Blowing Out Birthday Candles Increases Cake Bacteria by 1,400 Percent

I can identify the exact moment when my relationship with birthday cake changed forever, and it was last week, when I read a study titled “Bacterial Transfer Associated with Blowing Out Candles on a Birthday Cake.” Of course, the more cautious (aka germophobic) among us have already thought about it in gruesome detail. One colleague said she scrapes off the top layer of frosting, a habit that suddenly made perfect sense but which I for some reason had never before considered. I had been living in ignorant, saliva-splattered bliss. Intellectually, I knew it was fine. I’ve consumed countless slices of sheet cake finely misted with spit and suffered no particular consequences—and yet, the thought of eating another now sent visceral disgust through my body. So I called up Paul Dawson, a professor of food safety at Clemson University and one of the authors of this study, to ask why someone would want to ruin birthday parties.

Dawson said the idea for the study came from his teenage daughter. But he’s also conducted a whole set of studies around common questions in food safety with his undergraduate students, as a way of engaging them in original scientific research. These questions are, often, the same ones your germophobic friend would obsess over, including the validity of the five-second rule as well as with how bacteria spreads with double dipping (a lot), sharing popcorn (very little), and beer pong (do you even need to ask). To simplify things for the study, Dawson and his students dispensed with an actual cake and frosted a piece of foil atop a cake-shaped styrofoam base. His students stuck candles in, lit them, and blew. Oh actually, they did something before blowing out the candles: They ate pizza. “We also wanted to simulate a birthday party,” says Dawson. “We thought it might help the salivary glands get going.” “You have one or two people who really for whatever reason ... transfer a lot of bacteria.” Next came the bacteria counting. The team diluted the frosting with sterile water and spread it out on agar plates for bacteria to grow. Each colony that ended up growing on the agar represented one original bacterial cell from the frosting. (Not all bacteria will grow on agar plates, and there are now sophisticated and expensive ways to count bacterial cells more comprehensively, but this is a classic method that gives a baseline for comparison with past studies.)