Sunday, June 28, 2009

Follow-up on canned food production.

In the last posting, I was writing about canned foods and the consequences of their going out of their “best before” date.

I was confident that the young man in question was not at risk. Here’s why (sorry for the lecture).

Canned foods have been made commercially in significant amounts since about 1874, when Schriver invented the closed kettle – a device that allowed processing at temperatures above that of boiling water. Before that, commercial production was carried out, but the products sometimes spoiled. (We tend to forget that up until 1860 it was not known that bacteria cause food spoilage. The spoilage was almost certainly the result of failing to deliver a harsh enough heat process to destroy bacterial spores. Some spores can survive more than 4 hours in boiling water).

In all that time, manufacturers have developed reliable processes by experience. Scientists have been able to explain why the heat treatments work and thus design new processes with confidence. In the case of low acid canned foods (LACF) we have to rely totally on the process. We cannot test for safety.

You might ask “Why would it be difficult?” The problem is that we demand a high level of security i.e. we set the acceptable risk of food poisoning from LACF as around 1 in a billion (I mean a million million) i.e. only one can in a billion would contain a viable spore. There is no testing regime that could detect the presence of viable spores even ten times that limit.

The LACF process is designed, using a combination of knowledge of the type of microorganisms likely to be found in the food (and able to grow under those conditions) and the measured death rate of these bacteria, to calculate a process that will guarantee to destroy them. In the case of LACF, we are interested in a process that will destroy the heat resistant spores of Clostridium botulinum, leaving only one survivor in a billion. We call this a "12D" or 12 decimal reduction process. (See also the previous posting).

So when my reader contacted me and asked me to test the food for her, I knew that it was pretty much pointless. If the food had been processed properly and the seal remained unbroken, then she could rely on the food being safe for her son.


Anonymous said...

Please check the math:
1 million million is not a billion, but a trillion (10 to the 12th power)

1 thousand million = 1 billion (10 to the 9th power)
thanks for your attention
your integrity may be at stake

John said...

Thanks for your comment. I clarified my definition of 1 billion because where I come from (UK) a billion is 10 to the 12th power. The Americans, as far as I am aware, use a billion to mean 10 to the 9th power.

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