http://www.rawfoodexplained.com/selection-and-storage-of-foods-part-i/does-freezing-harm-foods.html :
"When a food is frozen, its water expands. This causes two immediately destructive occurrences:
1. The cell walls burst and the cell contents are spilled due to the internal water expansion; hence the cell's life is lost.
2. Oxidation occurs where air reaches the frozen foodstuff; hence nutrients are lost.
In addition to bursting the cell walls of foods and thus allowing oxidation to occur, two other things happen:
1. When the cells burst, certain of their organelles release self-destruct enzymes called lysosomes. While these enzymes are not active during freezing (and some are even destroyed), those which remain intact will speedily decompose the cell contents upon thawing. Lysosomes are in cells for the purpose of self-destructing dead cells so the dead cells will not create problems for the organism.
2. Whether oxidized or deranged by its own lysosomes, dead cells become soil for bacteria and fungi when the temperature becomes favorable again—bacteria are active at just above freezing up to temperatures around 160 degrees.
Oxidation of burst cells is the foremost cause of food deterioration during frozen storage. Frozen foods never taste as good to an unperverted palate as their fresh counterparts, even if no additives and pre-freezing treatments are employed. This is, of course, due to their deterioration while frozen.
While microorganisms such as bacteria are also inert during freezing, they become active just as soon as they are thawed. Hence, frozen foods, once removed from the freezer, decompose much more rapidly than do fresh foods. As mentioned, this is because of the bursting of the cell walls of the food when its own water expands and because of the subsequent decomposition through oxidation, self-destruct lysosomes and the final cleanup crew, bacteria.
It is well to repeat that food is rapidly destroyed when cell walls are burst, whether by cooking, blending, juicing, mashing or freezing. Oxidation occurs when cell contents are exposed to the air, and if temperatures are favorable, the cells' own lysosomes self-destruct its components."