Preservative Percentage Calculation: Water Phase vs. Total Formula
Question
How should I calculate the percentage of preservative needed in a cosmetic formulation? Should the percentage be based on the water phase or the total weight of the formula?
For example, if a formula contains 70% Witch Hazel (which already has 0.7% preservative), how does this affect the calculation and the amount of additional preservative (like `Phenoxyethanol` or `Mild Preserve`) needed?
Does calculating based on the total formula weight eliminate the need for microbial testing, as suggested for simplifying the process?
Answer
Preservative Percentage Calculation: Water Phase vs. Total Formula
Based on the information provided by cosmeceutical7:
Ideal Method: The most accurate way to determine the correct preservative percentage is through microbial testing (MIC) to find the minimum effective concentration.
Manufacturer Recommendations: Typically, manufacturers provide recommended usage rates based on the percentage of the water phase in the formulation.
Practical Application: In practice, formulators often calculate the preservative percentage based on the total weight of the formula. This is sometimes done to simplify the process and avoid the need for additional microbial testing, especially when other ingredients already contain preservatives. By adding the preservative at the manufacturer's recommended percentage (interpreted as % of total formula), they aim to ensure sufficient preservation without further testing, even if the total concentration might exceed what's needed based solely on the water content.
This approach applies to various preservatives, including Phenoxyethanol and Mild Preserve (Ethylhexylglycerin + Caprylyl Glycol), as mentioned in the discussion.
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Phenoxyethanol (Extra Pure)
