I have a Blichmann conical fermentor. I keep the temperature constant in an upright temperature controlled freezer. I use a blow-off tube to dissipate the CO2 during the fermenting process. I lower the temperature to crash the fermenting process. This lowers the pressure within the fermentor vessel. When I open the valve to remove the trub yeast from the bottom of the fermentor, air is sucked into the fermentor which may oxidize or contaminate the beer. Air is also sucked in when I take a sample from the fermentor to measure the specific gravity. What can I do to alleviate the problem? Can the air being sucked into the fermentor spoil the beer?