By Matthew Stock

LEUVEN, Belgium (Reuters) - Belgium famously produces hundreds of different beers, but that is nothing compared to the varieties of yeast used to make it - around 30,000 are kept on ice at just one laboratory by scientists seeking the perfect ingredient for the perfect brew.

A team from the University of Leuven and life sciences research institute VIB are examining and cross-breeding yeast strains, adding modern genetics to a search for brewing perfection that dates back centuries.

"We're ... using robots to cross different yeast like farmers have been doing with cattle and livestock for centuries," genetics professor Kevin Verstrepen told Reuters.

"We're now doing the same with yeast on a massive scale, making millions of new strains or variants of yeast and testing which are the better ones."

By analyzing the chemical and genetic basis of a beer's flavor and aroma, the scientists say they are breeding yeast strains that promote the best characteristics for a good beer.

Their work has caught the attention of commercial brewers keen to tweak their recipes to eliminate, for example, a certain smell or to speed up the fermentation process.

"We take their yeast and try to keep as much as possible of the good things and then try and make it better," Verstrepen said.

As well as its yeast research, the lab is working on a beer database. In twice-weekly meetings, Verstrepen and his students sip and promptly spit out beers in a "technical tasting" to detect minute subtleties and differences between the taste and aroma of each brew.

Each drink, served in unmarked, identical black glasses, is evaluated and subjected to chemical analyses.

Their aim is to characterize some 250 commercially available Belgian beers, creating what Verstrepen calls a "scientific map" of beer to help drinkers select their next tipple. They plan to publish their findings in a book in the coming months.

(Editing by Robin Pomeroy)