The flavour benefits of S. cerevisiae & B. bruxellensis mixed culture fermentations

I've been catching up with some of the excellent Master Brewers Association of the Americas podcasts recently. I was delighted to spot one that involves Brettanomyces: Co-fermentation Trials with S. cerevisiae & B. bruxellensis and the Organoleptic Benefits of Mixed Culture Fermentations

The Orval Brett.getting ready for mixed fermentation

Here's my notes:

Nick Mader of Freemont Brewing Co. carried out a series of fermentation trials using Saccharomyces cerevisiae BSI 565, a highly attenuative Saison strain and BSI drie a Brettanomyces bruxellensis strain isolated from a lambic beer.


They were cultured in a wort of 13.4° Plato (1.054) and 5.2 pH which was oxygenated to 10ppm O2

Five trials were each carried out in triplicate. The cultures used five trials were:

1. Pure Sacch.
2. 75% Sacch. /25% Brett.
3. 50% Sacch. /50% Brett.
4. 25% Sacch. /75% Brett
5. Pure Brett.

Only 2.5% difference in apparent attenuation was found in the mixed cultures, so mixed culture fermentations had similar attenuation to pure Sacch.

Pure Brett. had 77% apparent attenuation (3.1°P FG)

Fermentations were carried out at 20°C



Laboratory (GC) and sensory analysis was carried out.

Esters:

Ethyl acetate (in high concentration solventy taste, in medium conc. fruity). Highest conc. in pure Sacch. Decreased at first with Brett. in mix and then increased as Brett. conc. went up, so at 75% Brett. it's almost as high as pure Sacch. Ethyl butyrate had negative linear relationship with % of Brett.

Ethyl hexanoate, oxanoate and decanoate similar. Decanoate 3x higher than anything else in pure Brett. fermentation though

Iso amyl acetate high in the pure Sacch. fermentation but even 25% Brett. is enough to hydrolyse it immediately. Not detectable in pure Brett. fermentation.

Negative correlation in higher alcohols with Brett. conc. so really low in pure Brett. fermentation.

Phenolic compounds:

4 Vinyl Guiacol, 4 Ethyl Guiacol and 4Ethyl Phenol (barnyard, horse blanket) were measured.

4VG lower in mixed cultures. 4EG and EP increased with Brett. conc.

Pure Sacch. had high 4VG, Pure Brett no 4VG detectable.

All mixed culture fermentations had all three (4VG, 4EG and 4EP)

Mixed fermentations: wide array of ester and phenol concentrations. Pretty pleasant taste.

Pure culture fermentations had compounds in high concentrations but fewer of them. Mixed fermentations had more compounds in medium concentrations which gave a more complex taste.

I thought that last point about mixed culture fermentations really interesting: less intensity but more variety in flavour compounds giving a more complex taste. It looks obvious now, but it was useful to hear it stated.