This is a well used method. Add 4 M NaOH dropwise until the almost all of the formaldehyde has dissolved, but the solution is still a little cloudy. Procedure Grow up a mid-log culture. Put flask back in water bath at culture temperature and agitate for 90 min.
Popular wisdom says it is important to use fresh formaldehyde.
This is in contrast to fixation for electron microscopy. Spin cells at 2000 rpm for 5 min. 3 mg Zymolyase. The time of digestion is not critical and about 15 min will probably be enough. Incubate 5 min and spin.