I had a great chat with a very good friend of mine in Phuket, we were discussing the nutritional values of fresh, canned, and frozen vegetables. His research showed minimal change, and I did some research today, same results, minimal change, and there is actually a very good reason for that. And canned does not necessarily mean processed, it means prepared.
For instance, if you take a bag of dry beans and cook them, add some tomato sauce and spice, you have baked beans, the same as a can, you prepared that, just like a cannery did. Major difference is the salt added. Canned corn is just cooked corn with salt added, nothing more.
For the difference between frozen and fresh. Frozen, the veggys have to be blanched which does reduce some of the vitamins, but this with quick freezing, that locks in the majority of the nutrients.
Fresh veggys on the other hand, normally are harvested before they are mature, and they mature on the way to the store or on the shelf, if the veggy is not mature, it also is missing some nutrients. Fair trade off.
Canned veggys are a good item to stock in case of emergencies such as loss of power for extended times, downed power lines, unrest preventing travel, frozen is good for a few days only unless you have a local ice supplier.