Specific gravity is the wrong way

Question

I am brewing my first lager, a German Oktoberfest kit from Brewer's Best, after 12 batches of ales. I fermented in the primary for 17 days at 58 degrees and then raised the temperature to 65 degrees for 4 days before racking to secondary. Everything seemed to be going well as I was decreasing the temperature until I hit around 40 degrees when suddenly instead of blowing out of my airlock it appears to be trying to suck air in. I am including my brewing record.

What is going on?

13th Brew

Type of Product: German Oktoberfest Lager

Date of Brewing: Apr. 20, 2012

Date of Bottling Jun. ??, 2013

Volume of Water Added: 2.5 Gallons
(Fermcap S added to prevent boilover @ 1.5 drops per gal)

Type and Amount of Sugars Added:
6.6 lb. Munich LME (Briess CBW) 1.0 lb. Amber DME
Specialty Grains
8 oz. Caramel 60L
4 oz. Caramel 20L

Production Date (on can): Jun. 25, 2012 (120725B1)

Hops Added: 1 oz Bittering - Willamette AA 4.7% 1 oz Flavoring - Willamette AA 4.7%

Fining Agent: 1 Whirlfloc Tablet

Yeast Code on Sachet: Brewferm Lager BBE 01/04/2014 050.030.6 20671

Temperature of Wort Before Adding Yeast:
(recommended <70º) 54º

Original Gravity (before pitching yeast): 1.054

Fermentation Gravity Check
First Check May 3, 2013 - 1.021 @ 58º Second Check May 5, 2013 - 1.020 @ 58º

Racking Gravity (start of Lagering)
May 11, 2013 - 1.019 @ 65º

Gravity Check (during Lagering)
May 20, 2013 - 1.020 @ 38º

Answer 1

This is quite normal. At the end of fermentation, there isn't much CO2 production - the yeast are conditioning the beer. Also, as the temperature falls, the pressure of the gas in the carboy decreases, which causes more air to enter from outside to equalize the pressure.

Regarding the increase in gravity, 1 point is well within the error tolerance, and nothing to worry about. Again, during lagering, the gravity typically does not change much. Did you correct for temperature? If not, then that's why the gravity appears to increase - gravity increases as temperature falls, so you need to compensate for that using a correction table.

Answer 2

I agree that the change from 1.019 to 1.020 is within experimental error. I'm more concerned that fermentation seems to have stopped at such a high final gravity. 1.054 to 1.020 is only 63% attenuation. That yeast should give closer to 80%, which would be a final gravity of 1.011.

Unless your intention is to create a beer with a high finishing gravity, I'd say you dropped the temperature too soon. My procedure for lagers, which I learned from the German Brewing Techniques site is to raise the temperature when 80% of the expected attenuation is reached. This lets the yeast reduce any diacetyl that may be present, and finish fermentation.

So, with your beer, when the gravity read 1.020, I would have brought the temperature up to around 65 F. for a diacetyl rest. Within a day or two, the beer will have finished fermenting at which time I would start dropping the temperature to begin lagering.

My advice is to slowly raise the temperature to the mid 60's to let the fermentation complete, and then ramp down to lagering temperature.