What was going on? It was impossible to contemplate that the whisky coming off the still had mysteriously changed, so something must be happening - or rather, not happening - during maturation.

Had there been an atmospheric change, imperceptible to humans, which disturbed the slumbers of the maturing spirit? Was it to do with global warming, or holes in the ozone layer, or nuclear testing? Could it be the casks themselves? "But", the blenders reasoned, "we supply our own casks to be filled with new spirit, and we know the exact provenance of each cask, so how can they be letting us down?...Call Dr. Swan."

Jim Swan is a leading expert on maturation and a pioneer in techniques for analysing flavour in spirits. With the help of a government grant, he and his colleague, Jim Gray (an expert on whisky production), set to work.

"We soon determined that the problem was in the casks themselves. For some reason a proportion of American oak hogsheads in current use were not maturing the spirit they had been filled with at the rate that we might have expected. Yet there seemed to be nothing wrong with the casks. They were in good condition; many were first-fill; their provenance was well documented."

Or was it? The researchers noticed that a number of casks had been purchased, at very competitive prices, from a couple of cooperages. They had been bought as newly re-made hogsheads - i.e. they had been used for maturing Bourbon, but not Scotch. Yet chemical analysis of the wood showed that they had certainly held Scotch - indeed, many had been filled more than once, and some had contained both malt and grain whisky at different times.

"It turned out that a couple of unscrupulous cooperages (they are no longer in business) had been buying spent casks from blenders, cleaning them up and selling them on to other blenders as new hoggies. Physical inspection would simply not reveal this. At first we thought we had cracked the problem, then we realised that these casks were just the tip of the iceberg: many others had come straight from Kentucky, and there was no possibility that they had been used for Scotch. So Jim Gray and I took our investigation to America.

"The original source of American white oak was the Ozark Mountains of Missouri, a remote district with poor soil which grew trees too small for most purposes other than making barrels. In recent times the Bourbon industry had begun to source timber from further east - more easily accessed, with better soils; where the trees grew faster and provided a greater yield of wood. This was our first clue.

"Our second was the fact that many cooperages now kiln-dry their timber, rather than leaving it out to season in the open air for eighteen months, as was the traditional way. Kiln-drying takes a mere twenty-three days.

"Now, kiln dried wood makes no difference to the maturation and flavour of Bourbon, but it has a drastic affect on the second incumbent - Scotch, Canadian or Irish whisky - which relies on the intricate chemistry of the wood itself to achieve the desired results".

In 1991 Doctors Swan and Gray published their definitive Specification of American Oak Wood for use by the Scotch Whisky Industry. In a nutshell, their recommendation is that at least a quarter of the timber used in whisky casks must be slow grown and air seasoned, if the desired end result is to be achieved. After publication, Jim returned to America to see whether the specification could be met. The Bourbon industry was very supportive. He was introduced to the few small stave mills which still seasoned their timber in the open air. He discovered that the only place where the growing conditions were perfect, where the trees grew slowly because the soil was poor and the climate arid was - you guessed it - the Ozark Mountains of Missouri.

* Dr. Jim S. Swan is a partner in R.R. Tatlock & Thomson, (Analytical and Consulting Chemists to the Distilling, Brewing, Wine and Water Industries).