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10% SLS at pH 5.5 passes microbial stability test. Why?
Hi, everyone!
Can someone advise, please?
I made a 10% SLS solution by dissolving the powder at 60 C in water and brought it to pH ~ 5.5 with citric acid. I did a microbial stability test on it, and it passed. I tried the same with 5% sodium coco-sulfate and got the same result. I bought a “pure” SLS from Sigma Aldrich, and the same thing happened. After a year on the shelf, the 10% SLS was still preserving. I figured I must be doing something wrong, or no stuff such as DMDM Hydantoin would be added to shampoos. Did you observe the same?
Thank you,
Irena
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8 Replies
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USP 51/ISO/EP style test is not that hard a challenge - it merely shows there’s some degree of hostility. It is not a validated to anything.
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Thanks, Phil. So, what test to do to show the difference in microbial stability between the preserved/unpreserved?
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The objective of product preservation is to protect consumers. The objective of raw material preservation is to get it to the customer clean.
For product, you might try the CTFA (PCPC) test - it’s not validated either but isn’t so much a pushover as 51. For either - you could use compete kill at 7 days as your standard rather than the current specs. There are also “validated” protocols but not within the capabilities of any but larger companies.
For a raw material - it’s a function of making/packing hygiene and some biocidal intervention (preservative, heat, etc.). For SLS, 70% material, high pH, or preservation works.
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Thank you, Phil. I appreciate it very much.
I’ll try one of the PCPC protocols; product preservation is my goal.
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and look for complete kill (not just reductions), hopefully by day 7 and certainly by day 14.
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Got it! Thank you very much again, Phil!
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My 10% SLS power needs 16% of a 50% citric acid solution to reduce the pH to 5.5. how much citric acid did you use?
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Nowhere nearly as much. It takes about 0.4% of 50% citric acid.
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