A bulletin for the Australian Food Industry November 1999
Contents: Genetically modified foods | International Food Safety Conference | Foodborne outbreaks in Australia | Salmonella in unpasteurised orange juice - US | Preliminary treatment of fruit for fresh juice | High pressure processing of foods | Listeriosis from fruit salad | Cold Chain Guidelines
Listeriosis from fruit salad
The reported occurrence of three deaths of susceptible patients in health care facilities in the Hunter Valley of NSW highlights the challenges faced by food and health professionals.
The deaths have been attributed to infection with Listeria monocytogenes and a commercially prepared fruit salad has been implicated as the vehicle for infection. The deaths were separate events so the conclusion can be drawn that the fruit salad has been at least intermittently a carrier of L. monocytogenes. L. monocytogenes is widespread in the environment and has usually been associated with animal products. The first well documented incident of human listeriosis, however, was attributed to cole slaw. This led to a number of studies on the behaviour of L. monocytogenes on vegetable substrates with pH values down to around 5.0. Studies on the organism on fruit products or at pH values comparable to fruit products have been relatively infrequent.
It is clear, however, from those studies available that L. monocytogenes can survive for periods of days or even weeks at pH values which might be encountered in some fruit products especially fruit salad with a high proportion of melon products (Food Safety and Hygiene, March 1997). Another factor to be considered with these products is that the opportunity for cross contamination during preparation is greater than with many other products (Del Rosario and Beuchat Journal of Food Protection 1995. 58. 105).
In one study using commercial tomato juice (pH 4.21) as substrate, L. monocytogenes survived at the inoculation level (106 cfu/ g) for 14 days at 5°C. Numbers declined at 21°C but after 10 days there had been only an approximate two decimal reduction (Beuchat and Brackett, Applied and Environmental Microbiology 1991. 57. 1367).
Another study using orange serum as the growth medium (Parish and Higgins, Journal of Food Protection 1989. 52. 14) indicated that survival at 4°C could occur for extended periods at moderately low pH values. Viable cells of one strain of L. monocytogenes were reduced from 106 cfu/ml to fewer than 25 cfu/ml in 25 days at pH 3.6, 43 days at pH 4.0 and 81 days at pH 4.6. Viable cell populations remained at levels about 102 cfu/ml after 90 days in orange serum at pH 4.8 and 5.0. These pH values are not unusual in some ripe fruits which are common ingredients in fruit salad.
Studies in pH adjusted laboratory media confirm these results with two further points of note. The first of these is that the common fruit acids, citric and malic, are amongst the least inhibitory for L. monocytogenes at a given pH level (Buchanan and Golden, Journal of Food Safety 1998. 18. 37). The other, and perhaps more important point, is that prior adaptation of L. monocytogenes to mildly acidic conditions can greatly enhance the survival of the pathogen in low pH foods (Graham, O'Driscoll and Hill Applied and Environmental Microbiology 1996. 62. 3128; Lou and Yousef; Applied and Environmental Microbiology 1997. 63.1252). Both groups of researchers recommended this adaptive stress response or `stress hardening' be taken into consideration when protocols for food processing are being developed. This applies especially when technologies are being modified or developed.
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