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When your preservative prefers your packaging to your product

September 10, 2021

Preservatives are surface-active.

What you are hoping (or aiming) for is them prefering the inter-face (or surface) between your oil and water phases but that isn’t always what happens.

Behold, exhibit A:

A couple of weeks ago a help desk client sent an enquiry about a preservative they wanted to use but that was causing issues in their formula. I could see the current problem was one of solubility as the preservative that had been chosen was just not mixing into the main formula. I confirmed my suspicions with a few lab tests and also tested out a couple of formula adaptations to provide it a more accomodating environment but then kept the original samples to evaluate over time.

On returning to the samples this week I noticed that a couple of the samples had developed etchings on the side of the bottle, the rest of the formula being crystal clear now and with no sediment. Clearly in these samples the preservative was prefering the packaging to the product and had found a way to precipitate out onto the side of the bottle. I do see this behaviour from an ingredient from time to time and it can happen with other ingredient types too such as essential oils, fragrances and colours.

This situation takes a bit of time to develop. In this particular case we could immediately see the preservative was not in solution so the etching effect developed quickly. However, this situation can and does happen more slowly and quietly, showing up as stability issues down the track either in terms of microbial failure (when the offender is a preservative) or a premature loss of colour or fragrance of a product (for dyes or aromatics).

I often hear people talking about solubility as if it is an absolute thing but it is not. Solubility is always relative: this compared to that. It’s also applied – solubility of ingredients in the formula vs in the packaging, solubility under optimal storage conditions vs under stress. These are some of the reasons we take our time when developing new formulations. This is why stability testing is important and why on-shelf vigilance, especially of the first batch of new products comes in (as not all cases of packaging interaction happen during a standard shelf life test or in standard test conditions).

It can be daunting to know of all the ways your formulations can fail, especially when it’s to do with microbial stability. However, it’s also interesting and empowering when you can spot and fix a problem.

The key is to give yourself enough time and develop enough curiosity and knowledge to turn these unfortunate events into opportunities.

Amanda x

5 Comments leave one →
  1. September 11, 2021 7:49 am

    Good point…..we can become a bit overconfident sometimes and rush these tests under pressure from clients.

  2. Raveena permalink
    September 11, 2021 8:34 am

    Hi Amanda
    As an amateur formulator, your posts are always very helpful and informative. You mentioned there is a certain preservative that can cause the problem but you did not mention the product name. Also concerning solubility, it would be great if you could write another post and test under the stress conditions.

    • RealizeBeautyEd permalink*
      September 12, 2021 12:51 pm

      Hi there, Glad you find the posts helpful. I didn’t name the preservative as I didn’t want people to focus on that in any way as it’s irrelevant. I am planning to write a post with visuals showing solubility progressing through the stages but again, it’s relative not absolute so individuals would have to put their own formulations through the analysis to work out what step in the process they are up to.

      • Raveena permalink
        September 12, 2021 4:33 pm

        Hi Amanda, Thanks for replying. I look forward to your post on solubility. Very much appreciated.
        By the way, I really enjoyed watching your Youtube video on making a serum. You should make some more. Very well explained.

      • RealizeBeautyEd permalink*
        September 12, 2021 4:39 pm

        Thanks. I only made one or two videos like that as I wasn’t sure what I was aiming for with them. I’m much clearer on what I’m trying to communicate now and have more educational content in the pipeline. Stuff that I think is relevant and interesting but that encourages people to do their own work rather than just copy. That’s what I’m interested in 🙂

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