Vitamin stability, general

Pullman and Pullman (1963), in their seminal book on quantum biochemistry, noted the central role occupied in life processes by molecules possessing partly or completely conjugated systems. These molecules include the coenzymes, whose precursors are the water-soluble vitamins. Stabilization of transition states by electronic delocalization, transmission of electronic perturbations over several atoms, and facilitation of electron mobility were suggested as explaining, at least in part, the reaction capabilities of coenzymes. An additional role for coenzymes (Jencks, 1975) is the provision for optimal binding interactions with specific subsites on the enzyme, the so-called "anchor principle" discussed above. In this section, we shall consider the mechanism of action of the major coenzymes. A general description of coenzyme reaction mechanisms is available (Lowe and Ingraham, 1974). 

The stability of some vitamins is influenced by aw. In general, the stability of retinol (vitamin A), thiamin (vitamin Bj) and riboflavin (vitamin B2) decreases with increasing aw. At low av (below 0.40), metal ions do not have a catalytic effect on the destruction of ascorbic acid. The rate of loss of ascorbic acid increases exponentially as aw increases. The photodegradation of riboflavin (Chapter 6) is also accelerated by increasing aw. 

The well-known susceptibility of vitamin C to thermal and oxidative degradation has focused interest in derivatives of increased stability. In general, partial modification of the enediol system leads to two isomers, both of which have markedly lower reducing power and are therefore stabilized against oxidation. However, vitamin C activities tend to decrease in these substituted derivatives. 

In general, a planar method tends to have fewer sample preparative techniques than either gas chromatography (GC) or HPLC methods. The primary criteria for TLC is that the matrix should not distort or streak the analyte band or spot. One other concern should be the stability of the drug after sample application. For example, vitamin D, is stable on prewetted silica gel but decomposes quickly once the sorbent is dried. 

Multivitamin products for parenteral administration are available in a variety of compositions from different manufacturers. As different formulations are available generally, valid stability information cannot be provided. Stability data obtained on vitamins, derived from studies of a single vitamin, cannot be accurately extrapolated to all multivitamin preparations because of possible vitamin, preservative, and excipient interactions. 

Standard therapy Intensive care treatment and monitoring is accepted as standard therapy complications may appear unexpectedly and rapidly. In about 75% of patients with mild cholangitis, therapeutic success can be achieved by the substitution of fluid, electrolytes and zinc as well as vitamins, glucose (possibly also amino acids) and the (indispensable) administration of antibiotics. Analgesics and spasmolytics are generally necessary. In addition, the administration of fresh plasma is recommended to stabilize haemostasis. 

Because of their diverse stmctures, there are few common threads to vitamin chemical properties aside from their fat or water solubility. Of general concern in all appHcations, however, is vitamin stabiHty. Table 5 provides generic information regarding stability under several conditions. Levels of stability vary greatly and are impacted by acid or base strength, light intensity, etc. 

Lund (69) has published a table containing the D121 values for destruction of constituents in foods. From Table XII it is evident that vitamins are more thermally resistant than are vegetative cells, spores, or destructive enzymes. In fact, most vitamins are more thermally resistant than are the organoleptic qualities of color, flavor, and texture. As a general rule, one can process a food for optimal sensory quality and be relatively sure of optimal thermal stability of the vitamins contained in the foods. 

The use of purified vitamins and vitamin premixes as supplements to animal feeds has increased the economy of animal feeding. Vitamin premixes are composed of required vitamins and a suitable carrier and are given to animals together with feedstuffs providing energy and protein. Vitamin E is generally added to premixes for most domestic and laboratory animals (Combs, 1998). They are added as esters, because they are more stable than free tocopherols. Free tocopherols are known to stabilize other vitamins such as vitamin A in premixes. As an example, practical diets for chicks contain 5500 pg a-tocopherol equivalents/g (Combs, 1998). 

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