Outbreaks of foodborne disease from pasteurized dairy foods in the 1970-1985 period prompted the U.S. Food and Drug Administration (FDA) to launch the Dairy Product Safety Initiative in 1985. A part of this program was microbiological surveillance of finished products for pathogenic bacteria. Potential pathogens were isolated from samples collected in 70 (6.9%) of 1016 plants surveyed during the second year of the program. Among the isolates were Yersinia enterocolitica (3.2%), Listeria spp. (2.9%), and miscellaneous isolates of Salmonella, Aeromo-nas hydrophila, and other pathogenic species (0.8%). Positive test results were associated with postpasteurization contamination (U.S. Food and Drug Administration, 1987).
Klausner and Donnelly (1991) surveyed 34 dairy processing plants in Vermont, focusing on floors and other nonproduct surfaces. Y. enterocolitica and other strains of Yersinia were isolated from 10.5 and 2.5% of the sites, respectively. The incidence of Listeria innocua (16.1%) was high compared with that of L. monocytogenes (1.4%). Pathogens were significantly more likely to be found in wet than in dry areas (P < .05). This points to the importance of depriving microorganisms of water. Although sanitizing floor mats and foot baths are designed to reduce the incidence of transmission of bacteria by personnel, data from the study by Klausner and Donnelly indicate these devices may be sources of bacteria if they are not properly cleaned and refreshed with sanitizer.
A survey for listeriae in frozen milk product plants in California by Walker et al. (1991) revealed an incidence of 12% among 922 samples. Among the 39 plants sampled, L. monocytogenes and L. innocua were the single species recovered in 5 and 13 plants, respectively, and both species were recovered from 9 plants. No listeriae were isolated from 12 plants. A single species dominated at any particular site. Although floor drains have been major sources of Listeria in dairy plants, no isolates were made from drains in nine plants where they were present in other selected sites. The investigators suggested that increased awareness of high risks of drain-associated Listeria may have directed much attention to them even though other areas in the plant were neglected. Workers in Finland (Miettinen et al., 1999) monitored production environment, equipment, and ice cream from one plant during 1990-1997. Using pulsed-field gel electrophoresis, they identified 12 different endonuclease digestion patterns among the 41 isolates of L. monocytogenes. One strain persisted for 7 years. Samples became negative after the facility was modified structurally and cleaning and disinfection practices were improved.
Whereas confidential reports from industry laboratories indicate that it is not unusual to find listeriae in environmental samples, it is unusual to find them in finished product. The rationale for this is that hygienic practices common to the frozen desserts industry are effective in preventing transfer of pathogens from the environment to pasteurized product. FDA enforcement reports for the years 1997-1999 record six recalls of ice cream and frozen yogurt products because of potential contamination with L. monocytogenes (FDA Enforcement Reports, http://www.fda.gov/bbs/topicws/ENFORCE/ENF00498.HTML). In contrast, there were 14 recalls of cheeses and cheese products because of contamination with L. monocytogenes. During the same time, there were recalls of frozen desserts for the reasons cited and of the following numbers, respectively: undeclared or unspecified nut ingredient, 16; undeclared color additive, 7; undeclared egg ingredient, 6; undeclared wheat or corn flour, 5; environmental contaminants (metal, calcium chloride, and ammonia), 4.
Recent studies of microbiological quality of frozen desserts have revealed varying numbers of undesirable bacteria. For example, Nichols and de Louvois (1995) reported that the microbiological quality of commercially produced ice cream in the United Kingdom has been generally good with the occasional outbreaks related to ice cream usually caused by Salmonella Enteritidis from raw eggs in noncommercial ice cream. However, nearly one-third of 46 samples of ice cream from markets in Ankara, Turkey, failed to meet Turkish standards of quality, and fecal coliforms were found in 15% of them (Kocak et al., 1998). Masud (1989) found that among 50 samples of commercial ice cream in the Pakistani market, 72% had total viable counts of more than 106/g and 66% had coliform counts between 102 and 103/g. Of 122 samples of vanilla ice cream manufactured by eight firms in Caracas, Venezuela, 43 and 77% failed to comply with international standards proposed for the aerobic plate count and Enterobac-teriaceae count, respectively (de Tamsut and Garcia, 1989).
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