By Roger Brideau*
*Presented in part at the International Association of Food Preservation
‘IAFP’ Conference in Charlotte NC USA
Introduction- Pseudomonad organisms are a major cause of bacterial spoilage of pasteurized milk and dairy products due to post process contamination. Early detection of Pseudomonad’s can be a predictor of product shelf-life as they are the predominant psychotropic bacteria present. BioLumix has developed a rapid method for the detection of Pseudomonad’s in dairy products and the method is also applicable to their detection in process water.
Purpose- To evaluate the ability of the BioLumix system to detect Pseudomonad’s in dairy products, determine the speed to results, sensitivity, selectivity and ability to predict shelf-life.
Methods- the BioLumix system is an optical system that detects growth of Pseudomonad’s using a CO2 sensor in selective growth media. The BioLumix system was directly compared to the plate count methodology for milk samples stored at refrigerated temperatures and held overnight at room temperatures (enriched). Testing of water was also accomplished in side by side studies to show the capability of the BioLumix system for quantitation of Pseudomonads.
Results: Growth of Pseudomonad’s in the BioLumix vial
A growth comparison was made for detection of each Pseudomonad in the BioLumix system using PSE-B vials and on CFC (Pseudomonas agar) spread plates. Table 1 summarizes the growth of freshly diluted samples of organisms that were enriched in TSB during the prior 18-24 hrs. The PSE-B vials are selective, as shown by not allowing growth of unrelated gram positive and gram negative bacteria, yeast or mold. Four different species of Pseudomonad’s grew in the PSE-B vial and on CFC plates.
Milk Sample Testing
Commercial milk samples were tested upon arrival in the laboratory. Five of twenty were positive for the presence of Pseudomonad’s using both PSE-B vials and CFC spread agar plates. After storage for 3-7 days, twelve of twenty samples were positive for Pseudomonad’s including after enrichment at RT for 16-18 hrs. Thus, refrigerated milk samples have varying incidence of Pseudomonad flora.
Milk Calibration Curve
Organisms from milk samples that grew in PSE-B vials and on CFC plates were used to generate the Calibration Curve shown in Figure 1. These data suggest that low numbers (~10) of Pseudomonad’s should detect within 24 hrs in the PSE-B vial. The Calibration Curve can be embedded into the BioLumix software on the instrument and used to generate a read-out of cfu per gram of milk. This enables quantitation of the milk sample for the presence of Pseudomonad’s before 24 hr; a distinct advantage over plate methodology taking 48-72 hours.
Distribution of Data
In the dairy settings the goal is to separate a “good” sample that has a potential to maintain quality over a product’s shelf-life from a “bad: sample that will have a shorter shelf life. Criteria for separation between a “good” and “bad” product based upon the Pseudomonad’s numbers can be established. If one selects a count of 1,000 cfu/ml as the separation point: the Histogram shown in Figure 4 indicated at 12.5 hrs all samples with higher counts (in red) detected, while all the samples below 1,000 cfu/ml did not.
Results: Testing of Process Water for the presence of Pseudomonads
Eight different types of process water samples were found to be free of Pseudomonad’s after testing using PSE-B vials and CFC spread plates (data not shown). Clean process water samples were then inoculated with individual isolates of Pseudomonad’s were used to generate a calibration curves for water, similar to the milk calibration curve. Pseudomonad growth in inoculated process water was measured using PSE-B vials and PA spread plates and was used to generate the Calibration Curve. Detecting vials were confirmed to contain Pseudomonas by the Oxydase test.
The data presented show equivalency between the BioLumix PSE-B vial and CFC (Pseudomonas agar) plates for the detection of Pseudomonad’s found in commercial milk samples and in inoculated process water samples. PSE-B vials detected as little as 1-3 organisms (data not shown).
The number of organisms in commercial milk was found to increase over time at refrigerated temperatures and this agreed with a previously published report (Burdova et al 2002) showing the affect of storage temperature on milk shelf-life.
The BioLumix assay is completed in 18 hours and offers an advantage over spread plate methods for time to results and ease of calculation of cfu per gram of milk or water. A single vial is all that is needed and thus both time and material costs are reduced. Calibration Curves were easily generated for both milk and water sampling and can be used to generate a cfu/ml of sample in less than 1 day to yield an estimate of cfu/gram.
REFERENCE: Burdova, O. et al (2002). Bulletin Vet Med. Poland. 46:325-329. Hygiene of Pasteurized Milk Depending on Psychrotrophic Microorganisms.