Eggstrapolate please
Question: Does anybody have a formula to calculate the correct boiling time
for a soft-boiled egg, given its weight and initial temperature?
And which gives better results: starting with an egg that is at room
temperature or one at 5 掳C, and does putting it into cold water or boiling
water make a difference?
I imagine that to get the hardest white and softest yolk one would put an egg
from the fridge into boiling water, but they are likely to crack. How do you
avoid this?
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Answer: Frustrated by the somewhat random results of boiling my obligatory
Sunday morning egg, I began a series of controlled egg-boiling experiments. For
putting eggs at room temperature into boiling water, my results suggested the
surprisingly simple formula:
Boiling time (in minutes) = egg weight (in grams) x 0.1
Eggs prepared following this protocol show exactly the desired specification
of hard white and soft yolk (provided the yolk is positioned in the centre of
the egg, which is not always the case). However, this formula is an
approximation of a more general egg-boiling formula and is valid only for a
standard weight range of between 60 and 70 grams. Those gourmets who to try eggs
from less usual sources (and of extreme sizes) such as wren or ostrich will,
most probably, have to add some nasty mathematical terms to take into account
the nonlinearity of volume and body surface as well as the variety of different
egg shapes.
Achim Fischer
Freiburg, Germany
Answer: Chris Finn asked for a formula to calculate the cooking time for a
soft-boiled egg given its mass M and initial temperature Tegg.
If the egg is placed straight into a pan of water at Twater, it will be
ready when the centre temperature of the yolk has risen to Tyolk.
Tyolk 鈮� 45 掳C is about right.
If one makes some reasonable simplifying assumptions about the cooking
process and the egg, then the cooling time t can be deduced from the
solution to the heat-diffusion equation for a spherical object. Thus:
t is proportional to M2 (2梅3) 脳 loge [2 脳 (
Tegg 鈭� Twater)
梅 (Tyolk 鈭� Twater)]
where the constant of proportionality depends on the specific heat capacity
and thermal conductivity of the egg.
According to this formula, a medium egg (M 鈮� 50 grams) that takes
around 4 minutes 30 seconds to cook when it has come from the fridge T
egg = 4 掳C, would have taken a few seconds less than 4 minutes to cook if it
had been stored at room temperature Tegg = 21 掳C.
If all the eggs are stored in the fridge, then a small 47-gram egg will
require about 4 minutes 15 seconds to cook, and a large 67-gram egg will take
almost 5 minutes 30 seconds.
I believe that mass-market eggs crack more easily these days because the
proportion of grit in chickens鈥� diets has been reduced: it is heavy and
expensive to transport.
The result is shells that are rather thinner than they were 20 years ago. My
experience is that steaming eggs (use boiling water about 1 centimetre deep in a
pan with a close-fitting lid) reduces the chance of the eggs cracking as well as
saving time and energy.
Charles Williams
Department of Physics, Exeter University
Answer: The time required to boil the perfect soft-boiled egg depends on the
initial temperature, the size of the egg and its age. Fresh eggs require longer
than older eggs. As eggs age, proteins stiffen and the air bubble at the blunt
end of the egg expands, lowering the egg鈥檚 density and pushing the yolk closer
to the pointed end. This results in a shift of the centre of gravity towards the
tip.
Therefore, the freshness of an egg can be measured by its buoyancy or its
tumble: older eggs are more buoyant and they spin more smoothly if placed on a
table, held between thumb and index finger and spun in a horizontal plane.
Fresh eggs tumble to a quick halt because the yolk wobbles in the liquid egg
white in its position away from the spinning axis. With a little experience,
both methods allow an estimate of the freshness of an egg. But I doubt that
either has been described in an equation yielding cooking time.
Ulrike Krauss
Hu谩nuco, Peru
Answer: If you want a hard white and a soft yolk, it would be better to cook
at a temperature between 62 掳C and 68 掳C, for a very long time.
This means the temperature is higher than the coagulation point of the white,
and lower than the coagulation point of the yolk.
Herv茅 This-Benckhard
Coll猫ge de France, Paris
Answer: Piercing the more rounded end of an egg with the rapid application of
a pin before placing it in boiling water prevents the egg from cracking. This
allows air to escape from the egg into the water.
Jennifer Matthews
Milton Keynes, Buckinghamshire
The true connoisseur of the boiled egg may purchase a small plastic
device which drives a pin into the end of the egg with the required
蹿辞谤肠别鈥抬诲
This week鈥檚 questions
On your bike: The combined brainpower assembled in the Wheatsheaf this week
was unable to answer the following question: When a motorcycle is cornering at
speed does it have more or less of its tyre surface in contact with the road
than when it is travelling in a straight line?
John Vickers
Louth, Lincolnshire
Wave power: What mechanism tranforms gusting wind energy into the regular
wave train of ocean swells and what determines their amplitude and
frequency?
Frank Scahill
Eastonville, New South Wales
Numerical differences: On a trip to Saudi Arabia I expected to find 鈥淎rabic鈥�
numerals in use (as we call them in the western hemisphere).
Arabian numerals are entirely different, so did ours become known as
Arabic?
Jerry Kluza
Brookfield, Wisconsin
Global village: I have heard it said that every person on Earth knows
everybody else through a chain of no more than seven people so I could trace
everybody through a friend of a friend of a friend of a friend of a friend of a
friend of a friend. Is this true?
Glynn Harrington
Brough, East Yorkshire
Fruit ID: Why do items of fruit in supermarkets bear individual labels with
numbers on them?
Do the numbers denote variety so that, for example, every Cox apple is a
4105? If so, who allocates the numbers and where are they listed?
Bruce Anderson, Wivenhoe, Essex and
Fran Rawlinson, Lewes, East Sussex