It has been some weeks since I first posted about Vitamins and so it is time to go onto the second part of Vitamins in Food Chemistry.
The second module on vitamins in food chemistry is stability. Vitamins, especially Vitamin C (ascorbic acid), thiamin (B1) and riboflavin are exceedingly unstable. The final amount of vitamin in a food is affected by growing conditions, post harvest changes, initial treatments (washing, milling), blanching and other thermal processing, and storage. In addition to the effects above, the food itself and its chemical composition will influence vitamin stability and different vitamin structures have different stabilities. Most of the information has come from the various Food Chemistry textbooks I own and from information I have gathered over the years studying ascorbic acid reactivity (some references listed below). Most of this discussion is in reference to fruit and vegetables, which are one of the major sources of vitamins. They are also extensively studied.
Typical reactions that vitamins are susceptible to are:
Oxidation: Fat soluble vitamins, vitamin C, thiamin
Degradation and other reactions (non-enzymic browning) – ascorbic acid, thiamin
Photochemical reactions – riboflavin, beta-carotene, folic acid
Food chemists are primarily concerned with maintaining the maximal vitamin content of foods, but sometimes this is difficult. Even if we eat freshly picked produce that is then prepared at home, water soluble vitamins are lost by leaching into the cooking water. Thermally unstable vitamins are destroyed by cooking and fat soluble vitamins are susceptible to oxidation. Some vitamins, riboflavin in particular, are sensitive to photochemical changes and are destroyed by the presence of light. It has been shown that milk left of the doorstep (there are still home deliveries in the UK) had less riboflavin than milk stored in the dark. I remember also being told (personal communication with Dr T. Labuza) that pasta stored in clear plastic bags had less riboflavin than pasta stored in boxes. Also pasta at the back of the shelf, where less light got to it, had more riboflavin than the pasta at the front. Beta-carotene, provitamin A, is also sensitive to light-induced oxidation.
Growing conditions; soil, water, sunlight, as well as variety can alter the vitamin content of a food. In addition, the vitamin content of fruit changes during in maturation. Once the produce has been harvested metabolic reactions continue and vitamins can be destroyed. For example, potatoes stored through out the winter have less ascorbic acid than new potatoes. This used to be a problem in Britain as potatoes were the major winter source of this vitamin. By spring, the incidence of scurvy, Vitamin C’s deficiency disease, would increase until fresh produce was ready. Correct storage and treatment can reduce vitamin loss during post harvest.
After harvesting, food under go preliminary treatments such as trimming washing and milling. Thiamin loss is particular serious in the milling of rice. In fact, the cause of thiamin’s deficiency disease, beriberi*, was discovered due to a comparison of diets containing white rice (low in thiamin) and brown rice (high in thiamin). The loss of vitamins during milling lead to the legalization of enriched cereal grains (see part 4).
Thermal processing is an important step in food processing. At its simplest, blanching is used to inactivate enzymes and reduce micro-organism levels. Loss of vitamins occurs both by oxidation and by leaching into the cooking water. While the effect of heat on vitamins is important, during blanching the product is heated is for a very short time. Other processing steps with elevated temperatures can cause significant loss of vitamins and this loss is dependent on the chemical nature of the food. Acidity (pH), relative humidity, dissolved oxygen, presence of transition metals and other reactive compounds all influence vitamin stability during processing. Vitamins are also lost during storage.
*Wikipedia states that Christiaan Eijkman (1890) and Casimir Funk (1912) were the discoverers of the link between thiamin and beriberi, but in my notes, I have Kanehiro Takaki (1880) as the first person to identifiy it on Japanese warships.
Fennema, O. Food Chemistry, 3 edition.; Marcel Dekker, 1997.
deMan, J.M. Principles of Food Chemistry 3rd edition, Aspen 1999
Christen G.L. and Smith, J.S. Food Chemistry: Principles and Applications, Science Technology System 2000
Davies C.G.A. Ascorbic acid degradation and its inhibition by sulphur (IV) oxospecies. PhD thesis, Leeds University 1993