This section is from the book "Monograph on Flavoring Extracts With Essences, Syrups, and Colorings", by Joseph Harrop . Also available from Amazon: Monograph On Flavoring Extracts With Essences, Syrups And Colorings.
The depths of low degree in quality or attenuation of strength, we do not pretend to delineate.
Invest ten cents in a small bottle of extract, say of Lemon, and test for proportion of oil present, by taking one hundred (100) parts of the extract, also one hundred (100) parts of different strengths or dilutions of your own make; add definite amounts of water to each, and observe the relative extent or density of milkiness. This, in a crude way, will give you a fair estimate of the amount of oil present in the sample purchased.
This method will apply only to any extract made directly from the oil, as a source of flavoring. Those made from the fruit, seed, leaf or plant direct, cannot be examined in this way. These may be diluted with water in definite amounts, and compared carefully as to odor, with samples purchased, for relative strength.