杏吧原创

Budding flavours

Question: Why and how do we 鈥渁cquire鈥� taste? I don鈥檛 know anyone who liked
the taste of Marmite, Fisherman鈥檚 Friends or olives at first sampling, but after
a few attempts they begin to taste pleasant. Is it anything to do with our taste
buds becoming less sensitive and therefore needing stronger flavours to excite
them?

Answer: Although there may be changes in taste or smell receptor cells as we
age, in general the acquisition of most individual food preferences does not
relate to any changes in these sensory systems. Most individual likes and
dislikes are probably best explained by a form of Pavlovian conditioning.

The brains of humans and other animals have the capacity to link the
consequences of eating a food鈥攎etabolic, physiological, psychological and
social鈥攖o its associated sensory characteristics. This learning is
probably most closely linked to aromas (which are unique to each food) rather
than tastes, because many foods share being sweet, salty and so on. Numerous
animal experiments have shown how the physiological consequences of eating
particular foods, including subtle nutritional and metabolic effects, can
markedly alter sensory preferences.

Three points should be borne in mind. The first is that the sensory quality
or flavour of the food influences how much we enjoy it, apart from any
pre-existing feelings about it. The second is that it is not a conscious
process鈥攐ne cannot simply decide to dislike the flavour of a food. And the
third point is that it is not your nose or tongue which determines the response,
but the centrally mediated reaction to the sensations delivered by these organs.
With experience, therefore, it is not the actual sensation of a food flavour
which changes, but the affective response to these sensations.

A prompt and persistent dislike or aversion to specific sensory qualities of
foods can develop when they are associated with strong negative outcomes. In
humans, nausea and gastrointestinal upset appear to be particularly potent
initiators of aversions. For this reason, multiple food aversions may develop
with certain medical treatments, particularly cancer therapies. Aversions which
developed in childhood may persist many decades later, with dislike of a food
remaining long after the initial incident is forgotten, and despite the
knowledge that the item is known to be safe to eat or did not cause the
illness.

In an analogous manner, it appears that human sensory preferences can arise
after a taste is linked to positive physiological or psychological outcomes. It
seems reasonable to ascribe the development of liking for items such as coffee
and alcoholic beverages, which children have an innate distaste for, to their
psychobiological effects.

The biological advantage of the ability to modify food preferences is
obvious. Our enjoyment of fatty foods is a case in point鈥攖he evidence
suggests that this is secondary to their association with the metabolic
properties (perhaps energy density) of fat-containing foods. The example of a
liking for hot, spicy foods provides perhaps the most extreme example: chilli
peppers are universally rejected by young children, but are among the most
popular condiments in the world.

Interestingly, recent studies indicate flavour preferences may be acquired
early in life through exposure to volatile flavour compounds from the maternal
diet in the womb or during lactation.

Dave Mela

Institute of Food Research

Reading, Berkshire

Faint fingers

Question: Just after I wake up, my hands often feel very weak. A good stretch
is all I need to get rid of the feeling and, even without stretching it passes
off within a few minutes. Does everybody feel this and how does it happen?

(continued)

Answer: This effect is often noticed by children, less by adults. The
temporary hand paralysis or weakness experienced on waking is attributed to a
central nervous system inhibitory mechanism which may have evolved to reduce the
possibility of injury during dreaming. It is a real phenomenon and almost
certainly other muscles are affected, though less noticeably.

Martin Harris

Hawthorn, Victoria

Shafted

Question: If you find yourself in a free-falling lift is there any action
that you can take to reduce the effect of the collision? Would jumping just
before you hit the bottom of the lift shaft help?

(continued)

Answer: I can see three ways of increasing your survival chances, although
only slightly. One has already been mentioned 鈥攋umping as vigorously as
possible before you land in order to cancel out some of the upthrust. The second
is to get any soft objects you have with you, your clothes for example, and
place them underneath yourself prior to impact. This would increase the
deceleration time of the collision, and slightly reduce the amount of damage
done. If you aren鈥檛 bothered about your legs, I suppose you could try standing
up, and have them act as 鈥渃rumple zones鈥�, although this could be fairly messy.
The third is hardly worth mentioning. You could try to spread yourself out as
much as possible while holding on, in order to increase the lift鈥檚 surface area.
This should decrease its terminal velocity by some indiscernible amount.

David Foale

Tollerton, Nottinghamshire

Micro madness

Question: A colleague of mine is in the habit of heating bottled water for
his tea in a mug in a microwave oven. When the water is up to temperature he
removes the mug.

On several occasions, the water has started to bubble violently after he has
added a tea bag. On one occasion, the boiling started when he was removing the
mug. It was so violent that it blew 90 per cent of the water from the
mug鈥攚hich is obviously quite dangerous. What is happening?

(continued)

Answer: Superheated liquid can boil explosively if something is added, as in
the examples given by your previous correspondents, or if the vessel is moved. I
have seen a spectacular explosion of a bottle of liquid which had just been
removed from a microwave in a laboratory鈥攇lass and hot liquid were thrown
across the room. This can be avoided by leaving any liquid that has been heated
in a microwave to stand for at least a minute before touching it or opening the
door. This allows for slight cooling and for the heat to become more evenly
distributed. I recommend that everyone does this when heating liquids in a
microwave, even to make a cup of tea.

Diane Warne

Cambridge

This week鈥檚 question

Flight path: Why do rockets take off standing up? Why don鈥檛 they go along a
runway?

Toby Biggins-Gilchrist (aged 5)

Sydney, New South Wales