A bulletin for the Australian Food Industry February 2000
Contents: Safety of food derived from biotechnology | Acid tolerance of E. coli O157:H7 | Money handling in food service operations | Hand washing and the use of gloves for food protection | More on bare hand contact of ready-to-eat foods | Acute copper poisoning
Acid tolerance of E. coli O157:H7
We have discussed previously the acid tolerance of acid-adapted E. coli O157:H7 and Salmonella spp. (Food Safety & Hygiene May 1999). Evidence continues to accumulate to support earlier published studies.
In a study of the tolerance of acid-adapted and non-adapted E. coli O157:H7 to reduced pH, Deng, Ryu and Beuchat (J. Applied Microbiology 86 1999 203-10) have shown this is influenced by the type of acidulant. The order of sensitivity at a given pH is acetic > citric > malic acid. When performing acid challenge studies to determine survival and growth characteristics of E. coli O157:H7 in foods, the authors caution that consideration should be given to the type of acid to which cells have been exposed previously and possible differences in response among strains.
The use of studies with less emphasis on pH than type of acidulant or vice versa could result in an underestimation of the potential for survival and growth of E. coli O157:H7 in acid foods.
One food which has become a food poisoning concern comparatively recently is unpasteurised fruit juice. This follows salmonellosis incidents traced to unpasteurised orange juice in Australia, Canada and the United States. E. coli O157:H7 incidents have been associated with unpasteurised apple juice. The Food and Drug Administration (FDA) in the United States view this matter so seriously that they now require packaged juice products that are not pasteurised, or otherwise processed in a manner to produce at least a 100,000 fold reduction of the most resistant organisms of public health significance likely to occur in the product, to bear a specific label warning statement.
An option that is being explored by manufacturers keen to avoid thermal pasteurisation is high pressure treatment which has been shown to have less effect on vitamins, flavours and colour than heat treatment.
Researchers in Northern Ireland (J. Food Protection 58 (5) 524-29) have shown that a 15 minute high pressure treatment at 4500 atmospheres or 7000 atmospheres successfully achieved a 5 log reduction of Salmonella spp. and E. coli O157:H7 respectively in foods such as milk or chicken.
More recently, the same group (J. Food Protection 62 1999 1038-40) has shown that, not surprisingly, the survival of E. coli O157:H7 in orange juice at 4000 atmospheres is dependent on the pH of the juice.
The application of the high pressure prior to storage significantly increased the susceptibility of E. coli O157:H7 to high acidity. A 5 log (100,000 fold) reduction in numbers of E. coli O157:H7 was not achieved during the pressure treatment. During storage at 3°C, however, the time taken to achieve this level of inactivation was reduced by pressure treatment from 13 days to 3 days at pH 3.4, from 16 days to 6 days at pH 3.6 and from greater than 25 to 8 days at pH 3.9.
For further details on high pressure processing contact:
Food Science Australia, Sydney
Telephone 02 9490 8333
Food Science Australia
PO Box 52, North Ryde 1670. Tel +61 2 9490 8397 Fax +61 2 9490 8499
Email enquiries@csiro.au