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Why do candles turn yellow?

Vanillin is one of the components that contribute to the distinct aroma of vanilla.  It is an organic crystal that forms on the outside of the vanilla bean.  Most, but not all, Vanillin is synthetically produced.
Ethyl Vanillin is a synthetic chemical which is similar in structure to Vanillin. It is 3 times stronger in potency than Vanillin, but it is slightly different with regard to the note.

Fragrance oils that contain Vanillin or Ethyl Vanillin will oxidize faster than fragrance oils that do not contain this ingredient. Oxidation causes the fragrance oils to become darker in color over time.  To some candlemakers and soapmakers this isn't a big deal if you are using dye in your candles or soap or if you want to achieve a darker color naturally.  However, if you are not using dye and want your candles and soap to stay light in color, this could be a problem over time.  The darker color could show up after only a week of your candles or soap curing or it could take much longer.  It just depends on the chemistry of the specific products you are using.  Having your finished product appear darker in color does not affect its scent throw, it is simply important for the appearance you are trying to achieve.

Fragrance oils that are higher in Vanillin content may also crystalize when the weather is colder.  The Vanillin portion of the fragrance will appear as small white crystals.  All that needs to be done is to put the tightly sealed fragrance bottle in very warm water and the crystals will melt and mix back into the fragrance oil.  The integrity of the fragrance is not compromised; it simply separates if the temperature gets too cold.