Effect of packaging under carbon dioxide, nitrogen or air on the microbial flora of pork stored at 4 degrees C

Effects of Different Storage Temperatures on the Physicochemical Properties and Bacterial Community Structure of Fresh Lamb Meat

This study was aimed to compare the physicochemical properties and bacterial community structure of tray-packaged fresh lamb meat under different storage temperatures, such as 4°C (chilling), −1.5°C (supercooling), −4°C (superchilling) and −9°C (sub-freezing). The total viable counts (TVC), total volatile base nitrogen (TVB-N), bacterial diversity and metabolic pathways were investigated. The results indicated that the shelf life of superchilling and sub-freezing storage was over 70 d, which was significantly longer than that of chilling and supercooling storage. TVC and TVB-N values showed an increasing trend and were correlated well (R²>0.92). And the TVB-N values of lamb meat were exceeded the tolerable limit (15 mg/100 g) only found under chilling and supercooling storage during storage period. At the genus level, Pseudomonas was the core spoilage bacteria then followed Brochothrix for chilling and supercooling storage. Pseudomonas, Ralstonia, Psychrobacter and Acinetobacter were the dominant spoilage bacteria for superchilling and sub-freezing storage. Furthermore, the bacterial community diversity of lamb meat stored at chilling and supercooling storage decreased with the storage time prolonged, which was opposite to the outcome of meat stored under superchilling and sub-freezing storage. For chilling and supercooling storage, the abundance of main metabolisms (carbohydrate metabolism and amino acid metabolism, etc.) of bacteria increased with the storage time prolonged, which was opposite to superchilling storage. This may be related to the bacteria community diversity and the formation of dominant spoilage bacteria. In conclusion, this work provides data for the preservation of fresh lamb meat which will benefit the meat industry.

Effects of Intraoperative Insufflation With Warmed, Humidified CO2 during Abdominal Surgery: A Review

PURPOSE

During a laparotomy, the peritoneum is exposed to the cold, dry ambient air of the operating room (20°C, 0%-5% relative humidity). The aim of this review is to determine whether the use of humidified and/or warmed CO2 in the intraperitoneal environment during open or laparoscopic operations influences postoperative outcomes.

METHODS

A review was performed in accordance with PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines. The PubMed, OVID MEDLINE, Cochrane Central Register of Controlled Trials and Embase databases were searched for articles published between 1980 and 2016 (October). Comparative studies on humans or nonhuman animals that involved randomized controlled trials (RCTs) or prospective cohort studies were included. Both laparotomy and laparoscopic studies were included. The primary outcomes identified were peritoneal inflammation, core body temperature, and postoperative pain.

RESULTS

The literature search identified 37 articles for analysis, including 30 RCTs, 7 prospective cohort studies, 23 human studies, and 14 animal studies. Four studies found that compared with warmed/humidified CO2, cold, dry CO2 resulted in significant peritoneal injury, with greater lymphocytic infiltration, higher proinflammatory cytokine levels and peritoneal adhesion formation. Seven of 15 human RCTs reported a significantly higher core body temperature in the warmed, humidified CO2 group than in the cold, dry CO2 group. Seven human RCTs found lower postoperative pain with the use of humidified, warmed CO2.

CONCLUSION

While evidence supporting the benefits of using humidified and warmed CO2 can be found in the literature, a large human RCT is required to validate these findings.

The effective product life of vacuum-packaged beef imported into Saudi Arabia by sea, as assessed by chemical, microbiological and organoleptic criteria

Five transport and storage trials were conducted on commercial shipments of vacuum-packaged beef half striploins imported into Saudi Arabia by sea from Australia, New Zealand and the Republic of Ireland. The cold chains, from packaging to the end of chilled storage in Saudi Arabia, were monitored using miniature electronic data loggers placed in cartons of striploins. Development of spoilage microfloras was followed from packaging to trial end using a differential aerobic plate count technique. Changes in meat pH, total volatile nitrogen (TVN) and free fatty acid (FFA) content were determined over the storage period. Sensory evaluations to identify spoilage conditions and to determine consumer acceptability were also conducted over the course of storage in Saudi Arabia. Results indicated that aerobic plate counts, TVN and FFA were all unsuitable as indicators of the fitness, or acceptability of vacuum-packaged beef for human consumption. Under good but commercially realistic conditions (i.e. normal ultimate pH beef, initial microbial contamination at packaging of less than 10(3)/cm(2), packaging, film oxygen transmission rate less than 40 ml/m(2)/24 h/atm at 23°C and 90% R.H., and mean product temperature during transport and storage of 0°C), vacuum-packaged beef transported by sea has an anticipated product life of at least 90 days measured from the date of slaughter. Under the conditions just stated, deterioration of meat texture resulting from excessive aging rather than the onset of overt microbial spoilage will limit the effective product life of vacuum-packaged beef.

Carbon dioxide evolution of refrigerated meat

The carbon dioxide (CO(2)) evolution from pork loins stored in air or nitrogen at 5°C was measured. The loins were cut immediately after slaughter ('pre-rigor meat.'), or approximately 20 h after slaughter ('post-rigor meat'). CO(2) evolution rate was highest during the first day (2·3 × 10(-3)mlcm(-2)h(-1)) and then declined to approximately 0·5 × 10(-3)mlcm(-2)h(-1). The rate was fairly constant during this later phase, and there were no large differences between aerobically and anaerobically stored meat, or between pre-rigor meat and post-rigor meat. Four hypotheses for the origin of the CO(2) evolution are discussed: (1) diffusion from a pre-formed pool in the meat; (2) aerobic energy metabolism of the meat cells; (3) other biochemical reactions of the meat cells; and (4) microbial activity. None of the hypotheses completely supports the experimental results of the investigation. The impact of these findings on the vacuum packaging of meat is discussed.

Changes in the composition of the bacterial flora on tray-packaged pork during chilled storage analyzed by PCR-DGGE and real-time PCR

In this study, a polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis (DGGE) was used to investigate the changes in the composition of the bacterial population of tray-packaged pork during chilled storage. Relative quantitative real-time PCR was further used to evaluate the predominant spoilage bacteria obtained from DGGE analysis for their relative amount to the total bacteria in meat samples. DGGE analysis of the V3 and V6-V8 regions of the 16S rRNA gene showed that Pseudomonas were the predominant bacterial species at the end of the monitoring period. Real-time PCR expressed as the ΔΔC(T) method showed that the average 2(-ΔΔC)(T) values increased continually during the storage period from less than 0.001 at day 0 to 4.438 at the end of the monitoring, which indicated that the proportions of Pseudomonas within the total bacteria in meat samples increased. Both methods confirmed that Pseudomonas was the predominant spoilage bacteria. Practical Application: This study uses new techniques to identify bacteria in fresh retail pork and to follow changes in the bacterial population during 12 d refrigerated storage. Pseudomonads were found to increase with storage time, becoming the dominant flora after 12 d.

Starch-Ampicillin Agar for the Quantitative Detection of Aeromonas hydrophila

Interest in Aeromonas hydrophila as a food-borne and human pathogen is increasing. Isolation media from the clinical laboratory were evaluated for food use and either did not give quantitative recovery of A. hydrophila or did not permit ready differentiation of A. hydrophila from the background microflora. A new medium was developed which permitted quantitative recovery of A. hydrophila from foods. The medium consisted of phenol red agar base (Difco Laboratories), soluble starch (10 g/liter), and ampicillin (10 mg/liter). All foods surveyed contained A. hydrophila. Foods sampled included red meats, chicken, raw milk, and seafood (fish, shrimp, scallops, crab, and oysters). The count of A. hydrophila at the time of purchase ranged from 1 x 10/g (lower limit of detection) to 5 x 10/g. In most instances, the count of A. hydrophila increased during 1 week of storage at 5 degrees C. The starch-ampicillin agar developed permitted rapid quantitative recovery of A. hydrophila from foods in the presence of very large numbers of competing microflora.

Screening concepts for the isolation of biosurfactant producing microorganisms

This chapter gives an overview of current methods for the isolation of biosurfactant producing microbes. The common screening methods for biosurfactants are presented. Sampling and isolation of bacteria are the basis for screening of biosurfactant producing microbes. Hydrocarbon-contaminated sites are the most promising for the isolation of biosurfactant producing microbes, but many strains have also been isolated from undisturbed sites. In subsequent steps the isolates have to be characterized in order to identify the strains which are interesting for a further investigation. Several techniques have been developed for identifying biosurfactant producing strains. Most of them are directly based on the surface or interfacial activity of the culture supernatant. Apart from that, some screening methods explore the hydrophobicity of the cell surface. This trait also gives an indication on biosurfactant production. In recent years automation and miniaturization have led to the development of high throughput methods for screening. High throughput screening (HTS) for analyzing large amounts of potential candidates or whole culture collections is reflected in the end. However, no new principals have been introduced by HTS methods.

Preservation of raw milk by the addition of carbon dioxide

The effects of concentrations of 10–30 mM/1 of CO2on the growth of psychrotrophic milk spoilage organisms were examined, both in untreated whole milk and in pure cultures ofPseudomonas fluorescensgrowing in skim-milk and broth. Spoilage of refrigerated untreated whole milk was found to be inhibited by the addition of CO2and the effect was enhanced with increasing concentrations of CO2and decreasing storage temperature. Experiments with pure cultures ofPs. fluorescensshowed that the inhibitory effects of CO2were not due to increased acidity or to displacement of dissolved O2, but to the presence of CO2per sewhich induced an increase in the duration of the lag phase of growth and had only a small effect on the logarithmic phase.

The glutamate decarboxylase acid resistance mechanism affects survival of Listeria monocytogenes LO28 in modified atmosphere-packaged foods

AIMS

The contribution of the glutamate decarboxylase (GAD) acid resistance system to survival and growth of Listeria monocytogenes LO28 in modified atmosphere-packaged foods was examined.

METHODS AND RESULTS

The survival and growth of the wild-type LO28 and four GAD deletion mutants (DeltagadA, DeltagadB, DeltagadC, DeltagadAB) in packaged foods (minced beef, lettuce, dry coleslaw mix) during storage at 4, 8 and 15 degrees C were studied. Survival and growth patterns varied with strain, product type, gas atmosphere and storage temperature. In minced beef, the wild-type LO28 survived better (P < 0.05) than the GAD mutant strains at 8 and 15 degrees C. In both packaged vegetables at all storage temperatures, the wild-type strain survived better (P < 0.05) than the double mutant DeltagadAB. The requirement for the individual gad genes varied depending on the packaged food. In the case of lettuce, gadA played the most important role, while the gadB and gadC genes played the greatest role in packaged coleslaw (at 15 degrees C).

CONCLUSIONS

This work demonstrates that elements of the GAD system play significant roles in survival of L. monocytogenes LO28 during storage in modified atmosphere-packaged foods.

SIGNIFICANCE AND IMPACT OF THE STUDY

A better understanding of how L. monocytogenes behaves in modified atmosphere-packaged foods, and how it responds to elevated carbon dioxide atmospheres.

Composition of the bacterial population of refrigerated beef, identified with direct 16S rRNA gene analysis and pure culture technique

The composition of the dominating population of freshly cut beef, and beef stored at 4 degrees C for 8 d, was studied by direct analysis of the 16S rRNA gene (PCR amplification, cloning and sequencing) and compared with pure culture technique where the isolates picked from the viable plate count were identified by sequencing of the 16S rRNA gene. The composition of the bacterial population was recorded at two different time points, at the start when the viable plate count of the meat was 4 x 10(2) colony forming unit (cfu) per cm(2) and when it was 5 x 10(7) cfu per cm(2). Direct gene analysis by PCR amplification generated 30 clones, and 79 isolates were picked from the plate count, and identified by 16S rRNA gene sequencing. At the low initial bacterial load of the beef, the two sampling strategies showed variations in the composition of species. Direct 16S rRNA gene analysis revealed a domination of Bacillus-like sequences while no such sequences were found in isolates from the viable plate count. Instead the population of the plate count was dominated by Chryseobacterium spp. In contrast, the two sampling strategies matched on the multiplying beef population, where both methods indicated Pseudomonas spp. as the dominating group (99% of the population-sequences), irrespectively of sampling strategy. Pseudomonas panacis/Pseudomons brennerii was the dominating taxon (99% similarity to type strain), but sequences with highest similarity to Pseudomonas lundensis (99%), Pseudomonas beteli (99%) and Pseudomonas koreensis (100%) were also found.

Influence of packaging conditions on natural microbial population growth of endive

The influence of three packaging conditions, i.e., unmodified atmosphere packaging (UAP), passive modified atmosphere packaging (MAP), and active MAP, on the natural microbial population growth of endive was investigated at 20 degrees C. For UAP, endive was placed in macroperforated oriented polypropylene pouches that maintained gas composition close to that of air (21 kPa O2 and 0 kPa CO2) but also limited superficial product dehydration. For MAP, endive was placed in low-density polyethylene pouches that induced a 3 kPa O2 and 5 kPa CO2 equilibrium atmosphere composition. Steady state was reached after 25 h of storage with an oxygen absorbing packet (active MAP) compared with 100 h without the packet (passive MAP) and was maintained for 200 h. After 312 h of storage, both active and passive MAP reduced total aerobic mesophile, yeast, and mold population growth compared with endive in UAP. Active MAP accelerated and improved the inhibition of Pseudomonas spp. and Enterobacteriaceae, respectively, probably because of the rapid O2 depletion during the transition period. A shift in the Enterobacteriaceae subpopulation from Rhanella aquatilis to Enterobacter agglomerans was observed for both passive and active MAP.

Carbon dioxide inhibits the growth rate of Staphylococcus aureus at body temperature

BACKGROUND

Since the 1930s, carbon dioxide (CO(2)) has been combined with cold storage for the preservation of food. However, its use for the prevention of surgical wound infection was long considered to be impractical. Now CO(2) is widely used during laparoscopic procedures, and a method has been developed to create a CO(2) atmosphere in an open wound. The aim of this study was to investigate the effect of CO(2) on the growth of Staphylococcus aureus at body temperature.

METHODS

First, S. aureus inoculated on blood agar were exposed to pure CO(2) (100%), standard anaerobic gas (5% CO(2), 10% hydrogen, 85% nitrogen), or air at 37 degrees C for a period of 24 h; then a viable count of the bacteria was made. Second, S. aureus inoculated in brain-heart infusion broth and kept at 37 degrees C were exposed to CO(2) or air for 0, 2, 4, 6, and 8 h; then the optical density of the bacteria was measured.

RESULTS

After 24 h, the number of S. aureus on blood agar was about 100 times lower in CO(2) than in anaerobic gas (p = 0.001) and about 1,000 times lower than in air (p = 0.001). Also, in broth, there were fewer bacteria with CO(2) than with air (p < 0.01). After 2 h, the number of bacteria was increased with air (p < 0.001) but not with CO(2) (p = 0.13). After 8 h, the optical density had increased from zero to 1.2 with air but it had increased only to 0.01 with CO(2) (p = 0.001).

CONCLUSION

Pure CO(2) significantly decreased the growth rate of S. aureus at body temperature. The inhibitory effect of CO(2) increased exponentially with time. Its bacteriostatic effect may help to explain the low infection rates in patients who undergo laparoscopic procedures.

The bacterial flora of fresh and chill-stored pork: analysis by cloning and sequencing of 16S rRNA genes

The composition of the initial bacterial flora of pork and the development of the flora after storage at +4 degrees C for 4 days were analysed by amplification, cloning and sequencing of 16S rDNA. A total of 122 clones were obtained, with lengths of > or =400 nucleotides and > or =95% similarity to database sequences. Nineteen clones were similar to sequences in database not assigned to any genera. Fourteen different genera were represented in clones from fresh meat, with 36.5% of the clones most resembling Acinetobacter and 17.3% resembling Staphylococcus and Macrococcus. After storage, the clones were composed of six different genera, with 44.3% resembling Pseudomonas, 17.1% resembling Aeromonas and only 14.3% resembling Acinetobacter. This study shows that the overall pattern of the initial and chill-stored pork flora, as shown by a molecular approach, was in agreement with results obtained in previous studies using traditional cultivation methods.

Effect of oregano essential oil on microbiological and physico-chemical attributes of minced meat stored in air and modified atmospheres

AIMS

This study aimed to determine the combined effect of packaging (air, modified atmosphere) with or without the addition of essential oil not only on the selection of microbial association of meat but also to determine any significant difference in microbial metabolites produced from the prevailing bacteria.

METHODS AND RESULTS

Samples of minced meat were mixed with different concentration of oregano essential oil (0, 0.05, 0.5 and 1% v/w) and packed under aerobic or with modified atmosphere (Mixed Gas Modified Atmosphere--MGMA, 40% CO2/30% N2/30% O2; or CO2 Modified Atmosphere--COMA, 100% CO2) and stored at 5 degrees C. In all packaging conditions, only concentrations of 0.5% and 1% oregano oil were effective. Inhibition was evident in the order air < MGMA < COMA. Oregano essential oil delayed glucose and lactate consumption aerobically as well as under MGMA. pH changes were also evident. Furthermore, proteolysis was significantly inhibited in aerobically stored samples, and so was the production of acetate under MAP. Similar results were obtained for the other organic acids eluted from HPLC column.

CONCLUSIONS

Oregano essential oil delayed microbial growth and suppressed the final counts of the spoilage micro-organisms. It also caused a pronounced alteration in the physico-chemical properties of the minced meat.

SIGNIFICANCE AND IMPACT OF THE STUDY

Microbial analysis alone as spoilage index may misrepresent the effect of a hurdle such as essential oils on spoilage.

Review of centralized packaging systems for distribution of retail-ready meat

There is growing interest in centralized preparation of retail-ready meat cuts for distribution to widely dispersed retail stores due to the convenience of having high-quality ready-to-go products that are consistently provided to consumers at lower cost. Various centralized packaging techniques are described. Of all packaging techniques, master packaging is the most economical and shows promise for commercial application. Nevertheless, the master-packaging technique must be integrated with strict temperature control in a narrow range just above freezing (- 1.5 +/- 0.5 degrees C), good processing hygiene, and maintenance of a completely anoxic atmosphere in the package headspace throughout the distribution period to maximize storage life. Packaging using the CAPTECH process reduces the residual O2 present in the headspace to 300 ppm. Oxygen scavengers must be incorporated in the package to absorb the residual O2 and preserve the metmyoglobin reducing activity of meat tissues. Integration of all these technologies can provide a storage life of retail-ready meat up to 10 weeks in the master package followed by 3 days of retail display life. This extension of storage life is sufficient for transporting meat to distant markets.

A numerical taxonomic study of the Pseudomonas flora isolated from poultry meat

Pseudomonas strains were isolated from both fresh and cold-stored broiler skin. Phenotypically-based numerical taxonomic techniques were used to characterize the isolates and 36 reference strains. For this purpose, Biolog GN Microplates, API 20NE and a number of other biochemical tests were used. Jaccard clustering revealed the predominance of four major Pseudomonas groups: Ps. fragi, Ps. lundensis, strains belonging to Ps. fluorescens biovars and an unidentified group of strains displaying a high degree of similarity to Ps. fluorescens biovars. Within Ps. fluorescens, biovar A was best represented. The marked proteolytic character of members of Ps. fluorescens biovars A, B and C, as well as of members of the unidentified cluster, supports their possible role in the origin of organoleptic defects. In the Ps. lundensis cluster, a distinct group of Ps. lundensis-like species was found. Further genotypic studies should be carried out to clarify the taxonomic status of the Ps. lundensis-like strains and that of the unidentified group resembling Ps. fluorescens biovars A and B.

Predictive modelling of Escherichia coli O157:H7: inclusion of carbon dioxide as a fourth factor in a pre-existing model

Two models for Escherichia coli O157:H7 are compared, one with growth-controlling factors pH (4.5-7.0), temperature (10-30 degrees C) and NaCl concentration (0.5-6.5% w/v) and the other with the same factors and ranges, but with the addition of carbon dioxide (CO2: 10-80% v/v). Validation of the four-factor model, to include food packed in modified atmospheres containing CO2, was not possible due to lack of published data. However, where CO2 concentration was entered as 0%, only minor differences occurred between the predictions from the two models for the same conditions of pH, NaCl and temperature; consequently reliable, safe predictions using the four-factor model, with CO2 concentration recorded as 0%, can be made for foods packed in air. At temperatures from 10 to 30 degrees C, it was found that lower (10 and 20%) concentrations of CO2 had little effect on lag times and growth rates, and higher concentrations still permitted growth of E. coli O157:H7 under a wide range of conditions of NaCl concentration, pH value and temperature, suggesting that the organism is relatively CO2-tolerant.

Emerging pathogens: Aeromonas spp

Aeromonas spp. are Gram-negative rods of the family Vibrionaceae. They are normal water inhabitants and are part of the regular flora of poiquilotherm and homeotherm animals. They can be isolated from many foodstuffs (green vegetables, raw milk, ice cream, meat and seafood). Mesophilic Aeromonas spp. have been classified following the AeroKey II system (Altwegg et al., 1990; Carnahan et al., 1991). The major human diseases caused by Aeromonas spp. can be classified in two major groups: septicemia (mainly by strains of A. veronii subsp. sobria and A. hydrophila), and gastroenteritis (any mesophilic Aeromonas spp. but principally A. hydrophila and A. veronii). Most epidemiological studies have shown Aeromonas spp. in stools to be more often associated with diarrhea than with the carrier state; an association with the consumption of untreated water was also conspicuous. Acute self-limited diarrhea is more frequent in young children, in older patients chronic enterocolitis may also be observed. Fever, vomiting, and fecal leukocytes or erythrocytes (colitis) may be present (Janda, 1991). The main putative virulence factors are: exotoxins, endotoxin (LPS), presence of S-layers, fimbriae or adhesins and the capacity to form capsules.

Phenotypically based taxonomy using API 50CH of lactobacilli from Nigerian ogi, and the occurrence of starch fermenting strains

One-hundred and twenty isolates of lactic acid bacteria isolated from ogi and three traditional cereal-based alcoholic beverages from Nigeria, together with 18 reference strains from Swedish sour doughs, and 50 type-and reference strains of mainly Lactobacillus, were phenotypically classified on their fermentation ability of 49 carbohydrates, including soluble starch. Data were examined by Jaccard Coefficient (SJ), Simple Matching Coefficient (SSM), and unweighted pair group algorithm with arithmetic averages (UPGMA). Seven major clusters were defined at the 82% SJ-similarity level (corresponds to the SSM-level of 91%). Three were identified as Lactobacillus plantarum or L. plantarum-like (together 41% of the ogi isolates). The others were obligately heterofermentative; Cluster 7 was identified as L. confusus (11% of the ogi isolates). Three minor clusters were identified as L. murinus, L. agilis or L. gallinarium, and Leuconostoc mesenteroides subsp. mesenteroides, respectively. The phenotype of the L. plantarum isolates varied within wide limits. Seventeen isolates possessed starch fermenting capacity. Nine of these were identified as L. plantarum or L. plantarum-like (isolated from ogi); one was identified as Leuconostoc mesenteroides subsp. mesenteroides, and the rest were unidentified non-clustering strains.

Presence of Aeromonas hydrophila in slaughtered animals

The fate of Aeromonas hydrophila after a bacteriaemia in four healthy pigs intravenously inoculated with 10(9) and 10(15) cells of the bacterium was investigated. In two pigs slaughtered 30 min after inoculation the bacteria were found in blood (8 x 10(2)/ml and 4.2 x 10(2)/ml), meat and fatty tissues (1-10/g), but not in liver, spleen and superficial inguinal lymph nodes. A. hydrophila was not found in blood, meat, fatty tissue, liver, spleen, kidneys and superficial inguinal lymph nodes of other two pigs slaughtered 5.5 h postinoculation, with exception of one superficial inguinal lymph node of one pig.

The public health significance of Aeromonas spp. in foods

There is now evidence that some strains of Aeromonas species are enteropathogens. Such strains possess virulence properties, such as the ability to produce enterotoxins, cytotoxins, haemolysins and/or the ability to invade epithelial cells. Strains with these properties are common contaminants of drinking water and a wide range of foods. Contact or consumption of contaminated water, especially in summer, is a major risk factor in Aeromonas-associated gastroenteritis. Aeromonas-contaminated foods may also be vehicles of infection. Given the properties of strains that have been described in foods it has been suggested that food-borne illness could result not only from colonization and in vivo expression of virulence factors, but possibly also by intoxication following ingestion of foods that have been stored for a period of time, even under refrigeration. This paper reviews what is known about Aeromonas spp. in foods, their expression of virulence determinants, particularly at refrigeration temperatures, and the questions remaining to be answered to evaluate the risk they pose, so that an appropriate public health response can be determined.

Contamination of beef carcasses by psychrotrophic Pseudomonas and Enterobacteriaceae at different stages along the processing line

The extent of the contamination of beef carcasses with psychrotrophic Pseudomonas spp. and Enterobacteriaceae during slaughter, chilling and cutting was estimated by introducing a new analytical procedure; the contamination index. Comparisons were made between the initial viable counts and the contamination index. The contamination index was calculated as the sum of the bacterial counts obtained during aerobic cold storage of excised meat samples. The presence and composition of spoilage bacteria in the slaughter environment and on the carcasses was also determined at one plant. Rapid chilling was identified as a critical processing step by the contamination index. In addition to this, the dehiding and the chilling in cold storage rooms were implicated as critical operations, with respect to aerosol contamination and surface cross-contamination. Comparison of the composition of spoilage bacteria in the slaughter environment and the bacteria proliferating on the carcass surface samples taken at the corresponding steps showed similar distributions of the identified Pseudomonas spp. In five surveys at two plants, the contamination of beef carcasses along the processing line was estimated. Statistically significant variations between different processing steps were more pronounced for the contamination index than for the conventional counts. It was concluded that the contamination index could be used for identifying critical processing steps, with respect to the extent of contamination of carcasses by psychrotrophic spoilage bacteria.