Saturday, December 10, 2011

Is there such a thing as totally safe food?

The short answer is "No!".  Even if we eat only sterilised food, such as canned meat, we are reliant on the proper delivery of the sterilisation process and prevention of post-process contamination.  Low acid canned foods are essentially sterile - the food is hermetically sealed in a can, which is then processed under pressure to destroy bacterial spores, specifically, the process is designed to reduce the chance of survival of a Clostridium botulinum spore to less than 1 in 10^12.  On that basis, you have a better chance of winning the PowerBall lottery than getting botulism.

But nobody wants to live exclusively on canned food.  Are there any other ways that we can make food safe, or at least reduce the risks of food poisoning to acceptable levels?

Yes!  Food microbiologists and food processors study the bacteria that can cause food poisoning - pH range for growth, water activity range for growth, requirement for oxygen, growth temperature range, response to preservatives etc. - and they also look at the intrinsic parameters of the food - pH, water activity - and then consider the storage conditions, such as relative humidity, temperature and atmosphere in the pack etc.

Taking all these factors into account, foods can be designed to be safe under proper conditions of handling and storage.  A significant part of the training of  food technologists centres around study of these conditions and manipulation of the conditions in the food to achieve the manufacture of safe foods.

As an example, look at a tank of raw milk on the farm.  Immediately after it is drawn from the cow, the milk is chilled.  This reduces the growth rate of bacteria, which come either from the cow or the equipment.  When the milk arrives at the processing factory, it is pasteurised - the temperature is raised to 72C and held for 15 seconds.  This kills all the vegetative pathogens, that is, the bacteria that are not in the spore form.  The milk is now safe, but will not keep indefinitely, because heat resistant spoilage bacteria are still present.  We can make the milk keep for a much longer time by ultra high temperature processing (UHT).  In this process, the milk is heated to around 140C and held for a few seconds.  This sterilises the milk, so there are no viable bacteria to cause food poisoning or spoilage.

The milk can be dried to powder.  This lowers its water activity below the threshold for bacterial or mould growth.  Alternatively, the milk can be fermented to yoghurt or cheese by addition of starter cultures.  The starters convert lactose to lactic acid and reduce the pH of the milk until it curdles and sets.  pH is a measure of the acidity of the food; the scale runs from 0 to 14.  Numbers below 7 are acid, those above are alkaline.  As the pH falls, fewer and fewer bacteria are able to grow.  Below pH 4.0 no pathogens are able to grow.  Typical pH of yoghurt is 3.65 to 4.40

Other foods may be made safer by the application of hurdle technology, where several preservation mechanisms are brought together to prevent growth of pathogens in the food.  There is a number of new technologies appearing, such as high pressure processing, electron beam irradiation and ohmic heating.  None of these processes can guarantee safe food, but each has its advantages.  I'll look at some of these techniques in future postings.

Safety management systems, like HACCP can be used to control food processing operations to ensure that safe food is produced.  However, some foods are inherently less safe, such as raw vegetable sprouts, raw shellfish, or very unusual foods that require highly skilled preparation, such as fugu.
Consumers must take some responsibility in their choice of food and eating habits.  If you are doing the traditional turkey for Christmas dinner, ensure that it is properly cooked - use a meat thermometer - and consider cooking the stuffing separately.

If you are going to barbecue this Christmas, don't cook chicken legs on the BBQ without first microwaving them - they are an irregular shape and ensuring that they are properly cooked is difficult.  Don't use the same plate or utensils for cooked meat as for raw.   Don't keep foods beyond their use-by date and ensure that all raw foods are refrigerated during storage. 

Have a wonderful holiday and keep safe this Christmas.

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