Yeast, the essential microorganism for fermentation in the brewing of beer, converts carbohydrates into alcohol and other products that influence appearance, aroma, and taste. In a study published online today in Genome Research (www.genome.org), researchers have identified the genomic origins of the lager yeast Saccharomyces pastorianus, which could help brewers to better control the brewing process.
For thousands of years, ale-type beers have been brewed with Saccharomyces cerevisiae (brewer's or baker's yeast). In contrast, lager beer, which utilizes fermentations carried out at much lower temperature than for ale, is a more recently developed alcoholic beverage, appearing in Bavaria near the end of the Middle Ages. Lager beer gained worldwide popularity starting in the late 1800s, when the advent of refrigeration made year-round low-temperature fermentations possible. Saccharomyces pastorianus, the yeast used in lager brewing, is a "hybrid" organism of two yeast species, Saccharomyces bayanus and S. cerevisiae. It is thought that the contributions of both parent species resulted in an organism able to out-compete other yeasts during the cold lager fermentations.
Though early brewers understood that different brewing conditions would produce a unique beer, scientists are now unlocking the genetic differences between yeast strains that produce variation in flavor, color, and aroma. By comparing the genomic properties of yeast strains sampled from breweries around the world, Drs. Barbara Dunn and Gavin Sherlock of Stanford University have measured the genetic contribution of the parent yeasts to strains of S. pastorianus and revealed new insights into the events that brought about the evolution of lager yeast.
Surprisingly, the researchers found evidence that S. pastorianus strains used by brewers today may not have arisen from a single hybridization event, as was previously believed. "There were two independent origins of today's extant S. pastorianus strains," said Sherlock. "It is likely that each of these groups derived the S. cerevisiae portions of their genomes from distinct but related ale yeasts, and that these natural hybrids were then selected by brewers due to their abilities to ferment at cold temperatures."
While this work identified two distinct groups of S. pastorianus, Sherlock noted that they observed significant genetic variation and flexibility within the groups as well. Dunn and Sherlock speculated this genomic flexibility could have implications for the unique properties of each brewer's beer. "The fact that lager yeasts isolated from different breweries each seem to have a unique genomic make-up may indicate that the yeasts are adapting to the conditions specific to each brewery," explained Dunn.
Furthermore, this work paves the way for the characterization of specific genetic features of each strain that could aid in the brewing process. "Our discovery that unique genomic structures may be characteristic to each brewery and/or beer type could lead to insights on how to directly control flavor and aroma in beer," said Dunn.
Source: Cold Spring Harbor Laboratory