Weihenstephan 34/70 lager yeast and why brewers trust it

The strain comes from a Bavarian yeast bank

The 34/70 name refers to a culture held at Weihenstephan, the brewing research and teaching centre near Freising in Bavaria. Its yeast bank supplies strains to breweries around the world. The dry version most brewers know is Fermentis SafLager W-34/70. Liquid cultures such as White Labs WLP830 German Lager and Wyeast 2124 Bohemian Lager are widely understood to share the same origin.

Like all lager yeasts, 34/70 is Saccharomyces pastorianus, a hybrid of brewing ale yeast and the cold-tolerant species S. eubayanus. It belongs to the Frohberg group, the lineage that tends to ferment more fully and robustly than the Saaz group.

It ferments clean across a wide range

Classic lager fermentation runs at around 9 to 13 °C. At those temperatures 34/70 gives a clean, malt-forward beer with low esters and a soft finish. What sets it apart is how it behaves warmer. Many brewers ferment it at 15 to 18 °C. Some ferment under pressure at close to 20 °C. Both still get a beer that tastes like a lager rather than a fruity ale.

That tolerance is useful when glycol capacity is tight. In a brewery in Bengaluru or Pune where the chiller struggles through April and May, a strain that does not punish a two-degree drift is worth a lot. It is not a licence to ferment at room temperature. Esters and sulphur still rise as you go warmer. It gives you margin, not freedom.

Pitch rate matters more than the strain

Lagers need more yeast than ales. The common commercial target is about 1.5 million cells per mL per degree Plato. A 12 °P lager therefore wants around 18 million cells per mL at pitching. Fermenting colder pushes the need higher; fermenting warmer lets you pitch a little less.

With dry yeast, follow the manufacturer's dosage sheet for your temperature and rehydrate as it directs. With repitched slurry, count cells and check viability every time. Our guide to lager yeast strains compared sets 34/70 against the alternatives.

Sulphur and diacetyl need time to clear

Lager yeast produces sulphur compounds during fermentation. 34/70 is no exception. A young lager can smell of struck matches. Most of it escapes with the CO2 and fades during cold maturation.

Diacetyl, the buttery compound, needs active yeast to remove it. The standard approach is a diacetyl rest: once the beer is two thirds to three quarters of the way to final gravity, let the temperature rise to around 15 to 18 °C for two or three days, then confirm diacetyl is gone by a forced test before crashing to near 0 °C. Skipping that check is one of the most common reasons a lager tastes faintly of butter.

Pressure lets you run it warmer

Fermenting under pressure is one way breweries push 34/70 warmer without losing the lager character. A spunding valve on the fermenter holds back some of the CO2, commonly to around 1 bar. The dissolved CO2 suppresses ester and fusel alcohol formation. Fermentations at 15 to 20 °C under pressure can finish in a week or so rather than two to three.

The tank must be rated for the pressure. The valve must be reliable and calibrated. Pressure also carbonates the beer as it ferments, which can save a step later. Brewers who try it usually run a side-by-side with their standard cold fermentation first and taste the two blind before changing the house method.

It repitches well for several generations

Brewers trust 34/70 partly because it is robust in reuse. Harvested cleanly from the cone, stored cold and pitched within days, it holds attenuation and flavour across several generations. Most breweries set a generation limit for each strain and propagate or buy fresh when they reach it, or sooner if attenuation, flocculation or flavour drifts.

For a small brewery a box of dry yeast is a reliable restart. Some brewers buy dry yeast for the first pitch, then harvest and repitch two or three times before starting again.

Forgiving is not the same as foolproof

The reputation can make brewers careless. 34/70 will not rescue a wort low in free amino nitrogen, a fermenter that swings by five degrees overnight or slurry stored warm for a week. A strain that hides small mistakes still shows big ones.

Choosing between a dry and a liquid version is its own decision. Our guide to dry versus liquid lager yeast covers cost, cold chain and consistency.

A sensible first step: on your next lager, log gravity and temperature twice a day from pitch to crash. Plot them on one chart. Seeing where 34/70 slows, where the diacetyl rest falls and how long the beer takes to reach terminal gravity in your tanks is the start of running lager by data rather than habit.

Common questions

What temperature does W-34/70 ferment at?

Traditionally around 9 to 13 °C, though many brewers run it at 15 to 18 °C and still get a clean lager.

Is WLP830 the same as W-34/70?

WLP830 German Lager and Wyeast 2124 are widely understood to come from the same Weihenstephan strain, though handling and form differ.

Does W-34/70 need a diacetyl rest?

Yes. A rest at around 15 to 18 °C near the end of fermentation, confirmed by a forced diacetyl test, is standard practice.

Can I reuse W-34/70 yeast?

Yes. It repitches well for several generations if harvested cleanly, stored cold and checked for viability each time.