From the HBD Archive
Subject: Miller on Trub, O2, and Fusel Alcohol
Date: 1992-02-28 01:51:00 GMT

I went back to my Miller to try to figure out WHY he recommends leaving trub
behind, and what it had to do with oxygen, and what caused fusel production.
For brevity, I'll paraphase from his chapter on fermentation.

1) Oxygen is used during respiration to synthesize sterols and other complex
fatty substances that make up the cell wall and other structures. The
limiting factor in yeast growth is either amino acid or oxygen, usually the
latter. Yeast can grow without oxygen if the wort is rich in sterols and
unsaturated fatty compounds. Trub contains a lot of these.

2) Fusels are formed when amino acids are are broken down into keto acids,
then aldehydes and then alcohols. High temperature fermentations encourage
this. Esters are formed by combining these alcohols with fatty acids, which
is why warmer temperatures encourage esters. More fusels to combine with
fatty acids mean more esters. Esters will only be formed after all the oxygen
is used up, because if oxygen is present the fatty acids build up sterols
instead. The more aeration, the less esters.

- ----

In his section on procedural practices, he differentiates between commercial
and home procedures. But in re-reading this, I now understand a little more.
He recommends:

Pitching yeast as soon as the wort is at pitching temperature. Then, he
suggests racking within about 8 hours to get the wort off the trub.

The reasoning is that during respiration, the trub is good for helping to
build cell walls and other cell material. After that, the excess protein
content of the trub will build fusel alcohols by the breakdown of the amino
acids. If you remove the trub before pitching, you lose out on good yeast cell
building material. And, if you wait a few hours (like *I* do) for the trub to
settle before pitching, you also risk infection.

I'm going to change my procedure to match his recommendation on my next batch.

- -----------------------------------------------------------------
Josh Grosse
Amdahl Corp. 313-358-4440
Southfield, Michigan

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