Changes in the microbiology of vacuum-packaged beef

Evaluation of the Microbiological Status of Raw Beef in Korea: Considering the Suitability of Aerobic Plate Count Guidelines

This study was conducted to analyze the microbiological contamination status of raw beef distributed in Korea, and evaluate the suitability of current aerobic plate count (APC) guidelines. We analyzed five years (2010-2014) of microbiological monitoring data obtained from the Ministry of Food and Drug Safety and investigated the microbiological status of raw beef collected from meat packing centers and meat shops in the Seoul/Gyeonggi, Gangwon, and Chungcheong regions in August 2015. From 2010-2014, most raw beef (>94%) displayed APC levels of < 1.0 × 10⁶ CFU/g. However, raw beef samples collected from all three regions in August 2015 had comparatively higher APC levels than those reported in previous years. To evaluate the relationship between the APC level and quality, changes in beef loin were evaluated during cold storage for 15 days at 4°C. On day 11, the mean APC level (4.7 × 10⁶ CFU/g) conformed to current guidelines in Korea (1.0 × 10⁷ CFU/g) and the pH value was 5.82. However, the sensory evaluation score for color and overall acceptability was under 3.0, meaning that the beef loin was not acceptable for eating. These results suggest that current APC guideline for raw beef should be lowered to 1.0 × 10⁶ CFU/g to improve both the microbiological safety and palatability of raw beef.

Bio-protective potential of lactic acid bacteria: Effect of Lactobacillus sakei and Lactobacillus curvatus on changes of the microbial community in vacuum-packaged chilled beef

OBJECTIVE

This study was to determine the bacterial diversity and monitor the community dynamic changes during storage of vacuum-packaged sliced raw beef as affected by Lactobacillus sakei and Lactobacillus curvatus.

METHODS

L. sakei and L. curvatus were separately incubated in vacuumed-packaged raw beef as bio-protective cultures to inhibit the naturally contaminating microbial load. Dynamic changes of the microbial diversity of inoculated or non-inoculated (control) samples were monitored at 4°C for 0 to 38 days, using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE).

RESULTS

The DGGE profiles of DNA directly extracted from non-inoculated control samples highlighted the order of appearance of spoilage bacteria during storage, showing that Enterbacteriaceae and Pseudomonas fragi emerged early, then Brochothrix thermosphacta shared the dominant position, and finally, Pseudomonas putida showed up became predominant. Compared with control, the inoculation of either L. sakei or L. curvatus significantly lowered the complexity of microbial diversity and inhibited the growth of spoilage bacteria (p<0.05). Interestingly, we also found that the dominant position of L. curvatus was replaced by indigenous L. sakei after 13 d for L. curvatus-inoculated samples. Plate counts on selective agars further showed that inoculation with L. sakei or L. curvatus obviously reduced the viable counts of Enterbacteraceae, Pseudomonas spp. and B. thermosphacta during later storage (p< 0.05), with L. sakei exerting greater inhibitory effect. Inoculation with both bio-protective cultures also significantly decreased the total volatile basic nitrogen values of stored samples (p<0.05).

CONCLUSION

Taken together, the results proved the benefits of inoculation with lactic acid bacteria especially L. sakei as a potential way to inhibit growth of spoilage-related bacteria and improve the shelf life of vacuum-packaged raw beef.

Survival of Campylobacter jejuni on beef and pork under vacuum packaged and retail storage conditions: examination of the role of natural meat microflora on C. jejuni survival

The ability of Campylobacter jejuni ATCC 11168 to survive on beef and pork stored under chilled, vacuum packaged and retail display conditions were examined. In addition, the effect of natural microflora on commercial beef and pork on the survival of C. jejuni under these storage conditions was examined. When sterile cores of beef and pork were inoculated with ∼ 10(5) to 10(6) cfu cm(-2)C. jejuni, and were stored under aerobic or vacuum packaged conditions at -1.5 or 4 °C, its numbers dropped significantly and C. jejuni could not be enumerated by direct plating after 21 d of the 6 wks study. In contrast, survival of C. jejuni on commercial vacuum packaged beef and pork was significantly enhanced, resulting in only 1 log cfu cm(-2) reduction at the end of 6 wks. During 7 d of display in a retail case, numbers of C. jejuni dropped quickly, but could be enumerated by direct plating even after the 7 d. The presence of high numbers of inoculated C. jejuni on beef and pork had no significant effect on the natural microflora numbers compared to uninoculated controls when the meat was stored either in vacuum or in a retail display case. These results show that natural microflora on vacuum packaged meat afford enhanced survival of C. jejuni present on the surfaces of both beef and pork when stored at refrigeration temperatures. Hence, strict hygienic practices or the implementation of decontamination technologies are recommended to ensure safety of meat with respect to this pathogen.

Inhibition of the Anaerobic Growth of Brochothrix thermosphacta by Lactic Acid

Brochothrix thermosphacta can grow aerobically in the presence of 210 mM l-lactate and anaerobically in its absence at pH values down to at least 5.5. Anaerobic growth is, however, inhibited by l-lactate, the concentration of undissociated lactic acid being the governing factor. Postrigor meat usually contains sufficient lactic acid to select against the anaerobic growth of B. thermosphacta. At least some Lactobacillaceae strains are more resistant to lactic acid and so their growth is favored on vacuum-packaged meat.

Influence of oxygen exclusion and temperature on pathogenic bacteria levels and sensory characteristics of packed ostrich steaks throughout refrigerated storage

Ostrich steaks (290) were obtained from Iliofibularis muscles. For microbiological and pH determinations, samples were inoculated with Listeria monocytogenes NCTC 11994 (80 steaks) or Escherichia coli ATCC 12806 (80), then air- or vacuum-packed and stored at either 4±1°C or 10±1°C. Analyses were carried out on days 0, 3, 6 and 9 of storage. For sensory evaluation, samples (130) were air- or vacuum-packed and stored at 4±1°C or at 10±1°C. Sensory attributes (odour, colour, drip loss, texture and general acceptability) were scored by six untrained judges using an unstructured nine-point hedonic scale on eleven sampling days (0, 3, 6, 9, 12, 15, 18, 21, 24, 27 and 30). Increases in microbial counts (log(10)cfu/g) were observed throughout storage in all groups of samples for both L. monocytogenes (from 6.39±0.43-6.62±0.32 at day 0 to 8.87±0.19-9.64±0.43 at day 9) and E. coli (from 5.57±0.15-5.68-0.40 to 7.79±0.96-9.64±0.17). Gas atmosphere influenced microbial counts from day 3 of storage with lower (P<0.05) values observed in vacuum- than in air-packed samples at 10°C (L. monocytogenes) or at 4 and 10°C (E. coli). Storage temperature significantly influenced bacterial counts throughout storage, especially in air-packed samples. Lower pH values in vacuum- than in air-packed samples were observed from day 6. Both effects (gas atmosphere and temperature) influenced the hedonic scores, with higher values assigned to vacuum-packed samples for most attributes (with the exception of drip loss) and sampling days. A marked influence of storage temperature on sensorial scores was obtained in air-packaged ostrich steaks. The shelf-life (time until the average general acceptability score fell below 5) was 6 (air-packed samples), 9 (vacuum-packed, 10°C), or 12 days (vacuum-packed, 4°C). The results being reported here suggest the importance of both oxygen exclusion and storage at low temperatures to reduce microbiological risks and improve the acceptability of ostrich meat. However, the short shelf-life of this product highlights the need to keep the time between slaughter and sale to a minimum.

Effects of temperature, oxygen exclusion, and storage on the microbial loads and pH of packed ostrich steaks

Ostrich steaks (96) were packed (air or vacuum) and stored at 4°C or 10°C. Microbiological loads (total viable counts - TVC, psychrotrophic, Enterobacteriaceae, lactic acid bacteria - LAB, Pseudomonas and fluorescent Pseudomonas) and pH values were determined at 0, 3, 6 and 9 days. High counts (8.0-10.2 log(10)cfu TVC/g) were observed at day 9, possibly as a consequence of the initial high microbial load (range 4.9-5.4 log(10)cfu TVC/g) and pH (average 6.7). Temperature and sampling day significantly influenced all microbial counts. Gas atmosphere had a small or negligible influence on levels of Enterobacteriaceae and LAB. For most microbial groups temperature significantly influenced bacterial levels up to day 6 of storage, while gas atmosphere had a significant effect at days 6 and 9. Both effects (temperature and gas atmosphere) were significant factors from day 0 for fluorescent Pseudomonas. Samples vacuum-packed and stored at 4°C showed the lowest microbial loads at day 9. Only for these samples was no sensory rejection observed at the end of the experiment.

Effect of Lactobacillus-protective cultures with bacteriocin-like inhibitory substances' producing ability on microbiological, chemical and sensory changes during storage of refrigerated vacuum-packaged sliced beef

AIMS

To investigate the effect of applying two different Lactobacillus-protective cultures, with bacteriocin-like inhibitory substances' (BLIS) producing ability, individually or in combination, on microbiological, chemical and sensory changes during storage of refrigerated vacuum-packaged sliced beef meat.

METHODS AND RESULTS

Lactobacillus sakei CECT 4808 and Lactobacillus curvatus CECT 904(T), which were shown to be producers of BLIS, were inoculated individually or in combination on slices of beef M. semitendinosus. The samples were vacuum packaged and stored at 4 +/- 1 degrees C and were assessed during a 28-day storage period for microbiological [Enterobacteriaceae, Pseudomonas spp., lactic acid bacteria (LAB), Brochothrix thermosphacta and yeasts and moulds], chemical (pH, protein hydrolysis degree, lipid oxidation), sensory (abnormal odour) parameters and instrumental colour. Samples inoculated with the Lact. sakei strain and samples inoculated with the combination of the two strains had significantly (P < 0.05) lower spoilage microbial counts than those inoculated with the Lact. curvatus strain alone or the controls, while both chemical parameters (including lipid oxidation) and abnormal odour scores were also significantly (P < 0.05) improved by the former. Moreover, Lact. sakei alone showed a better preserving effect (P < 0.05) than the combination of both strains in the majority of the parameters tested. Instrumental colour measurements changed with storage time, but no treatment effects (P >or= 0.05) were observed during the whole 28-day storage period.

CONCLUSIONS

The BLIS producer Lact. sakei CECT 4808 strain may be used for improving preservation of vacuum-packaged beef slices, as regards spoilage microbial counts and the chemical parameters tested in this study.

SIGNIFICANCE AND IMPACT OF THE STUDY

Inoculation with the BLIS producer Lact. sakei CECT 4808 strain would provide an additional hurdle to improve storage life of refrigerated vacuum-packaged sliced beef. Furthermore, this strain demonstrated limited antioxidative ability, which could make a contribution to the prevention of lipid oxidation in meat and meat products.

Predictive modelling of growth and measurement of enzymatic synthesis and activity by a cocktail of Brochothrix thermosphacta

The possibility was examined of developing a predictive model that combined microbial growth (increase in cellular number) and extracellular enzyme activity of a cocktail of three strains of Brochothrix thermosphacta. Estimations of growth and enzyme activity were made within a three-dimensional matrix of conditions: temperature 2-20 degrees C, pH value 4.0-7.5 and water activity (a(w)) 0.95-0.995. A model which predicted growth based on increases in cell number was constructed. No extracellular lipases were detected, but slight proteolytic reactions were observed. Although it was not possible to model protease activity, the growth model and information relating to enzyme activity will be made freely available in a database on the Internet.

Presence of acylated homoserine lactones (AHLs) and AHL-producing bacteria in meat and potential role of AHL in spoilage of meat

Quorum-sensing (QS) signals (N-acyl homoserine lactones [AHLs]) were extracted and detected from five commercially produced vacuum-packed meat samples. Ninety-six AHL-producing bacteria were isolated, and 92 were identified as Enterobacteriaceae. Hafnia alvei was the most commonly identified AHL-producing bacterium. Thin-layer chromatographic profiles of supernatants from six H. alvei isolates and of extracts from spoiling meat revealed that the major AHL species had an R(f) value and shape similar to N-3-oxo-hexanoyl homoserine lactone (OHHL). Liquid chromatography-mass spectrometry (MS) (high-resolution MS) analysis confirmed the presence of OHHL in pure cultures of H. alvei. Vacuum-packed meat spoiled at the same rate when inoculated with the H. alvei wild type compared to a corresponding AHL-lacking mutant. Addition of specific QS inhibitors to the AHL-producing H. alvei inoculated in meat or to naturally contaminated meat did not influence the spoilage of vacuum-packed meat. An extracellular protein of approximately 20 kDa produced by the H. alvei wild-type was not produced by the AHL-negative mutant but was restored in the mutant when complemented by OHHL, thus indicating that AHLs do have a regulatory role in H. alvei. Coinoculation of H. alvei wild-type with an AHL-deficient Serratia proteamaculans B5a, in which protease secretion is QS regulated, caused spoilage of liquid milk. By contrast, coinoculation of AHL-negative strains of H. alvei and S. proteamaculans B5a did not cause spoilage. In conclusion, AHL and AHL-producing bacteria are present in vacuum-packed meat during storage and spoilage, but AHL does not appear to influence the spoilage of this particular type of conserved meat. Our data indicate that AHL-producing H. alvei may induce food quality-relevant phenotypes in other bacterial species in the same environment. H. alvei may thus influence spoilage of food products in which Enterobacteriaceae participate in the spoilage process.

Effects of diacetyl and carbon dioxide on spoilage microflora in ground beef

The effect of CO2 and diacetyl, alone or in combination, on spoilage microflora in ground beef was determined. Ground beef was treated with 20, 30, or 40% CO2 for 22 days (study I); 20, 50, or 100 microg/g diacetyl for 26 days (study II); or a combination of 20% CO2 and 100 microglg diacetyl for 40 days (study III). Antimicrobial effectiveness was determined by aerobic plate counts (log10 CFU/g) using plate count agar (total aerobic bacteria), deMan Rogosa Sharpe (MRS) Lactobacillus agar (gram-positive bacteria), MacConkey agar (gram-negative bacteria), pH, and informal organoleptic assessments (by appearance and by odor). In study I, total bacteria and pH increased by day 4 in control meat samples. For all CO2 levels, gram-negative bacteria decreased and gram-positive bacteria increased compared with untreated controls. The pH remained constant for CO2-treated meat. Control samples had an off-odor and a brown appearance, while CO2-treated samples had no off-odor but did have a brown appearance. For samples treated with diacetyl (study II), spoilage was evident by day 7 for samples treated with 0, 20. and 50 microg/g diacetyl for all parameters examined. Ground beef treated with 100 microg/g diacetyl was spoiled on day 12. Diacetyl was detected (by odor) in samples that were treated with 100 microg/g diacetyl and had a brown appearance. Meat samples treated with the combination of CO2 and diacetyl (study III) showed that the addition of diacetyl did not have an additive effect on microbial growth. Combination-treated meat maintained a red appearance and no off-odor. Diacetyl and CO2 could be used in combination to maintain a red color and inhibit spoilage microorganisms.

Microbiology of fresh and restructured lamb meat: a review

Microbiology of meats has been a subject of great concern in food science and public health in recent years. Although many articles have been devoted to the microbiology of beef, pork, and poultry meats, much less has been written about microbiology of lamb meat and even less on restructured lamb meat. This article presents data on microbiology and shelf-life of fresh lamb meat; restructured meat products, restructured lamb meat products, bacteriology of restructured meat products, and important foodborne pathogens such as Salmonella, Escherichia coli O157:H7, and Listeria monocytogenes in meats and lamb meats. Also, the potential use of sodium and potassium lactates to control foodborne pathogens in meats and restructured lamb meat is reviewed This article should be of interest to all meat scientists, food scientists, and public health microbiologists who are concerned with the safety of meats in general and lamb meat in particular.

Microbial ecology of fresh pork stored under modified atmosphere at -1, 4.4 and 10 degrees C

The prevalent bacteria on fresh pork packaged in modified atmosphere with elevated CO2 were determined by selection of representative colonies from the greatest dilution of meat samples. The pork samples were stored in two packaging films of different oxygen permeability at three storage temperatures. Strains were classified and those identified as lactic acid bacteria were screened for production of inhibitory substances. The types of bacteria isolated from samples stored in the two packaging films were similar. Storage temperature influenced the type of bacteria that dominated the microbial population. At 10 degrees C the prevalent microflora consisted of aeromonads, Enterobacteriaceae and lactic acid bacteria but at 4.4 and -1 degrees C, aeromonads, Brochothrix thermosphacta and lactic acid bacteria dominated. Listeriae were detected as part of the prevalent microflora on samples stored at -1 degree C, but not on samples stored at 4.4 or 10 degrees C. Species of lactic acid bacteria dominating the microflora were influenced by growth medium. The majority of isolates taken from Plate Count agar were carnobacteria whereas those from Lactobacilli MRS agar were homofermentative lactic acid bacteria. Of the 538 lactic acid bacteria isolates screened for production of inhibitory substances, 162 strains showed deferred inhibition toward a range of lactic acid bacteria and nonlactic acid bacteria indicator strains.

Bacterial sources of putrescine and cadaverine in chill stored vacuum-packaged beef

Of the meat strains of streptobacteria, leuconostocs, Enterobacteriaceae and Brochothrix thermosphacta tested, only Hafnia alvei and Serratia liquefaciens showed diamine-producing potential during growth in pure culture on beef stored in vacuum packs at 1 degree C. Both organisms produced cadaverine at concentrations similar to those reported previously in naturally contaminated beef stored under the same conditions. Putrescine concentrations produced by the two organisms, however, were an order of magnitude lower. During the growth on beef of either H. alvei or S. liquefaciens in mixed culture with arginine-utilizing strains of streptobacteria, putrescine as well as cadaverine concentrations were similar to those detected in naturally contaminated samples.

Relationship between lactic acid concentration and bacterial spoilage in ground beef

Lactic acid concentration correlated with organoleptic spoilage of refrigerated, coarsely ground beef stored in casings with low oxygen permeability. The samples were assayed over time for lactic acid concentration, total aerobic plate count, percentage of gram-positive organisms, and pH. Lactic acid increased in all samples, as did the bacterial counts and percentage of gram-positive organisms in the total microflora, the latter representing an increase in the lactic acid-producing bacteria. pH was found to decrease in all samples, with the smallest decrease in pH being observed in the meat sample which maintained the lowest proportion of gram-positive organisms. With samples evaluated by a sensory panel, lactic acid levels were found to correlate inversely with odor acceptability.