Tailoring your yeast performance
When talking about fermentation profiles or figuring out exactly how your yeast are going to affect your beer, there are a lot of variables. On the one hand, this can quickly get overwhelming and be quite challenging if you're one of those brewers who's prone to worrying about every little thing. On the other hand, it's a huge opportunity to tailor your fermentation exactly how you want it.
Let's start with the basics of what flavours are available and then we'll move on to the factors that will affect those flavours.
For those who like things simple, I'll divide the flavours in to two broad groups: growth factors and energy factors. The metabolic pathways split into these two large groups and generally speaking (with some exceptions) anything that positively affects one, will negatively affect the other.
Growth factors are: Organic Acids, Fusel Alcohols and Esters. Anything that promotes growth will increase these three.
Energy factors are Ethanol, Aldehydes and Diacetyl. Anything that promotes energy will increase these three.
In addition, Sulphur compounds and Fatty Acids also exist. Brewers yeast does not secrete fatty acid, but it is produced and will be present in the beer if yeast autolyses. Sulphur compounds can be lumped in with Growth factors, but shouldn't really be present unless something bad happens.
Now let's take a look at the factors that will influence these flavours.
FAN or Free Amino Nitrogen - Not really a concern if you're using all-malt and no adjunct. Do not add yeast nutrient unless you're using DME for a yeast starter or you're using adjuncts, as it acts as a pH buffer.
Oxygen - Low levels are ~4 ppm, which is what you'll get with filling from above with splashing. 8 ppm is more appropriate for ales and may require an oxygen stone, an aerator or a very vigorous shaking.
Pitch rate - Somewhere between 0.75-2 million cells/mL/°P. Less than that is asking for trouble.
Temperature - Between 10°C and 24°C (or higher for Kveik and some Saison strains)
Gravity - How much sugar is in your wort
Pressure - Is your fermenter capped or not (or pressurised)
Sulphate concentration - usually only a concern if above 150 ppm
Without further ado, here's a handy chart that briefly outlines the effects each factor has on each of these flavours.
Increased FAN will ensure a smooth growth phase. Without it, less alcohol (both ethanol and higher) are produced because resources are diverted elsewhere. This leaves more acetaldehyde floating around and diacetyl increases as a result of amino acid synthesis. There is a limit point, however, with zinc. Most amino acid formulas contain zinc and for some unknown reason, once you hit 2.5 mg/L, the growth phase becomes uncontrolled and you get an explosion of esters. Weirdly enough, zinc also works as a head stabilizer so if you add it after fermentation, you can increase your foam stand.
Oxygen acts as the "wake up" chemical for yeast, so it dismantles trehelose, which protects the cell membrane and glycogen, which is the reserve nutrient. It also diverts carbon from the acetaldehyde -> ethanol pathway to the sterols and lipids pathway. Redox levels must be balanced, however, so higher alcohols are formed. This is why it's crucial to match your oxygen with your pitch rate. Bombing your wort with oxygen and underpitching will give you a nail polish nightmare.
Pitch rate restricts growth in the same way that adding more mouths to feed results in less food for everyone. The fridge empties quickly, which is kind of what you want to happen in this situation. Quick fermentations (within reason) reduces the amount of time available for other microbes to gain a foothold, as well as increasing turnaround time so you can brew again.
Increased Temperature will give you more of almost everything. Diacetyl will actually fall because the conversion of acetoin to diacetyl will be quicker - assuming the yeast is healthy. However, to avoid more fusel alcohols or esters, you want to hold off increasing the temperature until the growth phase (aka log phase) is done. That way you get the best of both worlds.
Increased pressure moves things more in the direction of a clean fermentation, but it's a minor factor and can't compete with temperature, gravity, pitch rate, etc.
Most importantly, strain selection is king. You can't make a clean lager with a kveik yeast, no matter what the yeast lab or a home brewer says. You'll also notice that many of these factors have variables. For a simplified breakdown of fermentation profiles, see Yeast Pitching - Putting it All Together, but if you're trying to pinpoint the cause of a particular yeast-derived off-flavour, this chart can help.