10% SLS at pH 5.5 passes microbial stability test. Why?

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10% SLS at pH 5.5 passes microbial stability test. Why?

Posted by irenaj on March 9, 2024 at 5:29 am

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

irenaj replied 1 month, 2 weeks ago 3 Members · 8 Replies
8 Replies

PhilGeis

Member
March 9, 2024 at 5:51 am

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.
- irenaj
  
  Member
  March 9, 2024 at 6:29 am
  
  Thanks, Phil. So, what test to do to show the difference in microbial stability between the preserved/unpreserved?
  - PhilGeis
    
    Member
    March 9, 2024 at 7:23 am
    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.
    
    This reply was modified 1 month, 2 weeks ago by PhilGeis.
    - irenaj
      
      Member
      March 9, 2024 at 9:23 am
      
      Thank you, Phil. I appreciate it very much.
      
      I’ll try one of the PCPC protocols; product preservation is my goal.
      - PhilGeis
        
        Member
        March 9, 2024 at 1:32 pm
        
        and look for complete kill (not just reductions), hopefully by day 7 and certainly by day 14.
        
        irenaj
        
        Member
        March 11, 2024 at 4:04 am
        
        Got it! Thank you very much again, Phil!
Abdullah

Member
March 13, 2024 at 1:08 am

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?
- irenaj
  
  Member
  March 13, 2024 at 5:52 am
  
  Nowhere nearly as much. It takes about 0.4% of 50% citric acid.