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Natural effervescence

The labels on some bottles of sparkling mineral water inform us that its carbon dioxide content comes from the same source as the water. This is removed when the water is collected and added again to the packaged bottle when it is ready to be sold to the public. In what form is CO₂ found in natural mineral water springs, how does it get there and why is it removed and then added back before bottling?

• One good reason for not simply leaving the gas and water alone is that it is difficult to conserve dissolved gases while treating the water to make it fit for human consumption. It is easier to extract the gases, store them, and re-inject them during the final bottling stage. This also enables the bottlers to ensure a consistent concentration of gas in the product. Retaining the original CO₂ is partly good marketing and partly to avoid squabbles with advertising standards authorities. However, as long as it is clean it hardly matters to the consumer whether the gas is the original or not.

The CO₂ in the springs is just that: CO₂ plus some bicarbonates. It comes from carbonate rock under the influence of soil chemistry or geothermal heat. Volcanic gases are mainly steam plus CO₂ baked out of deep crystal rocks. Where this gas meets water below the surface, pressure keeps it in solution until it emerges or reacts with surrounding rock.

Water from deep boreholes commonly emerges cloudy, containing bubbles of gas that had dissolved under the pressure of the depths.

Jon Richfield

Somerset West, South Africa

• The source of the well-known mineral water Perrier is at Vergèze in southern France and it contains natural gas bubbles which come from deep beneath the spring. This gas is volcanic in origin, collected within the geological strata. Originally these gases met and mingled with the spring water underground, rising at a constant pressure and temperature. They formed a cool, bubbling pool in which locals, for pleasure or health, used to bathe. This pool was called Les Bouillons.

When the water began to be bottled, a desire for consistency led French scientists to devise a more efficient means to capture the water’s balance of minerals and natural carbonation in the bottling process. Today, the water and the natural carbonation of Perrier are captured independently, from isolated points within the same geological formation.

The gas is tapped at a great depth before it reaches the underground mineral water. The water and the natural gas are brought together again at the bottling stage. A filter removes any natural impurities in the gas before it enters the water.

This process guarantees consistent quality and ensures that the level of carbonation in every bottle of Perrier is exactly the same as it is when the water emerges from the spring.

Daphne Barrett

Infoplan International Public Relations (representing Perrier)

London, UK

Highland swarm

I was recently in the Scottish Highlands taking refuge in my tent while squadrons of blood-hungry midges circled outside. The glen was devoid of humans and sheep, so I wondered how the midges survived. What do they normally eat in an area where there is no other source of blood?

• Only the females of the biting species of midge suck blood. They need proteins and fats for making eggs. Males seek no such luxuries in barren glens, nor do they risk their lives attacking dangerous hosts: after all, sperm is cheap.

The behaviour of bloodsuckers is appropriate to the availability of hosts in their environment. Those that target birds and mammals track CO₂, smells, warmth, or visual clues to find say, grouse and rabbits, so they are major vectors of avian malaria, myxomatosis and other diseases. Others feed off reptiles and amphibians. No doubt some frogs and lizards return the compliment, but most midges are too small be attractive prey for large insectivores with big appetites.

On the other hand, where conditions favour the growth of fly larvae and myriad adults emerge ready for mating, it does not follow that each is guaranteed a meal. Only the lucky, the aggressive and the best bloodwinners survive to breed. But then, even a survival rate of one in a hundred should more than maintain the population.

Jon Richfield

Somerset West, South Africa

• Most midges do not bite. Of the thousands of midges that exist worldwide, only 34 species of biting midge reside in Scotland. Of these, only five are partial to human blood and only the female is equipped with the proboscis that enables her to bite. Even if the midges patrolling the air space around your tent belonged to the family of biting midges, Ceratopogonidae, there would doubtless have been enough mammals and birds in the glen to provide plenty of blood meals. To read the full story of the midge, you need to get hold of a copy of George Hendry’s definitive account, Midges in Scotland (Mercat Press, Edinburgh, 2000).

Mike Follows

Willenhall, West Midlands, UK

• We did get hold of a copy of Hendry’s work and found that midge swarms not only persecute tourists but are a regular cause of bad language at evening games of Highland shinty, and their attacks have brought to an end any attempt to perform The Tempest outdoors on Rannoch Moor.

The Highland midge is tiny, with just a 1.4-millimetre wingspan, but very dedicated scientists who have sat it out record that a swarm of midges can deliver as many as 3000 bites in a single hour.

Midges are attracted to mammals by the mix of octenol (which is a by-product formed by animals that consume large amounts of vegetable matter), CO₂, lactic acid and water vapour and released by sweating and breathing. If they can get away unnoticed, midges will feed for up to four minutes and take about 0.1 microlitres of blood. For everything else you might want to know, turn to Midges in Scotland – Ed

This week’s question

Tottering jumbos

My young daughter has been discussing pressure, stiletto heels and elephants. This set me thinking. If I plotted the weight of all known walking animals against the total area of their feet, would I find that they all exert more or less the same pressure on the ground?

H. Manwaring

Huntingdon, Cambridgeshire, UK