CA1097688A - Production of iso-alpha acid - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method of providing a high quality iso-.alpha.-acid preparation is described.
The method involves extracting a high quality primary extract containing .alpha.-acids from hops using liquid CO2 and isomerizing the .alpha.-acids in the primary extract. The liquid CO2 extraction is performed at a sub-critical temperature of not less than -5°C. The primary extract contains .alpha.-acids, .beta.-acids, hop oil and usually no significant amounts of other organic compounds originating from the hops. In the preferred method the primary extract is not purified and the isomerization is performed by boiling an alkaline solution of the extract. This boiling can be utilized to remove the hop oil. The .beta.-acids can be readily removed by acidifying the solutionof iso-.alpha.-acid and filtering off the precipitated .beta.-acids. The iso-.alpha.-acid preparation is of outstanding purity and can be added to beer after fermen-tation and in particular to bright beer after final filtration to provide bitterness.

Description

~his ln~ention relates to the mar.u~acture of iso-u-acids 9 suitable for use in beer maki~ from o-acids extrac-ted from hops.
It is co~mon practice to extract various constituents-from hops and to use these extracted constituents in place of ho~s themselves in makin~ beer. ~he principal coDstitue~ts which haYe been ~o extracted previously are the resins and in particular the ~-acid. fraction of the soft resins which on chemical rearrangement give iso-~-acids which are the main bitteri~g components of beerO ~he main advan-tage of extrac-ting o-acids is that the ~_acids can be used much ~ore effi-cientl~ than is possible in traditional beer m~ing. In traditional beer making typically only 25% to 35% of the ~-acids in the original hops are utilized. By extracting the o~acids, iso~erizing them separately from the beer and addi~g the iso~acids produced to beer a~ter fermentation o_acid utilization caQ be much higher, typically from 6~/o to 85~'.
further substantial ad~a~tage i~ using extracts~is that they can be ~tored for longer periods without the; presumably oxidative, degradation especially of the bitter pri~ciples includi~g the ~-acids that occurs in hsps under extended storageO
! ~ Conventional hop extractio~ procedures i~volve the use o~ organic sol~ents such as methylene chloride 7 trichloro-ethylen~, hex~e and~or meth~nol~ ~hese solvents will readily dissol-~re nvl, o~y the d~sired a~acids but relativel~ l&rge propor~ivns vf ~ acids~ tanni~,s, chloroph-gll and ~ariOllS othe .
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hop constituents.
A primary extract made by extracting hops with an organic ~olvent would typically ha~e the follo~Jin~ composi-tion:
wt. %
~-acids 8-45 ~-acids 8-~0 uncharacterized soft xesins 3-8 hard resins 2-10 hop oil 1-5 : fats and waxes 1-2 total~ resins 15-60 t~nnins 0.5-5 chlorophyll up to 1 fines (cellular debris) 2-5 inoxganic salts 0.5-1 residual solvent (usuall~
: C~2C12 or C~3o~) 1.5-2.2 water 1-15 ~o obtain high quality o_acids suitable for isomeri- -~ation the cxude extract has to be extensively puxi~ied ofte~
: involvin~ other organic solvents and invariably i~olving man~ and o~ten complex steps. ~urther, it is difficult to e~tirely remove the organic ~olvent from the extract, typical cs~mercial extracts can contain oYer 1% b~ weight solvent~ Whilst i~ is believed that residual. solvents par~i-cularly of methylene chLorid~ srd methQnol are lost entirely ~3 ' ' ' ,' ~':
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duxing b~er making3 it i.s not a wholly satisfactory position to rely on such 'accidental' elimina~ion of possibly noxious ~aterials during production of a foodstuff.
.Further5 although there is at present no great technical difficulty in meeting the public health requirements for levels of residual solvents there may well be considerable difficulties in the future,.
~n organic solve~t extract, purified to be suitable for isomerization to give an iso-~-acid preparatio~ suitable for use in bittering beer, would typicall~ have the ~ollowing composition: -wt . %
a~acids 60-80 ~-acids o.3 o,5 uncharacterized soft resins 1-2 hard resins 1-5 hop oil up to 1 fats and waxes caO 0~1 total resins 70_90 tannins ca. 0~1 chlorophyll ca. 0.1 fines nil inorganic salts æubsta~tially nil xasidual solvent~ 0.5-1 water up to 10 ~ I~ addition ~o CE?C12 and C~30H which are com~only used both i~ extract-io~ and purlfication, ~hese solvent residues .. .

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may include solvents such as ethyl acetate, butanol and tric~oroe-thylene.
Fluid carbon dioxide either in tne form of the liquid or the supercritical gas has been suggested as an extraction medium for hops. ~hus, ~ritish Patent Specification ~o. 1,~88,581 describes a method of making a hop extract b~
extracting hops with a variety of gases in the supercri'cical state with respect to temperature and pressure~ Carbon dioxide is stated to be the most preferred gas, ~xtraction under such supercritical conditions with C02 typically yields ~ an olive-green pasty product which contains o-acids, ~-acids, uncharacterized soft resins, hard resins and small quantities of t~nnins. The extraction conditions can be varied to give yields of o_acids proportionately higher than the concen-trations in the starting hops, the best extracts described~
however, containing onl~ about one_third a-acids. ~he optimum extraction conditiRns are stated to involve extraction under a pressure substa~tiall~ in excess oX the critical pressure9 which for C02 is about 72.8 atmospheres, preferably in excess of 100 atmospheres ~gauge) and temperatures of from 40 to 50C. It is stated that by appropria$e selection of the extraction conditions 3 it is possible to extract substantially all the ~oft resins and the essential oil of hops whilst mini-mizing th~ extraction of hard resins or that ~he proportion-~te extraction of ~acids can be reduced but at the expense of increasing the proportion of hard resin,s extracted~
~brther, it is ebated tc be pos,sible to ertract hop~ with 8i~

liquid C02 but this does not form part of the invention of this prior patent because 'its dissolving power is less than that of supercritical C02'.
The extract as described in Specification No.
1,388,581 compares favourably with t~pical primary extracts obtained using organic solvents and would appear -to be suitable for use in brewing beer, eOg. by addition to the copper~ However, the extract described would not be suitable for isomerization without considerable purification to remove components which would produce ad~erse flavours under typical isomerization conditions and also result in the formation of substantial quantities of haze when added to beer~ (It is not practical to effect purification aftPr isomerization because the mixture is even more complex than the impure extract). In the present state of extract technology, this would require the use of organic solvents, thus giving up one of the main advantages of using super-critical C02 in extracting hops.
Li~uid C02 is described as an e~traction medium for hops in USSR Author's Certificate ~o. 167,798 i~ the name of Pekhov~ Ponamarenko and Prokopchuk and by Pekhov in Masloboino-Zhirova~a~a Promyshlemnost VolO 34~ part 10 (1968), pages 26 to ~9. The product obtained by extraction of hops with liquid C02 at 20 to 25C is stated in the Author's Certificate to be a light brown viscous mass~ ~hafto~ and Naboka in ISU Sev-Kauk ~auchn ~sentra Ugssh S~, Ser ~ekh - ~auk 1975$ 3(3), 29-31 [Chem ~bs Vol 84 (1976) 120046a~

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describe C02 extracts of hops as comp]e~ mixtures of ~-, ~ and ~-acids, o~ a~d ~-soft resins and hard resins which are subject to substantial deterioration, especiall~
b~ fairly rapid autoxidation on storage. The purity and stability of such C0 extracts i9 not altogether certain but, although it ma~ be possible to use them in beer making by addition to the copper it is clear that they could not be isomerized to give an iso-o_acid preparation without sub-sta~tial purification and, in any event 9 are considerabl~
less stable under storage than conventional hop extracts made using organic solvents~ ~eretofore it has not been recognized that a primary extract of hops could be obtained which would be satisfactory for direct isomerization to give an iso-o-acid prepara-tion suitable ~or bittering beer. ~he present in~ention is based on the discovery that extracting hops with liquid C02 (i.e. under sub-critical conditions) can be carried out to ~ield a primar~ extract which can be satisfactorily isom0rized without needing prior purification.
; It is an object of the present invention to provide an improved process for the ma~u~acture of iso-o-acids pre-parations suitable ~or addition to beer and in particular to ~igh quality preparations suitable for additlon to bright beer after filtratio~.
~he present invention accordingly provides a method of making aQ iso-o_acid preparation which method comprises con-tacti~g hops with liquid carbon dioxide at a temperature of i not less than -5C thereby extracting at least a portion of ' ~ .

, the ~-acids contained in the hops into the liquid carbon dioxide 9 isolating a primary hop extract of high purity from the li~uid carbo~ dioxide and isomerizing the ~-acids present in the hop extract to iso-o-acids.
In the practise of the present invention the high purity primary hop extract isolated as an intermediate produot generally contains ~-acids, ~ acids,hop oil, usually small ~uantities of water and usually no more than trace quantities of the main impurities seen in primar~ organic solvent extracts vi~: uncharacterized soft resi~s, hard resins~
tannins and chlorophyllO
~able 1 sets out the purity of the primary extract as measured by the quantities of the impurities which interfere with the subsequent isomerization. Column A gives the maximu~
permissible proportion of the impurities abo~e which we have found that the impurities i~terfere substantially with subse-quent isomerization, Column B gives the expected maximum proportion o~ the impurities extracted b~ liquid C02 under the general conditions set out herein ~nd Column C gives figures typical of what we expeot to obtain.

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~ I-D~ L vt. % on extract uncharacterized , (2) soft resins 3 001 < 001 hard resins 0.5 0.1 none Tannins 0.5 0.1 none Chlorophyll 0.2 0.1 none fats and waxes 0.2 0~1 < 0.1 fines 005 none none inorganic salts O.5 O.1 ~ O.1 (2) _ l otal ! 4% 0.3 0.1 (1) See below for a sllmmary o~ the ~nalytical techniques used.

(2) These figures indicate that with some primary extracts the impurities were qualitatively just detected with the Analytical techniques used but that the amounts were too small to be estimated with an~ degree of accuracy, The figures given as maximum values represent the limit of the analysis; the actual quantities of the impurities present may be even smaller.
The main components of the primary extract are as indicated above, ~-acids~ ~-acids and hop oil extract has the following composition in respeot of these components.

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wt~ o~ (1) ~_acids 40 to 75 (~) ~-acids 20 to 40 total resins 70 to 98 hop oil up to 10 (3) water up to 5 ~1) These figures are based on analyses for the resins performed by the method described in Analytica ~BC, published by Schweiæer ~rauerei Rundschau, ~rd Edition, 1975, page E49 and analyses for the non-resin components especially the tannins and chlorophyll by the method described by JO Jerumanis in ~ulletin Association Anciens ~tudiants ~rasserie ~ouvain, 1969~ volume 65 page 113.
(33 The maximum analytical figures for o-acids and hop oil will only generally be attained with hops containing unusually high proportions of these materials. Most usually the upper limits will be about 65% for ~-acid.s a~d about ~% for hop oil.
By referring to the primary extract as being of high purity we mean that the undesired impurities as set out i~
Table 1 are present in qua~tities less than those given i~
~ column A of ~able 1. Under the conditio~s, as ,set out herein ; under which we have extracted hops with liquid C02 we ha~e obtained results which are much better than the maximum ~igures given in column ~. -For practical purposes the composition of the eYtract ,. . . .

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ca~ be summarised as wt. %
a-acids 40 to 75 more usually 40 to 65 ~-acids 20 to 40 more usually 25 to 35 total resins70 to 95 more usually 80 to 95 hop oil up to 10 more usually up to 3 water typicall~ 2 to 5 and substantially no organic impurities originating from the hops.
~he amount of water present in the extract is not, in itself, critiGal, but, as is discussed below, if substantial amounts of water are present in the carbon dioxide at the point of extraction then tannins ma7 be extracted from the hops~ It seems that, using suitably dry li~uid C02, the amount of water in the extract depends on the amount present in the hops and thus, whilst the figures given above are typical, amounts outside the range indicated ma~ be obtained n svme circumstances.
~he high ~uality primary extract has the golden yellow colour of ~-acids and, at ambient temperature is usually a solid or semi-solid cr~stalline material, the exact form depending on the particular conditions of extraction and the type of hop extracted. A further indication of the puxity is that typical primary extracts separate identifiable c~ystals of ~- and ~-aGids on cooling to caO 4C~ Gonven-tional primary solvent eY.tracts show no signs of such crystallizatio~.

It ~ill be noted that this primary extract is a ver~y much cleaner material than typical organic solvent primar~
extracts and especially with regard to the hard resin and uncharacterized soft resin components, is substantially purer than conventional organic solvent extracts even after purification. ~ principal difference is that the extract obtained using liquid carbon dioxide contains substantially more ~-acids than typical conventional purified extracts.
It seems that the ~-acids can be tolerated at these much higher levels because of the otherwise outstanding cle~nness and purity of the extract. ~hus, the high level of ~-acids does not prevent the satisfactory isomerization of the ~-acids in the production of an iso-~-acid preparation and the ~-acids can be removed from such an iso-~-acid preparation b~
filtration at a suitable pH.
As far as we are aware, this has not previously been reported, indeed the tenor of prior art discussions on the amounts of ~-acids tolerable in extracts to be isomerized indicate, on the contrary~ that the maximum tolera~le amoun-ts of ~acids were very small, typically less than 1%. In the present invention we believe that it is possible to isomerize the ~_acids because the levels of other materials which might interfere with isomerization are so low.
~he general physical conditions of temperature and pres~ure used in the extraction step are to some extent critical to the invention. If the extraction is performed at temperatures below -5~C undesired org~nic compounds te~d ,:

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to be extracted~ These compounds seem to be mainly fats and waxes but at lower tempera-tures increasing quan-tities of hard resin and probably tannins are extractedO At sub-critical temperatures above -5C, we have been able to obtain high purity extracts. The extraction temperature is sub-critical because, as i5 clear from British Patent Specification ~o. 1~388~581, the use of supercritical temper-atures results in the extraction of substantial quantities of hard resins 7 chlorophyll, tannins, etc.
W1thin this temperature ra~ge good results can be obtained but generally we prefer not to use temperatures close to the critical temperature in order to avoid the possibility of the C02 beco~ing supercritical accidentallyO
T~e generally preferred temperature range is from -5 to 20C.
~he pressure at which the extraction is performed must clearly, be sufficient to keep the C02 liquid and not ~o high that the C02 behaves like the supercritical fluid as described in British Specification ~o~ 1,388,581. Generally it is both convenient and preferred to operate under the vapo~ pressure of liquid C02 at the extraction temperature.
In orde~ to allow ~or minor temperature differences between different parts of the extraction apparatus, particularly when ~ub-ambient tempera~ures are used for extractionl ~nd also to ~llow for hydrostatic pressure differences within the apparatus, the pressure will normall~ ~8 slightly, e.g~
up to 1~/o~ i.n excess of the vapour pressure. The variation of the vapour pressure of liquid ~2 ~iith temperature is shown in ~able 2~
__ ~able 2 ~emperature Vapour Pressure o~ Atmospheres _ (absolute) 31 (crit. temp.) 72.8 71.2 56.5 44.~
O 34.4 _ _ 30.1 ~rom an engineer~ ng standpoint it is desirable to h~ve the pressure as low as possible and to have the temperature as near ambient as possible~ ~hese requirements are not strictly compatible and i~ practice a compromise taking these considerations into account as well as the rate of extraction, selectivity and purity of the extract will usually determine the optimum opelating temperature i~ any particular caseO
~he form in which the hops are extracted is not especially critical in that efficient extractions can be achieved from green or dried hops~ hop cones, milled, powdered, pelleted powdered hops or crushed hop pelletsO
However~ the bulk density o~ hop cones is very low 2~ and as a practical matter it is preferred to use powdered or pelletted hops in the extraction. Powdered hops seem to be s~tisfactory for extraction in the inventio~ a~d the further processing required to pellet hops see~s to provide no ~14~
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particular advantage in extra~tionO ~he optimu~
extraction conditions may vary depending on the particular form used but determination of precise optima seems to be a matter of straigh-tforward chemical engineering.
Of course, the hops should themselves be of suitably high qualityO Inferior or deteriorated hops may not produce satisfactory extracts. ~he particuiar ~ype (cultivar) of hop used does not seem to be critical to the extraction itself and both seeded and seedless hops can be satisfactorily extracted.
~he particular composition of the extract and ~ield obtained is a function of the hop cultivar extracted and whether it is seeded or seedless. We have produced satisfactory extracts from the following cultivars: Wye ~orthdown, W~e Saxon~ Northern ~rewer, Wye Challenger, ~ullion, Comet, Pride of Ringwood and Styrian Golding.
~he amount of liquid C02 needed to extract the hops appears to be a fu~ction of the solubility of a-acids in the C02, the t-gpe of extraction system and of the precise conditions employedO We have not yet determined the li~its of sol~bilit~ of a_acids in fluid C02 but have experienced no difficulties in achie~ing concentrations of 3O7 grams per litre and higher concentration~ could well be possibleO
Extraction efficiency both in terms of the proportion of acids extracted and the amount of C02 nbecessary to extract them i5 i~creased by increasing the effective contact time between the C02 and the hops. This can be done for example by ~imple recycling of ~he C02 through a~ extraction bedO

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~' Alternatively, this can be done b~ using a semi-continvous or continuous extraction procedure, e.gr employing counter-current techniques. Although it is desirable to increase the effective contact time between the liquid C02 and the hops we have found that substantially complete extraction can be obtained in the laboratory in times comparable to the contact times used in conventional comr~ercial solvent extrac-tions. ~le believe that, i~ fact, liquid C0z will extract hops substantiall~ more rapidly than conventional organic solvents and that this does not specifically appear in the relatively small scale experiments we have performed to date because of the relative and absolute flow rate limitations imposed by the small scale of the apparatus used for extraction.
~he degree of extraction of o-acids is a function of the precise method adopted. We have successfully extracted more than 9~/o of ~vailable o_acids without difficulty and believe that even higher rates of extraction are possible without great difficulty.
~he e~tracted o_acids can xeadily be recovered from the liquid C02 extract by 'boiling off' the C02 as gas.
~his can conveniently be done by heating the solutio~ under constant pressure so that the liquid C02 boils. ~his approximates to isothermal conditions~ ~lternatively, the evaporation of the ~aseous C02 can be perfo~med by reducing or releasing the pressure, iae. approximately adiabaticallyo Both of ~hese metho~s are satisfactory on a small scale, the , . .. . ~ . . . . .. .. ... ... .

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quality of the product not being afiected4 It is preferable 9 particularly in commercial operation, to recover the C02 and recondense and reuse it as the extractin~
liquid. It is easier and thermodynamically more efficient and thus more economic to perform this recycling under approximately constant pressure thus taking advantage of the low specific heat and laten-t heat of vaporization of C02 to accomplish the necessar~ changes of state The large pressure and temperature changes on adiabatic evaporation make it rather less preferred in this respect.
When the extract is isolated by warming the liquid C02 to evaporate it, it i~ desirable to use a heat source at a moderately high temperature, e.g. 30 to 40C, in order to ensure adequate heat transfer. ~urther~ because the extract tends to bacome fairly viscous and may even start -to c~ystallise out at low tempeIatures, e.g. below about 5C, it is thus desirable to isolate the extract at near or slightly abo~e ambient temperature to avoid these potential problems.
~he primary hop extract thus obtained is a virtually pure mixtl~e o~ o-acids and ~acids toge~her with part of the es~e~tial oil ~'hop oil7) present in the hops1 ~ut not u~ually any hard resins, chloroph~ll or ta~nins. We ha~e had no difficulty in isolating crude extracts which are ~ubstantially free from hard resins, chlorophyll, tannins~
e~c~ ~is purity of the product is a mar~ed ad~ance over the products obtained by previously reported hop extraction techniques~ A quantitative exslmination of the crude product indicates that there is a rather higher proportion of ~acids as compared with ~ acids in the product than would be expected solely on the basis of the relative proportions of a- and ~-acids in the hops. ~he degree of this selectivity is significantly higher thSan that reported in Specification ~o. 1,388,581 and we cannot explain why the conditions we use for extraction are particularly adva~tage-ous in this way as well as in other aspects of purity.
As has been set out abovez the primary extract produced i~ the method of the present invention is usually a mixture ; - of o_acids, ~-acids and at least a substantial part of the essential oil of hops. ~he mixture will usually contain moisture but this is not regarded as san impurity for further processing S~lthough it ms~y be convenient and/or desirable to dry the extract if it is to be stored over long periods. ~he high purity extract is yellow in colour9 i.e. the colour of the o-acids, and not green, brown or other~ise dark in colour as is usual with conven~ional primary hop extracts including those described in Specification No. 1,388,581. ~ypically the extract is solid or viscous paste or liquid, the precise ~orm depending lslrgely on the temperatureO
The extract includes some hop oil and since this can be a ~alua~le product in its own right it can be removed from ~5 the primary extract by stes~m distillation under vacuum, e.g, u~der the general conditions of temperat~e and pressure set ` out in the specification of United Sta~es Patent :, , ~ --18 ;~,3 ~.- , , ' ,, ~ ' . ' . , ~ - . , ij: '-' ' . ' ' ' ' ,,; ' ' , ' ' .
h ;, ~ ' , ' , : , ' ~o. ~,979~527 to ~aws and PickettO Thus, convenientl~ the separation can be effected by mixing the extract with water and distilling the mixture under vacuum at a te~perature of less than 50C, typically from 20 to 25C~ ~he distillate, an emulsion of hop oil in water, can be collected as a dispersion in ice by cooling the vapour to below 0C, more usually to -20C or below. Xop oil distilled from the extract in this wa~ can be used to impart hop aroma and flavour to beer. However, we have found that using liquid C02 substan-tiall~ all of the hop oil is removed ~rom the hops so, as described in the above specification, care must be taken to select only the desired fraetion on steam distillation as described. ~urther, it seems that some of the more volatile components of the hop oil are extracted very rapidly by the C02 and unless care is taken these components may be lost, even during the time when residual air is being flushed out of the extraction apparatus. ~or these reasons it seems ; preferable, if hop oil is desired as a separato product, to extract the hop oil from the hops prior to the extraction with liquid C02. In any event, it is probable that the liquid C02 e~tract will co~tain at least some hop oil components.- Since, during isomerization some components of the hop oil may be degraded and contribute to o~f flavours and aromas t it will usually be desirable to remove the hop oil from the extract.
hs discussed in United States Patent ~o. 3,979,527, the qu~rltities of hop oil which will normally be desired in beer to give hop aroma a~d fla~our such as is obtained by dry hopping, are in the range of 0.5 to 2 ppmO The amou~t of hop oil ln the primary C02 extract is usually such that, if all the hop oil were re-tained, the ~nount of tbe oil added to beer by using the final product iso-a-acid preparation would be considerably in excess of what is desirable to give the beer hop aroma and flavour. This will be particularl~
true when the type o~ hop extracted is one having a high proportion of available hop oilO It is thus desirable to remove at least sufficient hop oil to prevent this becoming a serious problem.
O~e further practical point favouring separation o~ the hop oil before use of the iso-o_acid preparation is that a brewer will generally prefer to use e3sentially pure materials.
~hus, where he wishes to use the iso-~-acid preparation ~5 produced by the invention and also hop oil isolated ~rom the primary extract, he will prefer to have these materials supplied separatel~ for use rather than to be supplied with a pre prepare~ ~nseparated mixture.
When the preferred isomerization technique, described ; 20 in more detail below, is used it is not necessary to, and i~ indeed it i~ preferred not to~ speci~ically remove the hop oil from the extract before i~omeriæation~ ~owever, if it i~ desired to so purif~ the extract then this can be do~e by ; ste~h distilli~g all the hop oil off. HoweYer, care should be taken to avoid excessive heating as this can damage or destro~ the bittering potential of the extract. Such damage can be avoided by perforning the steam distillat~o~

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at suitably reduced pressure, e.g. such that the temper-ature is not more than 50C.
In connection with the purity of the extract we have noted that the extract will readily form complexes ~lith a number of me-tals such as iron and copper. ~hus, passing extract containing liquid C02 through mild steel or copper tubing particularly in the presence of moisture results in iron or copper respectively being leached from the walls of the tubing, ~his is undesirable because it can give rise to corrosion problems in the extraction apparatus~ salt crystallization in the extraction apparatus~ especially with copper which form blue crystalline slats with the soft resin acids and conta~inates the product with heavy metals.
Although iron is not toxic, as is copper, it is undesired becaus~ other heavy metals which may be toxic may be carried with the iron and the presence of iron in the extract seems to reduce the stability of the product particularly with regard to oxidative degradationO ~2tracts contaminated with iron are typically bro~ rather than yellow in colour~ ~he problem of contamination by metals c~n be ovexcome by using a suitably inert material for, or to line the extraction equipme~t. We have found tha~ making the extraction equip-ment principall~ of stainle~s steel, glass (for ~iewing parts) and suitably i~ert plastics materials overcomes this problem.
Other materials are no doubt suitable a~d selection of suitable inert materials is within normal chemical engineering skill.
~s is mentioned briefly above, the quality of the extract may be spoiled if excess water is present in the C2 used as extraction fluid~ particularly insofar as tannins, being relatively soluble in water, may be extracted in the water carried with the liquid C02. Commercial quality liguid C2 in the United Kingdom~ such as is available from Distillers Company (Carbon Dioxide) Iimited, is a hi~h quality product whose water content i~ measured in parts per million (typically about 20 ppm)~ Such C02 is adequately dry for use in the present invention. r~he purity of such liquid C02 is well within the relevant British Standard (BS No~ 4105/1967~.
We believe that liquid C02 complying with this ~ritish StandaId is of sufficient purity for use in the present invention. ~he use of less pure liquid C02 may give rise to problems in the quality of the extract.
~he possibility of heavy metal contamination and the pre~sure of water and/or other impurities ln the liquid C02 may, we believe 9 be at least in part responsible for the failure of the prior art proposals on extracting hops with liquid C02 to produce a product approaching the purity of that of the primary extract in the present inventionO
~he isomerization step in the method of the invention - can be performed by an~ suitable method. However, some techniques such as photoisomerization methods may re~uire that the extract be purified before isomerization a~d even though with the high purity extracts obtained in the present method such purification is relati~ely straightforward it constitutes a further step which is not necessar~ when the , .

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&~3 pxeferred method of isomeriza-tion is employed" Other techniques frequently include the use of organic solvents and are thus not preferred. Organic solvents are not necessary in the preferred method of isomerizationO
~he preferred method is to isomerize the ~_acids by heating and preferably boilin~ an aqueous solution of the extract which has been renclered alkaline. ~ypically the extract is dissolved in alkali, e.g. sodium and/or potassium hydro~ide or preferably carbonate preferably to give a solution having a pH of f'rom 8 to 11. For solutions not requiring subsequent dilution (see below) the concentration of ~acids is typically from 0.5 to 5 gl 1 and the concen-tration of sodium and/or potassium carbonate is from 0.01 to 0.5~. ~he solution is then boiled, e.g~ for from 5 minutes to 2 hours to isomerize the ~-acids to iso-o_acids. During boiling hop oil is driven o~f with the steam. It is known that the alkali isomerization of ~-acids can be carried out in relatively concentrated solution, e.g. up to about 35% by weight ~-acids. Such high concentrations can be used i~ the isomerization stage of the present invention if desiredO ~he U8e of high concentrations can be advantageous in reducing the heating re~uirement during isomerization and the subse~
quent cooling requirement and in that the total amount of i alkali needed to maintain the desired pH of 8 to 11 is some-t 25 what less than is necess~ry with more dilute solutions. ~his latter point may be relevant7 as a xeduction in the level of inorg~nic matter reduces the likelihood of inorganic s~lts~
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e.gO KCl or NaCl crystallizing out if the eventual iso-a-acid preparation is concentrated substantially, eOg. to between 30 and 4~/o by wei~ht. ~hus, the concentration of ~-acids during isomerization is not limiting on the present invention, and concentrations up to e.g. 400 gl 1 are contemplated. The particular concentration chosen is a matter for selection depending on the particular circumstances and we do not expect the skilled man to have any serious difficult~ -in this regard. ~ypically the concentration will be in the xange of 0.5 to 300 gl 1, more usually ~ to 100 gl 1~
~he solution is then cooled preferably at least to ambient temperature and optimally to between O and 10C. ~h~
pH of the solution is then made acid, e.gO from 20 5 to 5~
preferably 3.5 to 4.5, aQd optimally about 4Ø This can convenientl~ be done by adding a ~uitable quantity of acid such as hydrochloric acidO ~t this acid pH the ~ acids are much less soluble than the iso-~_acids and are precipitated and can be removed, e~g. by filtration sr centri~ugation.
~ere the isomerization has baen carried out with a relatively concentrated solution of o-acids then, because the solubility of iso-~-acids at the acid pHs used to precipitate the ~aGids-i~ fairly low5 the isomerized alkaline extract solution may have to be diluted before acidification in order to avoid undesir~d loss of iso~ acids. ~hus, at the optimum acidi-fication pH of about 4s iso-~-acids are soluble to about 0~5%
weight (ca. 5 gl 1) 7 and alkaline isomerization solutions more concentrated than this should be diluted before acidi~ication :

, .
'.. , ''', ' ~: ' ' ' : . - . ' .: ,:
:. -, . : .
. .
- . '' 6B~3 to avoid precipitation and thus possible loss of iso-u-acids. ~he preferred concentration of iso-u-acids at this stage is pH dependent but is generally i~ the range 0~5 to 5 gl 1 and optimally between 3 and 5 gl 1.
Con~eniently the solution is filtered with a filter aid such as kieselLuhr. After filtration the iso-extract can be added directly to beer, as is described in detail below, to bitter it. However, the iso-extract is not very stable on storage at acid pHs and 9 especially if the extract is not to be added to beer immediately, it is thus preferable to make the p~ moderately alkaline, e.g. to pX 8 to 10, preferably about 9.0 by adding a suitable alkali, eOg~ sodium a~d/or potassium hydroxide a~d/or carbonate, and the solutio~
concentrated to a desired extent (usually 10 to 4~/0 iso o_ acids).
We have found that if the isomerization is completed rapidly then a small proportion of the hop oil may not be boiled of~ with the steam~ If complete removal of the hop oil is desired then this can be done by continuing boiling the alkaline mixture~ ~owever, when the iso-extract is co~centrated before use as described below we have fou~d that any remaining hop oil ca~ co~veniently be remo~ed ~ith the water evaporated from the iso-extract~
ConcentraJGion can be carried out to obtain solid crystalline iso-a-acids as the appropriate alkali metal salts if desired. Since iso-a~acid can be lost by decompositio~
by overheating especially during concentratio~ by evaporatio~

~25--:
.
. . . .

~ ~7~

it is preferred that in the evaporation the solution is not ,heated to more than 50C and more preferably not more than 40C. We have experienced no difficulty in performing the evaporation at 35C~ Conveniently rapid evaporation can be performed under a partial vacuum~ ~'he concentration of the iso-extract need not, of course, be carried out to give solid iso-~-acids~ ~or storage and transportation concentration to form 10 to 4~/o w/~ is generally satisfactory. ~urther, provided that there is no undue delay the step of making the iso-extract solution alkaline may be per~ormed after, or even during concentration i~ desired.
~he use o~ the preferred isomerization technique is especially advantageous because it enables the method of the invention to be carried out in a particularly elegant manner.
~hus, in an especially highly preferred aspect the invention provides a method of making an isomerized hop extract which method comprises the steps of :-~i) extracting hops, preferably in crushed pellet ~orm, with liquid C0~ at a te~perature of from _5C to 30C, preferably from -5C to 20C and under a pressure slightly greater tha~ that of the corresponding vapour pressure of liquid C2 at the extraction temperature;
, (ii) recovering a primary hop extract of high purit~
by evaporating the liquid C02;
(iii) without further puri~ication, preparing an alkaline aqueous solution containing the . .

-26;

. ' .

.
.

extract preferably in an ar~ount co.rre~ponding to from 1 to 1Q0 gl 1 of ~-acids, containing sodium and/or po-tassium carbonate, preferably in a concentration of from 0.01~ to 0.5N and preferably havin~ a pH of from 8 to 11;
(i~) boiling this solution to convert substantially all the cx-acids present therein to iso-o-acids and to remove hop oil components, preferably for a period of from 5 minutes to 2 hours;
(v) cooling the solution preferably to a temperatuxe of from 0 to 10C;
(vi) if necessary dilutin~ the alkaline solutio~ to a concentration of not mo.re than about 5 gl 1 iso-~-acids and adjusting the p~ of the solution to from abou-t 2 to about 5, preferably about 4.07 by adding acid;
(vii) filtering this acidified solution to remove ~-acids precipitated therefrom whilst retai~ing the iso-o-acids in solution; a~d optionally (viii) concentrating the solutio~ of iso-~-a~ids thus obtai~ed to R predetermined extent by evaporation, preferably at a temperature of not more tha~
40C, and : (ix) either before, during or after concentration, adjusting the p~ of the filtered solutio~ to from 8 to 101 preferably about 9.0, by adding alkali.

-27~-: -. .. . .
- - , : , , , ~he isomeriæed extract ~roduced by the method of the invention is of excellent quality and can be obtained in high yield. We have been able to extract 85 to 95% of the ~-acid of hops without any substantial difficulty and we expect that higher levels of extraction are possible. Typically up to about 7~0 of the ~-acids are also extracted. Although the soft resin acids content of different varieties of hops varies 7 we have not experienced difficulties in extracting a number of different hop cultivars as set out above.
The conversion o~ the ~-acid content of the extract to iso-o-acid can be performed substantially quantitatively by the preferred isomerization method used in the invention, losses in the filtration to remove the ~-acids being slight.
~osses during evaporation are not normally significant provided the temperature is kept below about ~0C. ~hus the effective yield of the method of this invention can be typicall~r as high as 9~/o and higher yields are probably obtaina~le without very great difficulty~ -It is an outstanding advantage of the iso~-acid pre-paration obtained by the method of the invention that it does ~ot cause measurable haze on addition to beer~ This co~pares highly ~svourably with isomerized extracts available com-mercially~ ~he isomerized product of the i~ve~tion is of a sufficiently high quality that it can be added to bright beer ~5 after filtration without significant hazingO ~his has not - heretofor~ been practically possible; even the best com~nercial isomerizates ha~ing to be adde~ before fi~al ~iltration a~ ~he .

-2~

.

latest because of their tendency to cause haze~ The effective utilization of the bittering potential of the hops is tnus reducedO With commercial isomerized extracts the utilizatio~
obtainable under normal conditions is between 70 and 75% and the maximum under carefully controlled and optimised conditions about 85%; with unisomerized extracts added to the copper utilization is typically 27 to 35% compared with about 25 to ~/g when the hops are used without extraction. We have had no signi~icant difficulty in obtaining utili~ation values of better than 8~/o, without specifically optimizing the conditions, and believe that values of 9~/o and more will be obtained by optimizing the methods o~ addition.
When commercial isomerized extracts are used to bitter beer there are sometimes problems with gushing~ The main gushing promoters are oxidation products of hop resins which are present in the hops and/or formed during processingO
~nother advantage of our new process is that the resulting isomerized extracts show no tendency towards gushing. Hence gushing promoters are not extracted Xrom the hops or formed during processing~
The quality of the iso-a-acid preparations of this ~ tion is evidenced by analysis. Examination by column chromatography, as described by O~ter, Silvester and ~aylor, J. Inst. ~rew~ ~ (1972) 57, on typical samples produced as described above shows the presence of iso-a-aoids but fails to show any u-acids~ ~-acids or humulinic acids~ ~hese - compcnents are generall~ just detecta~le ~y thin l~yer .

chromatography. Commercially available iso-extracts generally r-eveal much higher levels of these undesirable compounds. Further the iso-~-acids of the present invention do not contain any detectable amounts of polyphenolic com~oulld~. These are usually present in commercial extracts and are thou~ht to contribute to the inferior properties of the prior art materials, particularl~ with regard to haze formation.
The invention will be descr-ibed further in connection with the accompanying drawings, in which :-~igure 1 illustrates apparatus for batch extraction of hops with liquid C02, and Figure 2 schematicall~ illustrates apparatus for semi-continuous extraction of hops with liquid C02~
Figure 3 schematically illustrates apparatus for con- -- verting a primary extract into an iso-~-acid preparation suitable for addition to beer.
In Figure 1, hops 1 are contained in a press~re cell having inlet 3 wlth associated tap 4, outlet 5~ including sintered metal filter 67 with associated tap 79 observation windows 8 and press~e gauge 9. To extract the hops, tap 7 is closed and liquid C02 is introduced into the cell ~ by way of inlet 3 and tap 4. ~ap 4 is closed and the cell 2 rocked to mix the hops and liquid C02 to extract the soft resins (~-acids and ~-acids) into the liquid C02, When extractio~
is completed the extract can be transferred to a second pressure cell 10 containing flask 11 and including observatio~

-3-, .

7~

wi~dow 8 and exhaust line 12 provided with heater 13 and tap 14~ ~o transfer, the extract tap 14 is closed and tap 7 is open and the extract flows through filter 6 (thus prevent-ing any fi.nes being carried with the extract) into flask 11 in cell 10. After completio.n of the transfer, tap 7 is closed. This extraction and transfer procedure can be repeated as necessary or desired to extract the hops. ~he extract can be separated from the liquid C02 by opening tap 14, thus controllably venting cell 10 to the atmosphere by way of heater 13.
In ~igure 2, liquid C02 in storage tank 20 is pumped by pump 21 at a pressure monitored by indicator 22 through heat exchanger 23 in which the temperature of the liquid C02 is adjusted to that desired for extraction~ In a com~ercial extractor the storage tank 20 will normally be refrigerated to a temperature of between -15C and -20C. ~or small scale extraction.s the storage tank can conveniently be replaced b~
a number of liquid C02 cylinders arranged in parallel~ When C2 cylinders are used, because of the small scal.e 9 these will ~` 20 nQt usually be deliberately cooled and, being at ambient temperature~ they will have an internal pressure greater than . the vapour pressure o~ liquid C02 at the extraction tempera-tur~ and this excess pressure in the cylinders can be used to force the liquid C02 through the system at a ~uitable flow - 25 rate thus obviating the need for a pump.
As a practical matter it is convenie~t to adjust the :, .
temperature of the liquid C02 at the outlet of the heat . -31-; . :
.,. , ~.
., .
,:
'' ~: . ' exchanger 23 to allow for any temperature differences between ambient temperature and the actual temperature of extraction. lJe expect that the extraction in 11 generally be carried out at sub-ambient rather than super-ambient temper-atures and the liquid C0~ will thus be cooled to compensate for the absorption of heat from the environment, which being at ambient temperature, ma~ be up to 20C warmer than the liquid C02 in the extraction column. To minimize such undesired heat transfer to the liquid C02, the extraction column and associated pipework valves etc~ will normally be lagged to give a suitable degree of thermal insulation~ With a suitably insulated column the liquid C02 will not usually need to be more than about 2~ or 3C cooler than the desired e~traction temperature. We have fo~nd that about 3C is generally a suitable margin in small scale extractio~ apparatus where heat pick up problems are more likely to arise tha~ orL
commercial equipment because of the proportionately larger surfa&e area.
~rom the heat exchanger 23 the liquid C? is passed through the extractio~ column 24 containing hops in suitable form, e.g. as a powder or crushed pellets~ In the apparatus illustrated the colu~n is arranged vertically with the liquid C2 passing upwards through the column.
Although onl~ one extraction column is illustrated in ~igure 2, others may be placed in parallel with it~ e.g.
between B and B' and by switching the flow of C0~ through such a plurality o~ columns (the neeessary taps etc. ~re not shown) it is possible to run the apparatus semi-continuous~J, e.g. when columns containing spent hops are isolated from t~e pressuxe circuit to refil them with fresh hops whilst con-tinuing the extraction through the other column(s) thus enabling extract to be obtained continuously.
From the top of the extraction column the liquid C02 containing the extract is fed to an evaporator 25 where the C2 is boiled off through pipe 28~ The evaporator is heated, e.gO by warm water entering at 27a and leaving at 27b. This warm water can conveniently be at 40C, although the temper-ature of the heating medium will depend to some extent on the extraction temperature and will be chosen to ensure adequate heat transfer and to avoid isolation of the extract at a temperature so low that it is very viscous of solid or so high that it may be degraded or decomposed. The ex~ract, which will typically be a liquid at the contac-t -temperature within the evaporator, is collected within the evaporator or a collecting vessel connected thereto and can be removed from time to time via tap 26.
r~he ~a~eous C02 coming from the evaporator Z5 at outlet 28 i5 passed to condenser 29 in which the gaseous C02 is cooled to reliquefy it. The liquid C02 is passed to storage - tank 20. From tank 20 the liquid C02 is pumped by pump 21 through heat exchanger 25 and thus recycled through column 24 to evaporator 25. Fresh liquid C02 can be introduced into the circuit via valve 30 to replace C02 lost when removing ex-~ract via valve 26 or when refillinc extraction collunns~
.
~33~

-, . .
' ~
i. , : : ~

In ~iKure 2 the liquid C02 is showrl as passing npward1y through extraction column 24. Whilst we prefer for engineer-ing reasons to pass the liquid C02 upwardly it is possible to pass it downwarclly through a column of hops if desired.
Other forms of equipment m~y be used to effect the ex-traction with liquid C02 and the extraction step of the inven-tion is not limited by the particular form of apparatus in which the e~traction is performed.
In ~igure 3 high quality primary liquid C02 hop extract 40~ e.g. that produced using the apparatus of ~igure 1 or 2 as des~ribed, is fed together with ~n alkali, e.g. sodium or potassium carbonate solution, of a suitable concentration, e~g. 0.01N to 0.5N~ into a vessel 4~ provided with heating coil 4~ and condenser 44. ~he solution of hop extract in al~ali t~pically containing from 0.5 to 2 gl 1 of o-acids at pH 8 to 11 is boiled, eO~. for from 5 minutes to 2 hours, to isomerize the o-acids in the extract to iso-o-acids. The steam evolved during~this boiling carries with it hop oil present i~ the primary extract~ ~he water vapour and hop oil are condensed in condenser 44 and can be collected at 45. After completion of the isomerisation the alka~i~e solution is removed from vessel 42 ~ia valve 46 and passed through Gooler 47 where the solution is preferably cooled to between 0 and 10C. hfter cooling the solutio~ 48 is fed to vessel 49 equippea with stirrer 52 and p~ meter 51~ Acid 5Q~ e~g. hydro-chloric acid at a suitable concentration is fed into vessel 49 to acidify the isomerized extract, e.g. to a p~ of from .

7~

2.5 to 5O The acid mixture is stirred and the ~-acids precipitated. The mixture, containing a filter aid such as kieselguhr if desired is then passed via valve 53 through fil-ter or centrifuge 54. ~he filtered or centrifuged solution 55 is then fed to vessel 56 equipped with stirrer 59 and pH meter 58. Alkali 57, e.g. sodium and/or potassium hydroxide and/or carbonate is fed into vessel 56 to make the solution of iso-~-acid alkaline, e.g. to a pH of from 8 to 10. ~his alkaline solution is then passed via valve 60 to concentrator 61 which may co~veniently be a flash evaporator pro~ided with heating coil 62, vapour trap 63, vacuum pump 64 and condensate collector 65. ~he temperature inside the evaporator is preferably not more than 40C in order to avoid degradation of the iso-~-acids. The concentrat'ed iso-~-acid preparation can be removed from collector 65 via valve 66 Although the apparatus illustrated in Figure 3 is described as producing a concentrated iso-a acid prepara-tion it will be appreciated that, if desired~ the isomerized ~-acid solution may be taken from a previous stage of the prepara-tion, e.~ after filter ~ or from valve 60 if desired.
~urther, the apparatus has been described in batchwise operation; iX desired continuous or partly continuous operation may be effected if desired.
~he design of the individual pieces of equipment for use in such apparatus is straightforward; indeed suitable (although perha~s not optimum equipment) will, we expect, be available "off the shelf". ~ruly co~tinuous operation may -~5-' , :

require custom built equipment but we believe that design of such equipment would not present any subst~ntial diffi-cultry to the skilled chemical engineer. Equipment other than as illustrated may be appropriate to sui-t particular needs and the isomerization step of the method of the inven-tion i5 not limited by the particular form of appara-tus in which it is performed.
~he invention ~ill be described further in the follow-i~g examples.

Extraction of Hops~ Batch Process~ and Conversion to an Ir~ri~ei ~xtr~ ~.
100 g~ of powdered Wye Northdown hops, which contained 7O3% ~-acid~ were extracted in the apparatus shown in ~igure 1 generally as described below.
1.0 litre of liquid carbon dioxide was introduced to the cell 2 via inlet 3 and associated valve 4 at a pressure of 730 p.s.i.g. At this pressure the temperature inside the cell was about 16C. ~he cell was rocked for 15 minutes, during which the pressure increased to 745 po S~ g~ ~ap 7 was opened and the liquid was then passed through sintered filter 6 into flask 11 standing in second cell 10. Tap 7 was closed and tap 14 was the~ opened and the liquid carbon dioxide was evaporated as gas which was vented to the atmos-phere over a period of 45 minutes~ ~he hops were extracted a further seven times using this procedure and the extracts, which contained residual solid carbon dioxide, were bulkedr , .
-~6-"~ , .
. ' :

The flask was removed from the pressurized vessel and the residual solid carbon dioxide was evapora-ted, via a bunsen value, to leave a yellow viscous extract (15.8 g.).
The extract was examined by thin layer chro~atography using the procedure described by the European Brewery Co~ven-tion (J. Illst. ~rewi~g 1970~ ~, 386) and the presence of onl~
~-acids and ~-acids were revealed as two distinct spots when the plate was sprayed with ferric chloride reagent. ~en products obtained by extractin~ hops with organic solvents are examined by this technique, the chromatogram is normally complex and often consists of more than ten spots.
The extract was ~ho~n to contain 40,~/o of ~-acid when estimated by a conductometric procedure (J. Inst. ~rewi~g 1970, 76, 343) using methanolic lead acetate. ~ence 88.1% of the available a-acids were extracted from the hops using liquid carbon dioxide. Examination of the extract by col1~mn chromatography on Sephadex (J. Inst. Brewing, 1972, ~8, 57) revealed that 70.~/o of the ~-acid present in the hops had been extracted.
~ 10.0 portion of the extract was steam distilled using the procedure similar to that described by ~oward (J. Inst.
Brewi~g, 1979, 76, 381) and a total of 450 mg. of hop oil was collected. In this experiment the liquid carbon dioxide had extracted 83% of the available oil ~rom the hop~
Similar yields of oils a~d resins were obtained when powdered Wye Challenger hops were extracted by this process usin~ liquid carbon dioxide~ ~his solvent was used to ., , . :. - . .
" ' ' ~ ' ' ' ~ -. , - - . ~ , - ,.
.
.~ , ' , ,, , : . . , i ' :' ~ ~8~
.

extract whole hop cones from the variety Wye Challenger and also cones which had not been dried, iOe. green hopsO
Hop resins and oil were successfully extracted from these samples.
A 1~1 g. portion of the extract obtained from Wye Northdown hops and containing 40.~ acid was placed in a flask fitted to a condenser via a cohobation head. 500 ml. of 0.1~ sodium carbonate was added and the mixture was boiled for 15 minutes under an atmosphere of nitrogen. ~he hop oil was discarded and the mixture cooled to ~0C and the p~ was adjusted to 4.0 by the addition of 2N hydrochloric acid.
~he mixture was stirred at constant pH for 100 hour, kieselguhr ~10 g.) was added and a stream of nitrogen was passed througk the mixture for 5 minutes to aid flocculation of the a-acids.
The mixture was filtered through a bed of kieselguhr (40 gD ) which was washed with water. ~he filtrate and washings were combined and the p~ of the solution adjusted to 9~0 by the addition of 2~ potassium carbonate to give a dilute solution of the required isomerized extract. A qualitative examination of the extract by thin layer chromatograph~ showed the presence of iso-~-acids together with only trace quantities of ~-a~ids and ~-acids. ~ quantitati~e analysis of this extract using the method of Otter et al (J~ Inst. Brewing, 1972, 78~ 57) revealed that the yield of iso a-acids obtained from the hop extract was 79.~/o. Hence 70.4% of the available ~-acids present in the Wye ~orthdown hops were converted into iso-a~acids.

~38-~he extract was concentrated to 15% iso-~-acids (W/V) usiIl~ a rotary evaporator (both tempera.ure 35C, 15 mm/~lg) without loss of iso-~-acids.
EX~LE 2 PreParation of ~n Isomerlzed Extract (Semi-Continuous ~xtraction of Hops).
= ~ . . .
200 gO of powdered Northern Brewer hops which contained 6.1% of ~-acid were placed in the column of the semi-continu-ous extractor shown in ~igure 2. ~iquid carbon dioxide was circulated through the system at a pressure of 890-910 p.s~i.g.
for 5 hours. The pressure in the system was released and the extract which had collected in the evaporator (temperature 40C) was recovered.
A further five 200 g. samples of hops were extracted in the same manner and the extract obtained from the six extrac-tions bu7ked (15708 g.). ~he extract was examined using the procedures described in Example 1 and the following results were obtained:~
a) only ~-acid ~-acids could be detected by thin layer ~0 chromatography 9 b) 93% of the available ~-acids and 6~/o of the avail-able ~-acids were extracted by -the liquid carbon dioxide, and c) 70% of the available hop oil was present in the extract~
28.0 g. of the hop extract which contained 43% o-acid were isomerized in a similar manner to that described in . . .
, ~9 ,, .

:
;" . . : -,, .

-Example 1 and a yield of 98% iso-~-acid was obtained af~er filtration through kieselguhr. Only trace amoun-ts of ~- and ~-acid were detected in the extract.
~he volume of the aqueous iso-extract at pH 4 (8.2 l) was reduced to below 1 litre by eva~oration at 50/15 mm of Hg~ ~he pH was then adjusted to 9 with 0.2N sodium carbonate solution and the volume made up to 1 litre to give isomerized extract as a pale yellow solution (overall yield of iso-~-acid 88%, i.e. 1.06% W/V ) .
Additions were made to an l1n~opped beer by adding the isomerized extract (diluted to 0.5~/o w/v iso-~-acid) prior to racking at a rate of ~0 mg. iso-~-acid per litre of beer.
~he bitterness of the resultant beer measured by the recom-me~ded method of analysis (J. InstO Brew~, 1971, 77, 181) was 26 EBU. ~he utilization of iso-~-acid in the beer was thus ~6%.
Isomeri~ed extract was also added to beer in the bright beer tank prior to bottling at a rate of 42 mg~ iso-~-acid per litre of beer. ~he bitterness of the bottled beer measured as above was ~5 E3U. ~he utili~ation of iso-~-acid in the beer was thus 83%.
A commercial isomerized hop extract was added to another batch of the same beer in the bright beer tank at the same dilution and ra-te of addition as that used with the abo~e iso-extract. ~1he initial haze readings of the resultant bottled beers, as measured by the recommended methods o~ analysis (J. Inst. ~rew~ 1971, ~, 181), were 0.60 E~C units for the 1~0_ :

' beer bittered ~ith the iso-extract prepared in this ~xP~ple and 4~42 ~BC units for the beer co~taining the commercial isomerized hop e~tract. ~he haze reading of the beer bittered with iso-extract prepared by this Example did not exceed 105 EBC units after 15 wee~s storage. ~eer is reckoned to have satisfactory storage stability with reference to haze formation if the haze reading is not more than 205 ~B~ units after 12 weeks storage. Induced gushing tests ~Jere carried out on the bottle beers by the method of Laws and McGui~ness (J. Inst~ Brewing, 1972 7 78, 30~) when the ]oss of beer recorded was 0.4 g/~ pint for the beer containing iso-extract prepared in this Example and 117.~ g/~ pint for the beer containing -the commercial isomerized hop extract. For comparison, an equivalent traditionally brewed (i.e. not involving addition of extract or iso-extrac~ after brewin~) bottled beer loses typically from 005 to 5 g/~ point under similar testing~ Hence the beer bittered with the new isomerized extract did not have a tendency to gush.

~ s on the Pllot Plant Scale.
830 g. of powdered W~e Northdown hops containing 701% of o_acid were placed in the colum~ of the semi-continuous extrac-tion apparatus illustrated in ~igure 2. Liquid carbon di-o~ide was circulated through the system at a rate of 6,4 litres/hour and pressure o~ 870 p~s~i~go for 5 hours. ~he pressure in the system was released usin~ the vent valve ~d the ~xtract (114~8 g~) which had collected ln the evaporator ~1 -was isomeLiz~d using the procedure described in Example 1 and an.alysis of the product revealed. that the overall effi-ciency of tke extraction of ~-acid and isomerization was 88%
EX~MPLE 4 Samples of a variety of hop cultivars were extracted with liquid C02 and issmerized according to the procedures described in Example 2 above. ~he extracts and isomerize~
extracts were of a similar high purity. ~he results of the extractions and isomerizations are summarised in ~able 3 and the results of tests on haze formation, gushing, bitterness and utilization of these isomerized extracts when added to beer are given in Table 4.

!. . .
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r~ O ~ Is\ r 1 ~ ~ ~ r~ C~
h r~ d C~ ~D C~ C~
~ 0 -~
i~; r~ u~ 0 _ _ _ _ ~ _~___ rl ~
rd ~ OC~ O ~ 11~
~ r~ 0(~ C~ C~ (r~
~_ ~ _ ._ _ _ _____ ~ ~r/
Pl ~D ~ oo ~) ~r~ ~ _ C~'~ ~
P~ ~d ,~ C' O
o ~ t~ a) ~o co P~ ~ , ~ 0 l ~I h rl _ __ rl ~ ~DIs\ (~
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R~-- -------- - ~--.r 3 h ~ Lt~ K~ ~I
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a? P ~ ~ ~ ~

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H h ~ _ _.__ ___.... ___ _.___ a) 1~1 rl O u \ ~ ~1 pd h ~ IS~ o 00 ~_ l ~1 ~ ~,~ oo ~ ;t . a~
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H t~ h ~ h~ __ cll - m- 4~
n ~ ~d ~d ~ o p, ~ d ~rl h ~rl h O
rl h h C) ~ t~ ~ ~ ,q 0 a~ h ~C h ~ O ~ S:~
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Claims (22)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of making an iso-.alpha.-acid preparation which method comprises contacting hops with liquid CO2 at a temperature of not less than -5°C thereby extracting at least a portion of the .alpha.-acids contained in the hops into the carbon dioxide, isolating a primary hop extract of high purity from the liquid carbon dioxide and isomerizing the .alpha.-acids present in the hop extract to iso-.alpha.-acids.

2. A method as claimed in claim 1, performed so that the extract comes in contact with equipment made only of chemically inert materials.

3. A method as claimed in claim 1 wherein the temperature at which the hops are extracted with the liquid CO2 is in the range of from -5°C to 120°C.

4. A method as claimed in claim 1 wherein the primary extract is isomerized by boiling an aqueous alkaline solution of the said extract.

5. A method as claimed in claim 4 wherein the primary extract is not purified before the said isomerization.

6. A method as claimed in claim 4 wherein the alkali used in the aqueous alkaline solution is sodium and/or potassium carbonate.

7. A method as claimed in claim 4 wherein the pH of the aqueous alkaline solution is from 8 to 11.

8. A method as claimed in claim 4 wherein the solution is boiled for from 5 minutes to 3 hours.

9. A method as claimed in claim 4 wherein the concentration of .alpha.-acids in the aqueous alkaline solution is from 0.5 to 300 gl-1.

10. A method as claimed in claim 4 wherein .beta.-acids present in the said primary extract are separated from the iso-.alpha.-acid preparation by acidifying the isomerized preparation and filtering or centrifuging the mixture to remove precipitated .beta.-acids.

11. A method as claimed in claim 10 wherein prior to the acidification the isomerized preparation is cooled to a temperature of not more than 40°C.

12. A method as claimed in claim 11 wherein the temperature is from 0 to 10°C.

13. A method as claimed in claim 10 wherein the pH of the acidified solution is from 2 to 5.

14. A method as claimed in claim 13 wherein the pH is about 4.

15. A method as claimed in claim 10 wherein the concentration of iso-.alpha.-acids is not more than 5 gl-1 immediately prior to the acidification.

16. A method as claimed in claim 10 wherein a filter acid is included in the solution and the .beta.-acids are removed by filtration.

17. A method as claimed in claim 10 wherein, after removal of the .beta.-acids, the iso-.alpha.-acid preparation is made alkaline.

18. A method as claimed in claim 17 wherein the pH of the alkaline iso-.alpha.-acid preparation is from 8 to 10.

19. A method as claimed in claim 18 wherein the pH is about 9.

20. A method as claimed in claim 10 wherein, after removal of the .beta.-acids, the iso-.alpha.-acid preparation is concentrated to a predetermined extent.

21. A method as claimed in claim 20 wherein the preparation is concen-trated by evaporation at a temperature of not more than 40°C.

22. A method as claimed in claim 21 wherein the evaporation is carried out under reduced pressure.