杏吧原创

Hail the ale

When Britain ruled India, brewers developed a special type of beer known as India pale ale that, after being brewed in Britain, could withstand crossing the equator twice before arriving in India still in a drinkable condition. This was in the days before beer could be pasteurised and filtered, and so it was heavily hopped to keep it in good shape. What are the properties of hops that help them to preserve beer?

鈥� Hops contain a group of compounds called humulones or 伪-acids, that are very insoluble in water, but which undergo a chemical rearrangement during the wort boiling to form an isomeric group of compounds called isohumulones that are water soluble. The analogues of humulone (and isohumulone) also found in hops 鈥� cohumulone, adhumulone and prehumulone 鈥� differ only in the number of carbon atoms in the side chains. It is these iso 伪-acids that give beer its characteristic bitterness and also exert 鈥渂acteriostatic鈥� effects on most gram-positive bacteria. In other words, they do not kill bacteria but prevent their growth.

Before the advent of refrigeration and pasteurisation, the only way to prevent spoilage in beer was by using alcohol and hops. Alcohol provided an unfavourable environment for microbial action, and the isohumulone content of the hops inhibited the growth of bacteria such as Lactobacillus. Thus, high alcohol content (in German export beers, for example) and high hopping rate (as in Indian pale ale) could protect beer from the souring associated with long storage times.

India pale ale was 鈥渋nvented鈥� by George Hodgson, a brewer at the Bow Brewery in East London in the 1790s, who took his pale ale recipe, increased the hop content considerably and raised the alcohol content by adding extra grain and sugar. Hodgson also added dry hops to the casks at 鈥減riming鈥�, when sugar is added to allow secondary fermentation, and conditioned the beer with more sugar than was typical for pale ales. The high sugar priming rate probably helped keep the yeast alive during the voyage and resulted in a very bitter, alcoholic and sparkling pale ale that could withstand the rigours of travel while having a reasonable shelf life in India.

Stefan Winkler

Vice-Consul Science and Technology British Consulate-General, Boston

鈥� With such large amounts of hops and alcohol, what did it taste like? Hops are the most expensive ingredient in any beer and a modern brewery probably couldn鈥檛 brew a genuine 19th-century India pale ale economically. With so many hops, a young IPA would hit the cheek cells like paint stripper. However, the long voyage and the pale ale futures market meant the beer usually spent 12 months ageing. This turned the hop bittering from an aggressive taste to a fine bouquet, which some writers described as reminding them of a French white wine.

I found this description fanciful, but I have carried out research on IPA. In the brews I made, especially those based on Edinburgh recipes of around 1840, the taste after a year was unlike anything currently on the market. It certainly wasn鈥檛 overpoweringly bitter.

The combination of hops and alcohol provided a very powerful antibacterial environment, but there was still much that could go wrong. The ale was vented before the long voyage to prevent serious explosions during the crossing and this may have introduced bacteria. It was also not easy to sterilise the casks before filling them. However, it was a trade worth pursuing because of the huge volume of empty cargo ships returning to the colonies and the cost of carriage was very low.

Clive La Pensee

Author of Homebrew Classics IPA CAMRA Books 2001 Beverley, East Yorkshire

鈥� Many brewers who exported porter (or Imperial stout) to Russia during the 19th century increased the beer鈥檚 life by boosting its hop and alcohol content. These dark porters, brewed in England but popular with Russian royalty, were high in alcohol, sweet and dense, and survived the journey from Britain to the Baltic and across Russia. They are still popular in the Baltic states where they are still brewed. Samuel Smith鈥檚 of Yorkshire produces a fine example.

At the same time as IPAs were becoming popular, the first golden lagers of Plzen (known as pilsners) were spreading fast, making use of the new railways and liberal doses of Saaz hops.

Most IPAs currently sold in Britain tend to be bitters that are hoppier than the norm, although a handful of 鈥渉istorical IPAs鈥� weighing in at 6 per cent or more alcohol by volume (still less than the 10 per cent plus of the originals) are brewed by such micro-breweries as Burton Bridge and Freeminer.

Laurent Mousson

Berne, Switzerland

Pingu鈥檚 pleasure

Why is it colder at the South Pole than at the North Pole?

鈥� Much of the temperature difference between the two poles can be explained by their difference in elevation. The North Pole (with monthly average temperatures in winter of around 鈭�30 掳C) lies on sea ice on the surface of the Arctic Ocean while the South Pole (at around 鈭�60 掳C) is 2800 metres above sea level on the ice sheets of the Antarctic continent.

The background variation of temperature with height (in Antarctica about 鈭�6 掳C per kilometre gain in height) thus accounts for over half the difference. Also, the 鈥渢hinner鈥� (and hence colder, drier and less cloudy) atmosphere overlying the South Pole reflects less heat back to the surface than its northern counterpart. Much of the remainder of the temperature difference can be explained by the contrasting atmospheric circulation regimes in the two hemispheres.

The continents of the northern hemisphere drive quasi-stationary 鈥減lanetary waves鈥� in the atmosphere. These waves transport heat polewards and also 鈥渟teer鈥� mid-latitude depressions into the north polar regions. The continents of the southern hemisphere are smaller and lower than those in the north, so the southern hemisphere planetary waves (and associated heat transport) are smaller.

The high mountains of Antarctica also block the poleward movement of mid-latitude depressions, which rarely penetrate into the interior of the continent. Finally, the atmosphere at the North Pole receives some heat from the underlying Arctic Ocean. Although the heat conducted through the 2 to 3 metres of sea ice that typically cover the ocean is small, large amounts of heat can be exchanged over the narrow 鈥渓eads鈥� of open water that occasionally form between ice floes.

John King

British Antarctic Survey, Cambridge

This week鈥檚 questions

Burnt out

What controls the rate at which candles burn?

Mark van Rhee

Ajax, Canada

Shapely curves

Why are the walls of a barrel curved? Why not save the trouble of bending the staves and make it perfectly cylindrical?

Alan Starr

Cologne, Germany