杏吧原创

Heated hop

Question: I was sitting with a group of friends in the pub last night and, as
we contemplated our pints of bitter, we wondered: why does beer go flat when it
gets warm? Not only that, but the effect seems to be more pronounced with lager
than with bitter.

Answer: The answer lies in the behaviour of gases, and their solubility in
water. Most beers are dilute solutions of sugars, gases, organic acids and other
complex compounds, and (hopefully) alcohols.

The gas which gives fizzy drinks their distinctive fizz is carbon dioxide. In
the case of British-style real ales, the CO₂ is generated by the action
of yeasts on residual sugars in the drink, whereas in most beers, including
British-made lagers, the conditioning gas is added artificially at the brewery
or at the point of sale.

The problem arises because the solubility of CO₂ is related
to the temperature of the solvent in which it is dissolved, in this case the
beer. More gas can dissolve in cold beer than in warm beer. This is also why
fish such as trout and salmon, which need a lot of oxygen, live in cold mountain
streams and rivers, because the amount of oxygen dissolved in these environments
is so much higher.

When the beer is served from the pump it will contain a certain concentration
of dissolved CO₂, but, as the drink heats up under the influence of a
sweaty hand in a warm room, its ability to hold its CO₂ in solution
decreases.

This excess gas is then released into the atmosphere through the bubbles that
you see rising in the beer, and the drink consequently goes flat. Other volatile
compounds from the malt and the hops vaporise faster and you may notice that the
beer also smells different.

The difference your correspondent observes between lager and bitter is mainly
caused by two factors. The first is that lagers are generally served much colder
than bitters, to disguise the fact that they have less taste (this is because
they contain fewer fruity esters and longer-chain alcohols, as a result of the
lower fermentation temperatures and different yeast strains that are employed in
their manufacture). The bigger temperature difference between the beer and the
air means lagers warm up faster than bitters. Their rate of loss of CO₂
is consequently much higher, and they go flat faster.

Secondly, lagers are generally more carbonated than bitters, so are fizzier
at the point of purchase and therefore have more CO₂ to lose in the
first place. This extra carbonation, like the lower serving temperature, is
usually used to camouflage the lack of flavour that is found in most
British-brewed lagers.

The answer to the problem, of course, is to drink your beer faster, or sit in
cooler pubs.

Geoff Nicholson

Newcastle upon Tyne

Salt removal

Question: I have a survival handbook which explains that in the Arctic it is
possible to obtain fresh water from sea ice, even though seawater is itself
undrinkable. The trick is to cut blocks of sea ice and then leave them stacked.
After a while, the top layer of ice becomes free from salt. Is this true and if
so, what is the underlying mechanism?

Answer: When seawater freezes (and there are up to 20 million square
kilometres of sea ice in the Southern Ocean and up to 14 million square
kilometres of it in the Arctic each winter), the crystals of ice that combine to
form the floes are pure water. All the salts contained within the seawater are
expelled from the ice crystals to form a highly concentrated brine solution
which collects in a labyrinth of pores and channels that permeate the ice.

These brine channels vary in size from a few micrometres to a few millimetres
in diameter, their size being governed by the temperature of the ice. When it is
very cold the channels become smaller. If the ice warms up, the channels
enlarge. As the ice gets older the brine in the channels gradually drains from
the ice, sinking into the unfrozen seawater below, and as the sea ice thickens
this brine expulsion leads to the desalination of the ice.

Sea ice that is only a few months old has very little salt remaining in its
uppermost layers. If a lump of sea ice is taken out of the water, the brine
rapidly drains away and the remaining ice can be melted and used as a fresh
water supply. The least salty water can be obtained from the ice at the top of
the block. Of course, this process works whether or not you cut the ice into
blocks, so old sea ice also provides a good source of low-salinity water.

David Thomas

School of Ocean Sciences

University of Wales, Bangor

Answer: This method of desalting ice has been used for as long as anyone can
remember, but perhaps the first written description appeared in 1874 when John
Rae studied the process and theorised that gravity drainage and some 鈥渟olvent
action鈥� of the brine accounted for the reduction in salinity.

It actually depends upon the fact that salt doesn鈥檛 dissolve in solid ice.
Therefore it is found in pockets of highly concentrated brine amid a matrix of
salt-free ice, where it was rejected when the ice crystallised during
freezing.

Two mechanisms are believed to contribute to the loss of salt. The major one
is brine drainage out of the ice along the channels between ice crystals. The
minor one is brine pockets within crystals migrating downward under the
influence of the temperature gradient between the cold air above the ice and the
warmer ocean (or land) below it.

Sea ice also desalinates itself naturally. Pools of water which form on top
of sea ice in the summer are often fresh, unless they have been contaminated by
ocean waves washing over the ice. In addition, old sea ice which has
recrystallised to form 鈥減encil ice鈥濃�攊ce composed of hexagonal ice crystals
the size of pencils鈥攊s also fresh. All this was well known to 19th-century
whalers, who used sea ice to obtain drinking water.

Ross Firestone

Winnetka, Illinois

This week鈥檚 questions

Words in the sky: Pilots who write advertising messages in the sky using
smoke from their aircraft are a dying breed here in the US. Nonetheless, I鈥檝e
often wondered how they do it. How does a pilot manage to orient the plane in
three dimensions to write a message in the sky that can be seen clearly by
observers on the ground?

William Goodwin

Massachusetts

Getting about: Why do some birds walk and others hop? And are there any that
do both?

Sam

By e-mail, no address supplied