OK went Googling, it's encouraging:
Quote:Vitamin C in Our Food
The current RDA is 60 mg and can easily be obtained through diet. Rich sources are citrus fruits, broccoli, cantaloupe, Brussels sprouts and strawberries. Many drinks are now fortified with vitamin C. Vitamin C is most exclusively found in plants with a small amount found in milk. Vitamin C is delicate, and the content of the vitamin in a food can be enhanced with the following tips:
Store food in a cool, moist place.
Use as soon as possible.
Vegetables lose vitamin C during long storage, but not the root vegetables.
Harvest at the peak of maturity.
Limit exposure to air and sunlight.
Avoid soaking the food in water for long periods.
Cook food in minimal amounts of water.
Microwave cooking preserves the vitamin C because water is not added.
Cook food in largest pieces possible.
Heat reduces some of the content of vitamin C in food.
Freeze-drying of fruit cues minimal loss of Vitamin C.
Frozen and canned foods, picked at the prime and optimally processed, may have more vitamin C than fresh produce that has been stored for a long time with poor temperature and humidity conditions.
So both frozen and microwaved should be fine. Yay!
Not sure how authoritative that is, though.