Fatsoluble vitamins Vitamin A

Nutritionally, the human body can obtain its vitamin A requirements from two sources: from animal sources as forms of retinol, and from plant sources from b-carotene and related carotenoids. Both sources provide a supply of vitamin A, but by different metabolic pathways. In terms of stability the two sources are different from each other.

Vitamin A is one of the more labile vitamins and retinol is less stable than the retinyl esters. The presence of double bonds in its structure makes it subject to isomerisation, particularly in an aqueous medium at acid pH. The isomer with the highest biological activity is the all-irans vitamin A. The predominant cis isomer is 13-cis or neovitamin A which only has a biological activity of 75% of the all-irans isomer; and 6-cis and 2, 6-di-cis isomers which may also form during isomerisation have less than 25% of the biological activity of the all-irans form of vitamin A. The natural vitamin A sources usually contain about one-third neovitamin A while most synthetic sources generally contain considerably less. For aqueous products where isomerisation is known to occur, mixtures of vitamin A palmitate isomers at the equilibrium ratio have been produced commercially. Vitamin A is relatively stable in alkaline solutions.

Vitamin A is sensitive to atmospheric oxygen with the alcohol form being less stable than the esters. The decomposition is catalysed by the presence of trace minerals. As a consequence of its sensitivity to oxygen, vitamin A is normally available commercially as a preparation that includes an antioxidant and often a protective coating. While butylated hydroxyanisole (BHA) and butylated hydrox-ytoluene (BHT) are permitted in a number of countries for use as antioxidants in vitamin A preparations, the recent trend has been towards the use of tocopherols (vitamin E). Both retinol and its esters are inactivated by the ultraviolet component of light.

In general, vitamin A is relatively stable during food processing involving heating, with the palmitate ester more stable to heat than retinol. It is normally regarded as stable during milk processing, and food composition tables give only small differences between the retinol contents of fresh whole milk, sterilised and ultra high temperature (UHT) treated milk.1 However, prolonged holding of milk or butter at high temperatures in the presence of air can be shown to result in a significant decrease in the vitamin A activity.

A provitamin is a compound that can be converted in the body to a vitamin and there are a number of carotenoids with provitamin A activity. Carotenoids are generally found as naturally occurring plant pigments that give the characteristic yellow, orange and red colours to a wide range of fruits and vegetables. Some can also be found in the liver, kidney, spleen and milk. The provitamin A with the greatest nutritional and commercial importance is b-carotene. The stability of the carotenoids is similar to vitamin A in that they are sensitive to oxygen, light and acid media.

It has been reported that treatment with sulphur dioxide reduces carotenoid destruction in vegetables during dehydration and storage. A study with model systems showed that the stability of b-carotene was greatly enhanced by sulphur dioxide added either as a sulphite solution to cellulose powder prior to b-carotene absorption or as a headspace gas in containers of b-carotene. While it was found that the b-carotene stability was improved by increasing the nitrogen levels in the containers, the stability was even greater when the nitrogen was replaced by sulphur dioxide. Comparative values for the induction period were 19 hours for b-carotene samples stored in oxygen only, 120 hours in nitrogen and 252 hours in sulphur dioxide.2

Investigations into the effect of sulphur dioxide treatment on the b-carotene stability in dehydrated vegetables have given varying results and it has been postulated that the effects of the drying and storage conditions on the stability of the sulphur dioxide has a consequential effect on the stability of the b-carotene in dehydrated products.3 Studies on the heat stability of both a-carotene and b-carotene showed that the b-carotene was about 1.9 times more susceptible than a-carotene to heat damage during normal cooking and blanching processes.4 Products containing b-carotene should be protected from light and headspace air kept to the minimum.

10.4.2 Vitamin E

A number of naturally occurring substances exhibit vitamin E activity, including the a, b, g and S tocopherols and a tocotrienols. Dietary sources of vitamin E are found in a number of vegetables and cereals, with some vegetable oils such as wheatgerm, sunflower seed, safflower seed and maize oils being particularly good sources. Both synthetic and naturally-sourced forms of vitamin E are available commercially. Whilst the natural sources of the tocopherols, which also have the highest biological activity, are in the d form, the synthetic versions can only be produced in the dl form. Both the d and dl forms are also commercially available as esters.

There is a considerable difference in the stability of the tocopherol forms of vitamin E and the tocopherol esters. While vitamin E is regarded as being one of the more stable vitamins, the unesterified tocopherol is less stable due to the free phenolic hydroxyl group.

Vitamin E is unusual in that it exhibits reduced stability at temperatures below freezing. The explanation given for this is that the peroxides formed during fat oxidation are degraded at higher temperatures but are stable at temperatures below 0°C and as a consequence can react with the vitamin E.5 It has also been shown that a-tocopherol may function as a pro-oxidant in the presence of metal ions such as iron.

a-Tocopherol is readily oxidised by air. It is stable to heat in the absence of air but is degraded if heated in the presence of air and is readily oxidised during the processing and storage of foods. One of the most important naturally-occurring sources of tocopherols are the vegetable oils, particularly wheat germ and cotton-seed oils. While deep-frying of the oils may result in a loss of vitamin E of around 10%, it has been found that the storage of fried foods, even at temperatures as low as -12°C, can result in very significant losses.

dl-a-Tocopheryl acetate is relatively stable in air but is hydrolysed by moisture in the presence of alkalis or strong acids to free tocopherols.

10.4.3 Vitamin D

Present in nature in several forms, dietary vitamin D occurs predominantly in animal products with very little being obtained from plant sources. Vitamin D3

or cholecalciferol is derived in animals, including man, from ultra-violet irradiation of 7-dehydrocholesterol found in the skin. Human requirements are obtained both from the endogenous production in the skin and from dietary sources. Vitamin D2 (ergocalciferol) is produced by the ultraviolet irradiation of ergosterol, which is widely distributed in plants and fungi. Both vitamins D2 and D3 are manufactured for commercial use.

Both vitamins D2 and D3 are sensitive to light and can be destroyed relatively rapidly if exposed to light. They are also adversely affected by acids. Preparations of vitamin D in edible oils are more stable than the crystalline forms, and the vitamin is normally provided for commercial usage as an oil preparation or stabilised powder containing an antioxidant (usually tocopherol). The preparations are normally provided in lightproof containers with inert gas flushing.

The presence of double bonds in the structure of both forms of vitamin D can make them susceptible to isomerisation under certain conditions. Studies have shown that the isomerisation rates of ergocalciferol and cholecalciferol are almost equal. Isomerisation in solutions of cholecalciferol resulted in an equilibrium being formed between ergocalciferol and precalciferol with the ratios of the isomers being temperature dependent. The isomerisation of ergocalciferol has been studied in powders prepared with calcium sulphate, calcium phosphate, talc and magnesium trisilicate. It was found that the isomerisation was catalysed by the surface acid of these additives.6

Crystalline vitamin D2 is sensitive to atmospheric oxygen and will show signs of decomposition after a few days storage in the presence of air at ambient temperatures. Crystalline cholecalciferol, D3, is also destroyed by atmospheric oxygen but is relatively more stable than D2, possibly due to the fact that it has one less double bond.

The vitamin D3 naturally occurring in foods such as milk and fish, appears to be relatively stable to heat processing.

10.4.4 Vitamin K

Vitamin K occurs in a number of forms. Vitamin K1 (phytomenadione or phyl-loquinone) is found in green plants and vegetables, potatoes and fruits, while vitamin K2 (menaquinone) can be found in animal and microbial materials.

The presence of double bonds in both vitamins K1 and K2 makes them liable to isomerisation. Vitamin K1 has only one double bond in the side chain in the 3-position whereas in K2 double bonds recur regularly in the side chain. Vitamin K1 exists in the form of both trans and cis isomers. The trans isomer is the naturally occurring form and is the one that is biologically active. The cis form has no significant biological activity.

The various forms of vitamin K are relatively stable to heat and are retained after most cooking processes. The vitamin is destroyed by sunlight and is decomposed by alkalis. Vitamin K1 is only slowly decomposed by atmospheric oxygen.

Vitamin K is rarely added to food products and the most common commer cially available form is Kj (phytomenadione), which is insoluble in water. A water-soluble K3 is available as menadione sodium bisulphite.

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