Temperature control must be strictly adhered to ensure the safety of food on the receipt, storage, preparation, cooking and serving of ingredients. The temperature danger zone is between 5 and 63 degrees Celsius. Food, after preparation and cooking, must be kept out of this zone. When hot food has been cooked, it must be retained above 63 degrees to remain free from bacterial growth. Food Safety pathogens will not grow above this temperature. Once food is cooked to 75 degrees C it is safe to eat, all bacteria have been killed. If food is retained at the correct hot-holding temperature, it can be served indefinitely, as it is going to remain safe at that temperature. (Although food quality would be greatly affected by retaining it for that length of time!). No vegetative pathogens will survive temperatures above 60 degrees C, so perhaps cooking to 75 is slightly overkill.
If high risk food is being served cold, it must be refrigerated between 1 and 4 degrees C and held at that temperature until served. Sandwiches and baguettes with protein fillings, pork pies, pates, cold meats, seafood, etc can all cause food poisoning if left at warm temperatures, as any bacteria or spores present will start to grow to dangerous levels. (Spores would only grow as vegetative pathogens after germinating.) It is the vast numbers of bacteria that cause food poisoning. Foodborne diseases are caused by fewer bacteria, which are termed low dose pathogens.
Although there is no time limit to how long hot food can held, from a food safety point of view, cold food can prove to be a problem if held for too long at refrigeration temperatures. Moulds, for example, can start to grow on food within several days, even in very cold conditions.
In the UK, the Four Hour Cold Rule can be used to good effect if there is a rapid turnover of cold food or if refrigerated storage breaks down. If, for example, a batch of sandwiches are prepared for service to the public, and is held in refrigerated display, and that equipment was to subsequently break down, the temperature of the food would start to increase to within the temperature danger zone. In those circumstances, one would be allowed to sell the sandwiches for a maximum of 4 hours from the time of preparation. If any remain after the 4 hours, they must be disposed of.
Some baguette/sandwich companies, which have a quick turnover of product, such as in train stations and airports, are allowed to sell their wares without refrigerating their products, due to this ruling. This ensures the sensory qualities of the food are enhanced. (There is nothing worse than cold rolls or baguettes).
This 4 hour ruling can be considered to be a food safety hazard, however, especially if buffet items are festering for 4 hours, after being prepared, in a warm room. If there are already bacteria present, which there will be, (from handling, environmental particles, cross contamination), they can multiply quite significantly in 4 hours. Bacteria can double in size every 10 minutes. In four hours bacteria will double in size 24 times. If only 10 bacteria are present at the start of the 4 hours, 10 to the power of 24 bacteria will grow in that time. (That is quite a few! In the order of trillions. It only takes 10 to the power of 6 Salmonella to cause food poisoning).
For further information on food safety, the prevention of food poisoning, cleaning, pest control, personal hygiene and legislation go to Food Safety.