A Melbourne student has busted the schoolground ‘10-second rule’ of dropped food as a myth. According to the rule, food dropped on the floor or ground is edible if picked up within 10 seconds. But in bad news for hungry teenagers, Perry Reiter, 14, found that cooked chips dropped for even five seconds were contaminated, whether they fell on a vinyl floor or outside on concrete.

Perry said he decided to do the research after encountering the rule at school. “I thought that food dropped on the floor would pick up dirt, so I decided to test it, and see if it became more contaminated with time. 

The result was a microbiological study completed during the school holidays at an industrial canteen, and entered in Victoria’s Science Talent Search, where it earned a prize. 

Perry used cooked chips in his research ‘because they’re a food to which the rule is often applied, and the cooking should ensure they have no bacteria to start with.’  Frozen chips were trimmed to a standard size, deep fried by the canteen operator and then dropped for timed periods - five, 10 and 20 seconds, one minute, 10 minutes and one hour.  

Perry said, “The shorter times represent the periods you’d find at school and the longer times were to test whether food became more contaminated if left for longer.” 

The research tested the idea on two different surfaces – the floor of the plant’s canteen and the concrete porch where employees often eat outdoors. Perry said, “These surfaces were chosen to simulate the situation in schools, where food is often dropped inside the  tuckshop or outside on a concrete play area.” 

The dropped chips were swabbed and the samples plated on Petrifilm, then incubated for 48 hours at 25oC and the colonies counted. Control chips were sampled without being dropped. Perry said, “I had to decide whether to test numbers of a specific pathogen such as E. coli or Total Viable Count (TVC).

TVC was chosen because it was thought unlikely that pathogen-specific tests would show any bacteria at all. 

“We chose 25°C as the incubation temperature because many mesophiles grow well at this temperature and most pathogenic bacteria are mesophiles.” 

The results showed that the control chips had very little contamination – zero or one colony. The dropped chips generally had higher counts than the controls, and those dropped on the ground were more contaminated than those dropped on the canteen floor.

However, TVC generally did not increase with time, up to 10 minutes. Perry said, “In most cases, it did not seem to matter whether the chips were dropped for five seconds or 10 minutes – they were contaminated pretty well immediately. “Two of the samples left for one hour showed large numbers of colonies, possibly reflecting generation time and high initial contamination, but the counts were all lower than I expected. 

“The results showed that chips have a higher TVC if dropped than if not dropped. This is a potential health hazard depending on the type of bacteria, so dropped food shouldn’t be eaten.” 

Perry said more research was needed – particularly pathogen-specific testing - because, while TVC could show contamination, it could not show whether the food was actually unsafe. 

“The most difficult thing about the research was finding a statistical test to use, because of the distribution of results. I’m very grateful to (NSW scientist) Bill Spooncer for help with that.”

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