Packaged Ice Quality Raises Concern
Ice receives little or no attention as a possible source of foodborne pathogens. While numbers decline, some microorganisms survive freezing and, although cells may be injured, survivors may recover when the ice melts. Past studies have indicated that the ability of foodborne pathogens to recover from exposure to subfreezing environments makes ice a vehicle for transmission to food and beverages.
We did a study to determine the microbiological and chemical quality of ice produced and bagged on-premise in retail establishments and in self-service vending machines in Georgia. Results were compared to those from ice produced by companies that are monitored by the International Packaged Ice Association (IPIA).
Packaged ice (250 bags, 8 oz. each) made in retail stores and self-service vending machines, along with 25 10-lb. bags of commercially manufactured ice, were tested. Ice was melted at 24°C within 24 h of collection and water was analyzed for heterotrophic bacteria, coliforms, non-pathogenic Escherichia coli, and enterococci. Membrane filtration coupled with enrichment was used for detecting Salmonella and Listeria monocytogenes. Chemical analysis was done on samples collected from retail establishments and vending machines. Conductivity, pH, turbidity, and nitrate content were determined.
Approximately 6% of the ice samples bagged at retail sites and from vending machines contained unsatisfactory levels of heterotrophs according to maximum counts set by the IPIA (>500 CFU/100 ml). Twenty-six percent of the ice samples contained an unsatisfactory level of coliforms (>2.2 MPN/ml), 1% contained nonpathogenic E. coli, and 13% contained enterococci (>1.0 MPN/ml). One sample tested positive for the presence of Salmonella and another was positive for Enterobacter agglomerans. Listeria was not detected. Ice produced by IPIA-monitored companies did not have microbial counts above the recommended IPIA limits. The presence of microorganisms at unsatisfactory levels in the ice obtained from retail and vending machine sources indicates the need for improved sanitary practices during manufacturing and packaging at on-premises locations.
Of the 250 samples from retail stores and vending machines, 38% were outside the acceptable range for pH based on limits (6.5 - 8.5) set by the EPA, and about 1% of samples had turbidity levels that exceeded the recommended level (<1 NTU).
A second study was done to determine the prevalence of injured Salmonella caused by freezing inoculated water and holding it for up to 120 h at -10°C. Five Salmonella serotypes were separately inoculated into tap water, which was then frozen. After storing ice for 0, 24, 72, and 120 h, it was melted and salmonellae were enumerated on selective (XLD) and nonselective (TSA) media. The percentages of cells killed and injured were determined.
Within 24 h at -10°C, 74% of the cells were killed. Of the 26% of cells that survived, 73% were injured. There was an increase in the percent of injured cells (89%) between 24 and 72 h. After 72 and 120 h, there was a slight increase in the number of dead cells (77%). Injured cells may not be detected at the time of post-process analysis and may undergo repair if given an appropriate environment, thereby posing a health risk to consumers.