Food Safety Bacterial Growth Requirements
High risk foods are common in taste unlimited poisoning outbreaks. They are usually composed of protein, are ready to eat, stored under refrigeration and no further processing such as cooking is required. Examples include pates, sandwiches, filled rolls, fresh cream and artificial cream cakes, pies and cold cooked meats.
Raw foods are a major source of food poisoning organisms. High risk foods must be kept away from raw foods at all times or cross contamination will result, compromising food safety and food poisoning will occur.
Low risk foods do not pose a food safety hazard mainly because they lack one or more of the requirements for bacterial growth. They could be acid foods with a pH of less than 4.5, they may have a high sugar, salt or fat content which are good natural preservatives. They may be dried products or preserved foods not requiring refrigeration. They may be foods that require ambient storage.
The last category of food is ready to eat raw foods. These include fruit and salad vegetables and they must be thoroughly washed before consumption to minimise the risk from low-dose pathogens.
The maximum recommended temperature for freezers is -18°C. At this temperature there is no bacterial growth, bacteria remain dormant. Although some bacteria are killed during the freezing process, many remain alive.
Fridge temperatures should be between 1 and 4°C. Although the maximum legal fridge temperature in the UK is 8°C. At fridge temperatures some bacteria will grow very slowly including Listeria and C botulinum.
The next range of temperatures is the temperature danger zone. This is between 5 and 63°C. Once food is being prepared it must be kept out of the temperature danger zone. If food is left in the temperature danger zone bacterial growth will occur, affecting food safety and food poisoning will result. As bacteria enter the temperature danger zone at 5°C they start to grow slowly. As the temperature increases the bacteria will grow more profusely. Bacteria grow best at 37°C, which is body temperature. As temperatures increase from 37°C bacterial growth slows down. Most bacteria are killed by 52°C. The UK recognised safety cooking temperature is 75°C, although foods can be cooked at a low temperature over a longer period of time.
The hot holding temperature is 63°C, which is the top of the temperature danger zone. If food is to be served hot, for example in a carvery situation it must be retained above 63°C. If the food is above this temperature, there is no time limit to how long the food can be served for and there is no food safety hazard.
If cold food is refrigerated, it must be retained at 1 and 4°C. The time limit would depend on the food product. The food could not be retained without time limit as eventually there would be mould growth (mould can grow at very low temperatures).
The two hour hot rule allows foods that had been prepared and are meant to be served hot, to be served at ambient temperature for no more than two hours. For example if food is being retained at 63° C and suddenly the heating equipment breaks down then that food which is losing temperature can be sold for a further two hours. After two hours the food is allowed to be chilled and reused at a later occasion after refrigeration.
The four hour cold rule allows foods that are normally sold at refrigeration temperature to be served at ambient temperature for no more than four hours. For example if sandwiches are prepared and refrigerated, and the refrigerator breaks down, the temperature will rise. The sandwiches must be sold within four hours. After four hours any remaining sandwiches must be disposed of.
In order to check temperatures of freezers, fridges, cooking temperatures and delivery of foods, a temperature probe must be used. It must be a thermocouple temperature probe which is able to be calibrated. You can calibrate with boiling water, when the probe should indicate a temperature of 100°C. The bottom range of temperatures can be checked with mixture of ice and water. The probe should indicate a temperature of 0°C. Probes must be cleaned and disinfected between use. This can be achieved by using alcohol wet wipes or simply boiling water. The temperature probe must be inserted into the thickest part of the food which is known as the core temperature.