I hear this question frequently and refer to the manufacturer.  Medela recommends changing the tubing:

"What do I do if milk gets in the tubing?
Inspect tubing after each pumping session. Clean tubing if there is any sign of condensation and/or milk.

CAUTION: If tubing becomes moldy, discontinue use and replace tubing. Contact Medela Customer Service at 1-800-435-8316.
CAUTION: Do not store wet or damp parts as mold may develop.
NOTE: Tubing should be washed if dirty or milk is present.
1.     Turn off breastpump
2.     Unplug breastpump from power source.
3.     Remove tubing by pulling straight out of tubing ports. Do not wiggle or pull tubing at an angle.
4.     Remove tubing from breastshield.
5.     Rinse tubing in cool water to remove breastmilk.
6.     Wash tubing in warm soapy water.
a.     Rinse tubing with clear water.
b.    Hang to air dry"

see:  http://www.medelabreastfeedingus.com/faqs
(second question under "Double Electric - Pump In Style Advanced")

Ellen

Ellen Rubin, MA, IBCLC, LLLL

             ***********************************************

Archives: http://community.lsoft.com/archives/LACTNET.html
To reach list owners: [log in to unmask]
Mail all list management commands to: [log in to unmask]
COMMANDS:
1. To temporarily stop your subscription write in the body of an email: set lactnet nomail
2. To start it again: set lactnet mail
3. To unsubscribe: unsubscribe lactnet
4. To get a comprehensive list of rules and directions: get lactnet welcome