Vitamin C, otherwise known as ascorbic acid, is a water-soluble vitamin that humans cannot synthesize and therefore must consume from food sources. Citrus fruits like oranges and grapefruits, sweet red peppers and strawberries are all significant sources of vitamin C. However, keeping these fruits and vegetables stored for a significant period of time can potentially degrade the quality and quantity of vitamin C present before you consume them.
Fresh Foods
In 2007 the "Journal of the Science of Food and Agriculture" published a comprehensive review of nutrient degradation in stored foods. After reviewing the available research, Joy Rickman, et al., found that fresh fruits and vegetables have the highest concentrations of vitamin C but immediately begin to lose the nutrient after harvesting. A groundbreaking examination of fresh fruit and vegetable degradation by C. R. Fellers, et al., in 1935 discovered that green peas lose more than 50 percent of their vitamin C content in the first 24 to 48 hours after harvesting.
Canned Foods
Because vitamin C quickly degrades, preservation methods like canning can be helpful in maintaining the vitamin C in fruits and vegetables for shipping. According to Rickman, et al., canning is one of the best ways to preserve ascorbic acid in foods. Canned foods may retain all the vitamin C of their fresh counterparts or have a slight loss of less than 15 percent in the first year after canning. However, the USDA National Nutrient Database does report that canned tomatoes lose about 26 percent of their vitamin C compared to fresh tomatoes.
Frozen Foods
Freezing fruits and vegetables does not appear to stop the loss of vitamin C, though it can slow it down. A 2002 study appearing in "Innovative Food Science & Emerging Technologies" found that frozen fruits and vegetables lose approximately 20 to 50 percent of their vitamin C in the first year of storage. However, some hardy vegetables --- like asparagus and green peas --- are more resistant to vitamin C loss while frozen. Vegetables that are already high in vitamin C, such as broccoli, may have a negligible nutrient loss from freezing.
Product Variance
Research into the best forms of fruits and vegetables to consume for vitamin C content has produced varied results. While fresh fruits and vegetables are generally best, cooked frozen peas and spinach have been shown to have more vitamin C than their fresh counterparts. Additionally, fresh foods that have been at your grocer for a significant amount of time may have degraded before you even purchase them.
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