Observations on the succession dynamics of lactic acid bacteria populations in chill-stored vacuum-packaged beef

Beef longissimus eating quality increases up to 20 weeks of storage and is unrelated to meat colour at carcass grading

Optimal beef meat colour is associated with increased consumer acceptance, whereas dark or pale meat has a reduced desirability. Dark beef also has a variable eating quality and reduced shelf-life. We hypothesised that a poor meat colour at carcass grading would generate an unacceptable eating quality after vacuum-packed chilled storage for up to 20 weeks, due to the unfavourable pH conditions commonly associated with light and dark muscles. At three beef processing plants, beef longissimus muscles from 81 pasture- and grain-fed cattle (mix of Bos taurus and Bos indicus × Bos taurus) were graded at ~24 h post-slaughter for meat colour. The carcasses were allocated to light, medium and dark colour groups, with n = 27 carcasses per colour group. From the 81 carcasses, a total of 162 longissimus lumborum (LL) muscles was collected and half LLs were randomly allocated to three ageing times (2, 12, 20 weeks) within colour group and six half LLs were used per colour group within storage period and plant. Vacuum-packed muscles were stored at –1.0 ± 0.5°C for the designated period and sampled for biochemical and sensory assessments. The effects of colour group, storage week and carcass traits were analysed. Dark muscles had higher pH than the lighter ones (P < 0.05). The carcass trait dentition, feed type and fat depth did not influence the eating quality (P > 0.05). After 2, 12 and 20 weeks of vacuum-packed chilled storage; eating quality was similar for all 3 meat-colour groups (P > 0.05). With increasing storage time, all eating-quality attributes improved (P < 0.001 for all). Lipid oxidation increased with storage time and although values at 20 weeks were slightly above accepted levels for rancidity detection, MQ4 scores indicated that the meat would still be categorised as a three-star product, indicative of the opportunity to store the longissimus lumborum (LL) for this length of time, while maintaining an acceptable eating quality, regardless of meat colour at carcass grading.

Unusual compositions of microflora of vacuum-packaged beef primal cuts of very long storage life

Vacuum-packaged top butt cuts from a beef packing plant that does not use any carcass decontaminating interventions were assessed for their organoleptic and microbiological properties during storage at 2 or -1.5°C. Cuts stored at 2°C were acceptable after storage for 140 days but were unacceptable after 160 days because of persistent sour, acid odors. Odors of cuts stored at -1.5°C for 160 days were acceptable. The numbers of aerobes on cuts increased from <1 log CFU/cm(2) to 7 or 6 log CFU/cm(2) for cuts stored at 2 or -1.5°C, respectively. The numbers of Enterobacteriaceae increased from <-1 log CFU/cm(2) to 5 or 3 log CFU/cm(2) for cuts stored at 2 or -1.5°C, respectively. Bacteria recovered from initial microflora were, mainly, strictly aerobic organisms. Bacteria recovered from cuts stored for 160 days were mainly Carnobacterium spp. that grew on an acetate-containing agar generally selective for lactic acid bacteria other than Carnobacterium. C. divergens and C. maltaromaticum were recovered from cuts stored at 2°C, but C. maltaromaticum was the only species of Carnobacterium recovered from cuts stored at -1.5°C. No lactic acid bacteria of genera that usually predominate in the spoilage microflora of vacuum-packaged beef at late storage times were recovered from the spoilage microflora. The findings indicate that carnobacteria, initially present at very small numbers, grew exponentially to persistently dominate the spoilage microflora of vacuum-packaged beef cuts of unusually long storage life.

Storage life at 2 °C or -1.5 °C of vacuum-packaged boneless and bone-in cuts from decontaminated beef carcasses

BACKGROUND

The microbiological condition of beef produced at North American plants has been improved as a result of the use of effective carcass-decontaminating treatments. The effect of these treatments on the storage life of beef has not been established. In this study, beef primal cuts in vacuum packs stored at -1.5 or 2 °C for up to 160 days were assessed for their microbiological and organoleptic properties.

RESULTS

The odours of boneless cuts were acceptable after storage at either temperature for ≤160 days; and the flavours of steaks from boneless cuts stored at 2 or -1.5 °C for ≤70 or ≤120 days, respectively, were acceptable. The storage life of bone-in cuts stored at 2 or -1.5 °C was, respectively, shorter or the same as that of boneless cuts stored at the same temperature. More than 20 microbial species that were mostly obligate aerobes were present on both types of cuts before storage. After storage for ≥30 days, the microflora was dominated by carnobacteria and Enterobacteriaceae were present in the flora from early storage times.

CONCLUSIONS

A storage life of 120-140 days was attained by vacuum-packaged beef primals from decontaminated carcasses stored at -1.5 °C. The bone-in cuts stored at 2 °C were spoiled at earlier times, probably by Enterobacteriaceae.

Microbial growth, communities and sensory characteristics of vacuum and modified atmosphere packaged lamb shoulders

Packaging fresh lamb in a vacuum (VAC) versus a 100% CO2 modified atmosphere (MAP) may influence product shelf-life and the bacterial communities. While VAC is a common packing method and 100% CO2 MAP is used in some countries, there is little information about how these different techniques affect the growth of spoilage bacteria and sensory attributes of lamb. The aim of this study was to assess changes in microbiological and organoleptic properties, and determine differences in microbial communities by terminal restriction fragment length polymorphism (TRFLP) and 454 pyrosequencing, in bone-in (BI) and bone-out (BO) MAP- and VAC-packed lamb shoulders stored at -0.3 °C over 12 wk. VAC and MAP lamb shoulders were acceptable in sensory test scores over 12 wk of storage at -0.3 °C, despite total viable count (TVC) and lactic acid bacteria (LAB) levels increasing to 8 log10 CFU/cm(2) for VAC lamb and 4-6 log10 CFU/cm(2) for MAP lamb. Similar to the sensory results, there were no significant differences in microbial communities between BI and BO product. However, types of bacteria were different between VAC and MAP packaging. Specifically, while VAC shoulder became dominated by Carnobacterium spp. in the middle of the storage period, the MAP shoulder microbial population remained similar from the start until later storage times.

Antimicrobial packaging to retard the growth of spoilage bacteria and to reduce the release of volatile metabolites in meat stored under vacuum at 1°C

A nisin-EDTA solution was used for activation of the internal surface of plastic bags that were used to store beef chops at 1°C after vacuum packaging. The aim of the work was to evaluate the effect of the antimicrobial packaging on beef during storage. Volatile compounds and microbial populations were monitored after 0, 9, 20, 36, and 46 days of storage. The active packaging retarded the growth of lactic acid bacteria. Brochothrix thermosphacta was unable to grow for the whole storage time in treated samples, while the levels of Carnobacterium spp. in treated samples were below the detection limit for the first 9 days and reached loads below 5 Log CFU/cm(2) after 46 days. On the other hand, Enterobacteriaceae and Pseudomonas spp. were not affected by the use of the antimicrobial packaging and grew in all of the samples, with final populations of about 4 Log CFU/cm(2). Carnobacterium divergens was identified by PCR-denaturing gradient gel electrophoresis analysis of DNA extracted from beef after 36 days of storage. During beef storage, alcohols, aldehydes, ketones, and carboxylic acids were detected in the headspace of beef samples by solid-phase microextraction-gas chromatography-mass spectrometry analysis. The microbial metabolic activity was affected by the use of the antimicrobial film from the beginning up to 36 days with a maximum in the differences of volatile metabolites in samples analyzed at 20 days. The volatiles were also determined by electronic nose, allowing differentiation based on the time of storage and not on the type of packaging. The active packaging reduces the loads of spoilage microbial populations and the release of metabolites in the headspace of beef with a probable positive impact on meat quality.

Significance of heme-based respiration in meat spoilage caused by Leuconostoc gasicomitatum

Leuconostoc gasicomitatum is a psychrotrophic lactic acid bacterium (LAB) which causes spoilage in cold-stored modified-atmosphere-packaged (MAP) meat products. In addition to the fermentative metabolism, L. gasicomitatum is able to respire when exogenous heme and oxygen are available. In this study, we investigated the respiration effects on growth rate, biomass, gene expression, and volatile organic compound (VOC) production in laboratory media and pork loin. The meat samples were evaluated by a sensory panel every second or third day for 29 days. We observed that functional respiration increased the growth (rate and yield) of L. gasicomitatum in laboratory media with added heme and in situ meat with endogenous heme. Respiration increased enormously (up to 2,600-fold) the accumulation of acetoin and diacetyl, which are buttery off-odor compounds in meat. Our transcriptome analyses showed that the gene expression patterns were quite similar, irrespective of whether respiration was turned off by excluding heme from the medium or mutating the cydB gene, which is essential in the respiratory chain. The respiration-based growth of L. gasicomitatum in meat was obtained in terms of population development and subsequent development of sensory characteristics. Respiration is thus a key factor explaining why L. gasicomitatum is so well adapted in high-oxygen packed meat.

The effect of marination on lactic acid bacteria communities in raw broiler fillet strips

Marination with marinade containing salt, sugar, and acetic acid is commonly used in Finland to enhance the value of raw broiler meat. In this study, we investigated the effect of marination, marinade components and storage time on composition of bacterial communities in modified atmosphere-packaged (MAP) broiler fillet strips. The communities were characterized using two culture-independent methods: 16S rRNA gene fragment sequencing and terminal restriction fragment length polymorphism. In unmarinated broiler fillet strips, Lactococcus spp. and Carnobacterium spp. predominated at the early storage phase but were partially replaced by Lactobacillus spp. and Leuconostoc spp. when the chilled storage time was extended. In the marinated fillet strips, Lactobacillus spp. and Leuconostoc spp. predominated independent from the storage time. By mixing the different marinade components with broiler meat, we showed that marination changed the community composition and favored Leuconostoc spp. and Lactobacillus spp. by the combined effect of carbohydrates and acetic acid in marinade. Marination increased the maximum level of lactic acid bacteria in broiler meat and enhanced CO₂ production and acidification of meat during the chilled storage. Accumulation of CO₂ in package head-space due to the enhanced growth of Leuconostoc spp. in marinated meat may lead to bulging of packages, which is a spoilage defect frequently associated with marinated and MAP raw broiler preparations in Finland.

Vacuum-packed beef primals with extremely long shelf life have unusual microbiological counts

When vacuum-packed striploins and cube rolls processed by six Australian establishments were stored at 2 0.5°C to determine their shelf life, all product was acceptable organoleptically for at least 26 weeks. The aerobic plate counts and counts of lactic acid bacteria over the storage period did not accord with those established by previous studies, i.e., stationary phase attained at 7 to 8 log CFU/cm(2) after 5 to 8 weeks followed by the development of negative sensory characteristics around 12 to 16 weeks. Rather, counts rarely progressed to 7 log CFU/cm(2) even after 30 weeks. It is believed that the combined effects of meat pH, temperature, and CO(2) concentration may combine to create conditions in which little or no growth occurs.

Monitoring of microbial metabolites and bacterial diversity in beef stored under different packaging conditions

Beef chops were stored at 4°C under different conditions: in air (A), modified-atmosphere packaging (MAP), vacuum packaging (V), or bacteriocin-activated antimicrobial packaging (AV). After 0 to 45 days of storage, analyses were performed to determine loads of spoilage microorganisms, microbial metabolites (by solid-phase microextraction [SPME]-gas chromatography [GC]-mass spectrometry [MS] and proton nuclear magnetic resonance [(1)H NMR]), and microbial diversity (by PCR-denaturing gradient gel electrophoresis [DGGE] and pyrosequencing). The microbiological shelf life of meat increased with increasing selectivity of storage conditions. Culture-independent analysis by pyrosequencing of DNA extracted directly from meat showed that Brochothrix thermosphacta dominated during the early stages of storage in A and MAP, while Pseudomonas spp. took over during further storage in A. Many different bacteria, several of which are usually associated with soil rather than meat, were identified in V and AV; however, lactic acid bacteria (LAB) dominated during the late phases of storage, and Carnobacterium divergens was the most frequent microorganism in AV. Among the volatile metabolites, butanoic acid was associated with the growth of LAB under V and AV storage conditions, while acetoin was related to the other spoilage microbial groups and storage conditions. (1)H NMR analysis showed that storage in air was associated with decreases in lactate, glycogen, IMP, and ADP levels and with selective increases in levels of 3-methylindole, betaine, creatine, and other amino acids. The meat microbiota is significantly affected by storage conditions, and its changes during storage determine complex shifts in the metabolites produced, with a potential impact on meat quality.

Spoilage-related activity of Carnobacterium maltaromaticum strains in air-stored and vacuum-packed meat

One hundred three isolates of Carnobacterium spp. from raw meat were analyzed by random amplification of polymorphic DNA (RAPD) and PCR and were identified by 16S rRNA gene sequencing. Forty-five strains of Carnobacterium maltaromaticum were characterized for their growth capabilities at different temperatures, NaCl concentrations, and pH values and for in vitro lipolytic and proteolytic activities. Moreover, their spoilage potential in meat was investigated by analyzing the release of volatile organic compounds (VOCs) in meat stored in air or vacuum packs. Almost all the strains were able to grow at 4, 10, and 20°C, at pH values of 6 to 9, and in the presence of 2.5% NaCl. The release of VOCs by each strain in beef stored at 4°C in air and vacuum packs was evaluated by headspace solid-phase microextraction (HS-SPME)-gas chromatography-mass spectrometry (GC-MS) analysis. All the meat samples inoculated and stored in air showed higher numbers of VOCs than the vacuum-packed meat samples. Acetoin, 1-octen-3-ol, and butanoic acid were the compounds most frequently found under both storage conditions. The contaminated meat samples were evaluated by a sensory panel; the results indicated that for all sensory odors, no effect of strain was significant (P > 0.05). The storage conditions significantly affected (P < 0.05) the perception of dairy, spoiled-meat, and mozzarella cheese odors, which were more intense in meat stored in air than in vacuum packs but were never very intense. In conclusion, different strains of C. maltaromaticum can grow efficiently in meat stored at low temperatures both in air and in vacuum packs, producing volatile molecules with low sensory impacts, with a negligible contribution to meat spoilage overall.

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.

Effects on the development of blown pack spoilage of the initial numbers of Clostridium estertheticum spores and Leuconostoc mesenteroides on vacuum packed beef

Beef steaks were inoculated with Clostridium estertheticum spores and Leuconostoc mesenteroides cells at all combinations of numbers of 0, 10, 100 or 1000/cm(2) for each organism. After vacuum packaging the steaks were stored at 4, 1, or -1.5°C. Pack volumes were determined by water displacement at suitable intervals. Irrespective of L. mesenteroides numbers, for packs containing meat inoculated with 0, 10, 100 or 1000 spores/cm(2), 60, 16, 3 and 1 of 60 packs did not swell. The times of onset of swelling were twice as long at -1.5 as at 4°C, but they were not affected by the inoculated numbers of L. mesenteroides. Rates of pack swelling increased with increasing storage temperature and number of spores, but decreased with increasing numbers of inoculated L. mesenteroides. Lactic acid bacteria can apparently prevent development of blown pack spoilage of vacuum packs containing meat contaminated with low numbers of C. estertheticum.

Microbial deterioration of vacuum-packaged chilled beef cuts and techniques for microbiota detection and characterization: a review

Gas production from microbial deterioration in vacuum-packs of chilled meat leads to pack distension, which is commonly referred as blown pack. This phenomenon is attributed to some psychrophilic and psychrotrophic Clostridium species, as well as Enterobacteria. The ability of these microorganisms to grow at refrigeration temperatures makes the control by the meat industry a challenge. This type of deterioration has been reported in many countries including some plants in the Midwestern and Southeastern regions of Brazil. In addition to causing economic losses, spoilage negatively impacts the commercial product brand, thereby impairing the meat industry. In the case of strict anaerobes species they are difficult to grow and isolate using culture methods in conventional microbiology laboratories. Furthermore, conventional culture methods are sometimes not capable of distinguishing species or genera. DNA-based molecular methods are alternative strategies for detecting viable and non-cultivable microorganisms and strict anaerobic microorganisms that are difficult to cultivate. Here, we review the microorganisms and mechanisms involved in the deterioration of vacuum-packaged chilled meat and address the use of molecular methods for detecting specific strict anaerobic microorganisms and microbial communities in meat samples.

Antimicrobial activity of soy edible films incorporated with thyme and oregano essential oils on fresh ground beef patties

Antibacterial activity of soy protein edible films (SPEF) incorporated with 1, 2, 3, 4 and 5% oregano (OR) or thyme (TH) essential oils was evaluated against Escherichia coli, E. coli O157:H7, Staphylococcus aureus, Pseudomonas aeruginosa and Lactobacillus plantarum by the inhibition zone test. Effects of SPEF containing 5% OR and TH or a mixture of OR+TH (ORT) were also tested on fresh ground beef during refrigerated storage (at 4 degrees C). OR and TH incorporated SPEF exhibited similar antibacterial activity against all bacteria in inhibition zone test. While E. coli, E. coli O157:H7 and S. aureus were significantly inhibited by antimicrobial films, L. plantarum and P. aeruginosa appeared to be the more resistant bacteria. SPEF with OR, ORT, and TH did not have significant effects on total viable counts, lactic acid bacteria and Staphylococcus spp. when applied on ground beef patties whereas reductions (p<0.05) in coliform and Pseudomonas spp. counts were observed.

Effects of temperature and pH on the growth of bacteria isolated from blown packs of vacuum-packaged beef

Bacteria recovered from the microflora of blown packs of vacuum-packaged beef were identified as Leuconostoc mesenteroides, Lactococcus lactis, Carnobacterium maltaromaticum, and Clostridium estertheticum, with L. mesenteroides predominant. Isolates of these lactic acid bacteria all grew in peptone yeast extract glucose starch broth (PYGSB) at temperatures between -2 and 30 degrees C but generally grew more slowly and over a more restricted temperature range in meat juice medium (MJM). A C. estertheticum isolate and the type strain of C. estertheticum subsp. estertheticum (ATCC 51377) both grew in PYGSB and MJM at similar rates at temperatures between -2 and 17 degrees C and grew at 20 degrees C in MJM but not in PYGSB. Square root models of the variation of the growth rate with temperature indicated that the C. maltaromaticum isolate and the C. estertheticum strains grew at similar rates that were faster than those of the other isolates at temperatures between -2 and 0 degrees C. The L. mesenteroides and L. lactis isolates grew in PYGSB at pH 5.0, but the C. maltaromaticum isolate and both strains of C. estertheticum did not grow in PYGSB at pH <or= 5.3. C. estertheticum stopped growing in MJM buffered at pH 6.5 when glucose was exhausted, although these bacteria then utilized lactate. The findings suggest that, like carnobacteria, C. estertheticum may predominate during the early stages of development of the spoilage microflora of vacuum-packaged beef but that C. estertheticum will likely be inhibited by a falling pH and so may be only a minor part of the spoilage microflora when maximum numbers are attained.

Comparison of culture-dependent and independent techniques for characterisation of the microflora of peroxyacetic acid treated, vacuum-packaged beef

The diversity of microflora associated with peroxyacetic acid (POAA) treated and untreated beef was investigated by 16S rDNA gene cloning, DGGE analysis and conventional bacterial cultivation. Following vacuum packaging, POAA treated and untreated meat samples were stored for up to 18 weeks at -1.5 degrees C. Each culture independent method showed Carnobacterium spp. to predominate on both POAA treated and untreated meat. However, 16S rDNA gene analysis also detected the presence of psychrotolerant Clostridium spp. in the POAA-treated beef. Culture-dependent analysis did not distinguish Carnobacterium spp. from Lactobacilli. Although culture-dependent analysis showed an increase in the ratio of Enterobacteriaceae to lactic acid bacteria from weeks 6-18 in the POAA treated compared with the untreated meat, the numbers of Enterobacteriaceae were significantly less on POAA treated than on untreated meat. The combination of data collected by culture-dependent and independent techniques provided the most robust approach for elucidating the efficacy of chemical sanitization of chilled vacuum-packaged beef. If conventional cultivation is used for monitoring bacterial spoilage of vacuum-packaged chilled meats it is recommended that culture methods specific for Carnobacterium and Clostridium spp. should be included in order to provide a more complete indication of microbial diversity.

Screening of lactic acid bacteria from vacuum packaged beef for antimicrobial activity

The objective of this study was to isolate lactic acid bacteria (LAB) from vacuum packaged beef and to investigate their antagonist activity. LAB mean counts of 5.19 log cfu/cm(2) were obtained from five samples of vacuum packaged beef. Two hundred isolates were selected and screened for the inhibitory effect on five ATCC reference Lactobacillus strains. Thirty six isolates showed activity in the agar spot test against at least two of the indicator strains. However, only six cell free supernatants (CFS) from these isolates exhibited activity against the indicator strains using the well-diffusion test and conditions that eliminated the effects of organic acids and hydrogen peroxide. L. acidophilus was the most sensitive indicator tested, whereas L. plantarum and L. fermentum were the most resistant ones. Identification by MIDI system indicated that these LAB isolates were Lactococcus lactis subsp. cremoris, Pediococcus acidilactici, Lactobacillus delbrueckii subsp. bulgaricus and Lactobacillus casei GC subgroup A. The antagonistic factors produced by most of these LAB against L. acidophilus were resistant to heat treatment (100°C for 10 min) and stable over a wide pH range (4.0 to 9.0). These data suggest that these isolates could be used as promising hurdles aiming increased safety and extended shelf life of meat products.

A TaqMan-based real-time PCR assay for the specific detection and quantification of Leuconostoc mesenteroides in meat products

A new real-time PCR procedure was developed for the specific detection and quantification of Leuconostoc mesenteroides in meat products. It is a TaqMan assay based on 23S rRNA gene targeted primers and probe. Specificity was evaluated using purified DNA from 132 strains: 102 lactic acid bacteria (LAB), including 57 reference strains and 46 food isolates, belonging to genus Leuconostoc and related genera, and 30 non-LAB strains. Quantification was linear over at least 5 log units using both serial dilutions of purified DNA and calibrated cell suspensions from Leuconostoc mesenteroides ssp. dextranicum CECT 912T. This assay was able to detect at least five genomic equivalents, using purified DNA or 59 CFU per reaction when using calibrated cell suspensions. It performed successfully when tested on an artificially inoculated meat product, with a minimum threshold of 10(4) CFU g(-1) for the accurate quantification of Leuconostoc mesenteroides.

Carnobacterium: positive and negative effects in the environment and in foods

The genus Carnobacterium contains nine species, but only C. divergens and C. maltaromaticum are frequently isolated from natural environments and foods. They are tolerant to freezing/thawing and high pressure and able to grow at low temperatures, anaerobically and with increased CO₂ concentrations. They metabolize arginine and various carbohydrates, including chitin, and this may improve their survival in the environment. Carnobacterium divergens and C. maltaromaticum have been extensively studied as protective cultures in order to inhibit growth of Listeria monocytogenes in fish and meat products. Several carnobacterial bacteriocins are known, and parameters that affect their production have been described. Currently, however, no isolates are commercially applied as protective cultures. Carnobacteria can spoil chilled foods, but spoilage activity shows intraspecies and interspecies variation. The responsible spoilage metabolites are not well characterized, but branched alcohols and aldehydes play a partial role. Their production of tyramine in foods is critical for susceptible individuals, but carnobacteria are not otherwise human pathogens. Carnobacterium maltaromaticum can be a fish pathogen, although carnobacteria are also suggested as probiotic cultures for use in aquaculture. Representative genome sequences are not yet available, but would be valuable to answer questions associated with fundamental and applied aspects of this important genus.

Lactic acid bacteria associated with vacuum-packed cooked meat product spoilage: population analysis by rDNA-based methods

AIM

To determine the lactic acid bacteria (LAB) implicated in bloating spoilage of vacuum-packed and refrigerated meat products.

METHODS AND RESULTS

A total of 18 samples corresponding to four types of meat products, with and without spoilage symptoms, were studied. In all, 387 colonies growing on de Man, Rogosa and Sharpe, yeast glucose lactose peptone and trypticase soy yeast extract plates were identified by internal spacer region (ISR), ISR-restriction fragment length polymorphism and rapid amplified ribosomal DNA restriction analysis profiles as Lactobacillus (37%), Leuconostoc (43%), Carnobacterium (11%), Enterococcus (4%) and Lactococcus (2%). Leuconostoc mesenteroides dominated the microbial population of spoiled products and was always present at the moment bloating occurred. Lactobacillus sakei, Lactobacillus plantarum and Lactobacillus curvatus were found in decreasing order of abundance. The analysis of two meat products, 'morcilla' and 'fiambre de magro adobado' obtained from production lines revealed a common succession pattern in LAB populations in both products and showed that Leuc. mesenteroides became the main species during storage, despite being below the detection level of culture methods after packing.

CONCLUSIONS

Our results pointed to Leuc. mesenteroides as the main species responsible for bloating spoilage in vacuum-packed meat products.

SIGNIFICANCE AND IMPACT OF THE STUDY

Prevention of bloating spoilage in vacuum-packed cooked meat products requires the sensitive detection of Leuc. mesenteroides (i.e. by PCR).

Characterization of a Leuconostoc gelidum bacteriophage from pork

A new bacteriophage (phage ggg) and its host, Leuconostoc gelidum LRC-BD, were isolated from vacuum-packaged pork loins. Homogenates of pork loin tissue were enriched with L. gelidum LRC-BD to isolate phages. Cultural, biochemical and genetic methods were used to compare L. gelidum LRC-BD and the type strain, L. gelidum ATCC 49366. The phages were characterized by host range, morphology and phage-bacterial interaction in All Purpose Tween (APT) broth and on pork adipose tissue. With the exception of its inability to produce dextran from sucrose and the fermentation of l-arabinose, L. gelidum LRC-BD was culturally and biochemically similar to L. gelidum ATCC 49366. DNA-relatedness of the strains was confirmed by sequencing of the 16s rRNA gene. Electron microscopic observation revealed that phage ggg was a member of the Siphoviridae. The host range was limited to L. gelidum isolates from meats. Phages were able to replicate and limit the growth of L. gelidum LRC-BD in APT broth incubated aerobically and anaerobically at 4 degrees C, with a multiplicity of infection (MOI) of 0.001. When inoculated pork adipose tissue was stored at 4 degrees C in air or vacuum, phages could multiply but a higher MOI (0.01 to 1000) was necessary to limit the growth of L. gelidum LRC-BD. Naturally occurring phages may affect the numbers of L. gelidum and other lactic acid bacteria residing in meats and thereby alter the storage quality or the preservative potential of competitive strains.

The use of multiplex PCR to detect and differentiate food- and beverage-associated microorganisms: a review

Regarding food safety, rapid detection of microbial species is crucial to develop effective preventive and/or adjustment measures. Classical methods for determining the presence of certain species are time-consuming and labor-intensive, hence, molecular methods, which offer speed, sensitivity and specificity, have been developed to address this problem. Multiplex PCR (MPCR) is widely applied in the various fields of microbiology for the rapid differentiation of microbial species without compromising accuracy. This paper describes the method and reports on the state-of-the-art application of this technique to the identification of microorganisms vehiculated with foods and beverages. The identification of both pathogens and probiotics and the species important for food fermentation or deterioration will be discussed. Applications of MPCR in combination with other techniques are also reviewed. Potentials, pitfalls, limitations and future prospects are summarised.

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.

Intraspecific diversity of Lactobacillus curvatus, Lactobacillus plantarum, Lactobacillus sakei, and Leuconostoc mesenteroides associated with vacuum-packed meat product spoilage analyzed by randomly amplified polymorphic DNA PCR

The intraspecific diversity of Leuconostoc mesenteroides, Lactobacillus curvatus, Lactobacillus sakei, and Lactobacillus plantarum was analyzed by randomly amplified polymorphic DNA (RAPD) PCR with universal primers M13 and T3. The study included 100 reference strains and 210 isolates recovered from two vacuum-packed Spanish meat products, fiambre de magro adobado and morcilla, previously identified by rDNA-restriction fragment length polymorphism profiles. The RAPD-M13 profiles identified isolates at species level in L. plantarum and L. mesenteroides, while RAPD-T3 provided profiles in L. sakei. The combination of RAPD-M13 and RAPD-T3 fingerprints revealed a total of 17 profiles in L. mesenteroides, 6 in L. sakei, 12 in L. plantarum, and 6 in L. curvatus. Of these, six profiles corresponding to L. mesenteroides and one corresponding to L. sakei were found in both products. The Shannon-Weaver diversity index (H'), calculated according to RAPD-M13 and RAPD-T3 profiles during storage, revealed that most profiles appeared only in single samplings in both products, indicating a high strain substitution rate during chilled storage of vacuum-packed meat products. When bloating appeared, only one profile corresponding to L. mesenteroides subsp. dextranicum was present throughout the storage period.

Direct molecular approach to monitoring bacterial colonization on vacuum-packaged beef

Denaturing gradient gel electrophoresis allowed us to monitor total bacterial communities and to establish a pattern of succession between species in vacuum-packaged beef stored at 2 and 8 degrees C for 9 weeks and 14 days. Species-specific PCR was used to confirm the presence of Lactobacillus sakei and Lactobacillus curvatus. Multiplex PCRs using 16S rRNA-specific primers allowed differentiation between Leuconostoc species. These methods provided the desired information about microbial diversity by detecting the main microorganisms capable of colonizing this ecological niche.

Effect of high-barrier packaging films with different oxygen transmission rates on the growth of Lactobacillus sp. on meat fillets

The goal of this study was to determine the combined effect of (i) the oxygen transmission rate (OTR) of packaging film, often called oxygen film permeability or film permeability and (ii) temperature on the growth rate of the main prevailing organism, Lactobacillus sp., in 100% CO2-packed sterile meat fillets. Multifactorial experiments were designed to study the effect of OTR and temperature (0, 5, 8, and 10 degrees C) on the growth rate of Lactobacillus sp. inoculated on sterile meat fillets under 100% CO2 and aerobic conditions. The packaging conditions (air or 100% CO2) and the film OTR significantly affected the growth rate of Lactobacillus sp. only at temperatures higher than 0 degrees C. Low-permeable films with different OTRs did not affect the final population of the bacterium, but the growth rate was significantly changed. The correlation of an ephemeral microbial association with a low spoilage potential (e.g., lactic acid bacteria) or their growth retardation cannot always be assumed unless other determinants (e.g., OTR) of equal importance are taken into account. The present study provides information that can be of benefit to industry and the consumer.

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.

Carnobacterium divergens and Carnobacterium maltaromaticum as spoilers or protective cultures in meat and seafood: phenotypic and genotypic characterization

Carnobacterium, a genus of lactic acid bacteria, frequently dominate the microflora of chilled vacuum- or modified atmosphere-packed meat and seafood. In this study Carnobacterium isolates were characterized by phenotypic and molecular methods in order to investigate the association of species and intra-species groups with distinct kinds of meat and seafood. Of 120 test strains, 50 originated from meat (beef and pork products, including 44 strains isolated during this study and 6 strains obtained from culture collections) and 52 from seafoods (cod, halibut, salmon, shrimps and roe products). In addition, 9 reference strains of Carnobacterium spp from other sources than meat and fish and 9 reference strains of lactic acid bacteria belonging to other genera than Carnobacterium were included. Numerical taxonomy relying on classical biochemical reactions, carbohydrate fermentation and inhibition tests (temperature, salt, pH, chemical preservatives, antibiotics, bacteriocins), SDS-PAGE electrophoresis of whole cell proteins, plasmid profiling, intergenic spacer region (ISR) analysis and examination of amplified-fragment length polymorphism (AFLP) were employed to characterize the strains. The numerical taxonomic approach divided the carnobacteria strains into 24 groups that shared less than 89% similarity. These groups were identified as Carnobacterium divergens with one major cluster (40 strains) and 7 branches of one to four strains, Carnobacterium maltaromaticum (previous C. piscicola) with one major cluster (37 strains) and 9 branches of one to four strains and Carnobacterium mobile (three branches consisting in total of 4 strains). Branches consisting of references strains of the remaining Carnobacterium spp. were separated from clusters and branches of C. divergens, C. maltaromaticum and C. mobile. Isolates from the main clusters of C. divergens and C. maltaromaticum were found both in fresh and lightly preserved meat and seafood products. High phenotypic intra-species variability was observed for C. divergens and C. maltaromaticum but despite this heterogeneity in phenotypic traits a reliable identification to species levels was obtained by SDS-PAGE electrophoresis of whole cell proteins and by ISR based on 16S-23S rDNA intergenic spacer region polymorphism. With AFLP, two distinct clusters were observed for C. divergens but only one for C. maltaromaticum. The two C. divergens clusters were not identical to any of the clusters observed by numerical taxonomy. A limited number of C. divergens and C. maltaromaticum isolates possessed a biopreservative potential due to their production of bacteriocins with a wide inhibition spectrum. This study serves as a base-line for further investigations on the potential role of species of Carnobacterium in foods where they predominate the spoilage microflora.