Hanahan Method for preparation of Competent cell - a little problem (Jun/01/2006 )

Hi everybody!!
I always prepare fresh competent cells by Hanahan Method using TFB buffer and DnD (Sambrook). I would like to prepare frozen stocks of competent cells by this method, using FSB buffer and DMSO as the protocol describes. At the end the protocol, It says you have to snap-freeze the competent cells by immersing the tubes in liquid nitrogen and then store at -70°C until needed.
I don’t have liquid nitrogen.
Can I prepare them any way?
What happen if I omit this step and I store them at –70°C directly?
Is there any other way to prepare them without liquid nitrogen?
How much time can I keep them at –70°C without losing efficiency.

Thank you very much

U could put dry ice and pour some 100% ethanol in a box and then place the tube holder inside. Now aliquot the cells in the tubes placed in the holder. This would help to snap freeze the cells.

I have used cells frozen in -80C even after a year and it has worked fine.