In the Champagne region, the first "miracle" attributed to Dom Perignon was the creation of  white wine from black grapes. He accomplished this by drawing off the juice from perfectly formed grapes, and then using only the initial must, or first pressing.

     Patient but tenacious, he was ingenious in knowing what grapes complemented each other. He blended not only different grapes, but the juice from the same grape grown in different vineyards. This was a technical feat not for the faint of heart, even among the most experienced vintners.

     But his most amazing feat was yet to come.

    The conditions of the Champagne region made for a short growing season and a late harvest. Wine was bottled for fermentation in autumn, and when spring came and temperatures rose, the fermentation started again. Then just as suddenly it would go flat.

     The bottle stopper commonly used at the time - a wood and oil soaked hemp plug  - was the suspected culprit. But the old monk was puzzled. What would it take to capture the bubbles?
 

    The discovery was close at hand when two visiting Spanish monks visited the vineyard in 1698. After a warm greeting, and an exchange of news, their host suddenly noticed that Spanish water jugs were plugged with cork.

   Of course! Porous cork was strong enough to plug the bottle, but would allow in enough oxygen for the wine to breath. With careful reblending, and more experimentation, and allowing time for proper aging, the half blind Dom Perignon had what he was searching for - and the brethren were excitedly summoned in for a first taste.

    "Come quickly," he said.

    "I'm drinking stars!"

More resources to medieval wine on the Web:

Ancient & Medieval Wines

Medieval Drink

Omar Khayyam: In Praise of Wine, c. 1100 AD