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Hop-bitter acid appears to possess the character of an aldehyde and of a weak acid; for the present I am not in a position to state its constitution more clearly. Most of the oxidizing processes have an energetic effect on it, forming also considerable quantities of valerianic acid.
After this operation has been repeated three or four times, we obtain an almost colorless mass, consisting of hop-bitter acid, contaminated by small quantities of a fatty substance, and a substance which I could not isolate, and which I had at first great trouble in separating from the hop-bitter acid.
The mother solutions must be speedily evaporated if we still wish to obtain crystals; after a time they will only furnish a resinous residue. The hop-bitter acid melts at 92 deg. to 93 deg.. It is easily soluble in alcohol, ether, benzol, chloroform, sulphide of carbon, and vinegar; to a lesser extent in cold petroleum ether, and not at all in water.
Old hops furnish far less crystallizable acid than new hops; from some samples I have been able to obtain only a few crystals; the remainder had been transformed into the resinous modification. If pure hop-bitter acid be pulverized and exposed to the atmosphere, it soon turns yellow and the surface assumes a resinous consistency.
Six kilos of lupuline, which included a large proportion of sand, furnished 400 grammes of crude hop-bitter acid. The first experiments in crystallization with petroleum ether gave poor results; it is difficult to produce the acid pure in large quantities by this process, as a small quantity of the above substance obstinately clings to it, and it readily assumes a non-crystallizable form.
Still, the extraction of hop-bitter acid from hops is a troublesome and thankless job, the petroleum ether taking up certain substances which add greatly to the difficulty of purifying the crystals. On the other hand, we can readily and quickly attain our object, if we employ for our original material fresh lupuline from unsulphured hops. The mixture is shaken up from time to time.
I have already referred to the fact that solutions of hop-bitter acid, if left standing too long, assume a yellow color, and on evaporation leave only a yellow resinous residue. This, as its reaction shows, evinces a complete analogy with the crystallized acid.
This combination is readily soluble in alcohol, to a lesser extent in ether, and is insoluble in water. In the course of analysis, I obtained the following figures: C 69.4 per cent. 69.3 per cent. H 7.95 " 7.98 " Cu 7.20 " 7.18 " If we suppose that the copper combines with two molecules of hop-bitter acid, by the decomposition of one of its atoms, H, we obtain the formula C H O Cu.
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