Serving & Displaying
Foods should be prepared as close to serving as possible to prevent the risk of recontamination. If foods are prepared and reheated, special care must be taken with the in between hot and cold time frames and prevention of contamination.
If not eaten immediately but put on display, safe temperatures must still be maintained. Hot foods on display must be heated to 80°c before being put into the display case, which must keep the food at over 65°c. This must not be for longer than one day on display.
Cold perishable foods on display must be at less than 4°c or not out for longer than 2 hours before being disposed of as unsafe to eat. Many people spend lots of money of chilled display cases that are not designed to act as fridges and leave perishable foods in them all day at temperatures like 10 - 20°c. This could be really dangerous and is an offence under the Food Hygiene Regulations 1974.
It doesn't matter whether you use time or temperature to protect you food, but one of them must be used. Know what temperatures your display cases provide by checking them with a thermometer. Then you can assess how long you may display perishable foods in them. If they are not cold enough for all day display, keep a small amount on display and stock them up from the fridge (which you have already confirmed to provide less than 4°c!)
Lower Risk Foods
Some foods, which aren't readily perishable, such as sweet muffins and cakes, fruit, vegetable dishes, may not need chilled display. But all foods on display must still be covered and protected from contamination.
Ensure foods on display do not touch each other if they are of a different risk type. Do not allow the bottoms of containers to touch other foods and ensure serving utensils are readily available for each dish to prevent cross-contamination by them. In self-serve situations, your customers may have to be supervised to ensure they provide no risk to the foods by misuse of utensils or contamination by poor handling and sneezing and such like.
During preparation and especially once food has finished being prepared, handle the food directly as little as possible. Use clean utensils, tongs, scoops and the like, but make sure these aren't a source of cross- contamination between foods.
Left-overs must be thoroughly reheated to piping hot, but only once. Do not make left-overs out of reconstituted left-overs. The time/temperature ratio will have been tested too many times to guarantee safety. If left-overs are to be eaten cold, it should be within three days, assuming they were kept in a fridge at less than 4°c for that time and well protected from contamination.