Freezing is a very efficient method of preserving the nutritional value, texture and flavor of many vegetables. Most vitamins will keep well in frozen vegetables. Carotene (a compound that is converted to vitamin A in the body) may actually be better preserved in frozen produce because packaging keeps the vegetables away from light (which destroys carotene).
For example, frozen peas typically have about 60% more carotene than 'fresh' peas (that have been exposed to light during their trip to the market and while awaiting sale).
Some losses of vitamin C and folate (also known as folic acid) occur during commercial freezing. However, the vitamin losses associated with blanching and the thawing/cooking process are similar to those that occur during normal cooking of fresh vegetables. This means that, provided they have been stored and then cooked properly, frozen vegetables provide similar levels of nutrition to fresh vegetables. It is also worth noting that for cooking both frozen and fresh vegetables, microwave cooking and steaming are both superior (in terms of retaining nutritional value) than boiling in a large volume of water.
Other vitamins are generally fairly heat stable and are largely retained during the blanching process and subsequent period of frozen storage, or are not found in significant quantities in vegetables anyway. Nutrients other than vitamins are not significantly affected by the freezing process.
Collect your frozen foods last at the super-market and take them home in insulated containers.
Put frozen food in the freezer as soon as you arrive home.
Frozen storage for no more than about six months is recommended for most vegetables.
Steam or microwave frozen (and fresh) vegetables to retain a high proportion of the vitamins. If cooking frozen food in boiling water, the food should be taken directly from the freezer and placed in water that has already come to the boil.
Boil vigorously until ready, and then serve immediately (ie, do not leave the food 'simmering' for long periods). Do not use saucepans or other utensils made from copper or brass.Cooking utensils made from glass, stainless steel, aluminium or enamel do not affect the nutritional content.
Source: Nutrition Australia