Two options come to mind: 1) The beads may be breaking the tubes. 2) Tubes, or part of the mill, may be loose and the caps are being worn away. Are the tubes that break always in the same position within the mixer? I can think of two options:

1) Make sure all tubes are secure in the holder.

2) Use smaller beads.

Thanks much, Timothy. 

Apparently, I have put my tubes for too long in LN2, making tube caps to crystallize. If I reduce exposure time to 1-1.5 min, everything is O.K. with the tubes. 

You mentioned something important, though is not directly related to the original question: beads were too big for my 2ml tubes so they were not grinding the tissue properly. I have changed from 7 mm to 3.5 mm beads and now I have better results. 

Thanks again for your thoughts and time. Best, 

Melania

Hi Mela, the reason why this happens is that by keeping your tubes in liquid Nitrogen too long, the Nitrogen eventually filters into the tube. When you take the tubes out, they get warm and the liquid nitrogen expands in the tube, making them explode. This can be very dangerous. Indeed keeping them less time in the Nitrogen is a solution, also closing them very carefully. In theory the cold should not affect the tubes themselves. I hope this helps.