Heat does destroy some vitamin C while freezing does not seem to affect this vitamin in a negative way, as stated by Scientific American and The World's Healthiest Foods. Vitamin C is an antioxidant found naturally in many foods and also used commercially as a preservative.
Vitamin C is affected by temperature changes, which means that cooking can remove some of the vitamins naturally found in food. This vitamin is highly unstable, oxidizing quickly when exposed to air. Heating only increased the rate of oxidation. Additionally, cooking in a water bath can also dissolve the water-soluble vitamin away. However, the length of time food is exposed to heat and the type of cooking method used affects vitamin C in different ways.
Tomatoes cooked at 190.4 degrees Fahrenheit for 2 minutes lost about 10 percent of their Vitamin C content while those cooked for half an hour lost about 29 percent of this vitamin, as stated on Scientific American. However, the cooking method also affected how much vitamin C was lost, too. For instance, vegetables that were steamed or boiled lost between 22 and 34 percent of their vitamin C while microwaved or pressure-cooked veggies only lost about 10 percent of the vitamin C.
Vitamin C is definitely affected by temperature changes, breaking down the most when exposed to high temperatures for long periods of time.