杏吧原创

Icy sparks

Question: I often cook mixed vegetables from frozen in a microwave, on a
microwaveable plate. I鈥檝e noticed that carrots produce sparks during cooking
and, on closer examination, they display small burns. Why is this?

Answer: Several factors make frozen carrots susceptible to the effect
described (although other vegetables do this).

First, while microwaves are extremely good at heating liquid water, ice is
almost totally transparent to them, so it is actually quite difficult to get ice
to melt in a domestic microwave oven. The 鈥渄efrost鈥� option on a microwave oven
relies on intermittent heating of a small amount of liquid water present on the
food, and heat conduction from these areas into frozen material. By putting
frozen material into the microwave oven with continuous energy input, no time is
given for thermal conductivity effects, and therefore a colossal heating effect
occurs on a very localised surface area. These areas鈥攖ypically at the
extreme point of the carrot鈥攚ill dry out rapidly and then char,
essentially forming small carbon points.

Secondly, carrots are relatively large objects (compared to, say frozen peas)
and because microwaves are essentially varying high voltage fields, a large
alternating electrical potential exists between the highly conductive charred
sections.

Thirdly, carrots are generally given quite angular cuts, giving sharp points
which will yield the highest field gradients.

The combination of a large alternating field across a good electrical
conductor with sharp points causes electrical breakdown of the air and the
sparks which accompany this.

Depending upon the precise conditions, it is equally possible for charring to
be a secondary effect, rather than a cause. In this case, the discharge may
originate from uncharred points, with charring only occurring as a huge current
passes through a relatively small point.

Gavin Whittaker

Heriot, Borders

Answer: There is a related phenomenon involving grapes which may cast
additional light on the subject.

If a grape is cut and opened, leaving a small hinge of skin joining both
halves, it produces sounds as well as bright flashes within seconds of being
microwaved. The brief show ends with two steaming pieces of cooked grape resting
at opposite sides of the plate.

This is, of course, a foolish and totally irresponsible misuse of kitchen
equipment and I am not suggesting that readers should try it.

Ian Russell

High Peak, Derbyshire

Rotten hoarders

Question: I have two red squirrels in my garden which I feed. Any nuts they
leave in their feed box eventually go mouldy. Yet I recently saw one of the
squirrels burying its nuts in the lawn. Why don鈥檛 the buried nuts go mouldy or
rot?

Answer: Some of them do. The dead ones definitely do. Are you sure that you
are feeding the squirrels live nuts?

If there is mould in the feeder, there is likely to be a bit of a buildup of
fungal spores, more so than in the ground. Saprophytic fungi (those that feed on
dead materials) would attack dead seeds as soon as there is enough moisture.
They would only tackle live seeds if the tissues weaken.

Moisture is one thing that weakens their tissues, age is another, so fresh
nuts do not usually go mouldy so soon.

For pathogenic fungi things may be slightly different but in suitable soil,
healthy seeds can stay unharmed, with most of the fungi in the ground being
saprophytes. Saprophytes are kept away from live tissues by defensive chemicals
and the like. So, while the seed stays alive or sprouts, it can defend
itself.

Remember too, that a nut going mouldy in air has room for the mould to show
as fibres or a crust. Tramped down in moist soil, the mould may be invisible
unless you have a microscope, or know what to look for, even though there may be
more actual fungus present. Again, soil contains many bacteria, some of which
will engage the moulds in silent, subterranean warfare, the various microbes
eating and inhibiting each other, while some of them eat the nut. Bacterial
rotting often does not look much like mould.

Jon Richfield

Dennesig, South Africa

Ivory poaching

Question: My wife recently had silver-coloured fillings in the back teeth on
both sides of her mouth. When she eats boiled eggs, she experiences a tingling
sensation on one side of her mouth and the filling itself becomes hot. This
occurs only with the white of the egg, not the yolk and only in one filling. Any
explanations?

Answer: Dental amalgam (so called 鈥渟ilver鈥� filling) is made from mercury,
silver, copper and tin. Saliva can behave as an electrolyte and allow
amalgam-filled teeth to generate an electrical current. Currents of up to 4
microamps can be measured from filled teeth. The current generated is specific
to the tooth and/or filling and it also tends to increase with the age of the
filling. Salty foods will produce a stronger current than non-salty foods
because salty foods produce a better electrolyte. If the filling is close to
nerves in the tooth then the current can excite the nerves resulting in a
tingling sensation or toothache.

Michele Hackman

Reigate, Surrey

Plank syndrome

Question: Several flies were gathered on a plank of wood in my garden this
summer. Strangely, they were all facing in the same direction. If one of them
took off and flew around for a while it would return to land near its neighbours
and, on landing, realign itself. At first I thought they were realigning
themselves with the Sun, but they didn鈥檛 alter position as the Sun moved across
the sky. Later, a larger fly of a different species landed on the wood and faced
the same direction too. Why did they all do this?

Answer: The flies are relaxing and their piece of wood is ideal because it
absorbs the warmth from the Sun. They face the same way because they have
detected a breeze or draught of air and have aligned themselves to face into the
air current for stability. This is less strenuous than being buffeted sideways,
or worse, facing downwind and having their wings ruffled.

Also, if they are disturbed by a predator, an emergency takeoff into the
breeze will give better lift than the less controllable downwind takeoff.

Resting pigeons can also be seen facing the wind to gain maximum stability
from their streamlining and feather positioning.

Edward Evans

Cark-in-Cartmel, Cumbria

This week鈥檚 question

It鈥檚 not: Sorry, but I had to ask, why is nasal mucus often green?

David Tanner

Felsberg, Germany