Germ Warfare

I’m sure that many people yearn for the glamorous life of the food microbiologist leading the war against microorganisms - yeah, right!  Perhaps those people visualise the microbiologist surrounded by modern laboratory equipment, such as PCR machines and gene sequencers, but the sad fact is that what is portrayed in films and television just isn’t reality.  It might be the case in forensic work or pathogen testing, but in the food industry, much of the work involves monotonous sampling and plate counting.

This is because there is a constant battle between food manufacturers and the microorganisms that can cause spoilage or food poisoning.  Food safety is assessed in relation to counts of specific bacteria, yeasts and moulds.  The International Commission on Specifications for Foods was formed in 1962 and wrote Microbiology of Foods Volume 2: Food Commodities in 1980.   This was updated in 1996 (1).  Our Microbiological Reference Criteria for Food were published by the Ministry of Health and Version 2 appeared in 1995.  The Reference Criteria are expressed in the ICMSF format as a guide to indicate when food can be considered unacceptable or unsafe.  

The result is that many food microbiologists and technicians spend a large part of their working day conducting plate counts on foods.  This is particularly so in the dairy industry, though large laboratories such as these may use automation for plating, such as spiral platers, robotic sample diluters, and image analysis for counting.  This is, of course, very expensive and involves a lot of plastic that probably cannot be recycled. Modern devices, such as the flow cytometer - a machine that counts particular cell types flowing through a tube by reflection of laser beams.  Against this background, it is disappointing that food-borne infections continue unabated in the USA, UK and Europe.  

Of course, some food microbiologists do lead an interesting life.  These are the Special Agents investigating food contamination, poisoning and spoilage.  While it might be argued that there is nothing new under the Sun, each case is different - human error, mechanical breakdown, packaging failures and poor equipment design may all figure in the systematic examination, leading to an understanding of what has gone wrong and a suggested solution.  Investigation often involves looking closely at the process and interviewing operators.  Access to a suitable microbiological testing laboratory is often essential in identifying the problem.

Looking back over many years as a consultant microbiologist, I can think of lots of investigations where the explanation was a surprise: 

A dried vegetable processing operation in which the product had spikes of contamination.  On this occasion, I was fortunate to see the problem immediately.  The vegetables passed through a steam blancher which was located under a cold air trunking.  Steam condensed on the trunking, which was covered in a thick black tarry layer, and every so often, condensate would fall onto the vegetables being conveyed to the drier.  A factory design fault.

Staphylococcus aureus contamination of canned product.  This was not a low acid food and therefore was not subject to a 12-D process.  The immediate suggestion was that the cans were leaking during cooling, sucking in cooling water.  However, it was found that the cans were dump loaded into a lidded vessel and hot water was introduced and held for the duration of the process schedule.  With a temperature above 75oC for a suitable time, the Staphylococci should have been destroyed.  We eventually discovered that the vessel was overloaded and some of the cans were above the water level, so did not receive the scheduled process.

In a small pie shop, minced meat for pie filling was cooked on a stove in large pans of about 35 L capacity, which were then put into a chiller to cool.  On occasion, the pies spoiled after the growth of Clostridial spores activated by the heating.  I found that the pans of meat took over 48 hours for the centre to cool to about 12oC.  The solution was simple - put the filling into shallow trays for cooling.

 Listeria monocytogenes was found in sliced meat.  The slicer was a standard design, but was very difficult to clean properly and dried meat residue was found in the transfer mechanism.  The slicer was mounted on a stainless steel table, and the whole area was meticulously cleaned.  However, the table surface was heavily scratched and two feet mounting spikes penetrated the stainless steel.  Unfortunately, the stainless steel was only a couple of millimetres thick and was laid on Medium Density Fibreboard (MDF).  The result was that the MDF got wet every time the table was cleaned and became colonised by L. monocytogenes.

These examples show that those working in processing facilities may be essentially blind to potential failures and a fresh set of eyes with appropriate experience may recognise the problems quickly. 



References

1.  ICMSF. (1996) Microoganisms in Foods 6:  Microbial Ecology of Food Commodities.
2.  Ministry of Health (1995)  Food Administration Manual S. 11: Microbiological Criteria