Yeast Flavours 2

As stated in Yeast Flavours 1, the flavour contribution of yeast is dependent on its metabolic balance. The most important factor in this balance is the strain chosen, but there are far more factors that can influence a yeast's excretion products than just the strain. We'll examine two of these factors - oxygen and trub today

Oxygen

Before we get too far into this, lets talk about DO (dissolved oxygen) meters. Although there are several brewers who hop on YouTube and demonstrate their portable DO meters to show the effectiveness of their chosen method of eliminating oxygen from a finished beer, they aren't designed for accurate measurements that low. Even then, it tends to be someone stirring a stick in an open glass that they'd just poured themselves. While this might, with a hefty amount of luck be a viable method of determining the worth of one procedure over the other, it will absolutely not give you an accurate measurement of dissolved oxygen in your finished beer. 

The portable DO meters that are expensive, but still within the budget of most home brewers are designed for testing dissolved oxygen in wort - in the mg/L range, not the ppb range, despite claims to the contrary. I'll write up another article about how to properly use a DO meter in a future article. 

At any rate, a simple test that you can do yourself is to shake a sample of wort before pitching in order to oxygenate it, which will put you in the 8 ppm range. Compare this to a second sample that you don't shake, which will put you in the <4 ppm range. The esters produced should be noticeably higher in the low oxygen environment than in the higher oxygen environment. 

During the log phase, yeast grow and use up the oxygen in the wort. They use the oxygen for a variety of purposes, but for our interests, it's being used to turn acyl-CoAs into various needed chemicals. If those acyl-CoAs aren't able to be used, they end up combining with alcohols to form esters. Exactly which esters and how big the acyl-CoA pool is is dependent on the strain. 

Trub

One of the big sources of oxygen use in wort is to form ergosterols which modulate yeast membrane fluidity, allowing it to survive a wide range of conditions. Ergosterols can be made by the yeast itself, or taken partially from trub. Although trub is widely vilified (and rightly so), there should be enough of it to provide some nutrients in the form of amino acids. High trub (within reason) will absolutely cause shelf stability problems in your beer, but it will also allow for a healthy fermentation. Too high will even cause excessive yeast growth which will lead to an unhealthy crop, but it will also limit ester formation as the oxygen needed to form ergosterols won't be needed and can be used to transform those acyl-CoAs into other needed chemicals. Trub however, is of secondary importance to oxygen if ester control is your goal. It can also be completely replaced by simply adding yeast nutrient, which is what any brewing school will recommend as shelf stability is a major concern.