Amazingly research has been meticulously performed to investigate the above question: Does the 5-second truth hold any truth? I think firstly it is important to determine what this golden rule of food hygiene is. In basic terms, it is claimed that if food is in contact with the floor for less than 5 seconds then you will be safe from harm against any of the bacteria transferring to your food.
The research was performed by a Professor of food science in the Journal of American Society for Applied and Environmental Microbiology. Four surfaces (Stainless steel, tiles, wood and carpet) with four different foods (watermelon, bread, bread and butter and gummy candy) in contact for 1 second, five seconds, 30 seconds and 300 seconds. All variables were tested for a total of 128 scenarios, each replicated 20 times to make 2,560 measurements!
Watermelon was found to hold the most amounts of bacteria and gummy candy holding the least. This effect may have been led by the level of moisture of the food. Also, the carpet surpace was found to have the lowest bacterial transfer rates compared to tiles and wood.
It was found that the transfer of bacteria is close to instant, although the type of food and the surface appear to have some influence, although small.
Take home point: If you drop food on the floor, there will be a transfer of bacteria and this will happen long before 5 seconds.