Green ham common?
Question: What causes the greenish iridescent sheen that I often notice on
bacon and ham? Is it harmful, and why does it vanish when the product is heated?
Does this occur on any other foodstuffs?
Answer: You are likely to find such a sheen on foods containing traces of fat
in water. When it is cool this mix separates out microscopically into a film,
like oil on a wet road.
In some types of cold meats, such as sliced silverside of beef or some hams,
you may see a handsome opalescence. The beauty of an opal results from light
being refracted and diffracted by arrays of microscopic beads of glassy material
in a matrix of a different refractive index. In the meat, the effect is caused
by microscopic spheres of fat dispersed in watery muscle tissue. Heat up the
meat and you destroy the droplets and change the optical character of the matrix
so that the effect is spoilt.
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Jon Richfield
Dennesig, South Africa
Answer: My father working alone in the Australian bush in the 1920s and 1930s
ate meat either fresh, soon after it was killed, or after it had been hung in a
tree long enough for it to turn a brilliant green. The meat was put into a bag
to keep the flies off it.
He claimed that the green colour showed that the meat was no longer dangerous
to consume, and it certainly never killed him. However, there is little doubt
that it did change the flavour considerably.
Jan Morton
West Launceston, Tasmania
Iridescence is caused by light striking a surface and being scattered. The
scattered waves interfere to produce a spectrum of colours which changes
depending on the position of the observer. However, if you see a bright green
colour rather than a mere iridescent sheen then your meat may be only for the
hardy stomachs of those who tramp the Australian bush鈥擡d
Answer: The green colour that is sometimes observed on bacon and ham is the
result of the action of nonpathogenic bacteria which break down the oxygen
transport protein myoglobin to produce porphyrin derivatives. These derivatives
are large heterocyclic compounds which can have greenish colours.
Stephanie Burton
Department of Biochemistry and Microbiology
Rhodes University, South Africa
Backwash brio
Question: When filling a can from a petrol station hand pump in preparation
for the summer grass-mowing routine, I have noticed that no matter how hard I
try, it is impossible to overfill the can.
The automatic stop mechanism on the petrol pump can detect the smallest
amount of back pressure (if that鈥檚 how it works). Can anyone explain how this
mechanism does its job so well?
Answer: Most petrol pumps have a small pipe inside the nozzle. This pipe is
connected to a vacuum at the other end of the hose and thus constantly sucks air
from the end of the nozzle. When the tank is full, some petrol will be sucked by
the vacuum pipe and create a sudden decrease in pressure, which triggers a valve
and stops the pump.
John Larsson
Copenhagen
Answer: As the flow of fuel through the pump to the nozzle (marked in green
on Diagram) passes the valve seat,
the flow passage narrows. This creates a
Venturi effect, a reduction in pressure, which draws air in through the sensing
tube next to the mouth of the nozzle and over the top of the diaphragm while
fuel flows.
When fuel rising up the vessel that is being filled reaches the sensing port,
air can no longer flow into the sensing port, creating a vacuum (marked in red)
in the sensing tube. This lifts the diaphragm, which releases a spring-loaded
poppet and pulls the valve closed, automatically shutting off the fuel flow.
The mechanism also closes the valve if the ball of the safety cutout rolls
back to obstruct the flow of air in the sensing tube. This can happen if the
nozzle is jolted or the lever pulled when the nozzle is facing upwards.
P. H. Doe
Permex
Hoddesdon, Hertfordshire
This week鈥檚 questions
On the turn: Why, when driving, does the steering wheel of a car straighten
itself if you remove your hands after turning it? It doesn鈥檛 happen on my
friend鈥檚 Lego Technics car.
Clare Sudbery
Manchester
It is, of course, very dangerous to remove your hands from the steering
wheel while driving鈥擡d
It鈥檚 the pits: How does the service fault machine at Wimbledon and other
major tennis tournaments work? It detects when a ball is out of play but doesn鈥檛
seem to react to players鈥� feet.
Ian Hunter
Oxford
Clotty spice: What is the substance in turmeric that causes blood to clot and
how does it work? Apparently chefs who cut themselves use it all the time.
Robert Rea
Edinburgh