Effect of Pitch Rate on Beer Flavor
Pitch rates, in addition to strain, temperature, and gravity, make a dramatic difference in the final flavor and aroma profile of any beer. The pitch rate will have a direct effect on the amount of cell growth during a fermentation. Cell growth decreases as pitch rates increase. Ester production is directly related to yeast growth as are most other flavor and aroma compounds.
A low pitch rate can lead to:
Excess levels of diacetyl
Increase in higher/fusel alcohol formation
Increase in ester formation
Increase in volatile sulfur compounds
High terminal gravities
Increased risk of infection
High pitch rates can lead to:
Very low ester production
Very fast fermentations
Thin or lacking body/mouthfeel
Autolysis (Yeasty flavors due to lysing of cells)
With some beer styles, where a complex ester profile is desirable (German Wheat) it can be beneficial to under pitch. Over pitching can often lead to a very clean beer lacking an ester profile (banana). This is a common problem with subsequent generations of Wyeast's Weihenstephan Weizen #3068. Conversely, beers that require a clean profile should be pitched an an increased rate.