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1. Cooling of Boiled Eggs
Catherine Gent, Carly Fitzmorris and Andrew LIn
BE 4120, Heat and Mass Transfer, Clemson University, Clemson, SC,
29634
Introduction
Materials and Methods
Mathematical modeling and
reactor design
Conclusions
References
Acknowledgements
Results
Abstract
This project investigates the rate of cooling of freshly hard boiled eggs with and
without the shell of the egg present on the outer surface during this process.
Newton’s Law of cooling, specifically the process of convection, was observed as
the boiled eggs returned to room temperature. The mode of convection investigated
in this experiment is natural convention. It was found that the eggs without the shell
cooled and returned to the ambient room temperature at a faster rate than the eggs
with the shell still on. It has been observed that the peeled eggs cool to room
temperature faster with a h value of 0.632 [W/cm^2K] and an overall amount of heat
transferred to be 1206.43 [J/s].
Eggs are a well balanced source of essential nutrients for the human diet and one
of the few products that are used throughout the world. The hard boiled, cooled and
peeled of eggs are often distributed to retail markets via food manufacturers (1). It is
important goal of cooling is to reduce the temperature of foods quickly to prevent
microorganism growth and to preserve food quality. This can be done via cool water
baths, fans or air cooling. There are three main components of an egg are the egg
shell, egg white and the yolk. The egg shell layer is a very thin layer and makes up
roughly 10% of the overall egg composition, therefore some consider the conduction
of heat transfer through the shell to be completely neglected (3).
Convection falls under Newton's Law of cooling and be described as the heat
transfer between surface and fluid at different temperatures, due to random
molecular motion in the fluid and bulk motion of the fluid and to bulk motion of the
fluid within boundary layer. There are two different type of convection: natural and
forced. Natural convection is caused by a difference in fluid density due to
temperature (2). For spherical shaped objects such as eggs, heat transfer is
occurring in a radial direction. As you move from the center of the sphere outwards
the area normal of the direction of heat transfer will increase.
To accomplish a rapid cooling the heat transfer coefficient (h) needs to be
increased. Throughout this experiment we measured the cooling of four hard boiled
eggs at room temperature. Two of these eggs had been peeled and two had not. We
compared the rate of cooling between these two groups, measuring the internal
temperature of the eggs through the process.
Materials:
❖ Six eggs
❖ Two 2000 mL beakers filled with 1100 mL of Water
❖ Hot plate
❖ Thermometer
❖ HOBO equipment
❖ Infrared thermometer
Methods
1) Weigh each egg individually and mark the eggs with a permanent marker
2) Measure the length and diameter of each egg in cm and record
a) Only four eggs are required for the duration of the experiment, using the recorded
measurement pick the four most similar in weight and diameter to use
3) Let 1100 mL of water boil or reach a temperature of 100 degrees celsius on a hot plate
4) Place all six eggs into the boiling water and leave them in their for 15 minutes and record
water temperature every 5 minutes
5) Remove the eggs and place them into a cold water bath at 20 degrees celsius for five
minutes and record the temperature after
6) Shell two of the four eggs
7) Insert the HOBO thermometer lengthwise into one shelled and one unshelled egg and
insert the thermometer widthwise through the other two eggs and record until they reach
a temperature similar to the air temperature
8) Simutantuelly record the infrared temperature of each egg every five minutes
The results display that increasing the layers on an object lower the show that as the layers
increase on an object, the heat transfer coefficient will decrease and in turn decrease the overall
amount of heat transferred (Table 3 and 4). This value then makes the heat harder to escape from
the center of the egg. This is visible in Table 2 where the eggs with the shells on took more time for
the surface of the egg to be comparable to the ambient air temperature. In Figure 1 it is also visible
that the eggs with shells remained at higher temperatures at the center for longer. This relationship
was modeled in COMSOL. Figures 2 and 3 show the thermal contour and graph for the peeled egg
while Figures 4 and 5 show the thermal contour and graph for the shelled egg. The graphs display a
parabolic relationship. In addition, the peeled egg shows a peak center temperature of 308.7 K while
the egg with the shell show a peak center temperature of 349.4 K. These graphs support the ideas of
a free convective cooling temperature at an ambient air temperature.
Our project displays the predicted results for the free air convection cooling
process of the boiled eggs after boiling. When a boiled egg is placed in
surroundings with a cooler temperature, the heat should dissipate into the
surroundings until the egg is similar to the temperature as the surroundings. The
room temperature of the lab room was approximately 21 degrees Celsius. The
eggs with the shell remaining on the egg for the cooling process did not return
to the ambient air temperature as quickly as the eggs with the peel removed.
For the peeled eggs it took approximately 20 minutes. This shows that
convective cooling processes have better effects on materials with higher
convection coefficients and less total resistance to release heat to the
surroundings. The hard shell on the egg acted as a layer of insulation, similar to
a person wearing a warm jacket in the winter. This inhibits the person from
feeling the cool air temperature and traps the heat of their body closer to them
instead of allowing it to dissipate to their surroundings. This is the same
concept when it comes to the eggs and their shells. Despite a person generating
his or her own heat, the example holds true. The hard shell holds in the heat as
much as possible, leading to a lower heat transfer coefficient, which in turn
keeps the unpeeled eggs a lot warmer when exposed to the same convective air
for the same time as the unpeeled eggs. After data analysis, it can be
recommended that for food processors and manufacturers it would be beneficial
to peel the hard boiled eggs to cool before packaging and distribution.
Acknowledgements: We would like to thank Dr. Drapcho for her insightful
knowledge and supplies. Thank you for supporting our research and
investigation efforts. Thank you to CCIT for producing our research display.
1. Erdogdu, Ferruh, et al. “Air-Impingement Cooling of Boiled Eggs: Analysis of Flow
Visualization and Heat Transfer.” Journal of Food Engineering, Elsevier, 28 Mar. 2006,
https://www.sciencedirect.com/science/article/pii/S0260877406002743.
2. Drapcho, C. 2022. BE 4120 Lectures 7 and 14. Unpublished course notes, Clemson University.
3. Williams , C.D.H. “The Science of Boiling an Egg.” Boiling an Egg, Exeter University , 2006,
https://www.newton.ex.ac.uk/teaching/CDHW/egg/.
❖ Wired drying rack
❖ Measuring tape
❖ Scale
❖ Permanent marker
❖ Tongs
Table 2: The surface temperature of the peeled and unpeeled eggs via the infrared
thermometer at 5 minute increments in the cooling process
Table 1: The weights, lengths, diameters and radii of the 6 eggs and the weight, radii and length of the peeled eggs
after boiling.
Figure 1: The HOBO graph of each of the 4 eggs as they cooled to the air temperature
Table 3: h values for eggs with and without shell Table 4: q values for eggs with and without shell
Figure 3: Peeled egg graph
Figure 2: peeled egg thermal contour
Figure 5: Unpeeled egg graph
Figure 4: Unpeeled egg contour
Introduction