Preservative Calculation in Formulations
Question
When formulating, if one ingredient (Ingredient A) already contains a preservative (Preservative E) at a certain concentration, and I add Preservative E separately to the final formula to reach a target total concentration (e.g., 1%), should I account for the amount of Preservative E already present in Ingredient A when calculating the total amount of Preservative E needed? I've observed that typically, formulators calculate the preservative amount based on the water phase only (e.g., 1% of the water phase), assuming other ingredients (like oils or those with preservatives) don't need extra preservation or already contribute enough. For example, in a formula with 30% other ingredients and 70% water, they might add 1% preservative based on the 70% water, resulting in 0.7% of the total formula. Is this common practice technically correct?
Answer
Preservative Calculation in Formulations
Regarding your question about calculating the total amount of preservative 'e' when one of your ingredients ('a') already contains it:
Technically Correct Approach: Yes, if you want to be precise, you should account for the amount of preservative 'e' already present in ingredient 'a'. You would calculate the total amount of 'e' contributed by ingredient 'a' (based on its concentration in 'a' and the percentage of 'a' in your final formula) and subtract that from the desired final concentration of 'e' (1%) to determine how much additional 'e' you need to add.
Practical Approach (as explained by cosmeceutical7): In practice, formulators often do not calculate this precisely. A common simplified method is to calculate the preservative amount based on the water phase of the formulation, assuming that oil-based ingredients or those already containing preservatives do not require additional preservation or already contribute sufficiently. For example, if your formula has 70% water, you might calculate the 1% preservative based on that 70%, effectively adding 0.7% of the preservative to the total formula.
In summary: While the technically accurate method is to include the preservative amount from ingredient 'a' in your calculation, the practical approach often involves simplifications, such as calculating based on the water phase. The best method depends on the desired precision and the specific requirements of your formulation.