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.
All the essential oils the bottler requires for compounding his syrups and flavoring his beverages are subjected to wholesale manufacture and are everywhere obtainable in commerce. When he nevertheless resorts to manufacturing his own essentials oils, and especially so in large establishments, and does not care for the tedious work of manufacturing most of his required flavorings, he probably is aware of the fact that freshly prepared oils impart a richer flavor to his beverages, and that most, if not all, essential oils of commerce are more or less adulterated, and yet are very high priced. But unfortunately the very finest, most expensive, and most frequently adulterated oils, he is bound to buy, and to rely on commercial resources having the material, the fresh plants, fruits or flowers, they are prepared from. While he is thus dependent upon the commercial products for the most fraudulent oils, he does not care to go into arranging an expensive plant for home-manufacturing, and prefers rather also to buy the cheaper oils, which are less apt to be adulterated. These reasons we found predominating amongst the great majority of bottlers. However, there are quite a number of them who manufacture their own oils, and where a carbonator is located in a part of the globe that yields an abundance of plants, etc., the aroma of which is desirable for his beverages, he will find it to his advantage to make them himself.
Whether manufacturing himself or not, the carbonator should be acquainted with the methods, characteristics and origin of the oils he is handling as well as with its adulterations, in order to more easily detect frauds, and principally to guide him properly in all its compoundings. Only when he is thoroughly acquainted with all the resources and properties of the materials he is using, can success in manufacturing carbonate saccharine beverage be attained.
As we have already said, volatile oils are very easily altered; it is therefore necessary that they should be preserved with great care to keep them in good condition. They ought to be placed, when fresh, in vessels that are well filled, and closely stopped, and kept in the dark. It should always be a rule to keep no large stock of oils on hand, for most of them do not keep well, some being more durable than others. The oils of lemon, bergamot, neroli, orange, etc., are specially liable to rancidity. The oils should when possible be kept on a shelf in a closet, in a dry place. Nothing has a more injurious effect on essential oils than light and heat. For this reason it is also advisable to paste colored paper over the bottles in which the oils are kept, or to use amber-colored glass, which excludes the actinic rays, and also keep the bottles in an even, cool temperature, best in a dark cellar, at not above 58° F. When the contents of large bottles have been partly used, so that they are only about one-half full, they should be emptied into smaller bottles, for the air in the bottles has an injurious effect on the oils. It is evident that the corks should also fit tightly; ground glass stoppers are the best.
The addition of a small quantity of strong alcohol, about from one ounce to eight ounces (one-sixteenth to one half the bulk of oil) to sixteen ounces (one pound) of the oil, prevents deterioration or change, and may be considered a good method of preservation. Should it be necessary to filter essential oils there is no difficulty in doing so without dissolving them in alcohol. Use best white filtering paper, fold it and place it in a funnel, which serves as a support for it. The oil easily filters through, and should be protected from the atmosphere by covering the vessel and filtering it in a darkened room.