Serum Formulation Issue and Solution

Based on the details provided and the expert responses, the stinging sensation you experienced with your homemade serum is primarily due to a high concentration of preservative.

Cause of Irritation

The main issue stems from the fact that Galactomyces Ferment Filtrate (Pitera), which you used as the base at a high concentration (55%), already contains a preservative (typically around 0.5%). When you added an additional 1% of Phenoxyethanol, the total preservative concentration in your final serum significantly exceeded the recommended safe limit of 1%. Concentrations above 1% dramatically increase the risk of skin irritation and sensitivity.

While Alpha Arbutin can also be a potential irritant for some individuals, in this case, the excessive preservative level is the most likely culprit.

Mixing Steps

Regarding the mixing order and shaking the ingredients together in the bottle, for the ingredients in your formula, the order of adding water-soluble components does not significantly impact the final product's stability or effectiveness. The method of mixing by shaking is also acceptable for this type of serum.

Adding Allantoin

Adding Allantoin can help soothe the skin and reduce irritation, which is why you likely felt some improvement after adding the mixture containing it. However, it addresses the symptom (irritation) rather than the root cause (excessive preservative).

Correcting the Existing Batch (Dilution)

Since you have already mixed the serum, the recommended solution is to dilute it to lower the overall preservative concentration. As advised by the staff, you should add distilled water (or your mixture of distilled water, Allantoin, and Glycerin) to the existing serum at a ratio of no more than 1 part added liquid to 4 parts of the existing serum (1:4 dilution).

For example, if you have 100g of the original serum, add a maximum of 25g of distilled water or your water/Allantoin/Glycerin mixture.

Effect of Dilution on Effectiveness

Diluting the serum will inevitably reduce the concentration of all active ingredients (Galactomyces Ferment Filtrate, Copper Peptide, WaterLock™, Alpha Arbutin) proportionally to the dilution factor. This means their individual effectiveness will be reduced compared to the original, more concentrated formula. However, this dilution is necessary to make the serum usable and prevent irritation. The 1:4 dilution is suggested as a balance point to minimize irritation while retaining some beneficial effects from the active ingredients.

In the future, when formulating, calculate the total preservative concentration from all ingredients that contain it (like the standard Galactomyces Ferment Filtrate) and the preservative you add separately (Phenoxyethanol) to ensure the final concentration is within the safe and recommended range (typically 0.5-1.0% total preservative).