Microbiomes, Enzymes & Spray Package Corrosion
Written on: July 1, 2023 by W. Stephen Tait
Hello, everyone. “Natural, sustainable ingredients” and “Green ingredients” are the current guiding phrases when formulating new products and derivatives of existing product bases. We are all used to “natural” ingredients in spray products; indeed, the quintessential natural formula ingredient is a fragrance. More recently, probiotics (microbiomes) and enzymes, to name a few, have been discussed as new sources for natural, sustainable and Green ingredients.
I’m not a biologist or a biochemist, so I can only provide a very basic overview of these ingredients. There are also many different types of natural ingredients currently being researched for skin and personal care formulas, so I’ve narrowed down my discussion to probiotics (microbiomes) and enzymes.
Probiotics are various types of benign bacteria, algae and yeast/fungi. Older medical research on probiotics indicates they might improve gastrointestinal health; more recent research on both cosmetic and personal care products indicates that probiotics might also contribute to skin and oral health.
Both UV light and oxygen can damage skin, resulting in visible aging. UV light damages skin DNA. Skin oxidation is similar to metal package corrosion, but is also produces free radical molecules that can cause visible skin aging. Photolyase enzymes repair skin DNA damage by UV while antioxidant enzymes neutralize the free radicals produced during skin oxidation.
What do these natural and sustainable ingredients have to do with spray package corrosion? Microbe colonies on surfaces can induce corrosion referred to as MIC (microbial induced corrosion). MIC is a complex type of metal corrosion that occurs under microbial colonies when formed on both uncoated metal and coated metal surfaces.
MIC is typically very rare in spray packaging. Indeed, I’ve only seen a few instances of MIC that have caused spray package failures (perforations). The main attributes of MIC are:
• MIC-causing microbes are attracted by the electrical fields around metal grain boundaries.
• MIC microbes fit into the cavities between the metal grain boundaries, making them prime sites for microbes to attach on surfaces and subsequently grow into colonies.
• Microbial colonies produce organic acid metabolic wastes, such as lactic acid and hyaluronic acid. Organic acids are typically electrochemically active and thus could contribute to or cause package metal corrosion.
Consequently, the organic acid metabolic waste from colony-forming microbes could also contribute to or cause package metal and polymer corrosion via the MIC corrosion mechanism.
Skin is a polymer, much like the internal coatings in spray packages or the laminate used on aluminum foils to fabricate bag-on-valve packaging, ointment tubes and squeeze tubes. As a result, natural ingredients, such as enzymes that modify skin chemistry, might also modify the chemistry of internal coatings and laminated foil packages. In some instances, polymer/laminated film modification by natural ingredients degrades the barrier-protection properties of a polymer coating or laminated film, thus causing or contributing corrosion of both the coating/laminate film and/or the substrate metal/metal foil.
Research of probiotics and enzymes for personal care and skin care products is still relatively new. The technical issues with these new ingredients in spray packages include:
1. What are their effective concentrations?
2. How to keep the live natural ingredients, such as probiotics, algae and microbiomes alive in spray packages, particularly with formulas incorporating biocides that prevent growth of harmful microbes;
3. How to keep natural ingredients, such as enzymes, from reacting with other formula ingredients and degrading with age inside the spray package; and
4. When do these new ingredients cause or contribute to spray package corrosion?
Much like fragrances, other natural ingredients will not always cause corrosion. However, corrosion by natural ingredients is unpredictable, and corrosion testing with formulas using any new ingredient is essential to avoid unexpected spray package corrosion with subsequent package failure.
In other words, corrosion testing is recommended to determine long term product-spray package corrosion compatibility for all new formula ingredients and new formula ingredient chemistries.
The following shortlist provides a few, select papers and articles on natural biologicals for personal care and skin care products. The list is not intended to be comprehensive.
1. Mei-Chiung Jo Huang & Jane Tang, Microbiology Discovery, Volume 3, Article 5, 2015 (probiotics)
3. COSSMA Issues 04-2023 and 05-2023 (articles on probiotics and enzymes)