Staling vs. Rancidity in Roasted Coffee Antioxygens Produced by Roasting LUCIUS W. ELDER, JR. General Foods Corporation, Battle Creek, Mich.
oxidation of green coffee oil cannot be excluded, although it will be shown that certain pyrolytic products resulting from the roasting operation could alone account for the observed facts. Experiments involving heat treatment of the oil fraction of green coffee alone under carbon dioxide show that, if the stabilizing effect of roasting is to be accredited to antioxygenic products of pyrolysis, such products must be derived largely from that fraction of the green bean which is insoluble or poorly soluble in hexane. The same experiments can be interpreted to show that the degree of stabilization due to destruction of oxidases is not sufficient to account for the magnitude of the observed facts.
THE FIRING EXDOF COFFEEROASTERS
T
HE preceding paper in this series (1) presented data t o show that the fat fraction of roasted coffee was equally stable over a series of storage intervals whether the coffee was kept fresh by vacuum packing or allowed to be* come stale by exposure to air. Further evidence bearing on this point was obtained by comparing the effects of roasting on the stability of the oil fraction and of adding t o green coffee oil some of the pyrolytic products known to exist in roasted coffee. This paper presents the results of such tests together with an extensive survey of the antioxygenic activity of compounds related to those found in roasted coffee. The apparatus and procedure used to measure the stability of the coffee oils were fully described in the preceding paper ( 1 ) .
Effect of Known Constituents of Coffee on Stability of Coffee Oils Portions of green coffee oil were mixed with roughly 0.2 per cent additions of caffeine, sucrose, and chlorogenic acid, respectively; the mixtures were heated under a slow stream of carbon dioxide for 15 minutes in an oil bath a t 2OO0C., cooled, and tested in the absorption apparatus. Of this group, only chlorogenic acid shows any protective effect and that to a minor degree (Table I). Suspecting that sulfur compounds might be involved and lacking knowledge of specific sulfur compounds present in green coffee, we have used as plausible type compounds, allyl mustard oil, cystine, and methionine, in the same manner and concentrations as described for caffeine, etc. Of this group only methionine was effective. A decided darkening occurred in the oil containing methionine, and a disagreeable putrid odor developed during the oxidation.
Effect of Roasting on Stability of Coffee Oil The oils extracted from green coffee varied widely in stability as measured by the induction periods, depending on the method of extraction. In general, the oils extracted from flaked green coffee in all-glass apparatus were the most stable, and it is with such oils that we have compared the oils from roasted coffee. The data plotted in Figure 1 are representative of the relations inrolved. On the most conservative basis, which excludes the less stable green coffee oils, it can be said that roasting increases the stability of the oil fraction toward oxygen by a factor of approximately 3. Furthermore, oil extracted from a dark Santos roast shows an induction period nearly twice as long as oil from a medium or light roast of coffee from the same lot. Since respiration studies in this laboratory have shown that a considerable portion of even 2-year-old green coffee still contains living tissue, the possibility that enzyme activity is involved in the
TABLE I. OXYGEN ABSORPTION B Y OIL EXTRACTED FROM FLAKEDGREEN COFFEE, TREATEDWITH REAGENTS,THEN HEAT-TREATED 15 MINUTESAT 200" C. UNDER CARBON DIOXIDE Induction Period, Solubility ildded Reagent Hr.at 90' C. in Oil 13.5 None .411yl mustard oil0 13.5 Coh;&ete Sucrose) 13.0 Complete 7 Caffeineb 12.0 Chlorogenic acidb 17.0 1 I-Cystineb 11.6 Insoluble dl-LMethionineb 24.0 1 a 1 drop in 10 cc oil roughly 0.2 to 0.5% b weight. b 20 mg. in 10 cd. oii, approximately 0.2% g y weight.
798
Protection Factor
...
1.0 1.0 0.9 1.3 0.8 1.8
JUNE, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
Kone of the compounds listed in Table I is sufficiently effective by itself to account for the observed stability of roasted coffee oil.
GREEN
OIL FROM ROASTED
IY
a m 3
c
00
5
199
TABLE11. OXYGENABSORPTION BY OIL EXTRACTED FROM FLAKED GREENCOFFNE,HEATED15 MINUTESUNDER CARBON DIOXIDEAT 200" C., THENTREATED WITH REAGENTS Induction Period, Solubility Added Reagent Hr. a t 90" C. in Oil None (control not 14.0 ... heat-treated) None (control 16.0 ... heat-treated) Ethyl mercaptan" 46.5 Complete Furfural0 16.5 Complete Furfuryl alcohol@ 17.5 Complete Diacetyl' 13.0 Complete None 15.0 Pyrrole" 225.0 coh;I;lete N-Methylpyrrole" 16.0 Complete Pyridine0 14.5 Complete Catechola 56.0 1 0 1 drop in 10 cc. of oil roughly 0.2 to 0.5% by weight. b 20 mg. in 10 00. oi1,'approximately 0.2% by weight.
Protection Factor
... ...
2.9 1.0
1.1
0.8
...
15.0
1.0
1.0
3.7
10
TIME, HOURS
FIGURE1. OXYGENABSORPTION BY COFFEE OILS AT 90" C. AND ONE ATMOSPHEREOXYGEN PRESSURE
TABLE 111. 90" C. INDUCTION PERIODS AND PROTECTION FACTORS FOR PYRROLE AND HYDROQUINONE -reen Coffee OilInduction Protection Pyrrole Concn.. % period, hr. factor 0.0 10.5 33.0 0.1 3.1 0.2 11.4 120 2240 0.2-0.5" 15.0 Hydroquinone Concn., % 0.1 120 11.4 a Heat-treated 15 minutes a t 200' C. under COY. Heat-treated control has a 15.0-hour induction period.
...
The identification of a considerable number of compounds in the distillate from roasted coffee ( 2 , 4, 6) offers a wider choice of materials for test than is the case for green coffee constituents. Table I1 lists the results on a few of the compounds described, showing that furfural, furfuryl alcohol, diacetyl, IC'-methylpyrrole, and pyridine are substantially without effect. Although ethyl mercaptan is not listed among the compounds identified by Staudinger, both methyl and furfuryl mercaptans are. For reasons of convenience our preliminary test was performed with ethyl mercaptan and showed a marked inhibition of the oxygen absorption by green coffee
Comparison of the oxidation induction period of oil extracted from coffee before and after roasting shows that the roasting process increases the stability of the oil fraction toward the development of rancidity by a factor of approximately 3. The oil from green coffee when subjected to the roasting treatment by itself does not acquire increased stability. The addition to green coffee oil of certain pyrolytic compounds known to exist in roasted coffee, notably pyrrole and aliphatic mercaptans, greatly increases the oil stability. A study of thirty-two additional compounds including a series of heterocyclic imino compounds shows that the latter, subject to certain structural and solubility limitations, comprises a new comprehensive group of antioxygens. A practical test involving the storage of ground roasted coffee which had been treated with a small amount of pyrrole shows that the oil fraction is definitely stabilized, but resistance to staling is unaffected by such treatment.
oil containing it. Pyrrole is the outstanding antioxygen of the group so far tested. The concentrations of the mixtures listed in Tables I and I1 were adjusted only approximately; hence strictly quantitative comparisons are not possible, but pyrrole and ethyl mercaptan are, qualitatively a t least, as good as catechol, an antioxygen of well-recognized potency. Quantitative comparisons of pyrrole and hydroquinone led to the data listed in Table 111.
= . J
looh 80
0
2 E
20-
I
OF OILS FROM TREATED FIGURE 2. OXIDIZABILITY AND UNTREATED ROASTEDCOFFEES
Fat fraction from vacuum-packed coffee Fat fraction from untreated coffee stored in air Fat fraction from coffee treated with 0.05 per cent pyrrole and stored in air 9 Fat fraction from coffee treated with 0.10 per cent pyrrole and stored in air 0
Effect of Other Nitrogen and Sulfur Compounds on Stability of Coffee Oils
A series of thirty-two compounds chosen because of the presence of functional groups related to those present in natural coffee constituents were tested in green coffee oil at
800
VOL. 32. NO. 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
Although all possess the iniido hydrogen and a or 6 nuclear hydrogen of pyrrole, the condemed ring structure has a p parently nullified the antioxygenic activity otherwise manifested by these groupings. I n the xanthine group caffeine and theobromine are without antioxygenic effect. This may he accounted for by the facts that caffeine, though oil soluble, lacks an imido hydrogen, and theobromine, thongh possessing an imido hydrogen, i s oil insoluble. PHENOLIC AND OTHERCARBOCYCLIC COMPOUNDS.Caffeic acid, catechol, and chlorogenic acid all possess phenol groups and, with the exception of the last which is poorly soluble, all are good antioxygens. The solubility of chlorogenic acid i8 roughly intermediate between that of its constituents, caffeic and quinic acids. The latter is completely inactive as an antioxygen, perhaps because of ita low solubility. MISCELLANEOUS ALIPHATIC COMPOUNDS.The most effective compounds of this group are those containing the mercaptan group-thioglydio acid and ethyl mercaptan. Methionine was noticeably decomposed and may owe its slight activity to mercaptans in the decomposed products. My1 mustard oil and cystine are both ineffective, both lack free SH groups, and in addition cystine is practically insoluble in oil. Sucrose and diacetyl, lacking both sulfur and nitrogen, are ineffective.
Effect of Pyrrole on Stability of Ground Roasted Coffee A DIKMFOR UI.ENDIKD1.5 TOSSOP' COFFEEAT OSE TIME
the 0.2 per cent level, with results as shown in Table 1V. The compounds are listed in the order of decreasing antioxygenic potency. It will be convenient to group them according to structural relations for the purpose of comparing the results. FIVE-MEMBERED ~IETEROCYCLIC COMPOUNDS.Pyrrole is the outstanding antioxygen in this group. Among the substituted pyrroles moderate antioxygenic activity is not,ed for the 2,5-dimetliyl- and 2,4-dimet~hyl-%carhet.hoxyderivatives, hut no protection n'as obtained from the N-met,byl- or 2,4dirnethyl-3,5-clicarbetho~ pyrroles. These facts suggest that both t.lie imido hydrogen and at least one of bho nuclear hydrogens are essential for the antioxygenic effect. Proline, containing the pyrrolidine or reduced pyrrole nucleus, is more protective than any other componnd in the list except pyrrole itself. On the other hand, liydroxyproline and histidine, the latt.er containing aii imidaaole group, are ineffective, possibly on aceirunt of their ION oil duliility. Histidine was available only as the diliydrochloride. It is possible that the free amino acid nould be more oil soluble. The pro-oxygenic effect of 3,54imcthylpyra7.de was unexpected, and no explanation can bc offered a t present. The furan derivatives, furfural and furfuryl alcohol, though readily oil soluble, are both ineffective. Six-MEninEnm ~IETEIUXYCLIC COMPOUNDS.Piperidine and piperazine are both excellent antioxygens, the latter confirming the claims of Murrill ( 3 ) . l'yridine and tetramethylp~~r~ziiie, both lacking imido hydrogen, are without effect on t,he oil stability. CONDENSED RING HETEROCYCLIC Co3lPoUNDS. The pyrrole nucleus, modified by condensation u-ith a beiizene nucleus, is contained in indole and its derivatives b-indole-.?. propionic acid, tryptophan, and carbazole. All but tryptophan are completely oil soluble at the 0.2 per cent coilcelltration, hut none are significantly protective as antioxygens.
To determine the practical value of the results obtained with pyrrole as described in the preceding sections, a test
501"biiity in Oil
PYriOk
Pyxrule Pyrrole, l-l'mline
Thioglyoolio acid Cnffcie acid
0.2 U.2 i) 21
U.2
ZPU* 120 ala
80
11. a
2.2
s:
0.2 0.2 0.2 0.2
4s 43.3 39.0 56s
b
470 24a 13
0 .2
0 2 0 2 0.2 1) 2 0 2
PTeteFtiun r*ct.ur
37
li" 13
11 1s14*
0 2
0.2
140 171 119 13" 10.5
O.?
10
&Tryptophan
0.2
10
Histidine dihydmehiuiide
0 2
LO
Theobromine
0 2
10
Caibazolo
Trtrarneth yipyrszino
0.2 0.2
I-Cystine
0 2
0.2
cofTei:ins
0 2
9
9
13. 12-
13" 0.2
... ...
8.0 10.0 10.6
Data taken from earlier runs, rcftmied t o diffcront control ~hmplc s 6 hpproiimntrly 0.2%, addition. (Tableu I and 11).
JUNE, 1940
IKDUSTRIAL AND ENGINEERING CHEMISTRY
series involving a commercial vacuum-packed coffee was set up as follows: From a set of four dozen cans containing coffee which had been roasted and vacuum-packed January 29, 1936, the contents of thirty-six 1-pound cans were mixed and divided into three 12-pound batches on February 7 . One batch was held as a control which was loosely packed into a 25-pound slip-covered can. The second batch was treated with pyrrole amounting to 0.1 per cent of the weight of the coffee by first incorporating the weighed amount of pyrrole with a small portion of the coffee (about 2 pounds). This was done by stirring in a mechanical mixing bowl with dropwise addition of the pyrrole. The treated portion was then returned to the main portion, and the whole thoroughly mixed by rolling the loosely packed can. Storage was in a 25-pound slip-covered can as used for the control. A third batch was treated exactly the same way except that half the concentration of pyrrole was introduced-i. e., 0.05 per cent of the coffee weight. As an additional set of control samples, the remaining twelve cans were retained in the vacuum pack, one to be opened and sampled at each of the sampling intervals which have been a t 3, 14,21,35,56,91, 193,381, and 728 days after starting the test. At each of the sampling periods approximately 1-pound portions were withdrawn for extraction by hexane, and a t the same time smaller portions were taken for cup tests. The extracted oils were then submitted to oxidation a t 90" C. in the volumetric apparatus previously described for measurement of the induction periods. The variation between duplicate runs was usually less than 10 per cent. The results are summarized graphically in Figure 2 and show that the oil fraction of roasted coffee is definitely stabilized against oxidation by the pyrrole treatment described above.
80 1
Discussion The most striking fact which appears indisputably from the evidence of the storage tests is that the oil fraction of roasted coffee can be definitely stabilized without affecting the aroma stability. Specifically, a t the end of 2 weeks the oil from coffee treated with pyrrole is more resistant to oxidation than that from the vacuum pack, yet the latter is fresher in cup quality. Furthermore the oils from both the vacuumpacked and air-packed controls have nearly identical oxidation induction periods throughout 2 years of storage, whereas the cup qualities of the corresponding coffees are different after 2 weeks. It may be concluded with certainty that stabilization of the fatty oil fraction of coffee will not of itself accomplish the preservation of coffee freshness. The results of the extensive tests with heterocyclic nitrogen compounds indicate that this group may be classified with the phenols and amino phenols which have previously been described as comprising some of the most effective antioxygens. The results also show that, subject to limiting restrictions with respect to solubility and excluding condensed ring systems, the most effective heterocyclic nitrogen compounds are those possessing unsubstituted imino groups and a t least one unsubstituted alpha or beta nuclear hydrogen.
Literature Cited Elder, L. W., Jr., IND. ENG. CHEM.,29, 267-9 (1937). Johnston, W. R., and Frey, C. N., J . Am. Chem. Soc., 60, 1624-7 (1938). Murrill, P. J., U. S. P a t e n t 1,822,108 (Sept. 8, 1931). Prescott, S. C., Emerson, R. L., and Peakes, L. V. J.. Food Research, 2, 1-20 (1937). Staudinger, H., U. S. P a t e n t 1,696,419 (Dec. 25, 1926).
THE ALCHEMIST By VINCENT MILLER
No. 114 in the
Berolzheimer series of Alchemical and Historical Reproductions comes to us through the courtesy of the Sharples Corporation. The original painting is by a young Philadelphia artist, less than one year o u t of art school. It is quite obviously copied (by permission) from the handsome painting by W'yeth, owned by the Hercules Powder Company (No. 88 in our series), but the artist has succeeded in incorporating his own style in it. It appears that the alchemist is weighing a small globule of gold he has made, bur it must be quite light for the hand-scale is balanced without any weight being shown on the left pan.
D. D. BEROLZHEIMER 50 East 41st Street New York, N. Y . 4 list of thc first 96 rcproductionr appeared 10 our January. 1939,isruc, pagc 124. The list of rcproductionr 97 to 108 appcircd inlanuary, 1940,page 134, A n additional reproduction appcarr each month.
