Keeping the growth and spread of bacteria under control is essential in any commercial kitchen in order to avoid the risk of food poisoning.
Four factors for bacterial growth
There are four main things required for bacteria to develop and in order to stop their growth; one of these factors must be eliminated.
Like us, bacteria need nutrients to give them the energy to multiply. Foods that are moist and high in protein, such as meat, poultry, eggs, shellfish and dairy products, are ideal food sources for bacteria. Even cooked rice and pasta can provide adequate nutrients and bacterial growth can still occur even after food has been cooked.
Bacterial growth doesn't occur in dried foods, but as soon as liquid is added harmful micro-organisms can soon multiply.
The bacteria that commonly cause food poisoning, for example E. coli, usually multiply between 5oC and 63oC. This temperature range is typically referred to as the 'Danger Zone'. Above or below these temperatures, bacterial growth is much slower and may even stop. It is important to make sure food is always thoroughly cooked through because once an optimum temperature is reached again, growth can resume. There are even some more resistant bacteria that can survive harsher conditions outside of the danger zone.
Food is likely to be in the danger zone when it is kept at room temperature, if cooked or heated slowly, when it is left out in a shop window in the sun and when hot sauce is poured onto cold food. The key to eliminate bacterial growth is to minimise the time between preparation, cooking and serving. To ensure that food is cooked well enough, the centre of the food must be at 75oC for at least 2 minutes and it should be served within 20 minutes (unless it is stored out of the danger zone).
Given that the previous three conditions are in place, bacterial multiplication can occur very quickly. In as little as 2 hours, 1000 bacteria can increase to over one million.
How do bacteria multiply?
Binary fission, in very simple terms, is the process by which bacteria multiply continuously and exponentially. First bacteria will split into two, then will split again to give four bacteria progeny, then into eight, and so on. This method of multiplication means that bacteria increase in number very quickly, which is why it's so important to make sure you take the necessary precautions to minimise the risk of harmful bacteria spreading in your kitchen.
Some bacteria can produce spores, which provide a protective coating (almost like a shield) so that they can withstand even high cooking temperatures. Although bacterial growth does not occur when in spore form, as soon as the conditions become more favourable again, the bacteria will be able to continue multiplying. These types of bacteria pose such a risk because they can survive harsh conditions and are difficult to eliminate, even if disinfectant is sprayed.
Remember that it is important to minimise the time taken to get the food from preparation to service. Cooked rice should never be stored and reheated because the bacteria commonly found in it are great at forming spore resistance, so won't be completely killed before the food is eaten.
When bacteria are destroyed, toxins are sometimes released from their cell walls. Like spores, these can be heat resistant, so food may need to be cooked at a high temperature for a long time before it's safe to be served.
There are a number of steps you can take to minimise the risk of toxins in food. Use a reputable food supplier so that you know your food is fresh; reduce the time between preparation, cooking and serving stages; and ensure all food handlers have good personal hygiene.
High and low risk foods
High risk foods are those in which the growth of harmful bacteria is more common. They are generally full of protein and moisture. Lots of high risk foods are 'ready to eat', but because they don't require cooking immediately before eating, there has been time for bacteria to proliferate. Therefore, these foods should be left in the danger zone for as little time as possible.
Low risk foods are less likely to harvest bacteria that can cause food poisoning. Moisture is often removed from these sorts of foods by using salt or sugar, or by pickling.
A few examples of high risk foods are cooked meat and poultry, patés, savoury spreads, milk and dairy products, and cooked rice and pasta. Low risk foods include jam, biscuits and dry food. However, once moisture is added, bacteria can resume growth. It is therefore important to make sure food is cooked properly and is kept in the danger zone for as little time as possible to avoid the proliferation of harmful bacteria.
Paul Grantham is employed by Safer Food Handler, which has produced a basic UK food hygiene course. Safer Food Handler offers the UK's lowest cost L2 Food Hygiene Certificate that fully meets the UK legal requirements. For food handling businesses with 5+ employees needing training, there are bulk food hygiene course discounts.