Matilda

I believe the yeast Mr. Greve is talking about is the brettanomyces strain of yeast.

according to wikipedia:

"In most beer styles, Brettanomyces is viewed as a contaminant and the characteristics it imparts are considered unwelcome "off-flavours". However, in some styles -- particularly certain traditional Belgian ales -- it is appreciated and encouraged. Lambic and gueuze owe their unique flavour profiles to Brettanomyces, and it is also found in Oud Bruin and Flanders red ale. Commercial examples of these styles include Liefmans Brown Ale, Rodenbach Grand Cru, and Duchesse de Bourgogne. The Orval Trappist monastery is unique in crafting the only Trappist beer with Brettanomyces charateristics. In Orval;s case, the brewers add the yeast to the beer at bottling.

Several American craft breweries use Brettanomyces in their beers. This use began with a renewed interest in Belgian style ales and later formed new styles altogether (Brewers Association, 2007 Great American Beer Festival Style Guidelines, section 13a, 16). Some breweries use %100 Brettanomyces for the fermentation of some of their beers, and omit Saccharomyces from the recipe. It is common for American brewers that use Brettanomyces to also include lactic acid producing bacteria such as Lactobacillus, and Pediococcus in order to provide sourness to the beer. Examples of American breweries that use Brettanomyces in their beer include Russian River Brewing Co., Lost Abbey, New Belgium Brewing Co., and Allagash Brewery.

While most stouts achieve their sour tang through the use of acidulated malt, roasted barley, or -- in the case of "milk stouts -- lactose and incipient lactic acid, some use Brettanomyces for the same purpose. Prior to 1980s-era changes in its fermentation regimen, Guinness's Foreign Extra Stout is held to have been one such."

so there you have it. later