A bulletin for the Australian Food Industry May 1995
Special issue on Mettwurst and Escherichia coli O111
Introduction
The microorganism responsible for the recent food-poisoning outbreak in South Australia is a member of a group of bacteria called enterohaemorrhagic Escherichia coli or EHEC for short. These bacteria can cause severe human illness which includes damage to the renal system (HUS or haemolytic uraemic syndrome). The best known of the group is E. coli O157:H7. The number attached to the name of the organism describes what serotype it belongs to. It is a means of distinguishing between different sub-groups of E. coli. Throughout the world, a wide range of foods have been implicated in outbreaks of food-poisoning brought about by E. coli O157:H7.
The microorganism involved in the South Australian outbreak was E. coli O111. While it belongs to the enterohaemorrhagic E. coli, or EHEC, group it is a different serotype from E. coli O157. Though E. coli O111 has been responsible for isolated and sporadic cases of human illness before, this is the first time a food has been implicated in illness caused by this particular strain. E. coli O111 has been found in the faeces of calves and adult cattle. However, because of difficulties in detection and the general view that this serotype only relatively rarely caused human illness, there have not been extensive studies into its presence in animals.
Enterohaemorrhagic E. coli (EHEC)
What distinguishes enterohaemorrhagic E . coli from other strains of E. coli is the possession of a number of factors that are all needed for the microorganism to be pathogenic. In most other respects, EHEC strains are similar to other strains of E. coli, Enterobacteriaceae or of Salmonella. For instance, just like Salmonella, none of these EHEC organisms grow below 7°C. They are all prevented from growing by the same pH and the same concentration of salt (or aw ) that stops growth of Salmonellae and other E. coli. Similarly, EHEC strains are as sensitive to heat as are other E. coli and Salmonella. Thus the conditions governing the behaviour of these new food-poisoning bacteria in food are very likely to be very similar, if not identical, to those used to control normal, non-pathogenic E. coli or Salmonella. However there is evidence that some EHEC survive better than other pathogens in acidic foods.
Control or Safety
Methods to detect and isolate EHEC strains from foods are still being developed. While methods are available for E. coli O157, there are none for E. coli O111. Because of the great similarity of E. coli O111 to other E . coli strains, it is difficult to distinguish it from normal E. coli. Detection is also complicated by the fact that many strains of E. coli O111 do not belong to the EHEC group and so do not cause HUS. Thus, it is presently not possible for producers of fermented sausage, or indeed of any food, to use testing for E. coli O111 as a means of assuring the safety of the food.
Safety is achieved by adherence to good manufacturing practice and by the application of the principles of HACCP. In the production of fermented meats, safety is achieved:
- by preventing growth of pathogens during production, and
- by achieving as great a reduction in the numbers of pathogens as possible.
While the first of these aims is always important, the second is particularly important where the pathogen causes illness by infection. E. coli O111 causes human illness by live organisms infecting the human intestine and there producing a toxin. It is therefore necessary that the production system used not only prevents growth of any cells of E. coli O111 but also results in as much destruction as possible. The Critical Control Points to achieve this are outlined briefly below.
Manufacturers should set up HACCP systems for each type of product. All staff should be familiar with the HACCP elements for each product type:
- The microbial hazards associated with the production of fermented meats;
- The steps or processes where the microbial hazard will be controlled (Critical Control Points; CCP);
- The specifications of the criteria that will be used to know that the operation is under control (e.g. a pH fall to a certain level in a defined time);
- The monitoring system that is used to check that the procedure at each CCP has been properly carried out – the monitoring system must enable action to be taken to remedy an out-of-control situation either before or during the operation of a process;
- The corrective action to be taken when monitoring indicates that a particular CCP is beginning to get out-of-control ;
- Microbiological data need to be obtained that verifies that the HACCP system in place is in fact working (see Destruction of Coliforms, below).
Raw Materials
Materials of good microbiological condition should be used. Even when correctly done, fermentation and drying cannot be relied on to destroy a large number of E. coli,or Salmonella. If the starting raw meat has not been produced under good manufacturing conditions, it could have excessively large numbers of pathogens, and some could survive the process. The situation with E. coli O111 is not different from the considerations for Salmonella. In both cases, the organisms may occasionally be present, but numbers should be low.
Thawing or Tempering
The meat supplied may be microbiologically satisfactory, but care is needed in preparing the batter to be used in the sausage. Frozen meat should be thawed or tempered under conditions that prevent growth of E. coli or Salmonella. Recommended thawing temperatures are at or below 5°C. If surface temperatures of the thawing meat are above about 7°C, growth of these organisms can occur. Similarly, delays in preparing the batter may permit growth. The approximate times for E. coli or Salmonella to double in numbers on raw meat are: at 10°C, 7 to 10 hours; at 15°C, 2.6 hours; at 20°C, 1.4-1.6 hours. The time meat is out of refrigeration before fermentation starts should be limited. The approximate doubling times above indicate the extent of the possible hazard. There should be time and temperature specifications for thawing meat and the preparation of the batter. The action to be taken if time or temperature are outside those specified should be known and acted upon.
Fermentation
The microbiological aim of fermentation is to achieve a pH fall that prevents growth of pathogens and provides some of the conditions that cause pathogens to die off. The addition of salt to the batter slows the growth rate of E. coli and Salmonella, and so these organism are held in check for a short period. However, unless there is a pH fall soon after fermentation starts, the pathogens will grow. The rate at which the pH should fall depends on the temperature of fermentation. Higher temperatures require faster pH falls to prevent pathogen growth.
Because the rate and extent of the pH fall varies when "natural fermentation" (back-slopping or pre-salted meat) is used, the extent of control of numbers of pathogens can also vary. The variability can be so great as to sometimes allow growth of E. coli and Salmonella. Starter cultures should be used and there is some evidence that starter cultures are more effective than GDL (gluconodeltalactone) in reducing pathogen numbers. Small amounts of other acids (acetic, propionic, butyric) produced during fermentation are even more effective than lactic acid in the subsequent decrease in pathogen numbers. The correct starter culture for the fermentation temperature should be used. Some cultures are designed for temperatures around 25°C, others for temperatures near 35°C. The temperature of fermentation should be known and controlled. The success of fermentation should be assessed by measuring the pH at set time intervals. What action is to be taken if specifications of the rate of pH fall are not met should be known and implemented.
Drying or Maturing
After fermentation, the sausages are dried or matured at 10-15°C. Here the microbiological aim is to dry the product to the extent needed to give microbial stability (e.g. an aw ≤ 0.95 and a final pH ≤ 5.2). At this combination of aw and pH, pathogenic bacteria cannot grow even at ambient temperatures. Both the final pH and dryness (or aw ) will vary between products.
During this stage, factors such as drying temperature, the relative humidity of the air and the distribution of the air flow need to be controlled. Drying cannot be too fast otherwise case-hardening will prevent drying of the centre regions of the sausage. The rate and extent of drying is determined by measuring the weight loss. This weight loss can in turn be used to calculate the aw of the product. During the drying and maturing phase, a reduction in any E. coli or Salmonella present takes place. A greater destruction is achieved by the attainment of a lower pH. Additionally death is greater when the drying or maturing time is longer and the water activity lower. Therefore destruction is greatest in hard or dry salami types where considerable drying occurs over an extended period. In semi-dry products less destruction will occur. In the US and Canada, semi-dry products are dried for only a brief period and are then given a heat treatment at 55-60°C (to make them sliceable). This heating also destroys most bacteria present.
Release
No batch of final product should be released for sale unless measurements of pH and aw indicate that the batch has been produced within the plant's specification of rate of pH fall during fermentation, and drying during maturation .
Destruction of coliforms
Since there are a great variety of fermented products and since even the same product may be produced differently (e.g. different fermentation temperature) at different establishments, it is not possible to specify the extent of destruction of coliforms, E. coli or Salmonella that may occur. It would seem prudent that manufacturers should know the extent of coliform reduction that their particular process achieves when it is carried out according to their specifications.
It has been known for at least 20 years that strains of Salmonella differ considerably in the extent of their destruction during the production of fermented sausages. It will not be surprising therefore if there are differences between EHEC strains in the extent of their survival.
Food Science Australia
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