Solving Stability and Rancidity Issues in Silk Protein Cream

Question

I am formulating a thick silk protein cream. The original formula used to be stable for a long time, but recently, it has been reverting to a liquid state and developing a rancid odor after 3 months of storage at room temperature. The current formula is as follows:

Distilled Water: 79.60%
Silk Protein: 10%
Comedian SP (Pro Polymer™ Silk): ?
Cetiol OE: 3%
Argan Oil: 2%
Allantoin USP: 2%
Zinc PCA: 0.2%
Alpha Arbutin: 0.2%
Fragrance: 1%
Preservative: 0.5%
Vitamin E: 0.5%

I don't have specific details on the production process (pH, temperature, sterilization) or batch size yet.

Questions:

Should Disodium EDTA be added to improve stability?
What are the recommended ways to fix the stability of the cream texture and the rancidity issue?

MySkinRecipes Expert · Accepted Answer

Analysis of Formulation Stability and Odor Issues

The issues you are experiencing—the cream turning liquid and developing a rancid smell—are common challenges when working with complex active ingredients and natural oils. Here is a detailed breakdown of the causes and recommended solutions:

1. Texture Loss (Reverting to Liquid)

The primary reason your cream has thinned out is an incompatibility between your thickener and the electrolytes in your formula.

Electrolyte Sensitivity: If you are using Pro Polymer™ Silk (ID 832), it is important to note that this specific polymer is not electrolyte-resistant. Ingredients like Zinc PCA (ID 75) are electrolytes that disrupt the polymeric network of Pro Polymer™ Silk, causing the gel structure to collapse and turn liquid over time.
Solution: Switch to Pro Polymer™ (ID 234). Unlike the Silk version, the standard Pro Polymer™ is highly electrolyte-resistant and can maintain a stable, thick viscosity even in the presence of Zinc PCA.
Allantoin (ID 184): Note that if Allantoin is used at 2%, it exceeds its typical solubility limit (~0.5%). This can lead to precipitation or "grittiness," which may also affect the overall stability of the emulsion.

2. Rancid Smell (Oxidation and Spoilage)

A rancid smell after 3 months usually points to two possibilities:

Oil Oxidation: Argan Oil (ID 65) is a natural oil prone to oxidation. To prevent this, you must include an antioxidant like Vitamin E (Tocopherol) (ID 68) at 0.1-0.5% to protect the oil from turning rancid.
Protein Spoilage: You mentioned using Silk Protein (ID 89) at a high concentration (10%). Proteins are highly susceptible to microbial growth. If the preservative system (ID 995) is not robust enough or the pH is not optimal, the protein can spoil, leading to a foul odor.
Solution: Consider reducing Silk Protein to a more standard range (1-3%) or significantly strengthening your preservative system.

3. The Role of Disodium EDTA

Adding Disodium EDTA (ID 252) at 0.1-0.2% is highly recommended for this formula:

Prevents Oxidation: It chelates metal ions that catalyze the oxidation of Argan Oil, helping to prevent rancidity.
Boosts Preservation: It improves the efficiency of your preservative (ID 995), which is vital when using high amounts of protein.
Stabilizes Polymer: It helps protect the polymer structure from mineral interference, further ensuring the cream stays thick.

Summary of Recommendations

Replace Pro Polymer™ Silk with Pro Polymer™ (ID 234) for better electrolyte resistance.
Ensure Vitamin E is present to protect the Argan Oil.
Reduce Silk Protein to 1-3% to minimize spoilage risk.
Always include Disodium EDTA (0.1-0.2%) to enhance overall stability.