A bulletin for the Australian Food Industry August 2000
Contents: Bacteria in minimally processed lettuce | More on monosodium glutamate | Wallis Lake oyster contamination | Salmonellae in fruit juices | Algal blooms | Safe Food Australia | Refrigerated retail cabinets | Ciguatera poisoning
Thermal resistance of Salmonellae in fruit juices
We have discussed previously (Food Safety & Hygiene, March 1997 and Food Safety & Hygiene, May 1999) food poisoning incidents associated with fruit juices.
While relatively uncommon, these incidents have highlighted that acid tolerant pathogens such as Salmonella and enterohaemorrhagic Escherichia coli may survive for extended periods in fruit juices. In a paper published in Fruit Processing 10 2000 246, Parish lists 19 documented disease outbreaks associated with apple or orange juices and this list is already incomplete. There is also at least one earlier omission from the list, a Norwalk virus incident in Australia in 1991 associated with orange juice served to airline passengers.
Parish concludes his review by saying that normal pasteurisation regimes will eliminate these organisms from juices but that contamination after pasteurisation is still a concern.
It is against this background that ANZFA published a
preliminary assessment report in response to an application
from the South Australian Department of Health. The
application proposed that all orange juice, other than freshly
squeezed orange juice for immediate consumption on the
same premises, either:
(i) undergoes a pasteurisation process that kills pathogenic
bacteria; or
(ii) be labeled to ensure consumers are informed that the
product is unpasteurised and are made aware of the risks
associated with its consumption.
The applicant proposed, for further consideration, that:
(i) the juice be heated to not less than 72°C for 15 seconds;
or
(ii) the juice be heated to any other time and temperature that
can be demonstrated to produce a 5-log (100,000 fold)
reduction in the most resistant microorganism of public
health significance likely to occur in the product.
ANZFA has subsequently indicated it will undertake a full assessment of the application of the application following an initial round of public comment.
Doyle and Mazzotta have recently published a timely review of the thermal resistance of salmonellae (Journal of Food Protection 63 2000 779), one of the main acid tolerant pathogens of concern with regard to fruit juices.
The authors review published data on the thermal resistance of various salmonellae in egg products, milk and dairy products, poultry, other meats, chocolate, flour, shellfish, coconut, pecans, alfalfa seeds and some mixed hot dishes. Perhaps not surprisingly, they report that there are no data yet reported on decimal reduction times (D-values) for Salmonella in fruit juices.
This is important as relatively small changes in the time-temperature schedules to which fruit juices are subjected can have a pronounced effect on the acceptability of the juice.
The authors do note that the thermal process to inactivate salmonellae (in common with other microorganism) depends on strains of the organism used, their growth phase, the food composition or test media, other limiting physical conditions, e.g. water activity, and the presence of other microflora.
They also point out that certain serotypes of Salmonella enterica are notorious for resistance to heat treatments. The most prominent of these is Salmonella Senftenberg 775W. Although this organism is not usually regarded as an important foodborne pathogen, it is often used as a test organism. The assumption is that if a particular heat process is effective against Salmonella Senftenberg 775W, it will also be effective against more common salmonellae in foods.
After reviewing all their collected data excepting that for Salmonella Senftenberg and data collected under stress conditions, Doyle and Mazzotta conclude that the highest heat resistance for salmonellae is in liquid eggs and egg yolks. They calculate that a process temperature of 71°C will require 1.2 seconds to bring about a 1-log (10 fold) reduction of Salmonella cells.
They emphasise that the process required to inactivate Salmonella in specific products will vary depending on the substrate and the conditions listed above.
ANZFA in its preliminary assessment of the South Australian Department's proposal put forward three possible options. The first of these was to maintain the status quo i.e. no requirement to pasteurise or label orange juice or any other juice. The other two options, one specific to orange juice and the second a more general one to apply to all fruit and vegetable juices talked in broad terms of a "pasteurisation process that would kill pathogenic bacteria or another process which provides an equivalent safety outcome".
It would appear that considerably more data is required in this area before processors and regulators can make scientifically robust decisions on an acceptable pasteurisation procedure for orange juice. Because of the influence of food composition on heat resistance it will be important to develop different requirements for different types of products.
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