Olive Oil vs. Oxygen

Most home brewers and pro brewers have likely heard of the stories and experiments relating to using olive oil as a replacement for wort oxygen. You can easily find discussions and write-ups all over the internet. The overarching theory is that olive oil can directly supply unsaturated fatty acids (UFAs) that yeast needs and therefore make oxygen unnecessary. The added theoretical benefits are higher shelf stability and lower oxygen-induced staling effects. 

It is well established that yeast either need to biosynthesize their own UFAs and ergosterol or have them supplied by the medium. In the various write-ups, you’ll find statements like, ​“yeast use oxygen to synthesize sterols and unsaturated fatty acids for their cell walls.” This is true, but does that mean olive oil can help with both of these things? No! Olive oil supplies unsaturated fatty acids (UFAs) but does not supply sterols.

The most thorough experimentation of the olive oil-as-oxygen-replacement theory was conducted by Grady Hull at New Belgium Brewing.

“The theory is that the oleic acid in the olive oil will provide the UFAs necessary for yeast growth and proper fermentation, eliminating the need for wort aeration.” 

The famous Fat Tire was brewed and olive oil was used in lieu of oxygenation.

Key takeaways from New Belgium study:

1. 13 – 20% longer fermentation time with olive oil and no oxygenation

2. Terminal gravity within normal range with olive oil

3. Elevated esters detected by GC-MS but not always detected by tasting panel

4. No re-pitching of yeast from olive oil batches

Brulosophy also performed an ​“exbeeriment” by brewing a Kölsch with and without olive oil. Tasters were unable to distinguish between the two beers in a triangle test.

Key takeaways from Brulosophy Kölsch:

1. Tasters unable to tell difference between beers

2. Terminal gravity identical

3. Olive oil batch reported to start and complete fermentation faster

Reviewing the setup of the experiment, I think the faster start and completion can be explained by a critical design flaw. Independent packs of the inoculation yeast were used for each batch. Yes, they were the same strain and they may have even been the same lot, but it’s impossible to say how each pack was treated from manufacturer to pitching. The more sound approach would have been to mix the two packs and pitch an equal amount of the combination. As the experiment was performed, the yeast would also have to be considered a variable.

The Experimental Brewing team also got on the olive oil train and had multiple people brew an amber ale with and without olive oil in lieu of oxygenation and had taste testers see if they could tell the difference. Consistent with New Belgium and Brulosophy, tasters could not tell the difference between the two beers to a statistically significant degree.

So what gives? If multiple people, including New Belgium, reported that olive oil additions allowed brewers to successfully make beer with no significant off flavors, why would I say that oxygen is required when results suggest otherwise? It turns out that yeast can bud several times and pass enough ergosterol and fatty acid to their daughters to reproduce but they can’t do this forever. 

Dekker and colleagues did a series of experiments demonstrating the yeast supplemented with fatty acids and ergosterol in the growth medium could grow robustly in anaerobic conditions. Interestingly, they had to grow what they called ​“carry-over” cultures before performing the experiments to see if yeast could grow in the absence of supplementation. ​“Carry-over” cultures consisted of growing yeast in media lacking unsaturated fatty acids and ergosterol. During this period, modest yeast growth was observed. However, when transferred to fresh media lacking fatty acids and ergosterol under anaerobic conditions, there was no growth. In other words, the yeast had exhausted their stores of ergosterol and fatty acid and could grow no further without supplementation (or oxygen). Interestingly, when the authors added ergosterol to the cultures with stalled growth, the yeast started growing again — even though there were no UFAs provided. Thus, the yeast absolutely required ergosterol for growth in anaerobic conditions but could grow in the absence of UFAs.

The Dekker paper illustrates pretty clearly what brewers are doing when supplementing with olive oil and claiming ​“success.” To use the terminology from the Dekker study, brewers have only used ​“carry-over cultures.” In other words, they’re using cultures that already have adequate stores of ergosterol and fatty acids. If brewers tried to harvest from the cropped yeast in the olive oil-only batches and re-pitch, that’s where they’re going to start to see problems because ergosterol will be depleted and growth will cease. The New Belgium study did not mention repitching any of the olive oil batch yeast. From what I’ve been able to ascertain, no one has done multi-generation repitches of olive oil supplemented, non-oxygenated yeast. Based on the requirements of yeast, there is no reason to think it would work.

Ah, so we only have to supplement olive oil and ergosterol and we’re off the races? Yep! Except ergosterol is cost-prohibitive compared to oxygen. Sigma-Aldrich has a 75% pure ergosterol available for more than $275 for 25 grams. Dekker and colleagues used a supplementation rate of 10mg/L. Doing the math results in upwards of $11/bbl of beer for the ergosterol plus the added cost for UFA supplementation via olive oil. Oxygen, on the other hand, costs well below $1/bbl.

Finally, I think this also illustrates a somewhat recurring issue with brewing science — or at least brewer understanding of brewing science. S. cerevisiae is one of the most thoroughly studied organisms on the planet. In addition to its use as an industrial fermentation organism in bread, biofuels, and beverages, it is a model eukaryotic organism used to study any number of human health-related problems. The knowledge that oxygen is needed for both ergosterol synthesis and fatty acid synthesis is old — much older than the theory of olive oil supplementation in beer fermentations. A review of the scientific literature would have made clear from the start that olive oil would only provide UFA and would not supply ergosterol.

So does this mean you shouldn’t supplement with olive oil? It’s certainly not going to hurt anything but as I hope you can see by now, it doesn’t replace the need for oxygen. Unless you’re also supplementing with ergosterol, you’re still going to need to oxygenate.