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.

Thursday, June 25, 2009

Do consumers understand “Use-by” and “Best-before” dates?

I think that many consumers don't understand these package marks.

I recently received a message from a very distressed reader. She'll recognise herself, but I'm sure she won't mind my writing this.

The reader had a 16-month-old son and had fed him a jar of organic apple, pear, blueberry and blackberry with vitamin C. The young man had eaten it all and was apparently bouncing around, as 16-month-olds tend to do. The jar and contents appeared normal, with a suitable vacuum in the headspace.

After the meal was over, the mother looked at the jar and found the “best before date was 29th July 2008!” (her exclamation mark). She went on: “I am very concerned that he could get Botulism from this - is there a way that I could get the remains of the food in the jar tested to make sure that he is not in any danger? Please, I am happy to pay for this and your time.

“I have spoken to a few people and they have all said that we will have to wait the 10 days to be on the safe side. Is there anything else I can do to protect my son? or help to prevent anything really nasty making him sick from this?”

This mother was obviously very frightened. Her concern was triggered by the best before date, which is an advisory to the consumer about quality. Chemical changes can occur slowly in cans or jars of food during storage. These changes may be accelerated if the food is exposed to sunlight or high temperatures. These changes may make the food less appetising, such as giving a "cardboardy" or flat, oxidized taste. The changes are not harmful. If the food is outside the Best Before date, it does not necessarily indicate that it is unsafe to eat, though that might be the case with something like ham or egg salad, which are much more perishable.

I didn’t have any further information on the product, but would expect that it would have been processed either as an acid food (pH less than 4.6) and pasteurised, perhaps hot-filled, or processed as a low acid food (pH above 4.6) and thus be given a 12D process (sterilised). In either case, Clostridium botulinum toxin would not be produced. Any changes occurring after the best before date would be quality deterioration only, not safety issues. There are several other points to come out of this discussion, but I'll cover them in a future posting.

Extrapolating from this, it is probable that consumers and perhaps retailers don’t fully understand the significance of “use-by” dates. I have certainly seen products in supermarket display cabinets with the original “use-by” date covered with a second label bearing a later date. The latter is illegal in New Zealand.*

And the boy? I had a message from his Mum today - this is the 11th day since he ate the food, which had actually been made in 2006. He didn't get sick and is now toddling around the home at high speed. Well, some food stories have happy endings!

* You can find an article "Date Marking - Standard 1.2.5" on the NZFSA website at