Vitamin C (SAP) Serum Stability and Odor Issues

You're experiencing common challenges when combining Vitamin C SAP and L-Glutathione Reduced in a single formula.

Stability (Yellowing)

The yellowing, especially at the neck of the bottle, is a sign of oxidation. This is primarily due to the inherent instability when combining Sodium Ascorbyl Phosphate (Vitamin C SAP) and L-Glutathione Reduced in the same formula.

• Conflicting pH Requirements: Sodium Ascorbyl Phosphate (Vitamin C SAP) is most stable in an alkaline environment (pH 7-9). L-Glutathione Reduced, however, is most stable in an acidic environment (pH 3-6) and sensitive to heat above 40°C. When formulated together, the pH must be a compromise, which destabilizes both ingredients, leading to faster degradation and yellowing.
• Oxidation: Exposure to air and light accelerates the oxidation process, causing the yellowing, particularly noticeable at the bottle's neck where air exposure is highest.
• Solution: The most effective way to achieve stability for both active ingredients is to formulate them into two separate serums.
  • Serum 1: Vitamin C SAP formulated at its optimal pH of 7-9.
  • Serum 2: L-Glutathione Reduced formulated at its optimal pH of 3-6.
  • This allows each ingredient to be in its most stable environment.
• Packaging: Using opaque, airless pump bottles can significantly reduce exposure to light and oxygen, further improving stability.
• Chelating Agents: Including chelating agents like Disodium EDTA and Sodium Gluconate is beneficial as they help neutralize metal ions that can catalyze oxidation. Your current formula includes these at appropriate levels.

Odor

The strong odor is likely coming from the L-Glutathione Reduced.

• L-Glutathione Reduced has a natural sulfurous smell.
• While you tried using Odor-Kill 1%, it may not be sufficient to completely mask the odor, especially at a 1% concentration of Glutathione.
• Solutions:
  • Consider using a deodorized form of L-Glutathione Reduced if available.
  • Adding a small amount of cosmetic-grade fragrance can help mask the smell, but be mindful that fragrance can sometimes impact formula stability or cause skin sensitivity.

In summary, the primary issue is the incompatibility of Vitamin C SAP and L-Glutathione Reduced in a single formula due to their different optimal pH ranges. Separating them into two distinct products is the recommended approach for maximum stability and effectiveness.