Bacterial Transfer Associated with Blowing Out Candles on a Birthday Cake

Abstract

This study examined the potential spread of bacteria when blowing out candles on a birthday cake. Preliminary tests of blowing on nutrient agar indicated that bioaerosols in human breath expelled from the mouth may be a source of bacteria transferred to cake surfaces. To test aerosol transfer to cake, icing was spread evenly over foil then birthday candles were placed through the foil into a Styrofoam™ base. After consuming pizza, test subjects were asked to extinguish the candles by blowing. Icing samples were sterilely recovered then surface plated, to determine the level of bacterial contamination. Blowing out the candles over the icing surface resulted in 1400% more bacteria compared to icing not blown on. Due to the transfer of oral bacteria to icing by blowing out birthday candles, the transfer of bacteria and other microorganisms from the respiratory tract of a person blowing out candles to food consumed by others is likely.

Full text

Journal of Food Research; Vol. 6, No. 4; 2017
ISSN 1927-0887 E-ISSN 1927-0895
Published by Canadian Center of Science and Education
1
Bacterial Transfer Associated with Blowing Out Candles on a
Birthday Cake
Paul Dawson1, Inyee Han1, Danielle Lynn1 , Jenevieve Lackey1, Johnson Baker1 & Rose Martinez-Dawson2
1Department of Food, Nutrition and Packaging Sciences, 2Department of Mathematical Sciences, Clemson
University, Clemson, SC 29634, USA
Correspondence: Paul Dawson, Depertment of Food, Nutrition and Packaging Sciences, Clemson University,
Clemson, SC 29634, USA. Tel: 1-864-656-1138. E-mail: pdawson@clemson.edu
Received: March 24, 2017 Accepted: April 10, 2017 Online Published: May 22, 2017
doi:10.5539/jfr.v6n4p1 URL: https://doi.org/10.5539/jfr.v6n4p1
Abstract
This study examined the potential spread of bacteria when blowing out candles on a birthday cake. Preliminary
tests of blowing on nutrient agar indicated that bioaerosols in human breath expelled from the mouth may be a
source of bacteria transferred to cake surfaces. To test aerosol transfer to cake, icing was spread evenly over foil
then birthday candles were placed through the foil into a Styrofoam™ base. After consuming pizza, test subjects
were asked to extinguish the candles by blowing. Icing samples were sterilely recovered then surface plated, to
determine the level of bacterial contamination. Blowing out the candles over the icing surface resulted in 1400%
more bacteria compared to icing not blown on. Due to the transfer of oral bacteria to icing by blowing out
birthday candles, the transfer of bacteria and other microorganisms from the respiratory tract of a person blowing
out candles to food consumed by others is likely.
Keywords: birthday candles, aerosolized bacteria, blowing, bacterial transfer
1. Introduction
1.1 Blowing Out Birthday Candles
The tradition of blowing out birthday candles has different theories as to its origin. Some theorize the practice
began in Ancient Greece related to bringing cakes with lit candles to the temple of the goddess of the hunt,
Artemis. Other ancient cultures believed the smoke from candles carried their wishes and prayers to the gods. A
written account reported of birthday candles matching the age of Count Ludwig Von Zinzendorf being presented
at the Count’s birthday celebration in Germany in 1700’s (Frey, 1753). This tradition has become commonplace
in many parts of the world.
1.2 Spread of Bacteria
Bacteria are an unavoidable part of life, present in and on almost everything humans contact. Whether benign or
pathogenic, it is important to understand how bacteria are transferred and become familiar with measures for
avoiding contamination. Illnesses related to pathogenic bacteria, which can spread rapidly throughout the
population, are a major public health concern in today’s society. Bioaerosols and poor air hygiene can have
adverse effects on human health (Douwes, Thorne, Pearce & Heederik., 2003; Xu et al., 2011). Respiratory
droplets expelled by coughing and sneezing are sources of normal human flora, as well as pathogenic bacteria
(Obeng, 2008; 1970; Houk, 1980) and viruses (Loosli, Hertweck, & Hockwald, 1970). The respiratory tract can
be colonized with pathogenic organisms that can then be aerosolized in the breath of an infected individual
(Couch, Knight, Gerone, Cate, Douglas, 1969; Knight, 1973). The spread of respiratory diseases including SARS
(Yu et al., 2004) and H1N1 avian influenza (Baker et al., 2010) have been attributed to oral airborne transmission.
In fact, influenza virus particles were detected in the exhaled breath of infected individuals through coughing,
breathing and talking (Fabian et al., 2008; Stelzer-Braid et al., 2009; Huynh, Oliver, Stelzer, Rawlinson & Tovey,
2008; Lindsley et al., 2010). When respiratory droplets are released, they may spread infection directly from
person-to-person or by contamination of surfaces then touched by others (Obeng, 2008). The bacteria may have
originated from either respiratory droplets expelled directly onto surfaces or indirectly as droplets coating hands
that are transferred by hands to surfaces. In fact, exhaled breath contained 693 to 6,293 CFU of bacteria/m3 (Xu
et al., 2012) and Qian, Hospodsky, Yamamoto, Nazaroff & Peccia, (2012) reported that human occupants are

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significant contributors to indoor air bacteria and that humans emit bacteria at a rate of about 37 million gene
copies per person per hour. Thus when a person forcibly exhales, as with blowing out birthday candles, bacteria
or viral particles are aerosolized from the respiratory tract of that individual.
1.3 Research Objective
The purpose of this research was to evaluate the level of bacterial transfer transferred to the top of a cake when
blowing out the candles on a birthday cake. Scientific data from our investigation may help raise awareness of
possible health risks associated with birthday celebrations and encourage others to take steps toward preventing
the spread of bacteria.
2. Methods
2.1 Blowing Out Candles
A sheet of foil (Bakers & Chefs, Bentonville, AR) cut in the shape of a circle with a diameter of 149 mm placed
on a Styrofoam™ disc (Styrofoam Brand Foam, Floracraft, Ludington, MI), of the same size then 18g of icing
(Betty Crocker Rich & Creamy Vanilla Frosting, General Mills, Minneapolis, MN) was spread in a thin layer on
the foil. Seventeen candles (Best Occasions, Bentonville, AR) (3.2 mm in diameter, 50.8 mm high, and set in
plastic holders 19.0 mm high) were evenly spaced into the Styrofoam, passing through the icing and foil layers.
Each test subject was asked to smell and consume a piece of hot pizza to simulate a meal-dessert sequence. After
lighting the candles, test subjects were instructed to blow until all of the candles were extinguished on the mock
cake (Figure 1). For each testing session a control sample was collected where the procedure was followed for
the test sample except candles were not blown out.
2.2 Enumeration of Bacteria
After lit candles were blown out (blow) or not blown out (no-blow) the candles and holders were removed from
the Styrofoam™ base, without touching the icing. Using sterile forceps, the foil was folded in half with the layer
of icing inside. Then, the foil was placed in a stomacher bag (Classic 400, Seward, UK) and unfolded inside the
bag.
Figure 1. Styrofoam™ base and candle apparatus with icing used to test bacterial transfer when blowing out
candles
Fifty ml of 0.1% sterile peptone solution were poured into the stomacher bag over the iced surface of the foil.
The stomacher bag was placed in a stomacher (Stomacher 400, Seward, UK) at 230 rpm for 1 min. Duplicate
samples of 1 ml and 0.1 ml volumes were aseptically removed from the stomacher bag (Classic 400, Seward,
UK), serially diluted and surface plated on plate count agar (Difco Plate Count Agar, Sparks, MD) in petri dishes.
Samples were spread evenly on the agar and incubated at 37°C for 48 hours. Colony forming units (CFU) were
counted on plates containing 25-250 colonies and converted to CFU per sample and log10 of CFU per sample.

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2.3 Research Design and Statistical Analysis
The experiment was replicated 3 times on separate days by 11 subjects yielding 33 observations per treatment
(blow or no blow). The effect of blowing vs. not blowing candles out on bacterial counts in the frosting was
determined using the proc univariate command of SAS (2010) to obtain mean, median, range and standard
deviation. The student’s t-test was also performed and proc glm and pdiff commands were used to determine if
significant differences existed between the blowing and non-blowing treatments.
3. Results and Discussion
Blowing out candles over icing resulted in 15 times more and statistically higher number of bacteria recovered
from icing compared to icing that did not have candles blown out (Table 1). Also, the variation (range) in
bacteria recovered from icing was 100 times greater for icing exposed to the blow compared to the no blow
treatment. Furthermore, the median and maximum transfer of bacteria increased 300 and 12,000 %, respectively,
due to blowing out candles. Studies on airborne droplet size from the oral cavity are found as early as 1899
(Flugge, 1899) and by several others before the mid 20th century (Hutchison, 1901; Winslow, 1910; Strausz,
1922; Lange & Nowoselsky, 1925; Hamburger, 1944; Duguid, 1946). These early studies came to varying
conclusions but found droplets were released into the atmosphere surrounding humans that are breathing,
coughing and sneezing. One study reported that 90% of bacteria-carrying droplets remaining airborne for 30
minutes in still air and that some smaller droplets remained for up to 30 hours (Duquid, 1946). More recently,
Wan et al. (2014) established that up to over 2,000 moisture particles were released per breath, all less than 5 um
in diameter. The particle size is an important factor since bioaerosols will carry both bacteria and viruses in small
particle droplets generated by breathing, blowing and coughing. The average size of expelled particles generated
by couching and speaking was found to be much larger (13.5 um for coughing and 16.0 um diameter for
speaking) by measurement at the mouth opening thus minimizing the effect of evaporation on droplet (particle)
size (Chao et al., 2008) which may be a factor in other studies using droplet condensation methodology. Chao et
al. (2008) also found that there were between 1000 to 2000 in number and 2 to 5 ml in volume of droplets per
cough and even 0.2 ml of moisture droplets during speaking. Therefore the size of droplets in expelled air are
large enough to carry bacteria as well as viruses. Normal respiratory aerosols can include Staphylococcus spp.,
Streptococcus spp., Corynebacterium spp., Haemophilus spp., and Neisseria spp. (Madigan, Martinko, Dunlap,
& Clark, 2009). Madigan et al. (2009) also found certain pathogenic species, such as Streptococcus pneumonia
and Staphylococcus aureus, may cause illness when spread through surface contamination via oral aerosols.
Considering contagious diseases such as influenza, some researchers have concluded that airborne transmission
is a likely pathway (Weder & Stilianakis, 2008; Wein & Atkinson, 2009). Fabian et al. (2008) and Stelzer-Braid
et al. (2009) detected viral influenza in the exhaled breath of infected patients. To this point, Fabian et al. (2008)
reported that 60% of patients with influenza A had detectable levels of the virus in exhaled breath with 87% of
exhaled particles less than 1 um in diameter. In another study, Lindsley et al. (2010) reported that 81% of
influenza patients had influenza RNA in their breath and that 65% of the influenza were found in aerosol
particles 4 um in diameter or smaller.
Verifying that bacterial cells as well as viruses are carried on human bioaerosols, Fennelly et al (2004) reported
that 25% of tuberculosis patients exhaled from 3-633 CFU per cough of Mycobacterium tuberlosis in expelled
air particles.
Birthday celebrations routinely include the ceremonial blowing out of candles on top of a cake. Some food safety
concern exists in light of previous research on bioaerosols generated by breathing, coughing and speaking
supported by the results of the present study finding that bacterial levels averaged 15 times higher in icing due to
blowing out candles.
Table 1. Mean, median, range and standard deviation of the bacterial counts for cake icing exposed to blowing
out candles and not blowing out candles
No blow1
Blow2
Increase from No-blow to blow
CFU/sample3 (log CFU/sample)4
CFU/sample5
(%)6
Mean
183b (2.2) b
2889a (3.5 a)
2706
1479
Median
150 (2.2)
600 (2.8)
450
300
Maximum
300 (2.5)
37,450 (4.6)
37150
12383
Standard deviation
112 (2.1)
6620 (3.8)
6508
5811
1No-blow = cake icing not exposed to blowing out candles
2Blow = cake icing exposed to blowing out candles

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3CFU/sample = colony forming units per cake icing sample. N = 33.
4Log CFUsample = log10 of colony forming units per cake icing sample
5CFU/sample Increase = CFU/sample from samples blow on - CFU/sample from samples not blown on
a ,b means with different superscripts are significantly different (p ≤0.0001).
Acknowledgments
This research was supported by the Creative Inquiry Program at Clemson University. Technical Contribution
No. 6547 of the Clemson University Experiment Station.
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