Brewers have traditionally saved the yeast from one brew for use in the next brews, and we now know for sure that the yeast used to brew Guinness today dates back to at least 1903 when the Guinness yeast library was established here at St. James’s Gate.

Guinness yeast strains occupy a distinct group, separate from other historical Irish brewing yeasts. At least 38% of our yeast is of unknown origin which makes it truly unique and the only yeast that can create our signature Guinness flavour.

OUR YEAST: DID YOU KNOW?

Guinness has always been at the forefront of scientific discovery. Our records show that the first scientific brewer, Thomas Case, was appointed to the brewery in 1893, just 39 years after Louis Pasteur discovered that yeast causes fermentation. Shortly after that, the Watling 1903 yeast was first isolated and stored as Guinness stout production yeast. It was held at the Watling Laboratory, named after its location on Watling Street, here at the brewery. We’ve brewed with the same strain of Guinness yeast since then, but we believe it is much older than that.

Yeast is the engine of brewing, without it, there is no beer. It takes sugary, bitter water and transforms it into beautiful beer. You can’t say that about any other ingredient. The traditional strain of yeast that goes into every brew of Guinness, everywhere it is brewed, is incredibly important to us.

A small amount of yeast from each brew is transferred onto the next to ensure consistency. The yeast for Guinness is grown here at St. James’s Gate and is stored in liquid nitrogen tanks at -196 degrees celsius.

INGREDIENTS MASTERS

ANGELA LARKIN - QUALITY COMPLIANCE LEAD

“My job is to ensure the Guinness yeast is in perfect condition for fermentation. We carry out regular propagations where the yeast is grown up from pure culture slopes.

The master slopes are kept safely under lock and key in the yeast library, where they are stored in liquid nitrogen.

I lead the St. James’s Gate tasting panel, sampling fresh beer to ensure all of our ingredients have done their job properly. It’s probably the best part of my day.”

50 MILLION PINTS: SMALL BUT MIGHTY

To use our yeast, we take a small vial from our liquid nitrogen which holds roughly one million cells, and we grow it up.

We then add that vial into a small conical flask holding sweet wort and leave for 24 hours until it starts to grow. It’s then added to a larger flask where it has now grown to roughly 50-60 million cells. At that point, it is transferred into a flask, which holds 25 litres of liquid.

In the brewing process, we get through one of these flasks every three weeks. One flask helps ferment 28 million litres of stout, that’s about 50 million pints. From one vial – our yeast is small but mighty.