A method to assess the bacterial content of refrigerated meat

Aminopeptidase activity by spoilage bacteria and its relationship to microbial load and sensory attributes of poultry legs during aerobic cold storage

The shelf life of poultry legs stored aerobically and the possible role of the aminopeptidase activity of gram-negative bacteria (p-nitroaniline test) as a predictor of poultry spoilage were evaluated on the basis of microbiological and sensory parameters. Chicken legs (n = 30) obtained immediately after evisceration in a local poultry processing plant were kept under aerobic refrigeration (4 +/- 1 degrees C) for 7 days. Microbiological (counts of aerobic bacteria and psychrotrophs) and sensory (odor, color, and general acceptability on a hedonic scale of 1 to 9) parameters and aminopeptidase activity (absorbance at 390 nm [A(390)]) determinations were performed after 0, 1, 3, 5, and 7 days of storage. Aerobic plate counts of 7 log CFU/g and a score of 6 for general acceptability were used as indicators of the end point of shelf life. Strong correlations (r > or = 0.76; P < 0.001) were obtained between bacterial counts, hedonic scores, and A(390) values. Samples were judged as unacceptable (shelf-life end point) after 2 and 4 days on the basis of sensory and microbiological analyses, respectively. A(390) values of 0.52 and 0.89 (corresponding to p-nitroaniline concentrations of 6.25 and 10.7 microg/ml, respectively) are proposed as the upper limits for acceptability on the basis of sensory and microbiological determinations, respectively. However, these recommendations are based on a small set of samples, and their general application is yet to be verified.

A review of aerobic and psychrotrophic plate count procedures for fresh meat and poultry products

This is a review of reports that employed aerobic plate counts on fresh meat and poultry products since 1985; it lists synopses of 100 applications. A total of 15 different plating media were used, with 48 (48%) being either plate count agar (PCA) or tryptone glucose yeast extract agar. The temperature-time relations ranged from a low temperature of 20 degrees C for 120 h to 37 degrees C for 24 h. Some 29 different temperature-time combinations were used among the total of 109, with 21 (19.3%) being 35 degrees C/48 h, followed by 12 (11.0%) at 32 degrees C/48 h, 11 (10.1%) at 25 degrees C/48 h, and 9 (8.3%) at 25 degrees C/72 h. Fifty-four (49.5%) plate count applications employed incubation temperatures of 30 degrees C and below. From the 26 reports that employed psychrotrophic counts, 16 (61.5%) used PCA; 18 different temperature-time combinations were used, with 7 degrees C/10 d employed by only four. Twenty-one (80.8%) employed an incubation temperature at or <10 degrees C, and five employed an incubation temperature >10 degrees C. There is a serious need for some consensus on methodologies for aerobic and psychrotrophic counts on fresh meat and poultry products.

Comparison of different peptidase substrates for evaluation of microbial quality of aerobically stored meats

Different aminopeptidase and endopeptidase substrates were assessed for the detection of enzymatic activity of microorganisms collected from the surface of aerobically cold-stored pork and beef. The most sensitive substrates were fluorogenic Ala-7-amino-4-methylcoumarin (Ala-AMC) or Leu-AMC and colorogenic Ala-p-nitroanilide (Ala-pNA). Activity on natural oligopeptides, e.g., bradykinin or alpha(s1) casein fragment 1 to 23, was very low. The correlation coefficient (r) between log surface counts of 66 meat samples and log fluorescence or absorbance after incubation of surface microbial cells for 2 h with Ala-AMC, Leu-AMC, and Ala-pNA was 0.89, 0.83, and 0.82, respectively. A distinct yellow color was obtained with Ala-pNA when the surface count was approximately 10(6) CFU/cm2. Although correlation and sensitivity was better, no clear advantage is obtained with the use of the fluorogenic Ala-AMC or Leu-AMC instead of Ala-pNA, a substrate proposed by Alvarado et al. (J. Food Sci. 57:1330, 1992) for rapidly assessing the microbial quality of refrigerated meat. The correlation coefficient (r) between time of cold storage and surface count was 0.69.

Chemical/biochemical detection of spoilage

Although sensory and/or microbiological analyses are widely relied on when assigning shelf-life of foods or trouble shooting problems with spoilage under storage, they do have drawbacks. Delay in obtaining results is one of them. The expense of the expert panels required to obtain meaningful sensory evaluations is another, while spoilage is not always of microbial origin. Even when it is, there are an increasing number of situations, including that of meats and fish packaged in modified atmospheres, where the relationships between microbial growth and spoilage onset is poorly defined. Chemical analysis has long been recognized as a means of circumventing at least some of the drawbacks and its potential is reviewed below. From the data presented it can be concluded that chemical characterization of spoilage processes is presently of most value in trouble shooting i.e. establishing the causes of spoilage. Its value in assigning total or remaining shelf-life requires more knowledge of the chemical processes leading to reduced acceptability/spoilage and of their correlations with sensory and microbiological changes.

Polyclonal antibodies against protein F from the cell envelope of Pseudomonas fluorescens for the detection of psychrotrophic bacteria in refrigerated meat using an indirect ELISA

An indirect enzyme-linked immunosorbent assay (ELISA) has been developed for detection of Pseudomonas fluorescens and related psychrotrophic bacteria in refrigerated meat. The ELISA uses polyclonal antibodies raised in rabbits against protein F from the cell envelope of Pseudomonas fluorescens AH-70. The anti-protein F antibodies were recovered from the crude antiserum by ammonium sulfate precipitation and conjugated to biotin. Commercial Extr Avidin-peroxidase conjugate was used to detect the biotinylated antibodies bound to their specific antigens. Subsequent enzymatic conversion of substrate gave distinct absorbance differences when assaying meat samples containing P. fluorescens strains of different origin as well as related psychrotrophic microorganisms. The detection threshold for the ELISA assay developed in this work was 10(4)-10(5) cfu cm(-2).

A substrate-mediated assay of bacterial proton efflux/influx to predict the degree of spoilage of beef mince stored at chill temperatures

A method was developed to predict spoilage of minced meat at chill temperatures, based on the difference in proton efflux from and influx into bacterial cells. This difference depends on the number of organisms present, the available glucose in the meat sample and the ability of the organisms to metabolize amino acids. The proton efflux/influx of a meat filtrate containing bacteria was measured at 25 degrees C with a pH/ion meter in the presence of peptone with or without glucose. There was a noticeable rate of change of mV h-1 of the meat filtrate prior to the organoleptic detection of spoilage which may be used semi-predictively to determine the remaining shelf-life of meat at different storage temperatures. The method could be investigated further, encompassing type and relative numbers of organisms, incubation temperature, meat type and composition (i.e. available glucose) to produce a spoilage prediction model. The method does not require sophisticated equipment, only a standard pH/ion meter, is cheap, needing only peptone and glucose, is relatively simple, and takes less than 2 h to perform.

A sandwich enzyme-linked immunosorbent assay (ELISA) for detection of Pseudomonas fluorescens and related psychrotrophic bacteria in refrigerated milk

A sandwich ELISA (enzyme-linked immunosorbent assay) was developed for detection of Pseudomonas fluorescens and related psychrotrophic bacteria in refrigerated milk. Polyclonal antibodies were raised in rabbits against protein F from the cell envelope of Pseudomonas fluorescens AH-70. The anti-protein F antibodies (anti-PF) bound to the wells of a microtitre plate were used to capture this protein from the micro-organisms on milk samples. Further immunorecognition of the captured antigen was attained with the same anti-PF antibodies conjugated to biotin. ExtrAvidin-peroxidase was used to detect the biotinylated antibodies bound to their specific antigens. Subsequent enzymic conversion of substrate gave clear absorbance differences when assaying milk samples containing Ps. fluorescens strains of different origin as well as related psychrotropic micro-organisms.