I am trying to determine what type of evaporator would be best for a small (40L max) bulk liquid (milk) cooler.

There will be a maximum of 17L poured into the refrigerator at one time, to be cooled with an approximate thermal load of 70W. The milk will have to be maintained at approximately 40F for three days.

I have searched high and low and have come to the conclusion that a direct expansion evaporator (stainless) would be most effective.
1. Can anyone comment on this speculation?
2. If this will be an effective solution, where would I find a supplier for such a small evaporator?
3. Are there many other solutions that would be more effective?

Thanks for your time and I look forward to the comments.