[Footnote: _The Brewers' Guardian_, from the _Zeit. f. d. gesammte
Brauwesen_.]
By DR. H. BUNGENER.
Little that is definite is known of the substance or substances to
which the hop owes its bitterness. Lermer has succeeded, it is true, in
separating from hops a crystallized colorless substance, insoluble in
water, an alkaline solution of which has a marked bitter flavor, and
which easily changes on exposure to the air, assuming a resinous form.
According to Lermer, the formula of this substance is C_{32}H_{50}O_{7};
it possesses the properties of a weak acid and forms a characteristic
copper salt, which is soluble in ether. This hop bitter is, however,
produced from the hop by a very roundabout process, by treatment of the
extract with alkalies; it is not therefore regarded by many as present
in this form in the hop, and they hold that it is only produced by
the action of the alkalies. On the other hand, however, Etti, by a
complicated extracting process, but without using an alkali, succeeded
in producing a bitter substance from hops, which is, however, soluble in
water.
Several experiments convinced me that there really existed in hops a
crystallizable substance, insoluble in water, the alcoholic and alkaline
solution of which had a bitter flavor, in short, which possessed all
the properties of Lermer's hop bitter acid. Petroleum ether is the best
practical solvent in use for its isolation, as it does not dissolve
the majority of the remaining constituents of the hop, especially the
hop-resin, which they contain in considerable quantity. 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 following process has furnished me the best results:
The lupuline is first freed from gross impurities (hop-seed leaves,
etc.), and then covered with petroleum ether boiling at a low
temperature (40° to 70°) in stoppered flasks. The mixture is shaken up
from time to time. After twenty-four hours, by means of a Zullowsky
filter immersed in the mass, and with the aid of a suction-pump, the
dark brown solution is drawn off; then fresh ether is poured on to the
lupuline, and it is allowed to stand for another twenty-four hours.
After this process has been three times repeated, nearly everything the
petroleum will dissolve has probably been extracted. The solutions are
put together, and the petroleum ether distilled off _in vacuo_ at a low
temperature, until there remains in the flask a dark brown sirup, which
on cooling solidifies into a crystalline mass. This is pulverized and
turned on to a filter composed of a large funnel, in which a smaller
funnel covered with muslin is inserted. With the aid of a suction-pump,
the greater portion of the thick, crude solution can be filtered
through. There remains on the filter a highly colored crystalline
"cake," which should be pulverized with a small quantity of petroleum
ether and again filtered. 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.
If we do not wish to utilize this crude substance at once, it will be
necessary to melt it in the water bath and pour it into a bottle under
close seal, where it will at once crystallize and solidify. If it
remains exposed to the atmosphere, it will soon become sticky and
turn partly into resin. 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. Our
object is more readily attained if we crystallize it once from alcohol,
for which purpose we dissolve it in a little lukewarm alcohol, then
quickly cool the solution; flakes of a fatty substance will be
separated, which are removed by filtration with the aid of a
suction-pump. Then we throw a few small crystals of the acid into the
solution, and after a short time crystallization commences. As soon as
it appears to be ended, the mother solution is removed with the aid of a
platinum cone, and the crystals washed with a little cold alcohol. The
alcoholic mother solution, which still contains the chief part of the
bitter acid, must be quickly evaporated, and the residue consigned to
a flask. The acid crystallized from the alcohol is then recrystallized
several times from petroleum-ether. In order to quickly dissolve the
bitter substance, it should be carefully melted in a flask, and double
its volume of ether gradually added; on its cooling, we obtain beautiful
prismatic crystals, which attain a length of 1 cm., and become perfectly
pure after four or five crystallizations. 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° to 93°. 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.
In the analysis I obtained figures which correspond best with those
calculated from the formula C_{25}H_{35}O_{4}.
Obtained.
Calculated. ------------------------^-----------------------
-----^----- 2. Crystal. 3. Crystal. 5. Crystal. 6. Crystal.
p.c. p.c. p.c. p.c. p.c. p.c. p.c.
C 75.19 74.79 74.83 74.9 75.04 75.05 75.07
H 8.77 8.97 8.90 8.85 8.87 8.83 8.80
O 16.04
If we shake up the ether solution of bitter substance with an aqueous
solution of acetate of copper, the ether will assume a green color, and
gradually deposits a green crystalline powder, a cupreous combination
of the bitter acid. It is difficult to obtain in a pure state, as the
solutions are readily subject to slight decomposition, accompanied by a
small deposit of copper oxide. 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_{50}H_{68}O_{8}Cu. This combination will contain 69.87 per cent. C,
7.91 per cent. H, and 7.33 per cent. Cu. The figures obtained do not
perfectly coincide with those calculated; it is nevertheless probable
that the formula is correct, and the combined substance analyzed was not
perfectly true.
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. The dark-colored
mother solution, from which the crystalline cakes of bitter acid are
obtained, contains a large proportion of this resinous compound, which
can be isolated by treatment with a weak soda-lye; this substance, like
the crystallized acid, is soluble in alkalies, and can be precipitated
from an alkaline solution by an acid. 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. At
the same time, a more pronounced odor of fatty acids and aldehydes is
apparent. Still more rapidly will this oxidation occur if a thin layer
of an alcoholic solution of the acid is allowed to evaporate in the air.
On the other hand, we can allow hop-oil to stand for days without its
odor being perceptibly changed; it appears to me more than probable that
the peculiar smell of old hops is due far more to the oxidation of the
bitter substance than to the oxidation of oil.
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.
The question as to whether the hop owes chiefly to this acid and its
resinous modifications the property of imparting a pronounced bitter
flavor to a solution, I must for the present leave unanswered. The acid
and its isomer are both insoluble in water; they are, on the other hand,
very readily dissolved in hop oil; they also furnish a tolerably bitter
solution, if boiled for a long time in water, probably on their account
of their gradual decomposition. I will not for the present go further
into the subject, as I hope soon to be in a position to give more
definite information.
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