A. Chaston (Alfred Chaston) Chapman.

The hop and its constituents. A monograph on the hop plant online

. (page 7 of 9)
Font size
QR-code for this ebook

ISSLEIB also gives the analysis of a barium salt of lupulinic
acid, and of some other resinous compounds.

OTT/ in general, confirms LERMER'S findings concerning his
hop-bitter acid, but detects some difference in the reactions of the
ether soluble copper salt, which he finds readily decomposable by
hydrogen sulphide, whilst LERMER affirms it to be stable.

BuNGENER's 8 researches on hop-bitter acid marked a distinct

1 Archiv der Pharmacie, 1880, 216, 345-363.

2 Zeitschrift f. d. ges. Brauwesen, 1880, 201.

3 Bulletin de la Societe Chimique de Paris, 1886, 45, 487-496 ; also Zeitschrift
. d. ges. Brauwesen, 1884, 93.



75



advance in the study of the components of hops. He obtained, by ex-
traction of lupulin with light petroleum, a highly crystalline substance,
very soluble in most organic solvents, but absolutely insoluble in
water. Its alcoholic solution tasted intensely bitter. It melted at
92-93 C., and he assigned to it the formula CggHggO^. (This
ought to be doubled, because of the uneven number of H atoms.)
According to BUNGENER, the substance has the character of an
aldehyde and of a weak acid ; and he regards as not improbable the
view that it is a condensation product of five groups of the amyl
series, because, on oxidation, the substance yields valerianic acid.
This last point was, however, not proved experimentally.

GRESHOFF 1 repeated BUNGENER'S work, but was unable to
obtain a crystallisable body from lupulin by extraction with petro-
leum-ether. Working on the assumption that hop bitter is an alkaloidal
substance, he applied various methods (Stas-Otto, Dragendorff, &c.)
utilised in the isolation of alkaloids, but did not succeed in obtaining
a crystalline bitter substance.

HAYDUCK 2 extracted hops with ether, and isolated three readily
distinguishable resins, two of them bitter, the third tasteless. The
former he designated a- and /?-, the latter as y-resin. Under suitable
conditions microscopic crystals separate from the a- and /3-resins,
being evidently related to them in the sense, as HAYDUCK supposes,
that the resins are derived from the crystalline substances, the latter
being the oft-found bitter substances of acid character. The tasteless
y-resin is possibly derived from the hop oil. The crystalline bodies
from HAYDUCK'S resins are described as a- and /3-hop-bitter acids
respectively. N

LINTNER and A. BUNGENER S subsequently prepared the a-resin
by HAYDUCK'S method, and isolated therefrom, by repeated
precipitation and decomposition of the lead compound, the crystal-
lisable a-acid. In alcholic solution it was found to possess a strong,
bitter flavour and acid reaction. The acid differs in composition and

1 Inaug. Dissert, Jena, 1887, 36.

2 Wochenschrift fur Brauerei 1888, 937 et seq.

3 Zeitschrift f. d. ges. Brauwesen, 1891, 357.



76

solubility from the bitter substance obtained by H. BUNGENER, the
difference being particularly marked in the form of the crystals.
The a-acid crystallises in small rhomboids, whereas the /3-acid (H.
BUNGENER'S) forms long, massive prisms, or fine, felted needles.

SEYFFERT 1 who also investigated the hop resins later on, slightly
modified HAYDUCK'S method, and gave some details concerning the
ether-copper reaction of the a-acid, which he was able to obtain quite
pure by means of ice-cold petroleum. He proposes the same solvent
for the /2-acid (crystals from the /?-resin).

In addition, SEYFFERT refers to a crystalline body, which he
obtained from the y-resin, but gives no details as to its properties.

In continuation of these researches, and in collaboration with
V. ANTROPOFF 3 , he proposed an improved method for the preparation
of the /3-hop-bitter acid, particular stress being laid on the exclusion
of air (working in a current of carbon dioxide) and the avoidance of
high temperatures.

The different preparations obtained by the above, as also by
BUNGENER'S method, all exhibited very ill-defined melting points,
varying distinctly from that (92-93 C.) founded by BUNGENER.
Apart from this, the variations in the carbon and hydrogen per-
centages, indicated that impure substances were being dealt with,
According to these investigators LERMER'S bitter-acid is not identical
with the a-acid, but the body described by them as the a-acid,
although possessing faintly acid properties, to all appearances
belongs to a very different class of substances. According to
them the hop resins are a mixture of bodies undergoing continuous
and increasing decomposition.

If we stop now to consider the results of the preceding investiga-
tions, several inconsistencies become apparent. The chemical nature
of the bitter substances remains quite undecided, and not even the
elementary composition and molecular weights of the bitter acids are
known with any degree of precision. Only so much is certain, that

1 Zeitschrifi f. d. ges. Brauwesen, 1892, Nos. 4, 5 and 6.

2 Zeitschrift /. d. ges. Brauwesen, 1896, Nos. i and 2,



77



there are two crystallisable bitter substances of slightly acid
character ; further that hops contain at least three resins, of which
two are bitter and one is tasteless. It is highly probable that the
bitter resins are closely related to the crystallisable bitter acids,
which, therefore, for the present, may be distinguished as the a- and
y8-acids. H. BUNGENER'S acid is the (3 compound, and, in view of
the thorough nature of his researches, the empirical formula assigned
by him to the acid is in all probability correct. The a- and /2-acids
are undoubtedly dissimilar in composition.

This conclusion may be deduced from the work of LINTNER and
A. BUNGENER, also from that of H, BUNGENER, SEYFFERT, and
ANTROPOFF.

Further, it appears fairly certain that LERMER'S acid must be
identical with the /3-acid described by H. BUNGENER and SEYFFERT.

LiNTNER, 1 together with the author, instituted further researches
on hop-bitter, operating on lupulin, and directed particularly towards
the study of the a- and /3-acids. The method for preparing the
/2-acid was worked out so far, that it is now possible to obtain larger
quantities of this body with relatively little trouble. Methyl alcohol
was found to be a most useful solvent, and by its aid crystals up to
i cm. in length were obtained. The formula of lupulinic acid was
found to be C 25 H 36 O 4 , and BUNGENER'S formula C 50 H 70 O 8 is,
therefore, incorrect. Lupulinic acid, on exposure to air, develops an
odour resembling that of fatty acids, and is converted into a yellow
resin. That oxygen from the air plays a part in this change appears
from the following experiments :

Lupulinic acid was enclosed in small glass tubes, one of which
was filled with hydrogen and sealed, the other left open at both ends.
The tubes were then exposed to light, near a window, for several
months. At the end of this time, the acid in the open tube had
become quite yellow and " cheesy," whilst that in the sealed tube
remained perfectly white. In another similar experiment, the tubes
were preserved in a closed box. The result was the same, showing
that light is not an essential factor in the change. The acid was

1 Zeitschrift f. d. ges. Brauwesen, 1901, 500.



78

found to keep particularly well at low temperatures ; for which reason
the cold storage of hops is also to be highly recommended. Lupulinic
acid, when boiled with water, yields a bitter solution only when air
is bubbled through the liquid during the boiling ; evidently because,
by slight oxidation, a water soluble bitter resin is formed. The acid
does not, however, suffer any chemical change, but is polymerised in
the resinification process. The resinous products obtained by
VLAANDEREN are, doubtless, identical with the author's /?-resin.
Whether the " hard " and " soft " resins described by BRIANT and
MEACHAM 1 , belong to this class of resins, could not be decided ; since
it is questionable whether the products named are chemical entities,
or mixtures of various resins with CHAPMAN'S hop oil. On oxidation
lupulinic acid yielded valerianic acid ; but by reduction no definite
product could be obtained. As regards the salts of lupulinic acid, it
was found impossible to prepare a copper salt of constant composition ;
although preparations were obtained agreeing very well in copper
percentages with the copper resin compounds first described by
VLAANDEREN another proof that his resin was the fi variety. The
experiments relating to the benzoylation of the acid, its behaviour
towards acids and silver nitrate solution, and the action of alkaline
iodine solution, being of purely chemical interest, may here be omitted.
The action of sulphurous acid on lupulinic acid might be of greater
interest, as having, possibly a bearing on the " sulphuring " of hops.
It was found that sulphurous acid in all its forms is fixed by lupulinic
acid. The combination is, however, of so unstable a nature that it
is decomposed by water, recrystallisation from methyl alcohol, &c.,
and for this reason the study of the compound would present great
difficulties. It was hoped to obtain some idea of the constitution of
the bitter acid by acting on the substance with bromine and iodine ;
the main result being to show the existence of double carbon linkings
and of one methoxyl group. On prolonged boiling of lupulinic acid
with potash-lye, the solution, when acidified, yielded fatty acids
closely related to valerianic acid. The most useful indication of the
nature of the hop-bitter acid was furnished on fusing it with potash,
the results showing that lupulinic acid must be closely related to hop

1 Transactions of the Institute of Brewing, 1893, 149; 1894, 117. Journal of the
Federated Institutes of Brewing, 1896, 408.



79

oil. Products were obtained identical with those prepared from
hop oil by GRESHOFF 1 and WAGNER* and PERSONNEL

Further researches of the author's were made concerning the
a-acid. This acid, although it yields a well-characterised lead salt,
can be obtained quite pure only with great difficulty ; consequently
the results may be somewhat unreliable. The analytical data were
confirmed by M. BAMBERGER and LANDSIEDL*, but these authors
propose for the a-acid the formula C 20 H 28 O 5 or C 20 H 30 O 5 , basing
it on a determination of the molecular weight made with purer
material. The action of melting potash on the a-resin indicated a
relationship between that resin and hop oil, as also the /3-acid. Like
the latter, the a-acid is an unsaturated compound. When treated
with potash-lye and sulphuric acid, the a-acid furnishes a body of acid
character the nature of this substance could not be determined owing
to lack of sufficient material. Although the a-acid closely resembles
the /3-acid in many of its chemical properties, and apparently belongs
to the same class of substances, it is distinguished by physical
differences (melting point, solubility, crystalline form) ; by its
elementary composition ; and by the fact that it gives a crystalline
decomposition product when acted upon by potash solution and
sulphuric acid.

T. ScHNELL 5 has recently studied the a-acid, and his results
furnish additional information as to its constitution. Two decom-
position products are formed by the action of soda solution an
unsaturated oxyketonic acid (C 15 H 24 OJ and valeric acid, which
latter is also produced on the oxidation of the substance. The
oxyketonic acid is not identical with the product obtained by the
author in the same way, nor with that produced by ISSLEIB by the
action of sulphuric acid.

From the decomposition products of the a-acid SCHNELL ascribes
to it the formula C 2 H 3 2 O 5 , which differs only by two hydrogen

1 Inaugural Dissertation, Jena, 1887.

2 Erdmann' s Journal fur Praktisclie Chemie, 58, 351.
8 Comptes Rendus, 1854, 309.

4 Zeitschrift f. d. ges. Brauwesen, 1902, 461.

5 Inaug. Diss. Tech. Hochs. Munich 1904. Zeits. f. d. ges. Brauw, 1904, 666.



80

atoms from that hitherto accepted ; whilst, from its chemical nature,
he is of opinion that it contains a lactone group and belongs to the
class of pseudo-acids. These lactones, in aqueous or alcoholic
solution, can be titrated by alkalies, exactly as can the acids, except
that the reaction is slower. This peculiarity of the a-acid would
account for the author having obtained equivalent weights of 345 and

"^^

337 by titration with alcoholic potash, agreeing well with the

theoretical value of 352. The fact that this titration is possible
proves that in neutralisation there is no further breaking down of the
molecule but only a " rearrangement " ; otherwise values varying
largely from the theoretical would have been obtained.

Therefore, according to SCHNELL the a-acid is most probably a
lactone functioning as a pseudo-acid, and the name " a-acid " might
be replaced by " Humulone," and, correspondingly, the unsaturated
oxyketonic acid formed on splitting up this " humulone " should be
described as " humulinic acid."

Following upon these preliminary remarks concerning the
general chemical properties of the bitter resin and the crystalline
bitter principles present in hops, it remains to consider the distribution
of these bodies in the hop-flour, and their bearing in practical
brewing.

Lupulin must be regarded as the actual source of the bitter
principles of hops.

Lupulin, or hop-flour, consists of small organised granules or
lupulin grains. They form a part of the gland formation of the hop
cone, and HoLZNER 1 distinguishes

1. Isolated glands.

2. Head glands.

3. Glandular scales.
The last form hop-flour or lupulin granules.

Under the microscope they appear like multicellular, shield-
shaped scales, with conical or cup-shaped indentations, and

1 Zeitschriftj. d. ges. Brauwesen, 1877, 267 ; 1892, 337 ; 1893, 103 ; 1899,87.



81

resembling two cones placed base to base. During development of
the gland, a secretion collects in the indented portions, and the upper
cuticle breaks away and forms the tender, yellowish hood, forming
the upper cone. By the increase of the secretion, the gland, as a
rule, is burst open at the surface of contact of the two cones. The
glandular scales easily break off the stalk by which they are attached
to their bases ; and so it comes about that in the preparation of hops
for the market a part of the lupulin falls out. By lupulin, however,
is also understood the secretory contents of the gland.

The name has also been applied to an unknown alkaloid
recurring in hops. 1

The contents of the lupulin granules do not consist of a single,
definite, chemical compound, but are composed of a mixture of
different bodies, the bitter substances predominating. The last it is
proposed to consider at length.

Since very little is known concerning the chemical composition
of lupulin, it may be of interest to give here the analysis of the raw
material (commercial) employed by the author in his researches.

The lupulin was completely exhausted with ether, and the ether
extract and the insoluble cellular residue separately examined.

100 g. lupulin (i8'27 per cent, total ash) yielded on the average :
63*93 g. ether soluble extract.
36-07 g. lupulin cell walls.

The ether soluble extract comprised the following constituents :

Wax ... ., 0-18 per cent.

a-resin (by precipitation with lead salt

HAYDUCK'S method) I1 '55

/?-resin (by titration, the a-resin being

deducted) ... 43*31

Ether soluble ash 2 0-17 ,,

Other constituents (fat, oil, y- resin, &c.) ... 8-72 ,,

1 Griessmayer. Dinglers' Polyt. Journal, 1874, 67 ; Zeitschrift fur Analyt.
Chemie, 1880, 105.

2 Mineral substances contained in the ether extract.



82

The residue, insoluble in ether, comprised :

Ash, insoluble in hydrochloric acid ... 15*31 per cent.

Ash, soluble in hydrochloric acid 275

Albumenoids (N x 6-25) 478 ,,

Pentosans (ToLLEN's 1 method) 2-34

Nitrogen - free extract substance, crude

fibre, tannin and loss iO'8g

Extracts obtained by ether or petroleum-ether were always found
to be free from nitrogen.

The proportion of sand present in lupulin is very variable and
ranged in the samples examined from lO'gi to 20-24 per cent. (SiO a ).

Older analyses similarly show that the composition of commercial
lupulin is very variable.

For instance, IvES 2 gives :

Tannin, 5 ; extractive bodies, 10 ; bitter substance, 1 1 ; wax, 12 ;
resin, 36 ; fibre (lignin) 46, in 120 parts.

PAYEN, S PELLETAN S and CHEVALIER S find ethereal oil, 2 ; bitter
substance, 10*3 ; resin, 55 ; fibre, 32, in 100 parts.

AUBRY* gives :

Ethereal extract 82-5

Extracted by ammonia 7-5

Lupulin cellular substance 6-0

Ash, silica 4*0

Other analyses, to be found in the literature of the subject, are
quite worthless.

WiMMER, 4 for example, gives an analysis in which the
constituents add up to 111*15 per cent, obviously based on some
error.



1 Zeitsckrift f. d. ges. Brauwesen, 1898, 413.

2 American Journal of Science of Silliman II, 1820, 302.
8 Journal de Pharmacie, 8, 209.

4 Lintner : Lehrbuch der Bierbrauerei, Braunschweig, 1878, 96 and 98.



83

The great differences between the individual analyses may, no
doubt, be ascribed to actual variations in the material examined.
Lupulin, obtained by carefully brushing out the hop cones, contains
far less ash than the rough lupulin collected as by-product during
the preparation and handling of hops.

As regards the sub-division of the resins and decomposition pro-
ducts of the crystalline bitter principles into "hard" and " soft" resin,
numerous observations have been communicated by BRIANT and
MEACHAM, 1 who examined hops of diverse origins. REMY S investi-
gated the same subject, and for separating the hard and soft resins
proceeded as under :

" Twenty-five grms. of hops were completely extracted with ether,
and the ether residue treated with 90 per cent, alcohol to separate
the wax. The alcohol soluble portion after evaporation to dryness,
was again dissolved in ether, the solution repeatedly shaken out with
dilute sulphuric acid (0.5 per cent.) in order to remove certain basic
substances, evaporated, and the ether residue finally divided into
hard and soft resins by treatment with petroleum ether (b.p, up to
60 C.)."

BRIANT and MEACHAM'S* process is a good deal simpler than the
preceding.

" Five grms. of hops are placed in a Soxhlet extracter, and
completely exhausted with petroleum ether. The ether residue is
freed from wax in the way already described, the alcohol evapo-
rated, heated for six hours to 90 C, and weighed as " soft " resin.
The hard resin is then determined by exhausting the hops a second
time with ordinary ether."

The analytical error in these methods is, as a rule, as much as
i per cent. Nevertheless, the results serve to show that the better
kind of hops, as a general rule, exhibit a higher proportion of soft resin,
and a correspondingly higher percentage of total resins. According to
RE MY, the hops of the 1896 harvest were characterised by notably

31 Journal of the Federated Institutes of Brewing, 1897, 2 33-

32 Wochenschrift fur Brauerei, 1898. 530.

G 2



84

low content of soft resin. Certain experiments made by REMY as to
the influence of kiln drying on hops, showed that a considerable
proportion of the soft resin present in the fresh hops is converted
into hard resin during drying in the air.

In the course of investigations made by LINTNER and the author,
and relating to the bitter principles of hops, an attempt was made
to estimate the /?-bitter acid by titration of a hop extract. Since the
resins are themselves acid in character, it might be possible by this
method to find the total quantity of resin present.

LiNTNER 1 gives the following details: 10 grms. of hops are
placed in a ^-litre flask, graduated at 505 c.c. (5 c.c. is the volume of
the hops), and extracted for eight hours with 300 c.c. of petroleum
ether (b.p. 30-50 C.) under an inverted condenser. The time of
extraction eight hours was fixed upon as necessary from the
result of special experiments, although the bulk of the resin is
extracted in two hours. The flask is then filled up to the 505 c.c.
mark with petroleum ether at 1 7-5 C., and the liquid filtered as rapidly

as possible so as to prevent solution of the a-resin. 100 c.c. of the

N
extract are titrated with potassium hydrate. As the alcoholic

potash does not mix with the petroleum ether, 80 c.c. of strong alcohol
must be added before titration, together with 10 drops of phenol p-
thalein solution (i in 100) as indicator. A blank experiment must
be made to determine the acid in the ether and alcohol, and a cor-
rection applied. Since one molecule of alkali neutralises one mole-
cule of lupulinic acid (molecular weight 400), the volume of alkali
consumed multiplied by 0^4 equals the weight of acid present.

Dr. HEIM 2 examined 15 different hops by LINTNER'S method,
the samples being supplied by the firm of F. J. BARTH & WEIGMANN,
in Lauf. With most varieties the results ranged within compara-
tively narrow limits, 14*6 per cent, and 12*7 per cent. ; only one
variety, the Auschaer hops, yielding notably lower figures, 9*8o-7'O4
per cent. On comparing these figures with the percentage of soft

1 Zeitschrift /. d. ges. Brauwesen, 1898, 407.

2 Zeitschrift f. d. ges, Brauwesen, 1898, 409.



85

resin, which REMY 1 determined in individual cases, a quite
satisfactory agreement was observed.

In view of its simplicity of execution, LINTNER'S method un-
doubtedly presents some advantages over the usual processes for
determining resins.

The method received an interesting practical application at the
hands of REMY, 2 who endeavoured to judge the bittering and
aromatic value of hops by chemical means. REMY determined not
only the total quantity of bitter acids, but also the proportions of a-
and /3-acid, by precipitation with lead acetate ; and, as a matter of
fact, a certain connection was traced between the relative proportions
of the acids on the one hand, and the applicability of the hops on the
other. The results, although undoubtedly requiring further investi-
gation and confirmation, show that those hops, which in brewery
practice are employed on account of their greater bittering power,
uniformly contain a relatively large proportion of the a-acid ; whereas
with hops that are more particularly valued for mild beers, the /?-acid
predominates. Opinions, however, as to the value of hops, from the
point of view of bittering power diverge so widely, that it would be
rash to consider high content in a-acid either a good or bad indication.
For most purposes a hop showing a high percentage of total bitter
and a moderate proportion of the a-acid would probably be preferred.
In REMY'S experiments the total bitter acid ranged from 11*0 to
i8'5 per cent., the a-acid from 3-5 to 6*6 per cent., and the /?-acid
from 7*5 to 12*3 per cent.

As has been already pointed out, the hop-resins are extra-
ordinarily subject to changes which detract from the value of the
hops ; and it will be the endeavour of every brewer to prevent as
much as possible these alterations, which, according to BRIANT and
MEACHAM,* consist in a conversion of soft into hard resin. For this
reason the cold storage of hops, which is strongly recommended by
many scientists and practical men, is a step in the right direction.

1 Wochenschrift fur Brauerei, 1897, 513.

2 Wochenschrift fur Brauerei, 1902, 614.

8 Journal of the Federated Institutes of Brewing, 1897, 4^i-



86

For the benefit of those who do not care to erect a special store, it
may be mentioned that the temperature of an ice cellar is quite low
enough to preserve hops for some time, and to prevent changes
in the resins.

This brings us to a portion of our subject, closely related to
the employment of hops in the brewery ; and it may be pertinent to
ask: "Why are hops used in brewing?" Apart from the hop
tannin, which is said to effect a readier coagulation of the albumenoids
of the wort, it is particularly the hop resins that are to give the beer
a pleasant flavour. But in addition to this function of flavouring
agent, the hopping is a valuable means of increasing the stability of
the beer, since the hop resins influence the development of
bacteria.

HAYDUCK 1 was the first to prove the antiseptic action of the
hop resins, in experiments made with lactic, butyric, and acetic
bacteria, and the Pediococcus which forms lactic acid. Subsequently,
RiCHARDSON 2 investigated the influence of aqueous hop decoctions
on certain pathogenic organisms, and in nearly every instance
observed a germicidal effect, and he ascribed this action mainly to the


1 2 3 4 5 7 9