Food Protection Trends - August 2010 - 474

FIGURE 2. Air Boss Pedestal Fan Model #WATPF26 used in holding chillers

fans to minimize heat-induced refrigerant pressure build up. Heaters used were the Hotbox-Axial HBA Fan Blower Heaters HB90415 — 415V 9.0kW and HB15415 415V 15kW (Thermal Electric Elements Pty Ltd, Brisbane, Australia). The fans used were the Air Boss Pedestal Fan Model # WATPF26. Heaters were modified by mounting them onto mobile stands, fitting them with a 10 m × 3 phase cable and installation of a 20A plug top with thermostat/auto cut-off designed to switch the unit off at 50oC. Heat treatment of holding chillers commenced May 1, 2006. The temperature was maintained at 50oC for a minimum of 2 h at each treatment time.

Sampling and microbiological analysis
Sampling was performed on all chillers from January 2005 to December 2006. All chillers were still used during sampling and still run completely

for production. A mix of vegetables, starch (potato mash and rice), sauce and protein (beef and chicken) products were placed in the chillers. The amount of product passing through each chiller varied, but each blast chiller had an approximate 2,000 to 3,000 kg turnover per day. Holding chillers had twice as much turnover. Each of three blast chillers were sampled for Listeria species from Monday to Friday over a two-year period, 12 months pre- and 12 months postcommencement of intervention, amounting to 490 samples per blast chiller per year. Over the two years, a total of 2,940 blast chiller environmental samples were drawn. Similarly, each of two holding chillers was sampled for Listeria species from Monday to Friday over a two-year period,16 months pre- and 8 months post-commencement of intervention, amounting to 919 samples per holding chiller over the 16 months pre-intervention, and 551 samples per holding chiller over 8 months post intervention. Over the

two years, there was a total of 2,940 holding chiller environmental samples drawn. The general areas targeted were internal areas (floors, walls), seals and doors. Separate polyurethane sponges (Whirl-Pak Speci-Sponge, Nasco, Fort Atkinson, WI), moistened with Butterfield’s solution (25 mL; bioMérieux, Hazelwood, MO) were used to sample an area of approximately 25 cm2. Approximately 2,940 environmental samples were drawn. Sponge samples were tested for the presence of Listeria by use of the Listeria BAX Automated System (DuPont Qualicon, Wilmington, DE, USA). Each sponge was enriched in 225 ml of buffered Listeria enrichment broth (Amyl Media, Melbourne, Australia) for 24 h at 35oC. One ml of enrichment was inoculated into 10 mL MOPS-buffered Listeria enrichment broth (Amyl Media) and incubated at 35oC for 18 – 24 h. Enrichment cultures were analyzed using the automated PCR, following the manufacturer’s user’s guide for preparing reagents, performing the test, and reading the results. Specifically, enrichment cultures were lysed and the lysate was used to hydrate the PCR reagents contained within a proprietary tablet. Processing in the automated PCR unit took approximately 4 hours, and electronic results appear as positive/negative icons on the unit screen. Presumptive positive samples were confirmed following manufacturer’s instructions by streaking retained MOPS-buffered Listeria enrichment broth onto Oxford and PALCAM agar (Amyl Media) and incubating at 37oC for 48 h. Colonies surrounded by dark brown or black haloes were confirmed as per the Australian Standard method AS1766.2.16 (1). Results were reported as detected or not detected/25 cm2.

Statistical analysis
The relationship between Listeria prevalence and chiller intervention was analyzed using the CHITEST formula in Microsoft Excel 2003. Significance was indicated when P < 0.001.

RESULTS
The prevalence of Listeria in chillers pre- and post-chiller interventions is presented in Table 1. Chiller prevalence is the sum of holding and blast chillers

474 FOOD PROTECTION TRENDS | AUGUST 2010

Food Protection Trends - August 2010

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