Food and Drink: The magic that gives Marston's its Pedigree: Heath Robinson would have been proud of the 'Burton Union', a yeast cultivating system that gives a complex, fruity, dryness to Michael Jackson's beer of the month

NO OTHER beer is created in the way they make Marston's in Burton upon Trent. No other brewery still fills entire buildings with the enormous Victorian 'Burton Union' apparatus, a magic machine that looks as if it had been designed by Heath Robinson or some barmy professor in a Rupert Bear story. Other breweries have long since torn down their Burton Union systems as needlessly elaborate, expensive relics.

Next Friday, Marston's, a giant among independent breweries, will unveil a brand-new machine - a bubbling, foaming, dripping, fruity-smelling, hall-filling testimony to Marston's dedication to a particular aspect of its beer's aroma and flavour, even at a time when the firm is the subject of takeover speculation.

What the magic machine does is to breed the unique combination of yeast strains that give an inimitable complexity of fruity dryness to Marston's ales. It comprises a double-deck ironwork gantry. Into the lower deck are permanently bolted a series of huge oak casks, each about four times as big as a full-sized (288-pint) beer barrel. Along the top deck is a metal trough. Copper tubes rise from the casks and bend like swans' necks over the trough.

When barley malt from Lincolnshire, Nottinghamshire and East Anglia has been infused in the hard, faintly sulphury well water of the Trent Valley (in cast- iron tuns), and boiled with hops from Hereford, Worcester and Kent (in copper kettles), fermentation begins in open copper or steel squares for a couple of days, then moves on to the oak unions - perhaps so-called because they work in unison in sets of 20 or 24.

As fermentation reaches a peak in the unions, its natural forces send the foaming liquid up the copper swan-necks into the trough above. There, the foam subsides and the liquid is able to escape through ports that lead back into the unions. This circulation continues for three or four days before the beer has exercised sufficiently to go to the brewery's cellars. There it is racked into conventional casks for three or four days' maturation. Then more sugar and yeast are added before it leaves for the pub, where it has a further fermentation in cask before being tapped.

As a living organism, yeast adapts to its habitat. Just as a circus lion jumping through a hoop every night would develop ever more powerful hind legs, so the yeast that has been making the long journey through the Burton Unions at Marston's since the early 1800s has evolved its own characteristics. If this system were not used, the yeast would change.

For a long time, Marston's was able to expand without adding to its unions. In fermentation the yeast cells multiply. Even if the brewery runs less than half of its brew through the unions, enough of the distinctive yeast is produced to ferment the rest in more conventional vessels. In recent years that flexibility has been stretched to the point where the new unions were necessary.

The alternative would have been for the brewing scientists to try to train the yeast to live in other circumstances without being excessively influenced by its environment. That is what other brewers have done over the years.

Only in the past few decades have brewers begun fully to appreciate that, for all the importance of their particular water, barley-malt and hops, their house strain of yeast has a profound influence on the flavour of their beers. It is not much more than 100 years since the first pure-culture brewing yeast was isolated. Before the microscope, brewers did not know what yeast was; they simply scooped the foam off one batch as a 'starter' for the fermentation of the next.

With its good water, long brewing history, fine ales and central position, Burton became the beer capital of England in the canal and railway eras. Its system of fermentation in wood seems to have made a leap in scale along with the town's brewing industry. The fixed oak casks became 'Burton' Unions even though they were also used in other towns.

The most resilient oak traditionally came from Memel (now Klaipeda), in Lithuania, whose forests saw heavy fighting between the Russians and Germans in the Second World War. After the war the supply diminished (a cooper told me he often found shrapnel in staves) and the forests between Frankfurt and Heidelberg have been the source for the newest Burton Union apparatus. After some unforeseen problems, the wood eventually arrived at Buckleys, the 200-year-old firm of coopers now housed in an old cotton mill in Dukinfield, near Manchester.

The cooperage, which has about a dozen employees, had never made Burton Unions before. The job has taken a year or two, amid the strafing noise of electric wood-planes, the scream of saws, the hot surge of steam with which to bend the wood, the hammer of mallets and the fearsome squeeze of a machine called the iron man. Each of the unions made cost about pounds 1,000, and the new installation uses 120 of them. The oak shavings have gone to a man who smokes bacon to sell in the market at Ashton-under-Lyne.

After hearing that from Charles Howarth, the owner of Buckley's cooperage, I fancied a bacon sandwich for lunch with a pint of Marston's premium pale ale, Pedigree. Instead, he took me to the aptly-named Royal Oak, a Marston's pub at Didsbury, where the average sandwich seems to accommodate about half a pound of cheese.

Marston's is a larger-than-life company, right down to the chairman Michael Hurdle's taste in sporting tweeds. In the distant past it saw some of its stock acquired by Whitbread. The orders of the Mergers and Monopolies Commission now require a percentage of that stock to pass to other hands. It is to be hoped such hands are not hostile. Anyone trying to swallow Marston's might bite off more than they can chew. Meanwhile, I shall swallow another pint of Pedigree as my beer of the month.

(Photograph omitted)