As a general statement, if staff sharing credentials to 'help' each other is indeed a problem, then that also needs to be dealt with from a management perspective.
No credential method is foolproof, and unless the users respect control systems, they'll just prop freezer doors open or disable the door locks in rebellion. Personally, I would recommend using smartcards (13.56 MHz) long before fingerprints. I think the risk of sharing badges can be properly managed (it is even in high-security areas), and there's a real operational issue of needing to take gloves off and potentially throwing them away mutiple times just to use the system.
I'm personally not familiar with reading veins through plastic gloves, but that just doesn't seem reliable. Iris might be better (Stanley's Eyelock Nano comes to mind) but I'd still test before building a final proposal on it.
The readers inside the freezer need to be rated for cold temps and dynamic humidity. This may mean finding two different models - one for the outside, one for the inside.
We haven't yet discussed how you are going to physically 'lock' the doors, but I agree with the recommendations to use Genetec to knit together multiple systems here. By no means is that the only option, but getting everything in the same interface is likely important here, and Genetec does that nicely.