similar, but growth rates were lower in lactic acid acidified broth of pH 5.5 than in HCl acidified broth of that pH. The data for the growth of each organism in each medium were well described by the regression line of the plot of the square roots of growth rates against temperature. A hydrophila grew on fat tissue of pH 6.3 +/- 0.3, without lag, at 1.8 degrees C and higher temperatures at rates greater than the rates of growth in unacidified TSB. Numbers of the organism declined on muscle tissue of pH 5.6 +/0 0.2 at any temperature. L. monocytogenes grew on fat tissue without lag at -0.3 degrees C and higher temperatures, at rates which at lower temperatures were greater than the rates of growth in unacidified TSB. The organism grew on muscle tissue only at temperatures greater than or equal to 15.4 degrees C, at rates which were less than the rates of growth in lactic acid acidified broth of pH 5.5. Models derived from the cultivation of A. hydrophila and L. monocytogenes in commercial broths appear to be highly unreliable guides to the behaviours of those organisms on pork.

Effect of growth of selected lactic acid bacteria on storage life of beef stored under vacuum and in air.

The effect of growth of different types of lactic acid bacteria (LAB) on the storage life of normal pH beef was determined anaerobically (under vacuum) and aerobically. Four LAB from meat were inoculated separately onto sterile slices of lean beef. Inoculated samples were stored anaerobically at 2 degrees C for 10 weeks or stored aerobically in an oxygen permeable film at 7 degrees C for 10 days, with and without previous storage under vacuum at 2 degrees C. The LAB strains used were Carnobacterium maltaromicus (previously C. piscicola) LV17 and UAL26, Leuconostoc gelidum UAL187-22 and Lactobacillus sake Lb706. Storage life was determined by sensory panel evaluation of colour and odour. Under anaerobic conditions Lb. sake Lb706, inoculated at log 2 CFU/cm2, grew rapidly to reach maximum population within three weeks of storage. L. gelidum UAL187-22 also grew on the meat but at a slower rate. In contrast, growth of C. maltaromicus LV17 and UAL26 was unpredictable, achieving maximum population after 2 to 8 weeks. None of the test strains caused spoilage of the meat within the 10-week storage period under vacuum. When the test organisms were inoculated at an initial level of log 4 CFU/cm2, C. maltaromicus LV17 and UAL26 produced off-odours after 8 weeks of storage under vacuum at 2 degrees C. Under aerobic conditions at 7 degrees C, all four of the strains grew well on the beef samples. C. maltaromicus LV17 and UAL26 and Lb. sake Lb706 all caused off-odours and discoloration. The rate of aerobic deterioration in meat quality was faster with increased time of storage under vacuum. L. gelidum UAL187-22 could be a suitable antagonistic strain with the potential to extend the storage life of beef, stored anaerobically and packaged aerobically for retail sale, without producing undesirable sensory changes.

Lactic acid inhibition of the growth of spoilage bacteria and cold tolerant pathogens on pork.

The antibacterial effects of a 3% solution of lactic acid at 55 degrees C were assessed, by examining aerobic bacterial growth on artificially-inoculated pork fat and lean tissue. Discs of fat or lean tissues, each of 10 cm2 surface area, were aseptically excised from pork Longissimus dorsi muscle and inoculated with the cold tolerant pathogens Listeria monocytogenes 4b Scott A no. 3, Yersinia enterocolitica 0:4,32 or Aeromonas hydrophila ATCC 7966, or with the wild type spoilage bacteria Pseudomonas fragi or Brochothrix thermosphacta. After inoculation, each meat disc was immersed in water or lactic acid for 15 s and aerobic bacterial growth followed during 15 days of storage at 4 degrees C. P. fragi and B. thermosphacta grew on both fat and lean, but the pathogens grew on fat tissue only and A. hydrophila did not survive on lean. Lactic acid reduced all test bacteria on fat to below detectable levels within 4 days of treatment and no bacteria could be recovered from acid-treated fat surfaces for the remainder of the 15-day storage interval. Bacteria attached to lean were generally more resistant to lactic acid. In some instances the acid was bacteriostatic (P. fragi, L. monocytogenes) while in others the population declined at a greatly reduced rate as compared with a similar population on fat (B. thermosphacta, Y. enterocolitica). A. hydrophila was equally sensitive to lactic acid on lean and fat. Depending upon the tested strain, tissue type and storage time, maximum reductions in the number of bacteria recovered from acid treated pork ranged from 1 to 8 log cycles. The high bactericidal efficacy of lactic acid applied to pork fat was attributable to a low tissue pH, which varied from 3.49 to 4.41 during the 15 days of aerobic storage.

Bacteriology and Retail Case Life of Pork After Storage in Carbon Dioxide.

The effects of prolonged, anoxic storage, under CO2 at -1.5°C, upon the bacteriology and case life of pork on its subsequent transfer to the aerobic conditions of simulated retail display at 8°C was examined. Brochothrix thermosphacta , lactic acid bacteria, enterics, and pseudomonads were enumerated. Panel scores for odor and appearance acceptability were used to quantify retail case life. Lactic acid bacteria were the only bacteria found during loin storage in CO2 for up to 24 weeks. Those organisms reached maximum number of 107 CFU/cm2 within 9 weeks. The number of lactic acid bacteria initially found on the freshly cut surfaces of loin chops increased linearly during the first 9 weeks of loin storage in CO2. Thereafter, they continued to grow on the chops and dominated the spoilage flora during retail display. The pseudomonads grew rapidly and emerged as the next most numerous organism, while B. thermosphacta and enterics showed only limited aerobic growth. The acceptability of pork chop appearance and odor was adversely affected by loin storage time. Each 6-week interval of loin storage produced a 1 d reduction in case life. Should controlled atmospheres be a practicable means of meat distribution to the retail marketplace, efforts will be necessary to assure a maximum case life after their removal from preservative packagings.

Inability of a bacteriophage pool to control beef spoilage.

The biological control of beef spoilage, with a bacteriophage (phage) pool, was evaluated under simulated retail conditions. A pool of seven phages was selected with the potential to lyse 78% of 86 Pseudomonas test strains. Subsequent host range studies with 1023 pseudomonads from three meat species (beef, pork, lamb) and five abattoirs showed that 585 (57.2%) isolates were susceptible to the phage pool. Depending on bacterial origin, bacterial sensitivity to lysis by the phage pool varied from 25 to 72%. When added to ribeye steaks, the phage pool produced a significant reduction in Pseudomonas growth but this was not sufficient to produce any significant effect upon the retail shelf life of beef. The inability of phages to control beef spoilage was not attributed to a loss of phage virulence since sufficient densities (log pfu/cm2 = 5 to 6) of virulent phage could be re-isolated from beef, 14 days after treatment. It was concluded that the efficacy of the current phage pool was limited by a narrow range of specificity.