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 plants contain one or more principles, which, though originally colorless, are very easily altered under the influence of air a ad heat, acquiring a yellow or brown color. It is not known whether the so-called colorless extractive is alike in all plants, nor is its composition known or the nature of changes produced under the conditions mentioned, except that the heat of boiling water and the prolonged action of oxygen will convert it ultimately into a blackish insoluble substance, to which the name apotheme has been given, and which appears to be allied to humin. Extractive is almost insoluble in absolute alcohol and ether, but dissolves freely in weaker alcohol and water, and is removed from its solutions by animal charcoal and hydrate of aluminium, the more readily after it has become colored by oxidation. It is with difficulty freed from all admixtures, and the terms sweet, bitter, acrid, etc., extractives refer to the same body in a more or less altered condition, combined or intimately mixed with other principles, to which the peculiar taste is due. The injurious influence of air and heat upon the vegetable juice is mainly confined to the alterations of this extractive, and extends in a limited degree only to the majority of the well-defined principles. Its effects have often been much overrated, except as regards the appearance of the extracts. The color of the different extracts and fluid extracts varies with the nature of the drug from which they have been made, but should never be black. Fluid extracts should preserve the taste and also the odor of the drug, except in so far as both may be modified by the menstruum. The characteristic taste, and to some extent also the odor, of the drug should be perceived in the extracts, and these should yield a nearly clear or moderately turbid solution of the menstruum used in their preparation. Not to come up to these requirements is indicative of carelessness, and the presence of empyreumatic products proves the operation to have been slovenly". - N. D.