Stability of selected water-soluble vitamins in model systems.
The potential for regulated additions of L-ascorbic acid, thiamine, and riboflavin to
assist in maintaining vitamin stability when each formed part of a multivitamin
system was investigated in aqueous solutions. Screening studies were performed to
identify major factors that accelerated vitamin loss during aerobic storage at
ambient temperature. These showed L-ascorbic acid to be destabilised by cupric
ion. Light (at 339 lux) was further shown to affect the influence of cupric ion on Lascorbic
acid, whereby the catalytic activity of cupric ion at trace levels (0.5 ppm)
appeared to be concealed by the effect of light. Riboflavin degradation, by
contrast, resulted directly from the presence of light. Neither cupric ion nor light
affected the stability of thiamine. However, a change in pH from acidity (pH 3-6)
to neutrality (pH 7) caused a marked decline in thiamine stability. L-Ascorbic acid
was more prone to degradation at pH close to its primary pKa value. However,
there was no apparent relationship between riboflavin degradation and pH.
Investigations of between-vitamin influences showed riboflavin to accelerate Lascorbic
acid loss. This effect was apparent whether or not light was present.
However, L-ascorbic acid destabilised riboflavin only in the absence of cupric ion
To optimise multivitamin stability, study was made under acid conditions in
various types of citrate systems with cupric ion and light absent and under reduced
levels of oxygen. Input variables included the levels of vitamin additions and pH.
Using response-surface methodology, quadratic equations could be fitted
adequately to stability data obtained for L-ascorbic acid in citrate-phosphate,
citrate-sucrose, and citrate systems and for both thiamine and riboflavin in citrate-phosphate and citrate systems. L-Ascorbic acid and thiamine stability responses
(as % residue) were lowered by an increase in pH over the pH 2.6 to 3.0 range in
both citrate-phosphate and citrate systems. The stability of riboflavin with pH was
maintained solely in the citrate-phosphate system. Within the same system,
increased additions of both thiamine and riboflavin improved L-ascorbic acid
stability. In citrate, thiamine stability benefited from riboflavin additions only
when L-ascorbic acid additions were low. High levels of addition of L-ascorbic
acid were also directly responsible for thiamine instability. An improvement in
riboflavin stability with L-ascorbic acid additions was observed only with thiamine
added at high levels. In citrate-sucrose, L-ascorbic acid was more prone to
degradation with an increase in thiamine added level at low pH.
Levels of vitamin additions and pH that yield maximum responses in L-ascorbic
acid, thiamine, and riboflavin stability have been predicted using the set of
conditions given by the quadratic models.