Cold Fermentation is deemed better for the quality of the beer since the conversions, the pH drop and the excretions are proceeding at a slower pace and less extensively. The resulting beer shows a finer, nobler taste, a notable mouthfeel and good foam.

Modern breweries use heavily accelerated fermentation techniques.  These require considerable equipment for fermentation and analysis.  Although not completely out of reach for the home brewer, I found it best to concentrate on the classic cold fermentation schedule since it is within reach for everybody able to control temperatures as low as -1°Celsius.   Usually, a fridge and a chest freezer on a simple thermostat are sufficient to achieve this.

Fermentation is exothermic, it produces heat.  Large volumes of wort are often left to naturally increase the temperature before cooling begins.  However,  during lengthy experiments (they lasted nearly 2 years), I found that smaller volumes seldom achieve this and one can produce a similar effect by careful temperature control.  One could use software for this, for simplicity’s sake I opted for manual temperature adjustments of the thermostat.  Once in the morning, once in the evening.  This 12 hr rhythm produced very satisfactory results.

Cold fermentation needs a yeast with good vitality and the correct pitch volume.  Surprisingly, the pitch volume is much higher than manufacturers quote (although most recommend warm fermentation temperatures at 10°C — probably to avoid disappointment for novices).

Here are the generally used temperature steps used in Cold Fermentation.

Step Temperature/°C
Pitch 5 – 6
Target 7 – 8
End 3 – 5

It is worth mentioning that the low pitch temperatures are important for successful results.  Generally, an increase in pitch temperature results in

  • excess elimination of bitter substances and proteins
  • production of higher alcohols and a non-favourable ratio of higher alcohols to esters
  • less aromatic and more empty tasting beer
  • a reduction in volatile acids and an increase in fixed acids

Here is a graph showing the primary cold fermentation cycle.

When I researched cold fermentation I was surprised to note that

  • the rise towards target temperature is much slower than I previously thought
  • it only spends a couple of days at the target temperature
  • cooling to transfer (to secondary) temperature happens quite early and is almost linear by about 1°C a day
  • cooling starts at 40-45% EVG, for slower fermenting yeasts it starts at 60% — which is rare.
  • Transfer to Lager occurs with some fermentable extract left.  This naturally carbonates the beer with the added benefit that it also takes care of residual oxygen left in the receiving vessel

A few people I know a very nervous about fermenting at these low temperatures.  Indeed, it quickly turns into an unsuccessful experience unless the pitch rate is adequate.  Ideally the yeast starter should have been aerated with oxygen, but not on a stir plate, as it is propagated ahead of the brew day.

To be successful, here are a few things to keep in mind

  • Use healthy pitching yeast slurry (starter) usually 0.5 – 0.7 l/hl, which equates to 15 to 20 billion yeast cells per litre.  For e.g. the Wyeast Activator pack will provide a minimum of 100 billion cells per package.  For an average 25 litre brew, this means 4 to 5 packs!   For a warm schedule at 10° Celsius, the manufacturer claims one pack is enough.
  • It’s useful to be comfortable knowing how to propagate yeast in a good starter to make up the missing cell count before spending a huge amount of money on yeast
  • Yeast has to be present before introducing oxygen into the wort.  This is very important.  We know that oxygenation during the cold phase damages the wort considerably.  Yeast is an incredibly good and rapid antioxidant.
  • A Dissolved Oxygen Meter is a good investment.   They are expensive, but a humble field tool like the Extech DO600 has received good reviews from fellow brewers.  You may be able to estimate the required O2 level, but it’s a gamble.  Overshooting oxygen levels can be detrimental to the outcome.  Generally, levels of 6-8 mg/l are recommended
  • Once the lid is on do not remove it to see whether fermentation starts.   Signs of fermentation starting are best-monitored via the occurring pH drop.  Make sure you take a note of the pH of the wort in the fermenter before adding the yeast and take a measurement in the next 12 hours.  You may be surprised how quickly it drops before you can even detect visual signs.  Once the pH dropped you know the fermentation is progressing.

Once primary fermentation is complete then the secondary phase begins.  The secondary fermentation and conditioning phases are often referred to as the infamous “Lager” phase, but that’s a different chapter.

References

  • [GAHM] Gerolf Annemüller, Hans-J. Manger.  Gärung und Reifung des Bieres (2. Überarbeitete Auflage).
  • [LN] Ludwig Narziss, Abriss der Bierbrauerei (7. Auflage)
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