In cooking, aside from its obvious nutritional advantages, it is used for a very common purpose: to stop cut fruits and vegetables from browning. When you rub pieces of apple or a cut artichoke with a lemon, it is the ascorbic acid which is doing the work of stopping the cut surface from turning unsightly.
Technically, there are actually two forms of ascorbic acid: L-ascorbic acid and L-dehydroascorbic acid. L-ascorbic acid becomes L-dehydroascorbic acid when it reacts with oxygen in the air.
Plants and animals can synthesize vitamin C for themselves from glucose in their diets, but humans cannot.
We were all taught that boiling vegetables or fruit destroys the Vitamin C in them. That’s a very simplistic answer, though. Heat can destroy Vitamin C, and overly prolonged boiling will destroy a good deal of it. However, what happens first is that a good deal of the Vitamin C leeches off into the water. That’s not a problem if you are going to be using the water to make a syrup, stock or soup with it. Take rose hips as an example: after 80 minutes of boiling, about 85% of the Vitamin C that passed into the water will remain in the water — only about 15% is lost. Thus, a great deal of the Vitamin C will remain available to the body if the cooking liquid is used. That’s why rose hip syrup (the sweetened, thickened water that rose hips were boiled in) was so important nutritionally in Britain in the Second World War — it was a concentrated source of Vitamin C that didn’t have to be imported.
The assumption is that most people don’t use their cooking water is probably correct. Most people strain their vegetables in the sink and let the nutrient-rich water flow down the drain. If you freeze your cooking water for later use in stocks and soups, you need to worry far, far less about this Vitamin C loss as you will be using the Vitamin C in the water later.
The USDA gives the following advice for using ascorbic acid in preventing produce browning:
While preparing a canner load of jars, keep peeled, halved, quartered, sliced, or diced apples, apricots, nectarines, peaches, and pears in a solution of 1 teaspoon ( 3 grams / 3,000 milligrams) ascorbic acid to 1 gallon [Ed: 4 litres] of cold water. This procedure is also useful in maintaining the natural color of mushrooms and potatoes, and for preventing stem-end discoloration in cherries and grapes. You can get ascorbic acid in several forms:
Pure powdered form—seasonally available among canners’ supplies in supermarkets. One level teaspoon of pure powder weighs about 3 grams. Use 1 teaspoon per gallon of water as a treatment solution.
Vitamin C tablets—economical and available year-round in many stores. Buy 500-milligram tablets; crush and dissolve six tablets per gallon of water as a treatment solution.
Commercially prepared mixes of ascorbic and citric acid—seasonally available among canners’ supplies in supermarkets. Sometimes citric acid powder is sold in supermarkets, but it is less effective in controlling discoloration. If you choose to use these products, follow the manufacturer’s directions.  United States Department of Agriculture (USDA). Complete guide to home canning. Agriculture information bulletin No. 539. 2015. Page 1-11.
Our bodies use Ascorbic Acid to make collagen. Ascorbic Acid is also an anti-oxidant.
Freezing does not impact Vitamin C much, but canning does. In the canning process, some of the Vitamin C leaves the fruit or vegetable and passes into the canning liquid. The prolonged processing periods usually necessary for canning then destroys the Vitamin C. The little Vitamin C left in the canning fluid is often drained away when the food item is used.
It was only in 1932 that Vitamin C was isolated and definitively linked to scurvy (by researchers Albert Szent-Györgyi and Charles Glen King.) With this knowledge, efforts at synthesizing vitamin C industrially began.
A process for industrially synthesizing Vitamin C was developed shortly afterwards at the Swiss Federal Institute of Technology in Zürich by Tadeus Reichstein (1897–1996), a Polish-born chemist. He needed sorbose bacteria to help in the process of making Vitamin C from glucose : he got the sorbose from the legs of a fruit fly.
Reichstein patented the process, but a 1937 Nobel Prize in Chemistry for work on Vitamin C went to Walter Haworth of Birmingham instead. He did however finally get a Nobel prize in 1950 for work in other areas of medicine.
Using Reichstein’s process, the first industrially synthesized vitamin C sold publically was a product called Redoxon in 1934. It was sold by the Swiss pharmaceutical company Hoffmann-La Roche.
Literature & Lore
“Anti-Darkening Treatment. Some fruits darken during freezing if not treated. Several types of anti-darkening treatments are used.
- Ascorbic Acid. For most fruits that need treatment, ascorbic acid (vitamin C) may be used. This is very effective in preserving color and flavor of fruit, and adds nutritive value. Ascorbic acid in crystalline or powdered form is available at pharmacies. One teaspoon weighs about 3 grams. Dissolve ascorbic acid in a little cold water. Add to syrup or sprinkle over fruit and mix carefully.
- Ascorbic Acid Mixtures. Special anti-darkening preparations made of ascorbic acid and citric acid mixed with sugar are available. Follow the manufacturer’s directions.
- Citric Acid, Lemon Juice. Citric acid or lemon juice makes a suitable anti-darkening agent; neither is as effective as ascorbic acid. Citric acid in crystalline or powdered form is available at pharmacies. When using citric acid, dissolve it in a little cold water before adding to the fruit according to directions for that fruit.”
Lauritzen, Georgia C. Fruit Freezing Methods. Utah State University Cooperative Extension. FN 215. Page 2.
Paterson, David. The Fruits of Labor: The First Industrial Synthesis of Vitamin C. Basel, Switzerland: Karger Gazette. October 2010, No. 71.