Thursday, 13 June 2013

Yeast Propagation

Yeast Propagation - There and back again

Every so often we are reminded that we are in fact scientists as well as brewers. This was never more apparent than when we sat down for our first discussion on yeast propagation.

Side note: For the layperson; propagation is the principle of growing yeast from a small sample up to a large enough quantity to use in a production scale brew.

Our decision to brew a Saison style beer meant that we had to select a yeast strain that had the desired “yeast characteristics” and taste profile; namely a pronounced ester (fruity) and spicy flavour with a slightly acidic finish and a silky mouthfeel. Basically we are looking a whole lot out of these single celled fungi!!

We began trial batches with two strains; the first strain we used gave us a very enjoyable beer. Unfortunately, issues arose surrounding the high fermentation temperatures required and its slow fermentation rate; over 2 weeks!! We then brewed a batch using a second Saison strain; this gave superior flavours and aromas as well as fermenting in half the time, safe to say this was the strain for us!

With our yeast strain selected it was now time to grow it up from the somewhat measly liquid yeast samples provided by the yeast laboratory, to enough to pitch into around 20 barrels (30 hectolitres).

Cropping our yeast form ICBD fermentation vessel
Step one saw us taking the cropped yeast from our trial batches and pitching that into 150 litres of wort; which we brewed on the ICBD pilot brewery at Heriot-Watt University. To maximise our yield, we carried out this stage aerobically. By using an O2 stone in the bottom of the fermentation vessel, we were able to produce small bubbles of filtered compressed air; resulting in a plentiful supply of oxygen. The presence of oxygen means that the yeast will grow more readily and suppresses alcohol production (not normally what we are looking for!). We obtained a fantastic yield from this process; needing 4.5kg and ending up with around 7kg.

What we saw under the microscope
Bruce doing some microbiology on our yeast
We then cropped the yeast from the fermentation vessel ensuring to discard the first runnings (containing dead cells) and only saving high quality, healthy yeast. This was then checked under the microscope by Bruce and Pat for cell count, viability (number of living cells) and bacterial contamination.

Side note: As I mention the ICBD I would like to extend our gratitude to David Quain and Graham McKernan for their assistance and advice with the propagation plan.

Our bucket of yeast on the bus and some bus Orcs
With the yeast given the all clear, we began the perilous journey to Stewart Brewing. Feeling a little like Frodo Baggins carrying the One Ring into Mordor; we escorted our “precious” cargo in an inconspicuous black bucket. Two buses and a brief encounter with a few Edinburgh bus Orcs later we reached our destination.

Our yeast growing in a Stewart Brewing fermentation vessel
Upon arrival at Stewart Brewing, Iain (Stewart’s Head Brewer) and his team had just finished a batch of Edinburgh Gold (a fine brew in my humble opinion).

We diverted around 5 barrels (6 hectolitres) into a separate fermentation vessel and pitched our Saison yeast. We then let this ferment, growing our total yeast weight to around 18kg in a week.

This is more than enough for us to use for our first 20 barrel brew. This came as a huge relief; particularly for me as the man in charge of propagation but also everyone involved with Natural Selection Brewing. So we now have plenty of yeast for our 1st 20 barrel production batch and can crop and reuse this yeast for the 2nd 20 barrel production batch the following week.

Side note: Thanks go out to Steve and Iain at Stewart Brewing for trusting us to let our somewhat "funky" Saison yeast into the brewery!

I hope this has been an interesting insight into our experience of yeast propagation; my take home advice would be that hygiene is paramount; peracetic acid is truly a brewer’s best friend and is pretty handy against those pesky Edinburgh bus Orcs too!!

Bruce Smith

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