Expert: Keith Patton - 9/9/2009

Question
What factors affect the rate of fermentation?

Answer
The factors that affect fermentation in my experience are as follows:

The amount of yeast first pitched
The temperature
The type of yeast
The amount of complex sugars in the wort
The amount of yeast nutrient in the wort
Whether the wort is adequately aerated when the yeast is pitched
Gravity of the wort

A lot of these factors affect whether the ferment starts rapidly and stays vigorous or whether it starts slowly and never reaches high kreuzen.

Amount of yeast pitched can affect the fermentation rate initially as the ferment might not kick off rapidly as the yeast has to propagate and may become exhausted before the wort is fully attenuated.

Temperature.  Most any yeast will ferment more vigorously at higher temperatures, but a lot of byproducts are produced and will create flavors that may be off style.

Some yeast types are more aggressive than others and ferment more vigorously.

Complex sugars like honey and carmelized sugars take longer for the yeast to break down, and in some cases they never do.

To get a good vigorous start and continued ferment be sure that the yeast have the proper yeast nutrient by adding powdered yeast nutrient like Bruvigor or another additive.

Proper aeration of the wort when the yeast is pitched is critical.  The ferment might not kick off quickly and may remain tepid.  Yeast needs oxygen at the beginning of the ferment then quickly turns anerobic, but it does need the oxygen for a vigorous start.

Obviously a very high gravity wort may take longer for the yeast to adapt and begin to ferment, and will take longer for the yeast to fully attenuate.