Why do thawed, once-frozen berries taste different than always fresh?

When I thaw out my handi pak o’ frozen berries, high quality though the bag promises them to be, they have this strange different taste, I can describe only as lacking in freshness, like they had been cooked.

Why though do frzoen berries, such as strawberries, taste weird? Why would the freezing process make them different, once they thaw out?

Check the ingredients.
It also might be the plastic bag they put them in.

I don’t have my copy of On Food and Cooking here at work, but here’s the basic chain of events:

The texture changes because of the formation of ice crystals when the berries are frozen. The cells in a fresh berry are there to hold the watery contents of the cells (i.e., juice) in place. When water freezes, it expands and forms crystals; these ice crystals then cause some cells to burst, poke holes in the walls of others, and so forth. Since the walls don’t repair themselves when thawed, the thawed berry is mushier than the fresh berry.

As to why it seems to affect the flavour, let’s assume that your berries haven’t picked up any off-flavours from other things in your freezer (which is easier to do than you might think.) In that case, I would guess that it’s primarily that the mushy berries have a lot more sweet juice immediately available when you bite into them, while for a fresh berry you have to chew on the berry to release the juices. This would affect the balance of flavours in your mouth at any given moment, and the larger flood of sweetness at the beginning might wash out the more subtle taste components that you associate with a fresh berry.

The process of freezing damages the cells, resulting in a different (generally squishier) texture. If you took fresh berries and frozen-then-thawed berries and pureed them both, you probably wouldn’t be able to tell any difference in taste, but texture is a big component of what we think of as “flavor”.

After looking at my copy of Harold McGee’s On Food and Cooking, there’s another effect that’s important. Plant cells still have some biochemical activity in them; the proteins and enzymes are still performing their prescribed functions for some time after the fruit is picked. However, the action of these enyzmes depends fairly critically on how concentrated they are inside the cells. When the cells begin to freeze, the water molecules start to form into ice crystals; this means that there are fewer water molecules, but the same amount of enzymes, and so the enzyme concentration goes up. This increased enzyme concentration can then cause actual chemical changes in the cells, which would also affect the taste.

Finally, it’s possible that these fruits are cooked, at least briefly. Some of the enzymatic reactions that speed up during freezing cause the pigments and vitamins in the fruit to be broken down. These enzymes can be inactivated by blanching the fruit: boiling the fruit very briefly (for about a minute, give or take), and then plunging the fruit into cold water so that the cell walls aren’t broken down by any residual heat. This preserves the pigments and vitamins (which are what make the fruit sell, after all), but it does give the fruit a somewhat more cooked flavour. McGee says that frozen fruit is not as commonly blanched as frozen vegetables are, but it’s possible that this process might still be used in the berries you buy.

I think MikeS and Chronos have the most important points, but there could also be an effect of dessicating the berries - “freezer burn”. One test of this is whether the same kind of berries taste different depending on how long they have been frozen.