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Out of the frying pan, into the fire! Sorbic acid in stearate creams!
Hello everyone,
I am new to semi-solid formulations and recently I tried to change my paraben preserved cream formula into a “more natural” one by replacing parabens with sorbate. My emulsifier system is very simple: a combination of 0.5% polysorbate 80 and stearic acid/NaOH system. BHT 0.02% is used as an antioxidant. Phenoxyethanol is added as an anti-bacterial preservative. Final pH of cream is 6.0. These are the observations/problems I have come across:
(1) Potassium sorbate as well as sorbic acid has a dead mouse odour (I am using analytical grade material >99% pure). Fragrances mask this odour only for a few minutes after application. The dead mouse odor is very lingering on the skin
(2) Potassium sorbate causes perceptible yellowing of the cream within 2 months of manufacturing. The yellowing is slightly less when 0.1% citric acid is added but not totally absent. Will adding disodium edta help?
(3) Potassium sorbate (equivalent to 0.5% sorbic acid) completely changes the rheology of my cream. It used to be a pliable, soft non-flowing semi-solid with parabens or no-preservative at all. Now it is a thick flowing liquid (and I am worried about possibilities of phase separation). Using sorbic acid, the cream was less flowable than with the pot.sorbate formula, but still not as thick/stiff as it was in formulae with parabens.
Is anyone else facing this problem? And is there anyway to solve it without kicking sorbic acid out of the formula? Since the final pH is 6 (probably due to stearate/stearic acid buffer system), using benzoates is out of question.
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6 Replies
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Forgot to add: Sorbic acid has a more perceptible malodour as well as the yellowing effect that pot.sorbate.
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I just checked my potassium sorbate stock, pink granules, and it certainly does not smell like a dead mouse, in fact it smells slightly fruity.
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Sorry, I did not word it correctly. Formulae with sorbic acid have a malodour. This problem is especially noticeable with sorbic acid than with potassium sorbate.
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Just saw the five rules by “Iaskedbetter” about posting formulating problems. And remembered that I left out the processing details. Here they are:
Heating the oil phase and aqueous phase separately to 80 degC. Adding oil phase slowly to aqueous phase at that high temperature with stirring. Stirring till temperature reduced to 40 deg C. product allowed to set for 48 hours at 30 deg C before taking rheological/odour-related observations.
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1. Yes your system could be pH sensitive.2. pH 6 is too high for potassium sorbate to work effectively3. You could try the Euxyl 9010 instead since you already have phenoxyethanol in there.4. I have never experienced the smell you are describing -
Thank you Mr. David. I shall try it as soon as I can get hold of a sample. I hope the antimicrobial efficacy would be at par with a Methyl paraben and propyl paraben mixture (0.18 + 0.02%).
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