This section is from the "Elixirs And Flavoring Extracts. Their History, Formulae, & Methods of Preparation" book, by John Uri Lloyd. Also available from Amazon: Elixirs and flavoring extracts,: Their history, formulae, and methods of preparation;
In some cases it is desirable that a syrup should froth considerably. Judgment, however, must be employed in adding the frothing liquid, as well as drawing the carbonated water into the syrup, for some syrups are naturally inclined to foam too much. Among our formulae we occasionally direct the use of a frother, and the operator can select from the following that which best suits his taste.
S-67. The white of one egg added to a quart of the syrup specified.
S-68. One ounce of mucilage of acacia added to a quart of the syrup.
S-69. Two drachms of tincture of soap bark (quillaya) added to a quart of the syrup.
The first and second of these have been in use for a long time; the last is a comparatively recent addition. That the first and second are both harmless is evident, and we have as yet heard no complaints concerning tincture of quillaya.
Take of ground or powdered quillaya, 4 ounces.
Alcohol, water, of each a sufficient amount.
Moisten the quillaya with a mixture of alcohol two ounces, water fourteen ounces, and, having allowed the moistened powder to stand an hour to expand, pack it loosely in a percolator. Cover with menstruum, and when it appears at the exit of the percolator cork the exit and allow the mixture to macerate from twelve to twenty-four hours. Then continue the percolation until one pint of tincture be obtained.
This tincture is of an opalescent color and is likely to precipitate by age; it should be kept in a cool locality. It can be made clear by increasing the proportion of alcohol in the menstruum, but this increase of alcohol is at the expense of the frothing power of the product. The larger the amount of alcohol the less its comparative value as a froth producer. One ounce of the foregoing tincture is sufficient for a gallon of syrup.