Simon Jardine asks about health risks of drying cod at 20-25ºC, and mentions specifically S. aureus. He writes: 'Though it is clear that the surface of the product will have the lowest water activity and as this is also where the bacteria would be...'.
I don't think Simon should assume the surface of the product will have the lowest water activity, or significantly so, throughout the drying process. At the start of the process rate of removal of water is faster than rate of migration of water from the interior to the surface and the surface will be drier than the bulk of the muscle. The rate of evaporation of water molecules decreases as the surface becomes drier and there comes a time when the rate of diffusion of water within the muscle is the rate determining step and the surface is not drier than the bulk of material. The process is described in detail in Jason, A.C., 1958, 'A study of evaporation and diffusion processes in the drying of muscle', In: Fundamental Aspects of the Dehydration of Foodstuffs, Society of Chemical Industry, London, pp 103-135. There is a summary in Horner, W.F.A., 'Preservation of fish by curing (drying, salting and smoking)', In: Hall, G.M. ed. 1997, Fish Processing Technology. 2nd edn, Blackie Academical & Professional, London. Figure 18 of Jason's paper shows this first phase to be 10 hours at a dry bulb temperature of 30ºC. It will be longer at 20-25ºC, but for most of the drying time of 4 days, the surface will not be drier than the bulk of the material. Though contaminating bacteria will initially be on the surface, they will migrate into the interior of the fillet.
Horner, in the reference cited above, has a table of lag times and generation times for S. aureus and Salmonella typhimurium various temperatures in the range 10-30ºC and Aw's of 0.88-0.96. The Jason reference has a figure of water content against Aw for dried cod.
Is the product intended to be cooked before consumption? This will considerably reduce the risk to health.
What does the product taste like after 4 days at 20-25ºC? Spoiled, I would imagine.
Peter Howgate
----- Original Message -----
From: Simon Jarding
To: seafoodlist
Sent: Wednesday, May 24, 2000 7:34 PM
Subject: dried cod
Hi everyone!
I am currently making an risk assessment of a special product of which I am not sure how to establish the correct potential risks.
The final product can be described as dried cod with only a very few percent of water (5%) meaning that the water activity is very low. This should therefore not lead to any growth of pathogenic bacteria after the product has been dried.
However, my concern is based on the actual proces where the product is dried in a machinery designed herefore at app. 20-25ºC depending on humidity for 4 days.
I believe this temperature abuse in the begining of the drying period could create an envorionment for the growth Staphlycoccus aureus and the produce of toxin.
Though it is clear that the surface of the product will have the lowest water activity and as this is also where the bacteria would be...
I am looking for information or input on this issues with regard to completing a HACCP system for the proces.
I would be grateful for any comments.
Simon
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Simon Jarding
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