Yeast Pitching - Putting it All Together 

Having trouble wrapping your head around all of the yeast articles? No worries! Here’s a breakdown of fermentation profiles in order to get what you want.

Higher alcohols

(you probably don’t want this, but it will help with understanding.) Higher alcohols are caused by yeast growth, which we absolutely want to have happen, just not too much. For this to occur, we want:

  • Lots of nutrients (no adjuncts or add yeast nutrient)
  • Lots of oxygen
  • Higher temperatures
  • Low pitch rate

The oxygen will restrict ester formation and will push carbon toward growth, rather than ethanol production. The yeast will produce higher alcohols to maintain redox balance. Diacetyl may be a problem because of an older population of cells, but raising the temperature toward the end of fermentation should take care of that. For even more higher alcohols, brew at high gravity.

High Ester Profile

  • Lots of nutrients (no adjuncts or add yeast nutrient)
  • Low oxygen
  • Higher temperatures
  • Standard ale pitch rate (0.75 million cells/mL/degree Plato)

The lack of oxygen means that the terminal proton acceptor in the production of cell mass won’t be present and so acyls will remain, which will hook up with higher alcohols that are produced, which create esters (and limit free floating higher level alcohols). Diacetyl will be limited as long as nutrients are present and temperatures are raised toward the end of fermentation.

Diacetyl (for some reason)

  • Limited nutrients (use lots of adjuncts)
  • Lots of oxygen
  • Low temperatures
  • Low pitch rate

Your fermentation will be quite slow if you do all of these steps, so maybe choose only a couple and do this only as an experiment (it will be gross). For even more added buttery goodness, throw in some yeast nutrient after high krausen. Since valine (and isoleucine) are Class B amino acids, they won’t be taken up if Class A amino acids are present. Therefore, throwing in more nutrients after high krausen shuts down the uptake of Class B amino acids and forces the cell to make even more diacetyl as it’s internal valine concentrations deplete, right when it should be reducing diacetyl to maintain redox balance. The low temperatures will also slow the rate at which α-acetolactate is reduced to diacetyl and you’ll end up with a butter bomb.

It should also be noted that this situation can occur accidentally, such as with dry hopping. Unboiled hops contain the enzyme amyloglucosidase which reduces dextrins to fermentable sugars. In the package, there is a slow trickle of fermentable sugars for the small amount of yeast. Oxygen is limited, but more importantly, there are no nutrients. While you may think bottle conditioning is the same and should produce diacetyl as well, you avoid it because the process isn't dragged out (unless you bottle condition cold)

Clean fermentation

  • Lots of nutrients (no adjuncts or add yeast nutrient)
  • Moderate temperatures
  • High oxygen
  • High pitch rate
  • Standard gravity

Clean fermentation is basically doing everything the blogs tell you to do – the one thing to mention however, is that if you’re doing ultra high gravity brewing, you’re going to want to move your pitch rate even higher. This is because the growth rate will be higher, leading to a higher proportion of higher level alcohols and/or esters and so you need to compensate. You can use temperature as a lever to control these as long as you remember to do a ramp up after high krausen in order to take care of any residual diacetyl.

It should also be added that all of this advice only applies to small batch, single-pitch settings. Once you start getting in to having multiple fills, you have to either have extreme faith in your hygiene and pitch all at the end or stagger your pitching and oxygenation regimes. This is where having a detailed understanding of yeast cell biology really counts.