Fermentation and the mystery of oxygenation
By Thom Cannell
It was a typical home brew club meeting—18 brewers and 30 simultaneous conversations. I was cursing a slow fermentation when Scott Isham, then the head brewer at Harper’s Restaurant and Brewpub in East Lansing, Michigan said “Maybe you should have oxygenated better.” That led to the question of what really happens—or doesn’t—when you oxygenate.
The bigger question is oxygenations effect on flavor and what happens if you over-oxygenate or under-oxygenate your wort? Scott said, “The literature I’ve read discusses yeast health and yeast propagation, like the paper Maribeth Raines, PhD, presented at the AHA in 2001. It doesn’t say a thing about flavors in beer.” Scott was correct. Brewing literature references yeast health and growth, not how oxygenation affects flavors. Homebrewers agonize over wort composition, hop selection, dry hopping, fermentation temperature, and yeast strains. They seldom question yeast health and pitching rates despite frequent admonitions to pitch proper cell volumes and “oxygenate well.”
Proper pitching rates are well known; ales require 6-10 million cells /ml @ 1.048 OG or 10-15 million cells/ml for similar lagers. That’s at least 200 billion living cells for a typical 5-gallon batch of ale. Once again, established aeration guidelines address yeast cell growth, not flavor.
What happens if you don’t oxygenate your wort at all? Can a wort with no dissolved oxygen establish a clean fermentation? What happens if you aerate wort far beyond anyone’s wildest idea of proper? So, if we aerated for days would the resulting beer be oxidized (sherry-like flavors) and musty, or reek of diacetyl?
We consulted yeast experts of White Labs and The Brewing Science Institute in Denver, Colorado, about the construction of our experiment.
We were told that 10 ppm dissolved oxygen is optimum for the health of beer yeast, with slightly greater concentrations not a particular worry. They also said that “Shaking yields 1-3 ppm,” which is far below optimum. “An aquarium pump equipped with sterile filter should produce 8 ppm in an hour (longer if you use an air stone of some kind.) And pure oxygen should produce acceptable levels in 45—60 seconds.”
White Labs says less oxygen dissolves into higher gravity worts and suggests that for worts over 1.070 you should first aerate and pitch your yeast. Then 8-12 hours into fermentation, when all the O2 has been used, reoxygenate. The Brewing Science Institute (www.brewingscience.com) confirms this saying “If wort gravity has dropped less than 30% towards its final gravity, it is safe to reaerate. This is sometimes the only way to recommence fermentation activity. Yeast will remove added oxygen from the fermenting wort, and so the second aeration will not contribute to oxidative potential in the finished product.”
Elsewhere there is an interesting tidbit, “Dissolved oxygen levels in wort drop very quickly. After just two hours, the oxygen has almost entirely dissipated, even when yeast is not present!” Of course, the presence of yeast greatly accelerates the drop, and does so well before evidence of fermentation is visible.
Take away? If you wait too long to pitch your yeast, you'll be pitching it into unaerated wort.
As our intent was to see what, if any, differences are introduced by various levels of oxygenation we needed controls. Scott would brew 15 barrels of Pale Ale. Before oxygenating his wort with the brewery’s in-line pure oxygen system he would fill four carboys directly from the heat exchanger. Each carboy would be pitched with the proper level of very fresh first generation yeast cells and oxygenated differently. Every other aspect of brewing would be identical.
White labs agreed that one test wort should be completely unaerated, only shaken enough to suspend the yeast. Number Two would get four hours of aeration, Number Three 24 hours, and Four 48 hours. We used a clock timer for the 24 and 48-hour carboys, two hours on and two hours off for 24 and 48 hours.
Scott suggested filling each fermenter with 4.5 gallons to allow head space for foam production. One would be pitched and rocked for one minute to suspend yeast, then connected to a blowoff tube immersed in sanitizing solution. The other three would be pitched, blowoff tubes connected, and each of them would be connected to heavy duty dual outlet aquarium pumps and sintered stainless steel air stones.
We soaked three fermenters and all the hoses and air stones on stainless steel canes in Star San. I drilled three #7 stoppers with an extra hole for the air stones and inserted the stainless canes into the stoppers, then back into sanitizer.
We’d chosen a Pale Ale because most home brewers like and make Pale Ale and while Pale Ales are expected to have some fruity esters, not all do. The differences in taste between the four test brews should be readily apparent. Scott’s recipe used 2-row, Cara-Vienna, Cara-Munich, and Munich and produced a wort of 13.4°Plato, 1.0546 OG. (see recipe.)
Our yeast came from Brewing Science Institute, the regular supplier of the brewpub’s preferred Wyeast 1272. Brewing Science sent yeast for Scott’s 15 BBL batch and contributed four tubes of yeast containing pitchable volumes of equally fresh yeast.
After pitching yeast at 6:30 p.m., Scott returned that evening to observe and turn off the four-hour pump. By 11 a.m. the following day the brewery’s 18 BBL conical fermenter was blowing off scads of CO2 and, as expected, foam was exiting the blowoffs of all three aerated fermenters. What was not expected was the nice fermentation in the unaerated fermenter.
Scott thought the unaerated control was doing about as well as the aerated fermenters with a thick 1” cap of clean tan yeast/foam. “I used yeast nutrient in the wort as I always do,” he said. “I’ve noticed that commercial worts almost always begin fermenting very quickly and vigorously. I’ve attributed that to good (grain) crush and vigorous boils, whirlpooling for trub separation, oxygenation of the wort stream, and proper yeast pitching rates.”
Ten days later signs of fermentation had ceased in all four fermenters. Each had fermented vigorously, and we were vitally interested in tasting the results. We racked to secondary in freshly cleaned and sanitized kegs and the results were surprising.
The no-oxygen beer was barely attenuated at a gravity of 7.2°P/1.0286 SG. It had a taste we described as smoky/medicinal, hoppy/bitter, sweet, a Pale Ale nose and aroma. The 4-hour beer was said to be sour/bubble gum/vegetal aromas, thin, wet cloths, slightly bitter. Its gravity was better at 6.7°P/ 1.0266. The 24 hour beer was noticeably different. It lacked aroma (almost none, we noted, and was well attenuated at 2.9 P/1.0113. It was more medicinal, less hoppy/bitter, and thin in comparison to either predecessor.
The fourth beer, the 48-hour brew had less aroma than the four-hour beer, more than the 24 hour beer. It had solid bitterness with lingering after taste. It was the most fully attenuated at 2.5°P/1.0097
Scott’s beer was virtually complete at 2.1°P/1.0082. It too had modest Amarillo aroma with some very slight esters. It was fuller in body and had that nice Cascade grapefruit hoppiness in the finish.
We lagered the kegs at about 38°F for another two weeks before our next tasting. We failed to consider that the first two beers would continue fermenting while the second two more attenuated beers would not. So, upon tapping kegs, two were properly conditioned, two were lacking in CO2, which hurt their flavor profiles a bit.
Results and Judging
Nobody would throw any of these beers out despite the below-described comparative flaws. On their own each was judged an acceptable home brew. Asked if anyone would have thought that a beer oxygenated for 24 and especially 48 hours would be drinkable, let alone palatable, the answer was absolutely NO. All the beers were “clean,” and no one characterized any as undrinkable. This was due to good sanitary practices and using 0.2 micron sanitary filters in the air lines.
For our formal critical tasting we recruited Rex Halfpenny publisher of the now-shuttered Michigan Beer Guide and a BJCP judge. He and his wife Mary joined Scott and me, also a BJCP judge.
#1 (no oxygenation) surprisingly had the most going for it,” Rex said, “In spite of the fact it was obviously under-attenuated; with a lingering sweetness, you could sip it and lick your lips and taste sweetness. This means it will be unstable, too much biological activity remains.” For homebrewers that translates into drinking it up soon.
Scott ranked the beers; “I detected higher alcohols in 3 and 4, some green apple flavors though less in 4 (48 hours.) Mary said there was “wet cloth or bubble gum in # 3” and Scott agreed.
The 24 hour beer wasn’t well liked. Rex picked up a hint of vegetal and said that it was “over carbonated causing a CO2 bite; assertive hop bite, not as sweet, better attenuated.”
Nobody liked the 2 hour beer. “It tastes like a poorly made extract beer,” Rex commented. We agreed the off flavors could have come from the keg, though freshly cleaned (I noted some root beer aroma, surely that would be from the gasket.) “Acetaldehyde present; tart,” Rex said. Scott characterized as a “too-young” flavor.”
The beer common sense predicted would be thoroughly undrinkable after two days of aeration, only gathered comments like “tart, slight green apples and unpleasant drying” as a result of higher alcohol.
Our unanimous preference was for the brew pub’s version. It was better than any of the other four, though I though the finish a bit lacking and not enough hop aroma. “I thought your malt came through much nicer, and the Cara-Munich flavor came through as well,” Rex told Scott.
If there is one conclusion from this experiment, it is that when you pitch the proper amount of fresh yeast you will make drinkable beer. Few of us generate that quantity of yeast, so providing optimal environmental conditions—like oxygenated wort—is important. Oxygenation, while very important to attenuation (reduction of fermentable sugars and production of alcohol) itself does not predict drinkability. The results of our study show clearly that oxygenation does change flavors appreciably: keep your oxygenation to under four hours.
Extreme oxygenation does not—in any way experienced beer brewers and BJCP judges could discern—create completely unpalatable flavors in a beer. So perhaps this experiment raises as many questions as it answers.
50# each of DWC Cara-Vienna 20-30°L,
50# DWC Cara Munich 50-60 °L,
50# Cargill Munich 8-11 °L
Boil 75 minutes
Bittering hops Amarillo 3.5# 7.3 AA,
Flavor hops: Cascade @30 minutes remaining 4# 5.9 AA,
Aroma hops: End and whirlpool with Cascade 8#
Total water 300 gallons, pH 6.6, salts: 50 ml phosphoric acid, 8 oz. CaSO4
Kettle gravity (begin) 12.9°P, pH 5.5, 18.5 barrels. Sparge 500 gallons
Post Boil gravity 13.4°P/1.054 SG, pH 5.1.
Yeast, fresh Wyeast 1272. Terminal gravity 2.1°P
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