Bacteria reproduce by splitting into two. This process is known as binary fission and the time taken between each division (generation time) varies considerably depending on, among other things, temperature and the nutrients (food) available. In optimum (ideal) conditions some food poisoning bacteria can split in two every ten minutes, although at temperatures of around 10°c it may take up to ten hours or they may stop multiplying.
The average generation time of the common food poisoning bacteria under optimum conditions is usually considered to be around 20 minutes. When food bacteria are growing and multiplying this is described as the vegetative state.
If food is contaminated a common level of contamination may be around 1,000 bacteria per gram of food. Is this food provides optimum conditions then within one hour 40 minutes, these bacteria could double every 10 minutes and become 1,000,000. This number of bacteria is likely to cause food poisoning. The multiplication of pathogens in food is a hazard.
Food poisoning bacteria obtain their essential basic nutrients from amino acids, fats, vitamins and minerals. which are usually provided high protein food such as meat, fish and dairy produce. Foods with high sugar and salt content are usually unsuitable and therefore are unlikely to support bacteria multiplication.
Bacteria required water to transport nutrient into the cell and take away waste products. The water activity of food is the measure of the available water. With the expectation of dehydrated products such as milk powder, most foods contain sufficient moisture to enable bacteria to multiply. How ever, some bacteria can survive dehydration and when liquid is added to the dried food it once again becomes a high-risk food and must be stored under refrigeration.
Acidity and alkalinity (PH)
The pH of a food is measured on a scale of 1 to 14 (hydrogen ion concentration.) Acid foods have pH values below 7, alkaline foods about 7 and a pH value of 7 is neutral. Most bacteria will not multiply in pH below 4.0, i.e. an acid food such as fruit juice. However, if a large number of food poisoning bacteria are introduced into an acid food it may take some time for the bacteria to die. For this reason we must protect acid food from contamination at all times.
Bacteria have a maximum and minimum temperature for multiplication as well as an optimum temperature when multiplication is the most rapid. Most food poisoning bacteria grow best at around 37 ºC (body temperature) although clostridium perfringens prefers 46°C. The common food poisoning bacteria will not multiply below 5°C or above 52°C. However, many can survive outside this temperature range and start multiplying again when temperatures are suitable. The range of temperature, which is likely to encourage the fastest multiplication, is between 20°C and 50°C. Some pathogens will grown between 0°C and 20°C but they multiply more slowly at the lower temperatures. The lowest recorded temperature for the growth of pathogenic bacteria is -2°C and although pathogens can survive freezing they do not multiply. To minimize the risk of bacteria multiplication, food should be stored outside the range 5°C to 63 °C. This temperature range is called the danger zone.
Some food spoilage bacteria multiplying slowly under refrigeration, which is one of the reasons for food becoming unfit if stored longer than the recommended shelf life.
Presence of oxygen
Some bacteria can only multiply in the presence of oxygen and other can only multiply when there is no oxygen. The former bacteria are known as aerobes and the latter anaerobes. Many bacteria can multiply with or with out the presence of oxygen and these are called facultative anaerobes, for example, Salmonella.
Oxygen is normally present in food except in the case of liquids which have been thoroughly boiled. Cooking, for example, joints of meat, also drives off oxygen and the centre then provides ideal conditions for anaerobes, such as clostridium perfringens.
Food poisoning bacteria produce toxins (poisons) which cannot be detected by visual inspection. Toxins may be either exotoxins or endotoxins.
Exotoxins are produce as waste products during multiplication or sporulation of bacteria. Quite often Exotoxins are released into the food and many are heat resistant, so that even if cooking destroys the bacteria, the toxin may remain and cause illness if the food is eaten. This result in a short onset time. Toxins will usually not be produced if the food is kept below 5°C or above 63°C and at the time ambient temperature is kept short. Enterotoxins are Exotoxins that affect the gastrointestinal tract.
Endotoxins form part of the bacteria cell wall and are released on the death of the bacteria, usually in the intestines of persons consuming contaminated food. If the toxin is produced in the intestines, the onset period, for the first symptoms to appear, will usually be longer than if the toxin is in the food.
Some food poisoning bacteria, such as Clostridium Perfringens, Bacillus cereus and Clostridium Botulinum are able to form spores which are capable of surviving unfavourable conditions such a high temperatures, dehydration and the use of disinfectants. Spores are round protective bodies which form inside the bacterial cell and may allow survival for many years without food and water. They cannot be detected by visual inspection. Spores survive normal cooking and germinate during long, slow cooling. The vegetative bacteria released will then start multiplying and may produce toxins.
Temperatures in excess of 100°C are often required for long periods (as much as five hours) to destroy spores. The temperatures used to ensure the safety of low-acid canned food, by destroying spores of Clostridium botulinum, is the equivalent of 121°C for three minutes.
BACTERIAL SPORES DIAGRAM
When there are many different bacteria present, they will compete for the same food. Fortunately, most food poisoning bacteria are not as competitive as the normal flora found on food and unless present in high numbers, will usually die.
Destruction of bacteria
Food poisoning bacteria can be destroyed by using high temperatures for sufficient time. The higher the temperature, the shorter the time required. However, the type of bacteria (whether or not they produce spores), the number of bacteria present and the type of food also affect the time needed to kill bacteria and make food safe. Bacteria will start to die at around 55°C. However, a core temperature of greater then 75°C is required when cooking food to be reasonably confident that most harmful bacteria will have been destroyed.
Unfortunately, much higher temperatures are required to destroy toxins and spores produced by some food poisoning bacteria. Spores are much more resistant to drying, boiling, disinfectants and other conditions, which usually destroy vegetative food poisoning bacteria. The survival of bacteria, spores or toxins is a hazard.
Bacteria can also be destroyed by irradiation, chemicals such as nitrates, salt or sugar, u/v light, steam or the use of disinfectants such as bleach. Freezing cannot be used to destroy bacteria as most will survive the freezing temperatures.