There have been a few interesting videos and written articles about this subject. The following is one which captures the essence of what I wanted to convey.
The most important thing to say about fruit and vegetables is JUST EAT THEM!
Americans typically eat only one-third of the recommended daily intake (three servings instead of nine) of fruits and vegetables, so if you’re in a bind, a vegetable in any form is better than no vegetable at all.
And as winter approaches, fresh produce is limited—or expensive—in much of the country, which forces many of us to turn to canned or frozen options. While canned vegetables tend to lose a lot of nutrients during the preservation process (notable exceptions include tomatoes and pumpkin), frozen vegetables may be even more healthful than some of the fresh produce sold in supermarkets, says Gene Lester, Ph.D., a plant physiologist at the USDA Agricultural Research Center in Weslaco, Texas. Why? Fruits and vegetables chosen for freezing tend to be processed at their peak ripeness, a time when—as a general rule—they are most nutrient-packed.
While the first step of freezing vegetables—blanching them in hot water or steam to kill bacteria and arrest the action of food-degrading enzymes—causes some water-soluble nutrients like vitamin C and the B vitamins to break down or leach out, the subsequent flash-freeze locks the vegetables in a relatively nutrient-rich state.
On the other hand, fruits and vegetables destined to be shipped to the fresh-produce aisles around the country typically are picked before they are ripe, which gives them less time to develop a full spectrum of vitamins and minerals. Outward signs of ripening may still occur, but these vegetables will never have the same nutritive value as if they had been allowed to fully ripen on the vine. In addition, during the long haul from farm to fork, fresh fruits and vegetables are exposed to lots of heat and light, which degrade some nutrients, especially delicate vitamins like C and the B vitamin thiamin.
Bottom line: When vegetables are in-season, buy them fresh and ripe. “Off-season,” frozen vegetables will give you a high concentration of nutrients. Choose packages marked with a USDA “U.S. Fancy” shield, which designates produce of the best size, shape and color; vegetables of this standard also tend to be more nutrient-rich than the lower grades “U.S. No. 1” or “U.S. No. 2.” Eat them soon after purchase: over many months, nutrients in frozen vegetables do inevitably degrade. Finally, steam or microwave rather than boil your produce to minimize the loss of water-soluble vitamins.