Basic Brewing Radio is an outstanding informative podcast hosted by James Spencer (http://www.basicbrewing.com/index.php?page=radio). Between this and the content on the Brewing Network (http://www.thebrewingnetwork.com), it is nearly all that I listen to on my daily commute...
This week's episode of Basic Brewing Radio (3/11/10) featured a yeast starter experiment. I am always interested in this topic – my yeast are the best pets I've ever had! The original experiment can be found here:
http://seanterrill.com/2010/01/14/aeration-and-yeast-starters/
Mr. Terrill was thorough on his experimental design and procedures. The only piece that I had any questions about was his connecting regular agitation with “simulating a stir plate”. Experimentally, there is a huge difference between agitating or swirling the beer once per hour or every 15 minutes and the constant swirling caused by a stir plate. Everything else was wired tight, and there were some compelling results...
There was not much surprise that by constant aeration (filtered air from an aquarium pump) or agitation/swirling, the brewer can increase yeast production. I have always been paranoid about having that hose hanging out of my starter: with my luck, something would catch on the hose and spill the starter or I wouldn't be able to properly secure the cover with the tubing going in and my starter would get contaminated... When I began making starters almost two years ago, I would agitate the vessel every time I remembered – sometimes in the morning before work and once more at night, which was probably better than nothing; and sometimes it was every time I walked by the starter.
Eventually, I built my own inexpensive stir plate (like http://www.stirstarters.com/instructions.html, http://brewiki.org/StirPlate, http://www.byo.com/stories/projects-and-equipment/article/indices/20-build-it-yourself/401-build-your-own-stir-plate, http://www.swampwaterbrewery.com/index.php/do-it-yourself-diy-stir-plate.html, etc.). The first magnetic bar I got worked well in beakers & flasks, but not in my gallon jugs; the barbell type work well in vessels which are slightly convex at the bottom... I have never tried to quantify scientifically the differences I have obtained with the various techniques I have used, so this experiment grabbed my attention right away.
I don't want to rewrite this homebrewer's article, since anybody can access it at the link above. But I did want to highlight some of the things I found most interesting... First, he opted for a low-tech method for measuring the yeast at the end of the experiment: slurry volume settled and measured in a graduated cylinder. This is great, because it means nearly any homebrewer who is already doing starters can replicate this experiment for the price of an inexpensive graduated cylinder (1L for about $10). This doesn't measure actual viability, but he used fairly well established existing research for his estimates.
I was first excited when I saw the words “stir plate,” but as I mentioned earlier, there was no stir plate involved in this experiment – a point to which I'll return later. Even so, the wort variables he built in – still, frequent agitation/swirling, and constant aeration – provide insightful data for homebrewers who make starters. From his results, some aid in gas exchange (introduction of O2 and release of CO2), whether agitation or continuous aeration, resulted in better yeast propagation than the control that was left still.
The most striking of his findings, however, revolved around how the starter is capped. Most of the information I read when I decided to begin using starters directed brewers to use a fermentation lock. I never even thought to question this, given the source of the information (New Complete Joy of Homebrewing and other well-established texts). And the starters were yielding the intended results: more yeast, less lag time, and reliable fermentation. A few months ago, I was surprised to hear Chris White (of White Labs) talk about using loose foil instead of an airlock during primary fermentation... This went against nearly everything published for beginning homebrewers, but he's the yeast expert, right?
In Terrill's findings, there was about an 8% difference in volume between the airlock starter and the foil-covered starter. In the agitated/swirled starters, there was an astounding 17% difference between the airlock and the foil! Since there really is no way to aerate a starter and have a fermentation lock, there is no data to compare, but the aerated sample was within 5% of the agitated starter. In this experiment, there is clear evidence that an airlock suppresses yeast growth/reproduction and that the loose foil covering resulted in increased yeast propagation yields.
Bottom Line: I am going to change my yeast starter process based on these results!
(I actually have already changed my process on full-batches, opting for a blowoff tube into a pitcher without any liquid or a sanitized foil covering).
Within the next couple of months I plan on conducting a similar experiment, piggybacking on these results. I will drop the continuous aeration variable, and compare still, periodically agitation/swirl, and stir plate starters with both airlocks and foil covers. I will keep the other techniques, including the measurement procedures, the same for comparison.
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