Why is double pitching necessary?

[RedBarnGuy]

I have read that double pitching (even triple pitching) is necessary for high gravity beers. Alternatively, I know you can also make a starter (not an option for me b/c I travel during the week and brew on weekends).

Can someone please explain to me why this is necessary? Shouldn't the yeast from a single vial or smack pack multiply enough to consume all of the sugar in the wort? Why not? Is the problem that the yeast die off as the alcohol content rises, so you need more to start out with to finish the fermentation?

I'd really appreciate any thoughts on this topic. This has been really bugging me, and I can't seem to find any answers.

Thanks!
RBG

 

[terrapinj]

good answers here

http://www.mrmalty.com/pitching.php

http://www.mrmalty.com/starter_faq.php

you can always create your starter the week before you brew and put it in the fridge after a few days

 

[Shaneoco1981]

Because you are stressing the yeast. Stressing the yeast can cause off flavors. Yes, one pack will ferment a batch of beer, but it will take a LONG time to get going, and it will stress the yeast. That's the way I understand it at least.

 

[BigB]

Primarily due to undesirable flavor compounds... this is direct from Wyeast Labs:

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
Stuck fermentations
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.

 

[RedBarnGuy]

Great information, folks. Thank you for the quick replies.

RBG

 

[malkore]

Lazy answer for those who don't wanna research: yes yeast colonies grow, but a batch of higher gravity wort isn't the ideal growth environment.

Starters that can have air whipped into them, then chilled and decanted before pitching, IS the ideal environment.