I’ve come to realise a big difference between working in a microbrewery and homebrewing, which is in the amount of shepherding and monitoring of fermentation that goes on after the actual brew day.
At home, my fermenting vessels were buckets made from (food grade) high density polyethylene (HDPE) or polypropylene (plastic types 2 and 5 respectively). There was very little I could do to control the conditions of fermentation. I had a warming belt I could put around a bucket to keep it warm on cold winter nights, but I had no way of chilling the fermenters or of regulating temperature variation over the course of a twenty four hour period. So I had to give up brewing during the hottest part of the year or try brewing beer styles, like a Belgian saison, with yeasts that can tolerate high temperatures without spoiling the beer. In other words, there was a limit to what I could do and the course of fermentation was more or less in the hands of the gods.
In a microbrewery it’s a little different. We have large insulated fermenters equipped with cooling jackets which let me set a temperature for fermentation which will be maintained. I’ve got to decide what temperature best suits the yeast and the beer style. Ale yeasts prefer to work at warmer temperatures 18˚-24˚C and lager yeasts prefer cooler temperatures 12˚-14˚C. Warmer temperatures lead to an increase in metabolic byproducts of fermentation such as fruity tasting esters, so if you want a clean, neutral tasting beer, you need to choose a temperature at the lower end of a yeast’s range. On the other hand, the time it takes for the fermentation to be completed is also temperature dependent.
Judging the end of fermentation is a bit of a guessing game because you don’t know how fermentable the wort is. It might ferment out dry or it might stay comparatively sweet. In theory, by following the recipe closely and following the same procedure we shouldn’t have much variation from batch to batch, but you only know you are doing everything the same way when you get the same results and your wort is exactly as fermentable, dry or sweet as it’s supposed to be. Just thinking you’ve followed the same procedure doesn’t mean you have followed the same procedure, so you’re still in the same state of ignorance about when the fermentation is complete. There’s a test you can do to find the fermentability of wort, but it takes a couple of days and we’d need more lab equipment than we actually have to carry it out.
Judging the end of fermentation correctly is important because we need to move the beer into conditioning tanks. If the beer is moved to soon, there will be a lot of beer in suspension, which will be carried over into conditioning. This is problematic for two reasons. The first is that high concentrations of yeast combined with cold, nutrient depleted, mildly alcoholic conditions can result in yeast cell death which can give an off-flavour to the beer. The second is that high concentrations of yeast can make filtering the beer difficult.
Also, lighter flavoured beers, especially lagers, benefit from a diacetyl rest at the end of fermentation. This means keeping the beer on the yeast for 24 hours after fermentation has finished to allow the yeast to absorb diacetyl from the surrounding beer and metabolise it to a less flavour active compound. Diacetyl is a distinctly buttery or butterscotch off-flavour and is a normal byproduct of fermentation.
Add to all this the complication of managing the stocks and the tank space in the cold room and ensuring we always have a fermenter free to brew each week means the part of my job concerned with managing fermentation is much larger and more important than I realised. So far I seem to be doing alright. I’ve had one diacetyl-related disaster and one yeast cell death related disaster but I’m hopeful that lessons have been learned.