Adding the preservative

The preservative goes in late, in a small quantity, and it is the ingredient whose correct addition matters most. It is also the one most often added wrongly.

The last thing in, and the most important

By the time it is added the batch is cool, the pH is set, and everything else is present. That sequencing is deliberate: the preservative needs to enter a finished environment, because it is the finished environment it has to protect.

Temperature and pH first

Two preconditions, both of which are commonly violated.

Temperature. Add at or below the supplier's stated maximum. Adding to a hot batch because it is more convenient can degrade the preservative, and a degraded preservative is worse than none because the label says the product is protected.

pH. Set it first. Most preservatives are only active within a range, and adjusting the pH afterwards can move the system outside it. A preservative outside its range is present, listed, paid for and inert.

Getting it evenly distributed

A small quantity into a large batch needs proper distribution, and stirring it in at the top is not it. Pre-dilute into a portion of the batch or into a compatible solvent, then add that back with the mixer running.

Uneven distribution means some of the product is under-preserved, and it is not the part you will sample.

What this step cannot do

It cannot rescue a batch that is already contaminated. There is no kill step in skincare manufacture, and adding preservative to a compromised batch leaves you with a contaminated product containing preservative.

It also cannot substitute for evidence. The only thing that establishes whether this quantity, in this formula, at this pH, in this package, actually preserves is a challenge test on the finished product.

References