Fat-soluble and water-soluble vitamins need different handling, and different specs

Vitamin stability is not one problem. Fat-soluble vitamins fail by oxidation; water-soluble ones fail mostly through moisture and pH. The controls that protect one do very little for the other.

Fat-soluble: A, D, E, K

These degrade primarily through oxidation, accelerated by light, heat and metal ions. Commercial forms are engineered around that:

• Beadlets and spray-dried powders — the vitamin dispersed in a matrix with antioxidants, gelatin or starch. The matrix is the protection, and it is why these materials are typically 50% or less actual vitamin by weight.
• Esterified forms — retinyl palmitate and tocopheryl acetate are more stable than the free alcohols, which is why they dominate.
• Potency in IU or mg — vitamin A and D are still often expressed in IU, and the conversion differs by form. State both, and the form, or the arithmetic will go wrong somewhere downstream.

Water-soluble: B complex and C

These are dominated by moisture, and secondarily by pH and by interactions with each other:

• Vitamin C is the least stable of the common actives. Coated ascorbic acid and mineral ascorbates both trade some potency per gram for a great deal of stability.
• Folate and B12 are present at tiny inclusion levels and are light-sensitive and moisture-sensitive, which is why they are supplied as triturations — pre-diluted on a carrier so they can be distributed through a blend at all.
• Thiamine is unstable at neutral and alkaline pH, so its behaviour depends heavily on what it is blended with.

Interactions inside the blend

Vitamin C and copper together will destroy each other's assay faster than either degrades alone. Iron accelerates the loss of A, C and folate. Separating reactive components — by coating, by granulation, or by putting them in different layers or different products — is standard practice, and it is a formulation decision that has to be made before the premix is specified, not after.

What to require from a supplier

• The specific form and salt, not just the vitamin name.
• Potency with the assay method, and the IU-to-mg conversion where applicable.
• Coating or matrix composition, which becomes part of your ingredient statement.
• Recommended storage conditions and stability data supporting them.
• Packaging — foil-lined with a desiccant for the hygroscopic materials, and a statement of whether it was nitrogen-flushed.

Store to the specification you paid for

A well-chosen stabilised form is wasted if the drum is opened repeatedly in a humid warehouse. Whatever storage condition the stability data assumes is the condition the material actually needs — the specification is a promise about handling as much as about the ingredient.