When I'm cutting up tomatoes I look inside for any black rot and I also taste a small sliver of most of them. If the tomato tastes bad I compost it. You could try tasting your tomatoes before processing and also make sure all your equipment is very clean.

It isn't the freezing but the bacteria, molds and yeast that get a chance to grow when the tomato is frostbitten that are more likely to be present in the garden than in the freezer.

Can you make the taste any better by adding sugar? Or by cooking with green peppers, they might overwhelm the bitterness.

Reducing acidity means raising akaline levels, and it's the alkaline that makes it bitter. So frost damage would be a suspect, especially if it happened a while before you picked them (the tomaotes had more time to change while attached to the damaged plant).

I know sugar can cut a sour taste, but bitter is much harder to cover up. Maybe try ading a bit of lemon juice to see if that helps.

Got curiuos and found this. It's from a study about masking bitter in pharmacitals. This part gives some of the things they found that helped.

The spice note used in the flavor system may be selected from spices that are commonly known: preferably
ginger, clove, anise, cinnamon, nutmeg, or mixtures thereof. A preferred sweet note is vanillin, especially
ethyl vanillin. The fruity note may also be selected from among those known in the flavoring art, with
raspberry and lemon being preferred.

Another site said salt works well to cover bitter.
Here is the URL since I don't know how to list more than one. It's a pretty neat site.

http://www.exploratorium.edu/cooking/seasoning/kitchen/activity-quinine.html

Here is a link that might be useful: entire PDF

Agree that even "lightly" frost-damaged fruit will quickly turn bitter. That is one reason why canning them is not recommended by the guidelines.

But the primary reason is the change in pH frost causes. That makes it a safety issue rather than just one of flavor.

Dave

Found an interesting PDF here that says the following

Research has found several conditions that can reduce the acidity of tomatoes. These include decay or damage caused by bruises, cracks, blossom end rot or Insects, and overripening. Tomatoes grown in the shade, ripened in shorter hours of daylight, or ripened off the vine tend to be lower in acidity than those ripened in direct sunlight on the vine. Also, tomatoes attached to dead vines at harvest are considerably less acidic than tomatoes harvested from healthy vines. Decayed and damaged tomatoes and those harvested from frost-killed or dead vines should not be home canned.

But it doesn't mention bitterness.

The only time I have had tomato sauce taste bitter was because of the simmering down end of the process. I don't know if I simmered too long to reduce it, or simmered it too hot. It wasn't scorched, but it turned darker and bitter. I've tasted the same in some Italian restaurants. It's not 'bad' tasting, but it tastes off.