Most lab managers approach the task of defrosting a freezer with the same zeal you might apply to cleaning out a closet -- ugh! But both tasks represent an opportunity to get organized. Busy –20°C freezers are crowded with multiple users storing boxes of sample and reagents all competing against ever thickening layers of ice. A routine defrosting is a chance to create some space and organization. In order to minimize disruption and keep research on track, we’ve consolidated a variety of tips from lab managers below.
Notify, Alert, and Warn
Sample is precious – hard to obtain and often impossible to replace. As a courtesy, all lab members and PIs need to be notified of a freezer defrosting, alerted where their material will be temporarily transferred, and warned that loose unmarked tubes will be discarded. Email, post signs, and hold lab meetings to make sure all lab members are aware. It’s good to spread the word widely in case someone is out in the field, or just out of town.
Love Thy Label Maker
While you have everyone’s attention, this is a good time to train people to have better lab practice. You can now require that all users label sample and reagent boxes with their name, the PI’s name, and date -- especially important in shared and core facilities. Likewise, it’s an opportunity for lab managers to organize the freezer according to department or PI. Label makers are great here – the ink holds up to frost and the print is always legible. It’s also worth encouraging researchers to consolidate and cull their sample boxes. Anything no longer needed for projects should be transferred into deep freeze (–80°C). (Lab members should create and share an Excel file for anything housed in –80°C.) It’s best to have extra screw cap tubes on hand for ultra-cold storage as they better withstand condensation.
Minimizing Freeze-Thaw
Nearly all –20°C lab freezers have a manual defrost feature, which keeps temperatures more stable in comparison to automatically defrosting freezers. Every time a researcher opens and closes the freezer door, warm air enters forming condensation on the walls and coils that lead to ice build-up. In an active PCR set-up room, this can happen quite a bit. To minimize temperature changes, now might be a good time to establish a 4°C ‘working’ refrigerator in the same room for frequently accessed samples and (select) reagents. DNA is relatively stable in these conditions for roughly a week and storing it this way avoids shearing from repeated freeze-thaw cycles. (Note: small volume samples are most affected by temperature spikes.)
Preparing for Transfer
Finally, it’s time to defrost. The best time to do this depends largely on the amount of accumulated ice and activity in the lab. You want to schedule the defrosting when the lab is relatively quiet -- but this is often unpredictable -- and defrosting overnight is not ideal if excessive water is expected. Before you unplug the freezer to start the process, it’s best to alert facilities and place cautionary signs to avoid accidents. Then get a cart, a mop, empty containers to catch dripping ice and water, and plenty of rags. Before unloading materials, make sure back-up freezers are ready to receive all those boxes. If there’s no room available in the other –20°C freezers, then use recycled foam shipping containers. Load them with ice, transfer the materials, and cover.
Work From the Top Down
With the freezer empty, use squeegee bottles to squirt hot water on the coils and wire racks as those contain the heating elements. Sometimes ice will fall off in big chunks and this is where the cart and plastic containers come in handy. You’ll probably need to wear gloves depending on what room you’re working in. Mop around the freezer as ice melts and cart the water to the sink. Repeat. Never chip away at the ice with a sharp element as this may permanently ruin the freezer. Once the freezer is defrosted, the unit needs to come back down to temperature. Once it has reached the set temperature, and only then, the inventoried boxes can be transferred back into the freezer with correct labels.
Congratulations, you’re done!
Tips for continued maintenance
In the future, if you notice ice building more quickly than usual, you may want to have the freezer seal inspected. A poor seal will allow more outside air in. Expect ice to accumulate faster in the summer on humid days and when there’s less material stored in the freezer. As a final note, invest in a freezer alarm system if your lab isn’t already outfitted with one and have an emergency plan for a power outage or natural disaster in place.
The image at the top of the page are Puffer Hubbard High-Performance Laboratory Freezers from Thermo Scientific.