Science, Art, Litt, Science based Art & Science Communication

Qs people asked me on science and my replies to them - Part 267

Q: How can vitamin C be destroyed by heat in regards to its structure?

Krishna: You have asked me a technical question, therefore the answer can be technical too.

This is the structure of ascorbic acid or Vitamin C.

Image source: intechopen.com

In the molecular structure of vitamin C, there are two enol groups adjacent to the carbonyl group, which are easily oxidized. It can be oxidized in the absence of oxygen and oxygen. There are many factors affecting its stability. PH value, metal ions, temperature, water and light.

The vitamin C content of food may be reduced by prolonged storage and by cooking because ascorbic acid is water soluble and is destroyed by heat .

Vitamin C is a water-soluble and temperature-sensitive vitamin, so is easily degraded during cooking, and elevated temperatures and long cooking times have been found to cause particularly severe losses of vitamin C.

Steaming treatment significantly reduced the retention of vitamin C in all vegetables except broccoli; retention ranged from 0 to 89.24%. Microwaving had less of an impact on vitamin C content, with high retention (> 90%) observed for spinach, carrots, sweet potato, and broccoli. Steaming and microwaving retained higher concentrations of vitamin C than boiling because of the reduced contact with water at relatively low temperatures. Using minimal cooking water and cooking for shorter time periods should result in higher vitamin C retention(2)

Temperature determines the movement of molecules in food. The higher the temperature, the more atoms and molecules to meet one another and thus react. Therefore, a higher temperature often results in a higher reaction rate (3).

Ascorbic acid:

Melting point = 465.15 deg K, decomposes.

Stable to air when dry; impure preparation and in many natural products vitamin oxidizes on exposure to air and light. Aqueous solutions are rapidly oxidized by air, accelerated by alkalies, iron, copper.

Although vitamin C is the generic name of l-ascorbic acid, it has many other chemical names as ascorbate and antiscorbutic vitamin. l-Ascorbic acid molecule is formed of asymmetrical six-carbon atoms (C6H8O6) which is structurally related to glucose . Its molecular weight is 176 with a melting point of 190–192°C (with decomposition) and shows a density of approximately 1.65 g/cm3.

l-Ascorbic acid is freely soluble in water (300 g/L at 20°C), difficult in alcohol (20 g/L at 20°C) and insoluble in chloroform, ether and benzene. It forms a clear colorless to slightly yellow solution. It has two pKa values: 4.2 and 11.6. The pH of a 5% (w/v) solution in water is 2.2–2.5 (1) .

The chemical structure of ascorbic acid determines its physical and chemical properties. It is a weak, water soluble, unstable organic acid which can be easily oxidized or destroyed in light, aerobic condition (oxygen), high temperature, alkali, humidity, copper and heavy metals. Ascorbic acid is usually found in the form of white or slightly yellowish crystalline powder. Its crystalline form is chemically stable in dryness. However L-ascorbic acid is highly soluble in water, it shows great difficulty to be soluble in alcohol, chloroform, ether and benzene. In water, it forms clear colourless slightly yellow solution which is rapidly oxidized.

In pH 4.0, ordinary temperatures and aqueous media, dehydroascorbic acid can be oxidized irreversibly into 2,3-diketo-l-glutonic acid (diketogulonic acid). However, the dehydroascorbic acid oxidation begins in mild acidic media (pH 4.0), it requires a neutral or alkaline media to progress more rapidly. The resultant diketogulonic acid is a stronger reducing agent, not reduced by glutathione or H2S and not an anti-ascorbutic agent. It was found that below pH 4.0, diketogulonic acid losses its reducing property. In acidic media and the presence of H2S, dehydroascorbic acid can also reversibly change into ascorbic acid. Ascorbic acid and dehydroascorbic acid have the same anti-ascorbutic effect (1).

The third oxidation product is l-threonic acid and oxalic acid which proceed only in alkaline media (pH 7–9). All reversible changes can be done in the presence of H2S and glutathione in neutral or alkaline media. Sometimes, carbon dioxide may be the result of vitamin C oxidation at high doses .

In human beings, ascorbic acid is reversibly oxidized into dehydroascorbic acid, which can be reduced back to ascorbic acid or hydrolyzed to diketogulonic acid and then oxidized into oxalic acid, threonic acid, xylose, xylonic acid and lyxonic acid. Further oxidation (decomposition) may occur by the oxidizing agents in food. According to the oxidation-reduction reactions, ascorbic acid is the reduced form of vitamin C while dehydroascorbic acid is the oxidized form of vitamin C. The l-isomer of ascorbic acid is the only active form. Other isomers as d-ascorbic acid, d-isoascorbic acid and l-isoascorbic acid are present. These stereoisomers have no effect in the treatment of scurvy (1).

In research studies for the simultaneous determination of the degradation products of ascorbic acid was employed to investigate the degradation of ascorbic acid in aqueous solution at different pH values. After ascorbic acid aqueous solutions were heated at 100 °C for 2 h, four main degradation products, furfural, 2-furoic acid, 3-hydroxy-2-pyrone, and an unidentified compound, were separated and determined. In an acid aqueous solution, ascorbic acid was converted to 2-furoic acid and 3-hydroxy-2-pyrone via dehydroascorbic acid under aerobic conditions, whereas under anaerobic conditions, ascorbic acid degraded to furfural. Low pH conditions favored the formation of furfural, 2-furoic acid, and 3-hydroxy-2-pyrone; at extremely low pH (i.e., pH 1), the formation of furfural dominated. In an alkaline aqueous solution, the unknown compound became the main degradation product of ascorbic acid; at pH 10, only very small amounts of furfural and 3-hydroxy-2-pyrone with no 2-furoic acid were detected. Our results suggest that, in a hydrogen-ion-catalyzed environment, the anaerobic degradation of ascorbic acid to furfural is the main degradation pathway in an aqueous solution(4).