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Unknown colour instability in toilet soap
Hello chemists,
We manufacture a bar soap (85/15 tallow/coconut oil). The final product contains the chelator tetrasodium EDTA added. The desired colour is a peach shade, something like Pantone 148. The colours we would prefer to use are FD&C Red 4 and FD&C Yellow 5 to achieve this tone. The colour is initially a perfect match but in a period of <12h the colour turns almost a perfect yellow (close to Pantone 120). However, this only happens when exposed to the air. If the soap is in a clear, poly bag the colour will stay true. Normal colour instabilities are not observed if it is in the bag (ie, it is light stable and heat stable @44C, and is not drifting due to pH).
I have tried swapping out each colour individually for another colour (ie Red 40 instead of Red 4) and nothing has stopped this change. Other colours I have tried: Yellow 6 and Orange 4 instead of Red 4; and Yellow 8 instead of Yellow 5. All have the same result. I have also tried substituting the dyes for Lake colours Yellow 5 aluminum lake and Red 30 aluminum lake. This did not improve stability. The addition or absence of chelators from 0-0.5% does not appear to have any effect. Each colour on its own is stable and there are other products on the market that use this colour combination and appear stable. I have also tested this colour combination in a suppliers palm oil based soap and the same effect occurs so I’m sure it is not from our manufacturing process.
I am baffled as I have never seen anything like this happening before and really have no ideas what could be causing this or how else to proceed. Anyone experience or tips would be greatly appreciated.
I appreciate any help,
RDchemist15
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