This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
These artificial tinctures, if vanillin or coumarin of the proper strength has been obtained and employed, are about three times as strong as the ordinarily made tinctures from vanilla beans, besides being much cheaper in cost. It is then not advisable to make a stronger solution of vanillin or coumarin, as the admixture of small quantities of a too much concentrated flavor leads frequently to errors, and imparts often a too pronounced vanilla flavor, undesirable and not intended, where the harmonizing of flavors is the object of a combination. Weaker flavorings are for the latter purpose more desirable, and as vanilla flavor enters as an admixture into many others, we find a weaker solution more convenient. However, in case an inferior grade of vanillin should be employed, the strength will be reduced by its inferior quality, except the proportions of vanillin are doubled or tripled, which may be done if desired.
Examination of Vanilla or Vanillin Extracts, etc., for Coumarin. - Heppe proposes to add to an alcoholic solution a few drops of tincture of iodine, and let evaporate spontaneously. When coumarin be present a greenish-golden color appears.