My simple answer: You're low on time for a starter, but if you had a stirplate and pitched the entire starter then you'd be fine (and this would be what I would do). Since you don't seem to have the stirplate option, I would recommend 2 smack packs to ensure a plentiful pitch.
Edit: The very first time I used a stirplate I didn't have a stirbar so I made one out of a cut piece of nail, slid inside sterile tubing, and torched closed on both ends. I checked the tubing to ensure it didn't leak, and then put it to work on a liquid vial of yeast. It worked out fine - but I didn't like the discomfort of knowing whether or not my homemade stirbar was going to mess things up.
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My complex answer:
Yeast pitching/starting/rates/temperatures is a touchy subject with a lot of opinions. Some folks believe there's very little flexibility with it and others believe there's a fair amount of flexibility with it (<-me). One thing that seems fairly common amongst most folks is that properly handled and pitched yeast coupled with a good fermentation temperature regimen should lead to good beer - assuming a good process, proper sanitation, reasonable recipe, etc.
Recent re-discoveries on lab-grade yeast (i.e. White Labs and Wyeast) seems to indicate that it is indeed superior quality than repitched yeast, and doesn't follow the same guidelines that the pitching rate calculators estimate.
According to Palmer 2008: "
Fresh yeast from a well-prepared starter is at the peak of viability and vitality and only 5075% as many fresh yeast are needed to do the same job as re-pitched yeast." and "
Ales that are considered to have a very clean character, like American pale ale, blonde ale and Northern English brown ale should be pitched at the 0.751.0 billion cells per liter." (source:
http://byo.com/stories/item/1717-yeast-pitching-rates-advance-homebrewing)
I extrapolate this to include lab-grade yeast since it's grown in a much more yeast-friendly environment than our "well-prepated starters". It also gives the flexibility of 0.75-1.0 billon cells per liter (i.e. 0.75-1.0 million/ml/°P)
You are looking at needing about 211 billion cells of repitch yeast for a 5.5G batch of 1.055 wort (according to yeastcalc).
One packet of wyeast in the Activator packaging can supply "a minimum of 100 billion cells per package" (direct from Wyeast) during their Product Warranty period. Their Product Warranty period is 6 months from manufacture date.
Summarizing all of the above:
-Yeastcalc says you need 211 billion cells of repitch yeast (1m/ml/°P).
-Palmer says you need 106-158 billion cells of high quality starter (lab-grade) yeast (i.e. 50-75% the standard 1m/ml/°P).
-Palmer also says neutral ales should be pitched at the 0.75-1.0m/ml/°P for repitch yeast, which could mean a reduction by 25% of the high quality yeast cell figure.
-Wyeast is a supplier of lab-grade yeast with a warranty of a
minimum of 100 billion cells for 6 months from manufacture date.
My conclusions:
-Using <6 month old Wyeast, should give 100+ billion cells
-Using Palmers upper-end rates on high quality yeast, one Wyeast smack pack should be good for 5G of 1.040 at 1m/ml/°P (5G of 1.053 at 0.75m/ml/°P)
My recommendation:
-Use 2 smack packs and feel very comfortable that you supplied sufficient yeast (needed 158b; supplied 200+b)
-Use 1 smack pack and feel unsure on supplied yeast (needed 158b; supplied 100+b)
Any more than 2 smack packs is unneeded. The figures above basically scrape along the lines of safe rather than sorry. I was working on the premise of Palmers 75%, but he stated you might be able to go as low as 50% which would bring the 'needed' yeast count down to 106b cells thereby making 1 smack pack (100+b) seem like a very viable option.