Green eggs
Question: I love hard-boiled eggs. My mum makes me them for breakfast at the
weekend. I asked my mum why the yolk goes a funny green colour around the edge,
but she doesn鈥檛 know. It doesn鈥檛 happen with my sister鈥檚 soft-boiled eggs. Can
you tell me why my eggs change colour and why my sister鈥檚 don鈥檛?
Answer: The blue-green colour around the yolk of hard-boiled eggs is iron
(ferrous) sulphide. It is produced by a reaction between iron in the yolk and
hydrogen sulphide in the white鈥攚hich is why you see it around the edge
where the yolk and white join.
The iron in the yolk is attached to a yolk protein called phosvitin. Protein
molecules are long strings of amino acids and in raw eggs these long molecules
are folded into particular shapes. This gives them the physical properties for
doing the job that nature intended them to do鈥攁llowing a chick to grow
inside the egg. But if you boil an egg, these proteins partly unfold. This is
what makes both the yolk and white go hard, and it also releases iron from the
yolk phosvitin.
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Similarly, hydrogen sulphide is released from sulphur-containing proteins in
the egg white as they unfold during cooking. Smell a hard-boiled egg鈥攊t
has a very faint whiff of hydrogen sulphide.
Rotten eggs smell far more strongly of hydrogen sulphide because
microorganisms cause much more complete breakdown of the sulphur containing
proteins than occurs if you just boil eggs for 10 minutes.
As the egg cools, the yolk contracts slightly and pulls a little away from
the white. The blue-green iron sulphide forms at the boundary. You may have
noticed a darker colour in older eggs. This is because the yolks of older eggs
are slightly more alkaline, so more of the iron is released.
Your questioner wanted to know why you don鈥檛 see the colour form in
soft-boiled eggs? This is simply because they are cooked for a much shorter
time. They are not heated strongly enough or for long enough to release the iron
and hydrogen sulphide.
David Oakenfull and Ralph Burley
Wahroonga, New South Wales
Ralph Burley is co-author of The Avian Egg: Chemistry and Biology,
R. W. Burley and D. V. Vadehra, Wiley, New York, 1989
Sound holes
Question: Why are the sound-holes on violins and arch-top guitars 鈥渇鈥�-shaped,
and the sound-holes on flat-top guitars round?
Is there an optimum shape for the holes, or are they shaped the way they are
because of tradition?
Answer: The main considerations when looking at the sound-holes in guitars
and violins are their shape, size and position. Guitars with round sound-holes
are different instruments from those with 鈥渇鈥�-shaped sound-holes.
The fundamental problem with guitars is that the stresses of the strings
distort the instrument by both bending and twisting it. Flat-topped
guitars鈥攖ypically Spanish (or classical), flamenco and folk
guitars鈥攈ave round sound-holes. They are lightly built and usually have
gut or nylon strings. Distortion of the body is prevented by bracing structures
around the inside of the soundhole. In these guitars, a round sound-hole offers
the best structural solution while giving the best projection and volume.
Most flat-topped guitars are based on the designs of Antonio de Torres
Jurado, a famous 19th-century guitar maker. However, there is an alternative to
this type of guitar. The Ovation guitar has 11 small holes at each side near the
neck rather than one central round sound-hole. These holes eliminate feedback if
the guitar has an acoustic microphone.
The arch-top guitars with 鈥渇 鈥� holes use metal strings, are of heavier build,
and do not have the tonal qualities or projection of flat-top guitars. This is
because in the past, these guitars were mainly rhythm instruments that were
thumped rather than played. The 鈥渇鈥�-shaped sound-holes allow a solid strut to
extend the length of the guitar body, giving it the necessary strength.
However, this changed when electric guitars took over from standard acoustic
ones. Electric guitars have electronic sound pickups instead of sound-holes. The
body of an electric guitar must be rigid, because vibration will distort the
sound. When a guitar is played electrically sound-holes serve no useful
purpose.
Some players, typically jazz musicians, prefer the semi-acoustic guitar,
which has both pickups and 鈥渇鈥� holes. These can be practised on acoustically yet
performances can be amplified electronically. These guitars obviously still
require the sound-hole. However, the overall trend is towards solid guitars
which have no holes.
Violins do not have the option of a round sound-hole, as the pressure the
musician places on the bridge while holding the instrument would be likely to
distort the body if it had a central hole. The violin also requires internal
bracing that is better offered by the 鈥渇鈥�-hole design.
Some string instruments like the banjo have the sound-hole at the back. The
one-string fiddle has no body, but uses a horn and diaphragm.
As for the appearance of the 鈥渇鈥�-shaped design, this is probably related to
the musical character for fortissimo, rather than any acoustic benefit.
Stan Hayward
London
Answer: There are indeed optimum shapes for the sound-holes of the violin and
the guitar. These shapes differ due to the fundamental designs and objective
sounds of the instruments.
Arch-top instruments owe their stiffness to their arching, whereas flat
instruments with vibrating surfaces need bracing to impart stiffness and
distribute vibrations from the bridge. Even pianos have bracing on their
virtually flat, low-crowned sound boards.
A centred, circular sound-hole on a violin such as that found in a guitar
would defeat the purpose of the arching which is to introduce rigidity. This
would isolate the upper half of the top plate from the lower half, where the
bridge is mounted. In addition, the longitudonal stiffness of the instrument
would be severely compromised.
The slender 鈥渇鈥�-shaped soundholes on either side of the bridge of arch-top
instruments lessen lateral stiffness near the bridge to allow for greater
flexibility, while letting the longitudinal arching distribute the vibrations
and stresses from the bridge.
Austin Liu
Berkeley, California
This week鈥檚 question
Roman division: How did the Romans express fractions?
John Chappell
Grassington, North Yorkshire