Yeast needs to breathe

As readers may have guessed from previous posts, my brewing interests are minimally conventional. Fortunately, the Basic Brewing Radio podcast seems to regularly expand well beyond the usual “fermented malt flavored with a tisane of hops” thing (I need to try to make my own “Ginger Beer Plant” from scratch one of these days…). A couple of weeks ago, they did an episode covering an experiment on aeration methods which was very interesting. It does my ego good to know that I correctly guessed how the results would turn out. You can get a copy of the nice write-up of the experiment itself here, but here’s the simple version:

It’s usual to start brewing projects by mixing your ingredients together in a big pot and boiling them. In addition to potential effects on the flavor this also serves to sterilize what you’re about to ferment, which helps prevent having things growing in it besides the yeast (and possibly bacteria) that you intentionally put into it. Unfortunately, this also gets rid of most of the oxygen in your brew since gases don’t stay dissolved well in hot watery liquids. In order to allow your yeast to grow enough before settling down into the “fermentation” stage of the process, the yeast rely on oxygen to provide biological power through respiration plus a bit more oxygen which is used to generate cell membrane material in a separate process. That means that after boiling, you normally need to aerate your liquid somehow once it cools down.

Traditionally, this would be done by shaking or stirring the brew, or possibly by pouring it back and forth between two containers a few times. Humans being fundamentally lazy animals, however, a lot of people seem to like using air pumps instead. Fred Johnson decided to compare aeration with air pumps (with and without an “air stone”) with aeration by shaking.

I don’t think I can explain the results without giving away the punchline, so here goes: shaking works much better than air pumps. I recall reading the reason for this years ago in a book on keeping aquarium fish; the “air pump” is not actually putting substantial amounts of oxygen into the water. The reason the air pump results in aeration of the water is because the bubbles make the water circulate, drawing the water with less oxygen from the bottom of the tank to the top where it can exchange gases with the air across the surface of the water. This would also explain why the experiment seemed to show that just pumping the boiled test water into the container seemed to cause a substantial increase in the dissolved oxygen all by itself. In short, the more of the surface of the water that is exposed to air, the faster it will reach its normal oxygen saturation level.

Because I am extremely jealous of Fred Johnson’s ability to get his hands on peristaltic pumps and a dissolved oxygen meter while at the moment I can’t even afford a pH meter (let alone the microscope setup I want […sniffle…]), I’d like to take a couple of paragraphs to pick at a couple of possible nits in the experimental design to make myself feel better. At least one of those nits gives me an idea though…

First, after being cooled the boiled water in the experiment was pumped into the containers for testing through silicone tubing. Silicone is possibly the most oxygen permeable “rubber” in existence[1][4] – so much so that there are standard medical devices which act as artificial lungs, pumping blood through silicone membranes in air to oxygenate it[2]. If it works for blood, perhaps this principle might also be useful for other liquids. I’d love to see some experiments to determine how quickly water pumped through a length of silicone tubing is aerated (I would also hypothesize that putting lots of bends in the tubing would induce turbulence and increase the oxygenation rate over pumping through a straight or gently coiled set of tubing). I’d also like to see the same experiment done with butyl rubber tubing for comparison – butyl rubber is what we used for the stoppers on anaerobic culture tubes, since it’s got very low gas permeability by comparison[3].

I would also be curious as to whether sparging with nitrogen would be more effective at removing oxygen from the water than boiling would. This would probably have to be done with smaller samples, though – in the lab we were bubbling nitrogen through each small, narrow 10ml tube of culture medium for about ten minutes before capping the tube with a butyl-rubber stopper to do anaerobic culturing. I suspect it’d be a lot more difficult to scale the same method up to 6 gallons…

Anyway, here’s something that might be worth playing with if silicone is permeable enough for the purpose: one might get their hands on a long spool of silicone “air-line tubing” (smaller tube = greater surface/volume ratio and hypothetically faster gas exchange), then rig up a way to transfer your wort/must/whatever directly from the boiling pot to the fermenter through the coiled tubing to accomplish the aeration without having to shake, bubble, or otherwise disturb the liquid, thus reducing the threat of contamination.

A simpler hack might be to install a sprinkler head on the outlet end of the transfer tubing. Simpler still, perhaps crimping flat (but not closing entirely) the outlet end of the tubing so that the emerging liquid emerges spread out into a flat sheet rather than a stream. Either way, this ought to give a big increase in the amount of liquid surface exposed to air on the way out of the tubing and down into the fermenter, resulting in a substantial amount of incidental aeration without any additional labor or motorized gadgets.

There, aren’t I a know-it-allsmartass guy?

[1] Zhang H, Cloud A: “The Permeability Characteristics of Silicone Rubber
“; IN Sampe Fall Technical Conference Global Advances in Materials and Process Engineering: 38th International Sampe Technical Conference; Society for the Advancement of Material & Process Engineering ; November 9, 2006 ISBN 0938994727
(Article accessible as PDF from here as of 20080910)
[2] Carlson RG, Landé AJ, Ivey LW, Starek PJ, Rees JR, Subramanian VA, Twichell J,
Baxter J, Bloch JH, Lillehei CW: “Total cardiopulmonary support with disposable membrane oxygenator during aortocoronary artery-vein graft operations.”; Chest. 1972 Oct;62(4):424-32.
[3] Hungate RE, Smith W, Clarke RT: “Suitability of butyl rubber stoppers for closing anaerobic roll culture tubes.”; J Bacteriol. 1966 Feb;91(2):908-9.
[4] AZo Journal of Materials Online: “Silicone Rubber”; http://www.azom.com/details.asp?ArticleID=920 (accessed 20080910)