The staling of coffee I

7/27/2019 The staling of coffee I

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TH E STALING OF COFFEES A MU EL C. P RES C OTT ; ROBERT L. E M E R S O N , AND

L. V E R N O N PEAKES, J R .Department of Biology and Public Health,

Massachusetts Institute of Technology, Cambridge, Massachusetts(Received f o r publ ica t ion, December 4, 1936)

INTRODUCTIONBroad investigations on the chemistry of coffee from many angles

have been carried out by various workers and a vast literature hasaccumulated. The present work deals with but one phase of thestudy, namely, substances involved in changes which the roasted prod-uct undergoes when kept f o r extended periods whereby the modifica-tion of flavor known variously as ((flatness, staleness,) o r ran-cidity results. Since these changes are more marked in groundcoffee than in the roasted whole bean, and more readily observed incoffee freshly exposed t o air than in coffee not so exposed, they arecommonly regarded as oxidative in character. In recent years it hasbeen assumed in some quarters that it is oxidation or saturation of fatsnormally occurring in the bean which has been the principal cause ofstaleness o r rancidity. However, there are reasons for believingthat the oxidation theory is inadequate to explain all changes whichare brought about. Since the literature of coffee is so voluminous, ref-erence can be made to only a few papers which have dealt with theaspect of the work Kith Khich this article is concerned.

At the present time the knowledge as to chemical changes respon-sible f o r loss o r modification of flavor as exhibited i n freshly roastedcoffee is far from complete. We find statements such as that ofWendt (1930) who says that rancidity is due to oxidation of fa tof the coffee bean, and that of Trigg (1919) claiming that deteriora-tion is due to hydrolysis, alteration, and volatilization of its aromaticprinciples. More recently statements f r o m the Continental Can Re-search Fellowship at the Mellon Institute (Weidlein, 1934) attributestaleness to oxidation and volatilization of substances causing aromaand to oxidation o r rancidity of coffee oil. While such statements maycontain considerable truth, no experimental data are given t o supportthem, and they are too general to provide a satisfactory explanation.

A systematic investigation of the chemistry of deterioration ofcoffee has been carried on for many months by Bengis and Anderson(1932, 1934) and they have reported isolation of a constituent ofthe unsaponifiable mat ter which they term Eahweol. This is an

This inves t iga tion was mad e poss ible throug h a gene rous cont r ibut ion f rom th eAm er ican Can Company.FOODESEARCH,VOL. 2 , No. 1. 1


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2 S. C . P R E S C O T T , R . L. E M E R S O N A N D L. V P E A K E S , J R .unsaturated substance which changes very ra.pidly in presence ofair o r acids. They have also attempted to follow changes takingplace in coffee fat during a period of sixteen months, but coulddetect only very slight changes in the amount of extractible fat ,unsaponifiable content, iodine value, and saponification value of thefat or in the ratios of the various acid components of these glyceridesduring this period.

Several years ago some preliminary studies on rancidity of coffeewere undertaken by the authors, results of which appeared in aReport on an Investigation of Coffee published by the Jo int CoffeeTrade Publici ty Committee. After a considerable hiatus dur ing whichit was o u r intention to resume the investigation, and before theappearance of the papers of Bengis and Andersou (1932, 1934) onchanges in coffee fat, investigations were resumed in this labora-tory.At that time it seemed likely that fat played an important role instaleness of coffee, and this was the first aspect of the problem to beconsidered.

E X P E R I M E N T A L WORKDuring the earlier work carefully prepared samples of coffee ex-

tracts had been made. Instead of waiting f o r cofee to age, therefore,these samples, which were undoubtedly in a more advanced state ofdeterioration than those examined by Bengis and Anderson (1932,1934), were subjected t o cer tain chemical %tests. The samples gaveclear evidence of deteriorative change and were investigated withreference to acid value, saponification number, percentage of un-saponifiable matter, iodine value, and peroxide content. These werecompared with fresh coffee extracts prepared in a similar ma.nner,with extracts five and one-half months old, and also with freshlyprepared extract of ground coffee which had been kept in groundcondition fo r five and one-half months. The comparative results ofthese examinations are summarized (Table 1).I n order to place on record the methods followed in this investiga-tion the following descriptions of the extracts are presented :

Descr ip t ion of ExtractsExtract A was obtained by continuous extraction of freshly roasted and

ground coffee with petroleum ether which had been freed from unsaturated hydro-carbon by refluxing over concentrated sulfuric acid, washing with water, anddistilling over stick sodium hydroxide. Only that portion of the petroleum etherwhich boiled f r o m 42 t o 80C. (107.6 to 176F.) was used. Aft er extraction ofthe .fa t, the petroleum ether was removed by distilling o n a water bath. Thefinal stages of distillation were carried out under reduced pressure. The temper-ature of the water bath never exceeded 70C. (158F.). The weight of the extra ctwas 8.2 per cent of that of the coffee used. It was a dar k brown liquid of fa ir lylo w viscosity a n d surface tension and ha d a pleasant coffee odor.


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4 S. C . PRESCOTT, R. L. E M E R S O N AND L. V. PEAKES, J R .

one-half months. This fac t is quite contrary t o the opinion of atleast one writer, Bredt (1934), who claims that while extractedfats of coffee grow rancid rather slowly in the mass and underordinary conditions, the alteration occurs rapidly when they remainin contact with the great surface actually exposed in a large quantityof coffee beans.

In opposition t o this view our data on the lower iodine valueand the high peroxide value of the fa t kept five and one-half monthsafter extraction would indicate that decomposition took place morerapidly when the fat is in compact form than when it is dispersed.This difference is probably due to action of light in one case and in-accessibility of fat in the bean to atmospheric oxygen in the other.During a n aging period of five and one-half months only the acidand peroxide values changed sufficiently to give any promise as ameans of measurement of staleness o r rancidity. Peroxide contentshave been used by Greenbank and Holm (1934), Wheeler (1932),Lea (1931), and others to detect rancidity in fats and oils. AlthoughCoe and LeClerc (1934) have recently cast some doubts upon thevalue of such determinations as a measure of rancidity, peroxide con-tent is still considered the most reliable test f or rancidity of fats. Forthis reason i t seemed desirable to measure the peroxide contents of thesame coffee at different degrees of staleness to see if any relationexisted between this constant and the degree of deterioration. Af terconsiderable experimentation with various solvents and methods ofextraction the following procedure was found quite satisfactory.

Ten grams of ground coffee were placed in a ground glass-stop-pered Erlenmeyer flask with 50 C.C. of carbon tetrachloride whichhad previously been shaken with a little mercury t o remove traces ofchlorine. The mixture was shaken in a shaking machine for threehours. At the end of tha t time 25 C.C. of the solvent were filteredoff and placed in another ground glass-stoppered flask; 1 C.C. of asaturated solution of potassium iodide in water and 25 C.C.of glacialacetic acid were added. After the mixture had stood for 20 minutesin the dark, 100 C.C. of water and a little starch indicator solutionwere added. The mixture was then tit rated with 0.002 N sodinm thio-sulfate until the water layer became colorless. The peroxide contentin millimoles of peroxide per kilogram of coffee was computed a sfollows :

C.C. f 0.002 N thiosulfate10 = L peroxide contentX

The value may be somewhat in error, as the 25 C.C. of solventfiltered off probably does not contain exactly one-half of the peroxides


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THE STALING O F COFFEE 5in the 10 grams of coffee. However, the error is probably fairly con-stant and the values obtained comparable.The time of decomposition of the peroxides before titration wasvaried with the following results :

Length of time Peroxide value15 min. 0.4720 min. 0.4725 min. 0.5130 min. 0.54

The higher values obtained on 25 and 30 minutes standing areprobably due to oxidation of the hydrogen iodide by atmosphericoxygen. The differences are slight, and 20 minutes was chosen asthe best time.The amount of iodine adsorbed by the coffee fa t was estimated byextracting a sample of freshly roasted coffee containingno peroxides,exactly as in the determination above, and then adding a knownEffec t of Aging o n CoffeeTABLE 2

Description of coffeeCoffee 7 mo. old kept in the bean, freshly

ground just previous to testing................Same as above, but kept 7 mo . in

the ground state...........................................Coffee 2% mo. old, kept in the bean and

freshly ground before testing....................Same as above, but kept 294 mo. in

the ground state..........................................Coffee 1mo. old, kept in

the ground state..........................................

Peroxidecontent

00.5010.65

0

Trace

0

A c i dvalue

6.4

5.5

3.4

4.0

4.6

TastetestTasteless bu t

not rancidSlightlyrancid

Stale but notrancid

Rancid

Stale but notrancid

~

IDifferent parts of the sam e bag.amount of iodine (equivalent to 7.65 C.C.of 0.002 N thiosulfate), theacetic acid, and saturated potassium iodide solution; allowing it t oremain 20 minutes and completing the analysis as usual. An amountof iodine equivalent to 0.07 C.C. of 0.002 normal thiosulfate was ad-sorbed. This would lower the peroxide value by only 0.014 which isoutside the limit of accuracy of the determination.

That neither peroxide content nor acid value is a reliable measureof the staleness of coffee is quite evident from the summary of resultsof the application of this determination to various samples of coffee(Table 2). One cause of the low peroxide values of some stale coffees


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6 S. C. PRESCOTT, R. L. EMERSON A N D L. V. PEAKES, J R .

3.83.93. 94.0

might be the decomposition of the peroxides to give products of un-pleasant taste and smell. It seemed advisable, therefore, to study theeffect of moisture on the taste, odor, peroxide content, and acid valueof the coffee.

W F E C T OF M O I S T U R E ON COFFEEThe effect of moisture during the storing of coffee was determined

by keeping one lot of coffee in a very d ry atmosphere and anotherbatch in contact with air with a relative humidity of 75 per cent.The acid values, moisture, and peroxide values were determined atintervals. Taste tests were run at the same time. The humidity wascontrolled by placing small crystallizing dishes containing the groundcoffee in desiccators, one of which contained anhydrous calcium chlo-ride, and the other a saturated sodium chloride solution. The saltsolution gave a relative humidity of approximately 75 per cent overthe range of temperatures existing in the laboratory. The dishes

TABLE 3E f f e c t of Mo is tu re on Boastea Cof fee

p e t .0 2.380 .....

Trace .....0 1.4

Sample kept over anhydrous Sample kept over a saturatedcalcium chloride I Weeks I sodium chloride solution0369

Acid I Peroxide 1 Yoisturevalue content3.8 04.2 04.3 04.2 0

instorage I Acid I Peroxidevalue content Moisturep e t .2.38..........9.1

were placed so that free circulation of air within the desiccators w a spossible. The coffee was frequently stir red and positions of the disheschanged so as to obtain uniform aging. The desiccators were placedin a dark closet to eliminate any effect of light.

The first test was made after three weeks. It disclosed that thecoffee kept in a moist atmosphere was absolutely lacking in coffeeflavor. The sample kept in a dry atmosphere was but li ttle weakerthan fresh coffee and could not be classed as stale even after sixweeks had elapsed. The moisture contents, peroxide values, and acidvalues are shown (Table 3 ) . The acid value is that of the extractedfat and does not depend on the moisture content of the coffee.

While this experiment shows the importance of moisture in thedeterioration of coffee, it does not substantiate the hypothesis thatperoxides are intermediates in this degradation, for n o peroxidescould be detected in the sample kept in the moisture-free atmosphere.I n all this work no convincing evidence has been obtained t o showthat fat in coffee becomes rancid in the relatively short time that


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THE STALING O F COFFEE 7it take s coffee to become stale. F o r this reason it seemed wise t o t u r nattention to other consti tuents of coffee, especially to the aromaticoils which give it t a s te and odor. Another ind ica t ion th a t such aninves t igation might be advisable was t h a t th e salt so lu t ion in thedes icca to r used in the exper iment jus t descr ibed con ta ined smal lamo unts of fu rfu ra ld eh yd e; an d the volat i le o ils of coffee have beenfound to con ta in fu ra n der iva tives.

STUDY O F VOLATILE CONSTITUENTS OF COFFEEA num ber of inves t igators have analyzed th e volat i le oi ls f ro m cof-

fee. There is cons iderable d ivergence in the i r r esu lt s . A pa r t of th i s isprobab ly d ue to the use of d ifferent ty pe s of coffee. Oth er differencesma y be due to variat ion i n meth od of isolation of the oils. Sti l l othersmust be a t t r ibuted to mis interpreta t ion of exper imental resul ts .

Summary of Previous WorkT h e first system atic s tu dy of th e arom atic oils of coffee was un de r-

taken by Oscar Bernhe imer (1880). H e ob ta ined o il s by condens ingcoffee-roaster gases. F ro m these he isolated a mater ia l which he be-lieved t o be a methyl der ivat ive of sal igenin . The mater ia l which heobtained dis t i l led f rom 195 t o 198C. (383 to 388.4"F. ) . It w a s t h epr incipal cons t i tuent of the l iquid por t ion of h is product . The o thercomponen ts foun d which m ight affect arom a were acet ic acid , methyl-amine , pyr ro l , an d ace tone .

I f the mate r ia l ob ta ined by Bernhe imer was a methy l der iva t iveof saligenin it would havd had one of the fo l lowing form ulas :

o-methoxybenzylalcohol

o-hydroxybenzylmethyle t h e rT h e first of these compoun ds had been prep ared by C annizz arro andK o r n e r (1872). Bijtsch (1880) pointed o u t that th is could not be the

same compound th a t B ernhe imer had , fo r i t s phys ica l p roper t i es werequi te d if ferent. There is, for example, a difference of over 50 degreesin boil ing points. H e bel ieved th at it was p robab ly the compoundrepresen ted by Formula 11. This was shown not to be the case ,however , by Thiele an d Dim roth (1889) who synthes ized it. T h i scompound cer ta in ly cou ld no t be tha t i so la ted by Bernhe imer , as itcould not be dis t i l led a t a tmospher ic pressure without a lmost com-plete res in ification, while th at obtained f r om coffee dis t i lled a t a lmost200C. (392 F.) u nd er normal pressure . Nei ther of the two ethersof sal igenin which were prepared synthet ical ly resembled coffee in


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8 S. C. PRESCOTT, R. L. EMERSON A N D L. V . PEAKES, JR.odor, although Botsch thought tha t his crude methoxybenzyl alcoholhad coffee-like odor which disappeared on purification.

Although this work would seem to prove rather definitely thatBernheimer could not have had a methyl ether of saligenin, a numberof years later Sethness (1924) published a paper in which he speaksof methyl ether of saligenin being the chief constituent of the aromaticoils of coffee. He steam-distilled coffee on a large scale and obtaineda product of like boiling point. Apparently he ran no chemical testson the material. Sethness states that Lehmann and Wilhelm steam-distilled coffee and also found caffeol, o r chemically speaking, themethyl ether of saligenin to be the principal ingredient of the frac-tion possessing the powerful coffee aroma. The only paper that wehave been able to find by Lehmann and Wilhelm (1898) containedno statement o r experimental evidence as to the chemical nature ofsteam distillate of coffee. Bernheimer seems to have been the onlyinvestigator to present chemical evidence that the material boiling at195 to 198C. (383 to 388.4F.) was a derivative of saligenin. Thatit was a methyl ether of this compound seems impossible. It seemsprobable that Sethness and Bernheimer both obtained the same ma-terial, yet no other investigator has obtained any material resemblingthis product as the main constituent of the volatile oils of coffee.

Other investigators have obtained quite different results. A fernyears after Bernheimer, Jaeckle (1898) also analyzed the condensatefrom coffee-roaster gases. He identified acetone, furfuraldehyde,formic and acetic acids, pyridine, ammonia, and trimethylamine.This paper was soon followed by a report of the excellent work byErdmann (1902). Erdmann steam-distilled large amounts of coffeeand analyzed the steam distillate. He used a total of 225 kilogramsof coffee in his experiments. He was the first t o use the coEee itselfas a star ting material rather than the condensate from the roastergases. Erdmann found about 42 per cent of his product t o be acidic.This was mostly valeric acid but contained some acetic acid. Theneutral portion (58 per cent) was largely furfuryl alcohol but alsocontained s o m e phenols and an extremely small amount of a com-pound boiling at 93C. (199.4F.) under 12 mm. pressure. This frac-tion contained nitrogen.

Some years later Bertrand and Weisweiller (1913) analyzed asimilar fraction and reported that the nitrogen content is due t othe presence of pyridine. Grafe (1912) also analyzed the oils in coffeewhich were volatile with steam. He also reports valeric and aceticacids, fur fury l alcohol, phenols, fur fu ra n derivatives, and a pyridinederivative which gives coffee its aroma. Grafe was chiefly interestedin a comparison of normal and decaffeinated coffees and his work is


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TH E STALING O F COFFFEE 9on a comparatively small scale. He depended largely on color reac-tions and odors for his identification.

More recently Schmalfuss and Barthmeyer (1929) have shownthe presence of diacetyl in coffee.

A chronological tabulation of the findings f rom previous work onthe volatile oils of coffee is included here (Table 4 ) .

TABLE 4Bindings from Previous Work on Volatile Oils of Coffee

Investigator0. Bernheimer' (1880).........

A. Monari an dL. Scoeciantil (1895) ..........H. Jaeckle' (1898) ...............

E. Erdmann (1902).............

V. Grafe (1912)...................

G . Bertrand andG.Weisweiller (1913).........R. E. Sethness (1924) .........H. Schmalfuss andH. Barthmeyer (1929) ........

ComDounds identifiedA methyl de rivative of saligenin, caffeine, higherfatty acids, acetic acid, hydroquinone, methylamine,pyrrol, acetone.

Pyridine.Furfural, caffeine, pyridine, ammonia, trimethyl-amine, acetic and formic acids, and acetone.A valeric acid in large amounts, a little acetic acid,phenols, furfuryl alcohol, a fraction boiling 93" C.(199.4' F.) a t 13 mm., containing nitrogen andhaving t he odor of coffee. This last mate rial i n ex-tremely small amounts.Acetic and valeric acids, furfuryl alcohol, furfurancompounds, phenols, and a pyridine derivative.

Pyridine.Methyl ether of saligenin claimed t o be the prin-cipal constituent of coffee oils.

Diacetyl.Analyzed Condensate from coffee-roaster gases.

INVESTIGATION OF VOLATILE OILS IN COFFEEa. Extractiort of Coffee by M e t h o l

The problem of separation of volatile constituents f r o m the re-mainder of the roasted coffee is almost as d%cult and important asidentification of these compounds. The percentage of volatile oil inroasted coffee is always very small, e.g., Erdmann obtained 83.3 gramsof volatile oils from 150 kilograms of roasted coffee. This correspondst o about 0.05 per cent. Three methods of isolation have been used:


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10 S. C. PR.ESCOTT, R. L. EMERSON AND L. V. PEAKES, JR.(1) condensation of liquids and solids in the roaster gases, ( 2 ) steamdistillation of roasted coffee, and ( 3 ) extraction of roasted coffeewith ether. The first of these methods does not necessarily give thesame products as occur in the roasted coffee. The use of steam distil-lation is more reliable, but alteration of oils may occur owing to theaction of the water. As has been previously shown coffee aroma isgreatly affected by action of water vapor. The last method, extrac-tion by a solvent, seemed the most promising and was tried in thislaboratory, but it also had some disadvantages.

A series of preliminary tests indicated that methyl alcohol gavean extract having a very pleasant and distinct coffee odor, and thisalcohol was used as a solvent. It soon became evident that extractionwith this solvent gave an extract which upon distillation of the solventleft a residue which was too viscous to allow the removal of anyvolatile components by vacuum distillation. It mas found possible toprecipitate a par t of this viscous material by addition of ethyl ether.Taking advantage of this fact the procedure indicated (Table 5 ) wasdeveloped. It was necessary to carry out the last precipitation withpetroleum ether as ethyl ether was no longer effective. From the yieldsof volatile oils at the end of this procedure it is quite evident that someof the desired volatile components were occluded in the precipitatesthrown down by the ether. The ether precipitate was semicrystallinebut very difEcult to purify. All attempts t o filter it failed. Decanta-tion of the supernatant liquid was resorted to. The volatile oil wasshown to consist of furfuryl alcohol with a little furfuraldehyde,acetic acid, and some water-insoluble liquid which may have been anester or esters of furfu ryl alcohol. This method was finally abandonedbecause of the length of procedure and small yields of the desiredproduct.

A variation of the above scheme was to add basic lead acetate tothe methanol extract. Much of the viscous material in the extractwill precipitate with basic lead acetate and may be separated in thismanner. However, much of the volatile constituents must also beoccluded in some manner, fo r the yields of these compounds werepractically nil. N o r did this method serve as a method of purificationof the non-volatile materials precipitated by the lead acetate, forupon regeneration by hydrogen sulfide they were more tarry thanbefore.

A description of the four volatile fractions is given below.D e s c r i p t i o n o f V o l a ti l e M a t t e r E x t r a c t e d from Coffee by M e t h a n o l

Frac t ion boi l ing from 77C. (170.6"F.) at 40 mm. pressure. About 0.15 C.C.Pa r t ly m isc ib le wi th wa te r . Re f rac t ive index 1.4480 a t 25.5"C. (77.2"F.).


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TH E STALING O F C O F F E E

( a t t e m pt s a t pu ri fi ca -t ion f u t i l e )

11

Concent ra te in ana tm. of COz

Solid, m.p. 232-236' C.on an immers ionthe rmomete r(probably caffe ine)

Concent ra te in an a tm.of cozThick concent ra te

A d d e t h e rSolut ion (no sepa ra t ion)

Add pe t r o l e um e t he rIT a r r y m a t e ri a l IPe t roloum e the r + e the r + me t ha no l

Solut ionConcent ra te

Concent ra ted ext rac t { (Mine ra l oilVacuum a d d e d t o r e-di s t i l duce f ro th in g)IF our f r a c t i onsIRes idue(di sca rded)


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1 2 S. C. PRESCOTT, R . L. EMERSON AND L. V. PEAKES, JR.Gave a stro ng fuchsin aldehyde test. Vapors colored aniline acetate paperred. Formed a dimethon whose crystalline structure resembled that fromfurfuraldehyde when viewed under a microscope. It was mixed mith smalldroplets of oil.

From the above data it was concluded that this fraction mas largelyfurfuraldeh yde. The boiling point of furfuraldeh ydeis 77 to 78C. (170.6 to172.4'F.) a t 40 mm. pressure.

Fraction boiling from 88 to 94C. (190.4 to 201.2"F.) a t 40 m m . pressure.About one-half soluble in water. A s m a l l amount treated with a naph-

thylisocynate gave a urethane melting at 127 to 128C. (260.6 to 262.4"F.).The urethane from fqrf ury l alcohol melts at 129C.

After extracting with water this fraction had a refractive index of 1.4628at 25.5"C. ( 7 7 . 2 " F . ) . The refractive index of furfuryl formate is 1.4672;tha t of furfu ryl acetate, 1.4604; both at 25.5"C. (77 .2"F.) . This may be oneor a mixture of these esters.

Fractio n boiling 92 to 100C. (197.6 t o 212F.) a t 4.5 mm. pressure. About

About 0. 3 C.C.

0.1 C.C.Saponification value 47.

Fraction boiling a t 109C. (228.2"F.) a t atmospheric pressure. About 1 drop.Identified a s acetic acid by t he p-ni trobenzyl ester, m.p. 78C. (172.4"F.F.).

b. Extraction of the Coffee BrewFrom exper ience wi th ext ract ing coffee wi th methanol we fel tquite certain some method of separation of volati le consti tuents fromother extrac tible mate rial must be devised. A change in solvent wasalso considered, but even with a solvent l ike petroleum ether , whichextracts a comparat ively smal l amount of material , the rat io of non-volati le to volati le consti tuents in the ext ract is very high. One simplemethod of separating the desired volati le oils from other materialsextracted by organic solvents is to make f irs t a coffee brew (w ate rex t r ac t ) an d t h en ex t r ac t it with a n organic solvent. A n extract ionapparatus was devised in which the coffee brew was dropped in afine s pra y through columns of organic solvent five feet long. A t firstthree solvents were used in succession, ethyl ether , chloroform, and

ethyl acetate. Chloroform was then el iminated because it extractedthe same compounds as e thyl e ther an d necess ita ted a bothersome typeof a p p a r a t u s in which the brew passed up through the solvent ra therthan be ing dropped down th rough it as in the case of the l ightersolvents.

A test was a lso run to de te rmine the number of five-foot tubes ofether necessary f o r complete extraction of the coffee. Practicallynothing could be ext racted af ter the brew had passed through 27t u b es an d t h e amo u n t of volat i le mater ia l obtained f rom the nine-teenth to twenty-seventh tube was hardly suffic ient to w ar ra nt the in-


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TH E STALING O F COFFEE 1 3crease in time necessary f o r their use. The nature of the extractedoil seemed the same as in the previous tubes.

The scheme followed consists of making 10.5 kilograms of Xantoscoffee into 45 li ters of brew. The coffee is roasted in the laboratorywithin a few hours of the time it is used. A medium roast is em-ployed. The coffee brew is then passed through 20 tubes of ethylether, five feet long and about two centimeters in diameter. Afterpassing through the ether, the coffee is then dropped down 10 tubesof ethyl acetate. Impurities in the ethyl acetate interfered withidentification of any compounds extracted from the coffee brew, soit is uncertain at present if this extraction is worth while.

The ether in the tubes was changed af ter nine l iters of brew hadpassed through. The ether extract was dried over anhydrous sodiumsulfate and then anhydrous calcium sulfate (Drie rite) . TLe extractwas concentrated by distilling off the ether in an atmosphere ofcarbon dioxide gas. Usually the concentration was interrupted whenthe volume of the solution had reached about 200 C.C. and this wasthen combined with other similar batches. Af ter again drying overanhydrous calcium sulfate the greater part of the ether was removedf r o m the combined extracts by distillation o n a water bath. Carbondioxide gas was passed through the apparatus to reduce the amountof oxidation. The final stages of the distillation were carried outunder reduced pressure. The receiver was covered with dry ice so asto condense all the distillate. The pressure was reduced as low aspossible so as to reduce the temperature of distillation. Usually twoto four millimeters were obtained. No attempt was made t o determinethe boiling points, as superheating always occurred. The distillatewas carefully redistilled fractionally later. The temperature of the oilbath used for heating was never over 160C. (320 F.) .

c. A n a ly s i s of Ether Extract of Coffee BrewThe distillate from the ether extract was separated into the fol-A. That distilling at room temperature under 5 mm. pressure.

About 1 .2 C.C.B. That distilling up t o 70C. (158F . ) at 40 mm. About 2 C.C.C. That distilling from 70 t o 100C. (158 to 212F.) at 40 mm.

About 4 C.C.D. That distilling from 100C. (212F.) at 40 mm. t o 120C.(248F.) a.t 3 to 4 mm. About 1.5 C.C.

lowing fractions:

These fractions were treated as follows:Fraction A.Redis t i l led, this f rac t ion boi led from 70 t o 78"O. (158 to 172.4"F.) at a tmos-pher ic pressure . It was a lmost complete ly water soluble. It was ident i f ied as e thyla lcohol by th e a -naphth ylca rbam ate , m.p . 77 to 78C. (170 .6 to 172.4"F.).


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1 4 S. C . P R E S C OT T , R.. L. E M E R S O N A N D L. V . P E A K E S , J R .Fract ion B.

On redistillation nearly all of this fraction boiled at 110 t o 118C. (230 to244.4"F.). A little resinified material w a s left in the bottom of the distillingflask. It had an acid value of 124. Acetic acid has a theoretical value at 167.It was identitied as acetic acid by the p-bromophenacyl ester, m.p. 85 to 86C.(185 t o 1863F.) . The acetic ester of p-bromophenacyl alcohol melts a t 86C.(18623F.). A mixed melting point with the derivative prepared f rom acetic showedno depression. This fra ctio n is evidently essentially acetic acid.

Fract ion C .An acid value showed that this fraction still contained considerable acid.

The fraction gave about 0.5 C.C. of liquid distilling below 70C. (158F.) a t 40 mm.The remainder distilled fro m 77 to 100C. (170.6 to 212F.) under 40 mm. pressure.The last fraction was largely soluble in water. Af ter shaking with several timesits volume all but 0.5 C.C. dissolved. The water-insoluble portion was dried overanhydrous sodium sulfate and then over anhydrous calcium sulfate and distilled.A few drops distilled at 77 t o 85C. (170.6 to 212F.) a t 40 mm. This gave astro ng aniline acet ate reaction. This is probably largely furfuraldehyde.

The remainder of t he water-insoluble portion distilled at 95 to 98C. (203 to208.4"F.) a t 40 mm. A saponification value corresponded to about 50 per centester calculated as furfur yl acetate. B.P. of furf ury l acetate 95 t o 97C. (203 to206.6"F.) a t 40 mm. The water solution of the soluble components was saturatedwith sa lt and extracted several times with ether. These ether ex tract s mere com-bined an d d-ried over anhydrou s calcium sulfate.

After the ether wa s removed by distillation the residue distilled at 91 t o 94C.(195.8 to 201.29F.) at 40 mm. Fu rf ur yl alcohol boils at 92 to 94C. (197.6 to201.2"F.) a t 40 mm. This material was identified as fu rfu ryl alcohol by itsa-naphthylca rbamate, m.p. 127 to 128C. (260.6 to 262.4"F.). The melting po intof the a-naphthylcarb amate fro m fur fu ry l alcohol is given as 129C. (264.2"F.).A mixed m.p. with the urethane prepared from furfuryl showed no depression.The residue in th e flask (1 o 2 drops), af ter th e distillation of the material shownt o be furfuryl alcohol, smelled like phenol or th e cresols. The flask was washedout with water and a drop of very dilute ferri c chloride solution added. A deepviolet color appeared . To another portio n of th e water solution bromine water wasadded. A whit e turb idity appe ared. However, practically no precipitate settled outo n standing.The one-half cubic centime ter of mater ial d istilling below 70C. (168F.) at40 mm. was redistille d. It distilled a t 115 to 140C. (239 to 284F.) at atmosphericpressure. The greater pa rt of it distilled fro m 115 t o 120C. (239 to 248F.). Therewas a litt le ta rry material le ft in the flask. The distillable material was identified a sacetic acid b y making the p-bromophenacyl ester, m.p. 85 t o 86C. (185 t o 186.8"F.).

Fr om t he above we can conclude th at this f ract ion was one-eighth acetic acid.The remainder was nearly all furfuryl alcohol but also contained some ester,probably fu rfu ryl formate, and a small amount of a phenol.

Fract ion D .The material in this fr acti on was very viscous; it was redistilled. Only ab ou t1 x. distilled. Most of thi s boiled at 95 to 103C. (203 t o 217.4"F.) a t 1 o 1.5 mm.

There was considerable loss in manipulation owing to the high viscosity of thematerial. A sodium fusion followed by a cyanide test showed the presence of


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TH E STALING O F C O F F E l 3 15nitrogen. It had an acid value ,of 34.5 (ex. 0.1 normal alkali per g ram ). It wa sshaken with several times its volume of w ater. The insoluble oil was separatedan d dried over anhydrous sodium s ulf ate and the n over anhydrous calcium sul-fate . This water-insoluble portion still gave a positive but much weaker nitrogentest. It had an acid value of 34.7. Phenolphthalein was used as the indicatoran d the end point was sharp. The material was very viscous. It gave a lightviolet color with dilu te fer ri c chloride solution. There was too little le ft to t es tfurther.

The water solution of the water soluble components was satu rat ed with saltan d extracted several times with ethyl ether. These ex tracts were combined anddried over sodium sulf at e an d the n over calcium sulfate. Th e ethe r was removedby distillation. The residue was quit e fluid. It had a n acid value of 33.5. Th eend point was poor and the red color of phenolphthalein kept increasing as morealk ali was added. This residue contained nitrogen. The test was much strongerthan i n th e case of the water-solubl e portion. The color formed by the addit ionof a few drops of dilute ferri c chlorid e solution to a water solution of t he ma-terial was a very intense violet. This mas much stron ger th an in th e case ofth e water-insoluble portion. A water solution reduced Tollen's reagent immedi-ately at room temperature. It did not give Feigl's ( 1 9 3 4 ) test for primaryarom atic amines with 4-pyridyl pyridium chloride solution. Appa rent ly it isnot positive that the substance is n o t a primary aromatic amine, as methylant hra nil ate did not give the test, although anthranilic acid gave a very finepositive result.

From the above data it seems likely that the viscous, water-insoluble materidis an acid. It may be a fa tt y acid f rom the coffee fat. Its acid value is aboxtright. The water-soluble portion is probably a phenol. The indistinc t end pointcolor with fer ric chloride and reduction of Tollen's reag ent all point to this.

Redistillation of Last Portions of Ether WhichDistilled at Atmospheric Pressure

The las t f ew cub ic cen t imete r s of e ther t o d i s t il l a t a tmospher icpres sure had been saved f rom severa l runs. These were combinedand d i s t i l l ed th rough a good column hav ing a wa te r re f lux . A f te r asmal l quant i ty d is t i l led , the d is t i l la te , between 78 a n d 9 0 " C. (172.4and 194 ' F. ) , became very yel low and smel led of d iacetyl . A l i t t leo f the d i s t i l l a te was ex t rac ted wi th wate r and t r ea ted wi th n icke lsu l fa te , hydroxy lamine su l fa te , and ammonia . The b r igh t r ed n icke ld imethy lg lyos ime p rec ip i ta ted . Th e f rac t ion was near ly a l l solublei n wa ter a n d smelled of alcohol. The ex tracti on of th e coffee by e theran d the ident i f icat ion of the extracted com pounds is summarizedi n t a b u l a r form (Tab le 6 ) .

DISCUSSION O F PRESENT WORK A N D THAT O FPREVIOUS INVESTIGATORS

The resul ts of the present inves t igat ion do not whol ly agree withthose of an y previous inves t igat ion. We a g re e w i th G r a f e ( 1 9 1 2 ) a n dE r d m a n n ( 1 9 0 2 ) t h a t f u r f n r y l a lc o ho l i s t h e c hie f n e u t r a l c o n st i tu e n t


16/20

TABLE

EaooCoeBrww

Eth

15koamsoce-k4

eowe

P

hoeheh

Dayd

ebyow

coo

Bnra7

IS

bew

I

Vmdse

I

Ehea

I

P

ho

ehaae

Rse

C

aenaam.

Iocbdodg

IlaOeh

.1

J

FaoA10e

Beaom

tema4m

Rsei

bea7o7

C(1o14

F)aamopc

peueId

feaa

b

anhh

cbmae

-

FaoB2CC

BP1o1C.

(2

024F)

C(2

o24F)

w

rese

Id

eaac

adbhp

bom

ee

1

FaoC4CC

Rse I

FaoBP7o1C

(16o2F.)a4m

IAcad05CC

Wae

IWaeno05

CCBP9o

9C(2

o

24.)a

4mS

fcoshw

5%eea

fuuya

tae

I

WaesoBP

9o9C

(18o

22)a

4m

Id

ea

fuuyac

banh

cbmae

Rdae

ressme

op

Cowh

F8au

bdywhEn

.1

FaoD15CC

RseBP9o30C

(2

o24.)a1o15m

Wae

I

Waeso

SoNe

I

Waeno

WeNe

Weco

Ineco

whFC%so

whFCs

AV35

R

Vs

Tesre

AV35

Epnn

dsnPo

ayap

Fyfud


17/20

THE STALING O F COFFEE 1 7of a rom atic oils of coffee. E rd m a n n foun d a valeric acid, however, tobe the ma in acid component. Acet ic was found to be the pr incip alac id in the p resen t work . There can be l i tt l e doub t th a t th i s was theac id occur r ing in our produc ts , as both the odor a nd boi l ing points ofthe two acids could not be mis taken. I n addi t ion to th is we have theadd itiona l evidence of a mixed m elting of th e p-bromo phen acyl esterof t he acid f rom the coffee an d th at f ro m acet ic acid . San tos coffeewas used in both researches.

The method of isolation of the oils was different and may possiblyh a v e s o m e t h in g t o d o w i t h t h i s d i v e rg e n cy . G r a f e r e p o r t s t h a t h i sacid f ract io n smel led of acet ic acid b ut calculates it as a va le r ic ac idon t i t r a t ion . H e g ives no d e ri v at iv e s. B o t h E r d m a n n a n d G r a f eagree that coffee owes i ts odor to a ni t rogen-con ta in ing componnd .Sethness (1924) also repor ts n i t rogen in his caffeol f ract ions . Thehigh-boiling, water-so luble fra cti on which we fo un d had a f ine coffeearoma. It con ta ined n i t rogen an d may be the compound t o whichcoffee owes i ts del icate arom a. How ever , we have fo und th at it isv e r y d a n g e r o u s t o j u d g e b y t h e o d o r of m a t e r i a l s e x t ra c t e d. E x t r a c t sha vin g a good coffee aro m a m ay be alm ost entire ly composed of ma te-rials of different aromas o r of pract ical ly no aroma. Whatever com-pound gives coffee i ts p leasant odor must occur i n extremely smal lamo unts an d be capable of scent ing huge amo unts of mate r ia l .

As pointed out i n th e discuss ion of the p revious work, i t has beenra t he r well es tab l ished th a t Bernhe imer (1880) could not have had ameth yl der ivat ive of sal igenin . How ever , we examined our extrac tsfo r indicat ions of the presence of th is comp ound. It cer ta in ly w as no ta m ajo r consti t uen t of o u r volati le oils. Bo th ether s of saligenin boil20 to 40 degrees h igher than the mater ia ls obtained by us when dis-t i l led a t 40 m m . It is hard to bel ieve that a d if ference in the type ofcoffee used would make this difference, f o r Grafe gave ev idence tha tthe f u rf u ry l a lcohol probably comes f rom decomposit ion of the hemi-cellulose of t h e thi ck e nd ospe rm cells. It seems tha t th i s r eac t ionshould occur regardless Of th e typ e of coffee used. Of course th eamount would vary . It is really diff icult to see why Bernheimer andSethness did not ob ta in an y f u r fu ry l a lcoho l. -T he bo i ling po in t s offu r fu ry l a lcoho l and the ca f feo l r epor ted by Bernhe imer a re over 20degrees apar t , and Bernhe imer a l so ob ta ined sa l icy l ic ac id f rom h ismater ia l . Sethness checked his boi l ing point bu t gave no der ivat ive.

RELATION OF STALENESS TO CHEMICAL COMPOSIT~ONOF V O L A T I L E OILS IN COFFEET h e m i x t u r e of volat i le mater ia ls extracted f rom coffee is very

uns table . Th is ins tabi l i ty is probably d ue to the pecul iar composi t ion


18/20

18 S. C. PRESCOTT, R. L. E M E R S O N A N D L. V PEAKES, J R .of the mixture. It soon became apparent in this work that furfurylalcohol is unstable in the presence of even very small amounts of acid.If the mixture of volatile ingredients freshly extracted from the coffeebe kept for a short time, much darkening and finally resinificationtakes place. If exposed to air this change is very evident even over-night. Sealed in an evacuated tube the change is much retarded.After the acid is separated from the mixture by distillation, however,a much greater stability is noticed in the furfuryl alcohol fraction.

I n order t o test whether this decomposition could have anything todo with the unpleasant taste of stale coffee small portions of a freshlydistilled and of a dark sample which had been exposed to air forsome time following distillation were dissolved in equal amounts ofwater and tasted. The solution of the old sample left an unpleasantbitter taste in the mouth. This was entirely absent in the case ofsolution of the freshly distilled sample. Another test was run on amixture of furfuryl alcohol and its acetic and formic esters. Thesehad been carefully redistilled and were fairly free from acid. Aftereight weeks the sample in an unsealed tube had become quite brown,that in a sealed tube under an air pressure of two millimeters was avery pale yellow, while that placed in an atmosphere having a 75per cent humidity was a dark brown. Only the sample kept in amoist atmosphere had gone off appreciably in taste. It should benoticed that the relative stabilities of the three samples is the sameas that of coffee kept under similar conditions. Coffee kept in avacuum is the best preserved, that in air is not nearly so well pre-served, and that in a moist atmosphere deteriorates very fast. Ifnearly an equal volume of acid had been added to these samples andthey had been spread over a large surface, the differences and rateof deterioration would have been much greater. This is the condi-tion in the coffee bean and especially in ground coffee after thecarbon dioxide gas has h a d time t o escape.It is the similarity between the instability of impure furfurylalcohol and of roasted coffee that makes us believe that these mix-tures play an important part in the staleness of coffee. It is nearlyimpossible t o single out one group of compounds in such a complexmixture as coffee and to say that they are responsible f o r its deteriora-tion without first investigating other constituents. Repetition of thiswork on a somewhat larger scale will probably show up the presenceof other easily changed materials. F o r instance the fraction sus-pected of behg a phenol and reducing Tollens reagent is very likelyquite unstable in air and may be an important one of the group ofcompounds giving coffee it s odor and taste. Some of the non-volatileconstituents may also decompose in air to give products having un-


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THE STALING OF C O F F E E 1 9pleasant tastes and odors. A t present we can say tha t we have iso-lated a mixture of compounds which are unstable in air and wouldseem t o be in part responsible f o r the staleness of coEee, and we wishto point out the possibility of any group of compounds other thanthe fat being responsible f o r the staleness of coffee. This statementis based o n experimental observations and not on pure speculation.

REFEIRENCESBENGIS,. O., Ah?) ANDERSON, . J. , 1932. Th e chemistry of the coffee-bean. I. Con-

cerning the unsaponifiable ma tt er of th e coffee-bean oil. Prep arati on an dproperties of kahweol. J. Biol. Chem. 97, 99.~-1934. The chemi stry of t h e coffee-bean. 11. The composition of theglycerides of the coffee-bean oil. J. Biol. Ghem. 105, 139.BEENHDIMER,SCAR, 1880. Zur Een nt ni ss der Rostpro ducte des Coffees. Monatsh.f. Chemie 1, 456.Bmmh?), G., AND WEISWELLLER,G., 1913. Sur la Composition de l'essence deCaf6; Presence de la Pyri dine . Compt. rend. Acad. d. sc. 157, 212.

BOTSCH, . 1880. Z u r Kenntniss der Saligeninderivate. Monatsh. f. Chemie 1, 621.BRFIDT,UET,1934. Where does the gas in roasted coffee come fr om ? Food I ndus-

tries 6, 348.CANNIZZARROND EORNER,872. (Referred to in a communication by H. Schiff.)

Ber. d. deutsrh. chem. Gesellsch. 5 , 435.COE,M. R., AND LECLERC,J. A., 1934. Photochemical studies of ranc idity. Peroxidevalues of oils as affected by selective light. Ind. Eng. Chem. 26 , 245.ERDMANN,. 1902. Beitr az zur Ee nn tn iss des Kaffeeiiles. Ber. d. deutsch. chem.

Gesellsch. 35 , 1846.F ~ G L ,RITZ,934. Z u r Eenntnis der Photopyridinreaktion. J. prakt. Chem. (2)

139, 180.GRAFE, VICTOR,1912. Untersuchungen iiber die Herku nft des Eaffeals. Monatsh. f .

Chemie 33, 1389.GREENBANK,G. R., AND HOLM,G. E., 1934. Antioxidants for fat s and oils. Ind .

Eng. Chem. 26, 243.JAECKLE ,ERMANN,1898. Studien iiber die Produkte der Eaffeerostu ng, ein

Beitrag zur Keuutniss des sog. Eaffeearomas (Caffeol). Ztschr. f. Unter-such. d. Nahrungs- u. Genussmittel 1, 457.

LEA,C. H., 1931. The effect of ligh t on the oxidation of fats. Proc. Roy. SOC. ,106, 175 .

LEHMANN,. B., AN D WILHEIL.~,., 1898. Besit zt d as Coffeon und die coffei'nfreienEaffeesurroga te eiue kaffeeartige Wirknng. Arch. f. Hyg. 32, 310.

MON~RI, ., AND SCOCCIANTI,., 1895. L a pi rip ina nei prodotti della torrefagioliedel caffe. Annali di chimica e di farmacol gia 21, 70.

Miner Laboratories, 1934. The National Federation of CoffeeGrowers of Colombia.

SCEMAWUSS,ANS,A ND BARTHYEYER,HELENE,929. Deacelyl als Aromabe stand-teil von Lebens und Gunu bmitteln. Biochem. Ztschr. 216, 330.

SETENHSS,. E., 1924. Coffee's aro mat ic principles. Tea and Coffee Tr ad e J. 46 ,570.THIELE, ., A.ND DINROTE, O., 1589. Versuche mit 0- und p- Nitrobenzylchlorid.

Ann. d. Chem. 305, 110.

Acidity in coffee.New P or k City, 15 pp.


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20 S. C. PRESCOTT, R. L. E M E R S O N A N D L. V. P E A K E S , J R .TRIGG,. W., 1919. Coffee. R. K. D. Bul. 1,No. 3, 14. Ifellon Institute, Pitts-

burgh, Pa.WENDT, . L., 1930. New processes fo r liquid coffee. Glass Packer 3, 581, 601.WEIDLEIN, E. R., 1934. Research a t Mellon Insti tute dur ing 1933-34. Abstr.

Twe nty -fir st Annual Report of th e Director, E. R. Weidlein, to theTrustees of the Institution. Ind. Eng. Chem. News Edition 1 2 , 142.

WHEELER,D. H., 1932. Peroxide formation as a measure of autoxidative deteri-oration. Oil and Soap 9, 89.

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The staling of coffee I