A few months ago i bought frozen wheatgrass juice from a store, and i also wondered if nutrients are lost during freezing. I went to their website, and it says :
"Doesn't freezing destroy the nutrients in the wheatgrass?
Evergreen uses a special chiller that chills the juice in 1.5 minutes to 35°F. This preserves the quality of the nutrients during packaging and then it is frozen. Analysis of the product after freezing shows all nutrient are preserved, including the chlorophyll."
I'm guessing that your regular freezer wouldn't be any good, because it takes too long to freeze.