Double Dipping? 5-Second Rule? Scientists Separate Food Fact From Fiction In New Book

Download Audio
"Did You Just Eat That?" by Paul Dawson and Brian Sheldon. (Robin Lubbock/WBUR)
"Did You Just Eat That?" by Paul Dawson and Brian Sheldon. (Robin Lubbock/WBUR)

Subscribe to the Here & Now podcast on Apple Podcasts.

The holiday season is coming, and that means parties — where guests might double dip a chip, or pick up a dropped olive and pop it back into a glass.

But is that safe?

The new book "Did You Just Eat That?" looks at ways to avoid spreading illness through food, including results from detailed studies by food scientists and co-authors Paul Dawson and Brian Sheldon. They join Here & Now's Robin Young to discuss their findings on common scenarios people encounter — from blowing out birthday candles to handling a restaurant menu.

"Basically, the bacterial cells, viral cells, etc., microorganism cells, far outnumber the number of cells we actually have in our body," Sheldon says. "For most part, these organisms actually do a lot of good things. But then again, there are organisms that can cause food-borne illnesses or other types of illnesses."

The 5-Second Rule

Once a piece of food hits the floor, bacteria "almost immediately attach," Dawson says. That means "a lot can happen" in that famous five-second window, even though many might think it's harmless.

"We talk about the ... pounds of dog feces that are dropped each year, so the odds are there probably won't be a pathogen there. But there always could be."

"Biting the chip off and sticking it back in the dip, you're actually inoculating the dip with your oral bacteria."

Brian Sheldon, on double dipping

Blowing Out Birthday Candles

"Basically every time that we breathe, we are actually emitting bioaerosols — very small droplets, and these droplets can carry out microorganisms: flu virus, bacteria, tuberculosis organisms," Sheldon says. "In our study, we kind of comically came up with the scenario that you had a 90-year-old grandmother who was celebrating her birthday, and asked for the assistance of her grandchildren in blowing out these candles. So you can imagine just the number of organisms that could be spread by just the simple practice of blowing out birthday candles."

Double Dipping

"Biting the chip off and sticking it back in the dip, you're actually inoculating the dip with your oral bacteria," Sheldon says. "Salsa actually had more bacteria transferred than the cheese fondue or chocolate dip [in our study]. And when you think about it, it's common sense because salsa is much thinner, so more inoculated salsa, if you will, is dropping back off the cracker or chip back into that common bowl than with the other two."

Hand Dryer, Or Paper Towels?

"Just like we talked about blowing birthday candles out, the same thing would apply to a hand dryer or blower in a bathroom," Dawson says. "Just by flushing a toilet, you're emitting bioaerosols from the toilet or a urinal. You compound that by sticking your hands under a hand dryer.

"We actually tested not only the air coming out — we put a petri dish underneath where the air came out. We also tested the push buttons as well as the intake, so it could be that someone pushes a button before they dry their hands or wash their hands, you can imagine that's inoculating the surface there, so that might be one reason. Bacteria form biofilm, so actually dislodging or rubbing [your hands] actually helps. That might be a way that you're getting more bacteria off with a towel than you are [with] a hand blower."

Most hand dryers aren't cleaned very often, Sheldon says.

"Therefore they accumulate organisms and dust, and every time you cycle on the hand dryer, you are also basically taking that dust and that dirt and those organisms and propelling it out the exit vent," he says.

Handling A Menu

"In our study, we looked at a random sample of menus, as well as transfer of bacteria from hands to menus in an inoculated study in our lab," Dawson says. "Randomly we found significant numbers of bacteria on menus. Everyone, virtually, handles a menu — people can handle them, sneeze on them, cough on them."

Book Excerpt: 'Did You Just Eat That?'

by Paul Dawson and Brian Sheldon

Blowing Out Birthday Candles, or Spraying Germs on Cake?

Suppose you are attending Aunt Bessie’s ninety- first birthday party, and she’s ready to make a wish and blow out the candles on her cake. She needs some help with blowing out the ninety- one candles blazing away in front of her, so she enlists her three young great-grandchildren. All together now, the four of them blow, and blow, and blow again. It’s, well, a lot of blowing over the cake everyone is about to eat. Do you really want someone blowing on your food just before you eat it? (Or at all, for that matter?) Who started the practice of blowing out candles on birthday cakes?

Several theories are offered to explain when and where the tradition of blowing out birthday candles originated. Some think it began in ancient Greece when people brought cakes with lit candles to the temple of the goddess of the hunt, Artemis. Other ancient peoples believed that smoke from candles carried their wishes to the gods. One of the first documented accounts of using birthday candles appeared in the mid-1700s among the writings of Andrew Frey, who was traveling in Germany at that time. While visiting Count Ludwig von Zinzendorf in Germany, Frey reported that candles (one for each year of the count’s age) were placed on cakes for his birthday celebration.1

Spraying Germs, Much?

Droplets from your respiratory tract and mouth are expelled by coughing, sneezing, talking— and yes, even breathing. In fact, particles of flu virus were detected in the exhaled breath of infected individuals even during normal breathing and talking.2 When respiratory droplets are released, they may spread infection directly through the air or by landing on a surface.3 Other studies found that exhaled breath contained from 693 to 6,293 bacteria/cubic meter and that humans pollute indoor air by emitting bacteria at a rate of about 37 million gene copies per person, per hour.4 So when someone blows out birthday candles, it’s extremely likely that their breath carries bacteria or viral particles directly toward the cake that everyone else is about to eat! Let’s verify this idea with an experiment.

Experiment 4-1: Blow or No Blow

Materials and Methods

The objective of this study was to evaluate the level of bacterial transfer to the top of a cake when someone blows out candles. Since we were concerned only with what was landing on top of the cake, we designed a simulated cake consisting of a thin layer of icing spread on a sheet of foil and cut to fit over a circular foam base. Sixteen candleholders and candles were inserted through the icing/foil and into the foam base.

Each test participant was first asked to smell and consume a piece of hot pizza to generate saliva and simulate a birthday party atmosphere. After the candles were lit, test participants blew until all of the candles on the mock cake were extinguished. For each testing session, a control sample was collected: the same procedure was followed for the test sample, but candles were not blown out. After the candles were lit, the test participants either blew them out or did not blow them out. The candles and holders were then removed from the foam base without touching the icing. Using sterile forceps, the foil was folded in half with the layer of icing inside.

Science Stuff Ahead

The foil with icing was placed in a Stomacher bag (Classic 400, Seward UK), unfolded inside the bag, and then 50 mL of 0.1 percent sterile peptone water was poured into the Stomacher bag over the iced surface of the foil. The Stomacher bag was placed in a Stomacher (Stomacher 400, Seward) and mixed at 230 revolutions per minute (rpm) for 1 minute. A Stomacher is simply a laboratory tool that mixes samples within a bag that simulates the mixing action of your stomach; two paddles alternately squeeze a sealed bag in a stomach-like mixing action. Duplicate 1- mL and 0.1- mL volumes of the homogenate were aseptically removed from the Stomacher bag, serially diluted in 0.1 percent peptone water, and surface-plated in duplicate onto petri dishes containing plate count agar (Difco Plate Count Agar). Plates were incubated at 37°C for 48 hours. Colony- forming units (CFUs) were counted on plates containing 25–250 colonies and then converted to CFUs per icing sample and log10 CFUs per sample.

The experiment was replicated three times on separate days, using 11 participants and yielding 33 observations per treatment (blow or no blow). The effect of blowing versus not blowing out candles on bacterial counts in the frosting was determined using the proc univariate command of the Statistical Analysis System (SAS). Statistical differences between the blowing and not-blowing treatment groups were determined at a significance level of 5 percent. Descriptive statistics such as the mean (average), median, range, and standard deviation were also obtained.5

Results of Experiment 4-1

Some studies on airborne droplets originating from human mouths are found as early as 1899, and others were published before the mid- twentieth century.6 These early studies yielded varying results, but they all agreed that droplets were released into the air by breathing, coughing, and sneezing. Amazingly, 90 percent of the bacteria- carrying droplets remained airborne for 30 minutes, and some smaller droplets hung around for up to 30 hours.7 In our study, blowing out candles over icing resulted in a population of bacteria recovered from icing that was fifteen times higher than the bacterial population from icing where the candles remained lit. Also, the range (difference between the highest and lowest counts) in number of bacteria recovered from icing was 100 times greater for icing exposed to the blow versus no- blow treatment.

Furthermore, the average number of bacteria on the cake icing increased by about 2,700 bacteria, or 1.48 percent, due to candles being blown out. The sloppiest candle blower we tested increased the number of bacteria on the icing by over 37,000, or over a whopping 12,000 percent, compared to the icing not blown on.

Things to Consider

Bacteria are an unavoidable part of life. As nearly everyone knows by now, they are present in and on almost everything people touch or contact. Therefore, we believe it’s important to understand how micro­organisms move in our environment and to become familiar with ways to minimize contamination. Disease-causing microorganisms can spread rapidly through a human population by means of bioaerosols, and poor air hygiene can have adverse effects on human health.8

Ceremoniously blowing out the candles on a cake might be a problem if the person doing the blowing is sick, since we know that respiratory droplets expelled by coughing and sneezing can carry pathogenic bacteria and viruses.9 The spread of diseases including SARS and H1N1 avian influenza have been attributed to human bioaerosols.10 What’s more, microbes originating from respiratory droplets can be transmitted directly through the air or indirectly as droplets landing on surfaces that are then touched (or eaten, in the case of birthday cake). Now that you know the truth about blowing out birthday candles, do you think the tradition is safe in all cases? Well, you are now armed with the facts, so the decision is entirely up to you. Blow wisely.

Excerpted from "Did You Just Eat That?" by Paul Dawson and Brian Sheldon. Copyright © 2018 by Paul Dawson and Brian Sheldon. Republished with permission of W. W. Norton & Company.

Karyn Miller-Medzon produced and edited this interview for broadcast. Jack Mitchell adapted it for the web.

This segment aired on November 16, 2018.


Headshot of Robin Young

Robin Young Co-Host, Here & Now
Robin Young brings more than 25 years of broadcast experience to her role as host of Here & Now.



More from Here & Now

Listen Live