im distilling essential oil of syzgium campanulatum..it turn into blue colour so fast during hydrodistillation from yellow colour @Noel W Davies

interesting..i will send my sample for GC analysis and will confirm the compound in there and hopefully it will contain chamazulene that contribute to blue colour of the essential oil. Thank you so much.@Noel W Davies

Try to keep EOIs in nitrogen atmospher...

Hydrolysis of esters to alcohols and acids may occur during steam distillation. This will have serious implications in case of ester rich oils and special precaution have to be taken to prevent or at least limit the extent of ester degradation. It is common in Lavender. And Yess, I agree with the above comments on Chamazulene, because it is an unstable compound can be responsible for change in colour.

Good luck with your work.

Not all essential oils are yellow color. There are essential oils of blue or red color, it's depending on the chemotype

yes, I also observed this with lemongrass oil, however, its color changed after several days. Anis Asmi Azman

Check pH before and after hydrodistillation .