DISCUSSION

The 3 second rule: How children think about food safety

24 Oct 2013 09.42 am by Renny Wijeyamohan

I was at a family BBQ and as my niece, Sam, bit into a hot dog – the sausage part made a quick getaway - squeezing free of the bun (and Sam’s clutches) and onto the floor. It landed with a splash, in a pool of tomato sauce, as it settled near her feet. “3 second rule!” she squealed, grabbing the rogue sausage from the tiled floor, calmly returning it to captivity in her bun, before grinning widely and taking a bite.

The 3 second rule – that old schoolyard adage. In my childhood it had been expanded into the 5, 10, 15 and even 30 second rule depending on the urgency of the situation. This got me thinking – how do children think about food safety? Is there any scientific basis to the 3 second rule? And why would Sam – so quick to scoop up that sausage – not bother with beans, spaghetti or tuna if they spilled off her plate?

A child’s sensory experience

If it looks good and it tastes good – then it must be good right? Children seem to prefer two senses – sight and taste – over others when it comes to assessing food safety. Let’s go back to Sam and try to apply some logic to the way she responded to the situation. She dropped a sausage on the ground, it was still intact – its structural integrity had been maintained and it still had the appearance of edible food. By this same reasoning, I suspect she would be just as quick to pick up a piece of chocolate, a biscuit or some cake. But what if it was a squashed tomato, a pie or a bowl of spaghetti that dropped on the floor? Despite Sam’s generous nature, the less attractive (read: inedible) appearance of this food might prevent its rescue from the floor. 

Intuitively this makes sense – the fact that a piece of food still “looks” how it was cooked – means it must still be good – if it’s covered in dirt, less so. There are probably a few implicit assumptions that Sam is making too, though she might not even know it: the sausage’s skin is intact, it’s only had minimal contact with the surface area of the floor, it’s a dry food. Let’s compare this to spaghetti: it splits apart, it splatters over a wider surface area, it’s wet so it could be friendlier to bacteria.

High fat, high sugar more bacteria resistant

In a 2012 study by the Manchester Metropolitan University (MMU) a biscuit, dried fruit, ham, cooked pasta and bread with jam were dropped on the floor for intervals of 3, 5 and 10 seconds. Foods with a high fat and sugar content like the biscuit, ham and bread with jam fared the best – picking up little or no bacteria growth in 3 seconds. The dried fruit and cooked pasta on the other hand showed signs of Pseudomonas and Klebsiella – both potentially harmful pathogens.

The study points to some factors that can impact a food’s anti-microbial qualities when dropped on the floor:

• water activity level: high water activity level can support bacterial growth
• adhesion ability: how easy it is for bacteria to get a grip
• molecular make up: high salt, high sugar prevent the growth of most    bacteria
• contact time: longer contact time means more bacterial growth

Now that we know a little bit more about how foods pick up germs from the floor, we can see that Sam’s instinctive responses to what foods the 3 second rule applies is – according to the MMU study – not too far off the mark.

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