Geiger prawns
Question: I recently read an article about illegally irradiated food. Tests
showed that frozen supermarket prawns had been subject to this process. How is
it possible to test a prawn for previous exposure to gamma radiation?
Answer: The Food Standards Agency sponsors the development of detection tests
for irradiated food. Perhaps the best methods of detecting gamma irradiated
prawns are luminescence techniques: thermoluminescence (TL) or photostimulated
luminescence (PSL).
Many foodstuffs have small amounts of mineral grains associated with them.
Prawns, for example, have silicate material in their gut. These mineral grains
act as dosimeters. When they are irradiated, energy is stored as charge centres
trapped within the crystal lattice of the grains. If you 鈥渢ickle鈥� these minerals
with energy (heat in the case of TL or infrared light for PSL) the charge
centres can be released from their traps, giving rise to luminescence that can
be measured using light-detecting instruments.
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Several European standard protocols for the detection of irradiated food have
been produced, and in 1996 the Ministry of Agriculture, Fisheries and Food
undertook a survey: The MAFF Undeclared Irradiated Foodstuffs Surveillance
Exercise. A total of 789 samples were collected (307 dried herb and spice
samples, 286 chilled or frozen prawn and shrimp samples, 81 fresh fruit and
vegetable samples, 69 chilled or frozen chicken samples and 46 chilled or frozen
liquid egg samples). Four samples鈥攍ess than 1 per cent鈥攚ere
identified as irradiated or containing irradiated components but not labelled as
such. These four samples were chilli powder, paprika, hot curry powder and mild
curry powder.
The sale of undeclared irradiated food is not a food safety issue, but one of
ensuring proper food labelling. Regulations state that all foodstuffs treated
with ionising radiation or containing ingredients that have been treated with
ionising radiation have to be labelled using the words 鈥渋rradiated鈥� or 鈥渢reated
with ionising radiation鈥�.
Carl Blackburn
Radiological Safety Unit
Food Standards Agency
London
Answer: Others approaches besides TL and PSL have been explored. Although
shellfish have low lipid content it may be possible to detect cyclobutanones
using gas chromatography mass spectrometry. These are unusual radiolytic
products derived from triglycerides and should not be present in untreated
products. Electron spin resonance signals from these trapped radicals can be
detected in well-mineralised shells for a time after irradiation.
There are potential methods based on radiolytic damage to the cell nucleus
and DNA in the prawns themselves, although some of these effects can also be
mimicked by spoilage. There are also some intriguing microbiological approaches.
For example, the radio resistance of the microflora on irradiated prawns may
take on an unnatural profile. And there are techniques for demonstrating
possible excess populations of dead microorganisms in comparison to live
microbiological counts.
Finally, there is evidence that irradiated prawns might even taste better!
This may be due to a release of volatile compounds from ruptured cell membranes.
But the precise mechanism has yet to be determined.
David Sanderson
Scottish Universities Environmental Research Centre
East Kilbride
Vanishing birds
Question: What happened to the sparrows in London? There used to be millions.
Now I see maybe two a year. Did cats eat them all?
Answer: Estimates suggest the number of house sparrows in Britain fell
between 1972 and 1996 by 9.6 million birds from a total of around 17 million.
While the cause is unknown, the best theories suggest the demise of the urban
sparrow is either due to lack of food or the wrong sort of pollution. Food
sources in cities may be declining or it may be that sparrows are losing out to
other species in competition for food. Differences in the levels and types of
pollution鈥攊ncluding the introduction of unleaded petrol鈥攎ay also
have had an effect.
One other factor may be a lack of suitable nesting sites in modern towns and
cities. Modern houses are simply not sparrow-friendly, because they lack the
holes, nooks and crannies that make for good nesting positions.
The Royal Society for the Protection of Birds is currently backing research
which it hopes will provide the explanation. This is running alongside a survey
using reports from the public, which the RSPB is conducting in conjunction with
the BBC, to determine actual numbers of sparrows and nests in urban areas.
Rural sparrow numbers have also declined in recent years but the reasons
behind this seem easier to pinpoint. Modern farming methods have reduced the
number of weed seeds on which rural sparrows feed and this, along with a decline
in insects thanks to more effective pesticides, has led to a vast reduction in
countryside sparrows. This mirrors a reduction in around 20 small rural bird
species, some of which have declined by up to 90 per cent.
Sparrow populations can be supported by providing regular supplies of weed
seeds. Supplies of seeds suitable for birds can be obtained in supermarkets.
Mike Everett
RSPB, Sandy, Bedfordshire
Answer: Although we do not know why the house sparrow has declined so
dramatically in London, it is probably connected with a decline in the
population of invertebrates that are essential for the nestlings in their first
few days of life. The decline began in the late 1980s around the same time that
that unleaded petrol containing methyl tertiary-butyl ether was
introduced. An impact on the invertebrate population is possible but not
proven.
The decline in urban sparrows is not confined to London. It has also occurred
in other large cities in Europe, though not in small towns where traffic density
is lower. Interestingly, the decline appears smaller in Paris where there is a
proportionately higher use of diesel fuel.
J. Denis Summers-Smith
British Trust for Ornithology
Thetford, Norfolk
This week鈥檚 questions
Perspiring patterns: Men not only seem to sweat more than women but they seem
to sweat in different places. Is the pattern of sweat glands different, and what
distinguishes a sweaty person from a non-sweaty one?
Pat Endacott
London
Fungal frolics: In continental Europe, gathering wild mushrooms is a popular
pastime and knowledge of types and their culinary virtues is widespread. In
Britain few people can tell one mushroom from another. Why? Surely there was a
time when Britain鈥檚 rural population ate anything edible. Has mushroom lore
simply been lost?
Ann Cooke
University of Cambridge