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Literature Cited (1) Adadurov, I. E.,and Guminskaya, M. A., J. Applied Chem. (U.S.S. R.),5, NO.6-7, 722-35 (1932). (2) Comey, A. M., and Hahn, Dorothy, “Dictionary of Chemical Solubilities,” 2nd ed., 1921. (3) Conidelon Soci6t6 Anonyme, British Patent 5174 (May 29,1913). (4) Fairlie, “Sulfuric Acid Manufacture,” p. 43. (5) Haen, C. J. E. de, U. S. Patent 687,834(Dec. 3, 1901). (6) Jaeger, A. O.,IND. ENQ.CREM.,21,627-32 (1929). (7) Keffer, R.,“Methods in Non-Ferrous Metallurgical Analysis,” 1928. (8) Kharmandaryan, M. O . , and Brodovitch, K. I., Ukrain. Khem. Z ~ U T8, . , NO.1, 49-57 (1933). (9) Knietsch, Ber., 34, 4069-115 (1901).
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use
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(10) Monsanto Chemical Works (by A. 0. Jaeger and J. A. Bertsch), British Patent 266,007 (May 14, 1928). (11) Neumann, B.. 2. Elektrochem., 35, 42-51 (1929). (12) Neumann, B., and Juettner, H., Ibid., 36,87-96 (1930). (13) Nickell, Chem. & Met., 35, 153 (1928). (14) Phillips, Peregrine, Jr., British Patent 6096 (Sept. 14, 1831). (15) Scott, W. W.,“Standard Methods of Chemical Analysis,” New York, D. Van Nostrand GO., 1925. (16) Scott and Layfield, IND. ENQ.CHEW,23,617-20 (1931). (17) Selden Go., British Patent 314,858 (June 19, 1930). (18) Slama, Franz, and Wolf, Hans, U. S. Patent 1,371,004(March 8, 1921; reissued Sept. 21, 1934). RECEIVED June 9, 1936. Submitted in partial fulfillment of the requirementa for the degree of doctor of philosophy in the Faaulty of Chemical Engineering, Polytechnic Institute of Brooklyn.
RANCIDITY IN
ROASTED COFFEE Oxygen Absorption
by the Fat Fraction LUCIUS W. ELDER, JR. General Foods Corporation, Battle Creek, Mich.
T
HE development of stale flavor in coffee as a result of its exposure to air has been the subject of much study (2, 3, 17-22, 28). Staling has frequently been attributed to the autoxidative’ development of rancidity in the fat fraction of coffee. This is perhaps a natural assumption in view of the important role played by rancidity in the field of vegetable oil technology. However, some investigators have questioned the participation of fat oxidation in the staling of roasted coffee (17, 19, 21). Part of the confusion which has appeared in this connection is due to a loose definition of the term “rancid” and to the failure to distinguish between the properties of an unrefined fat extract and those of its component fatty acid glycerides. In the following presentation the term “rancid” is used to designate the odor and flavor characteristic of the products resulting from the autoxidation1 of the glycerides of unsaturated fatty acids. This is the sense in which the term has been most commonly used (96,27). The experimental data reported here show that the development of stale flavor in coffee is not identified with the development of rancidity in the fat fraction.
Analytical Methods Available Fatty oil technologists have developed a variety of tests to appraise the degree of oxidative deterioration which may have occurred in processing or storage. None is infallible and none is significant without prior experimental correlation with organoleptic tests. Of the more common tests the Kreis I The term ”autoxidation” is now generally accepted as a n abbreviation of the more cumbersome but more oorreot term “autocatalytic oxidation” [of. Milas, N. A., Chem. Rea., 10, 296 (1932)l.
Analytical methods commonly used for evaluating the tendency of fats t o become rancid are discussed in relation to their applicability to the fat fraction of roasted coffee. Many such tests are based on the detection of products of fatty decomposition. In the presence of the highly reactive aroma constituents of coffee fat, these tests cannot be interpreted in the conventional manner without misleading conclusions. The method involving measurement of the oxygen absorption induction period is reasonably free from such objections. On the basis of oxygen absorption measurements it is shown that the fat extract from coffee stored in air for periods up t o 13 weeks has an induction period identical with that of the fat extract from vacuum-packed coffee in spite of the fact that differences in cup quality in favor of the vacuum-packed coffee were apparent after 2 weeks. I t is concluded that the staling of coffee proceeds by a reaction or group of reactions which are not identified with the development of true rancidity in the fat fraction.
test (8, 16)) peroxide value (11, 16, 29), Schibsted fataldehyde value (24), von Fellenberg test (6), and methylene blue fading test (12, 23) which involve color comparisons or colorimetric titration are not directly applicable to the fat extract from roasted coffee on account of its deep pigmentation. The ordinary fat constants such as free acidity, iodine value, etc., are not usually employed because significant changes do not occur until some time after changes detectable by organoleptic tests are well advanced (26).
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Samples which could not be tested at once were stored in stoppered Basks in a refrigerator at 4' C . Storage at this temperature can be extended over periods of 2 or 3 months without significant change in oxidation induction period.
Oxidation Induction Measurements The apparatus employed is illustrated in Figure 1.
Of the renmining tests available, those which would seem to be applicable to the fat extract from roasted coffee are tlie Issoglio value and its modifications (S, 7, 14)' Bailey value ( I ) , and the oxygen-absorption induction period (6,9, 20,18, f l ) . Bengis also reports the use of the Reichert-Wollny value in the study of coffee fat (3). Chemical tests for measuring fat stability oan be broadly divided into two classes-(u) those which measure the products of dccomposition and (6) those which measure some change in property of the fatty acid glyceride itself. I t is apparent that tests of the first kind applied to coffee fat extract may be misleading since there are present nonfatty materials of a high degree of reactivity which are not fat degradation products. The oxygen absorption method (6,9, I S ) was ehoseii for the study described in this communication, sirice it. belongs to the second class of test. Furthermore, it has been ohserved repeatedly and consistently in the case of the mcasurements reported here that at the end of the induction period at 90' C. a pronounced organoleptic rancidity bas developed. Just before the end of the induction period, hovever, no rancid taste or odor can be detected. This experience con&ms the observations of others (4,9) to the effect that the oxygen absorption method is a reliable measure of the susceptibility of a fatty extract to the development of organoleptic rancidity.
Fat Extraction The solvent employed was redistilled Skeliy-Solve B (boiling range, 60-70" C.). Roasted coffee was ground to the commercial "universal" grind and steeped in successive batches of solvent at room temperature. The main portion of the solvent was removed by vacuum distillation under a water-pump vacuum, and all hut a trace of the remainder was removed with the aid of a Hyvac pump protected by a suitable tr+p. The last traces of solvent, water, ete., were removed in preparing the individual samples for oxidation, as described later. The distillations were all performed below 30" C .
Samples (10 ec.) were pipetted into the bulbs which were made by fusing the necks of 50-cc. murid-hottom Pyrex flask.. tosuitablelengths of 12-mm. 0. d. tubing. The ipete were shielded by means of sleeves made of Bmm. 0.d. &ing which were slipped into the bulb necks to prevent the fat extraot from coming in contact with the ends which were later to be fused onto the gas burets. Each buret was mounted on an individual frame welded from half-inch cold-rolled steel rod and was provided with capillary connections to the mercury reservoir and to the sample hulb as shown in Figure 2. After fusing the sample bulbs in place, the apparatus was evacuated through the T-connection shown in Figure 2 by means of a Hyvac ump until the fatty oil ceased frothing. Oxygen from a eylin&r was then admitted through a Y-connection to restore one atmosphere pressure, and the units sealed off with a glass blowers' torch. Sufficientmercury was left in the bottom of each buret to permit a readjustment to atmospheric premure after immorsing the bulb and connecting tubing in the thermostat at 90" C. This was done by draining off mercury through the buret stopcock until a flow of mercury from the constant-level reservoir was started. After waiting 1 or 2 minutes for e uilibrium to he established, the Eem level was marked on the'bwet and subsequentobservationsof the rise in level (volume oi oxygen absorbed) were recorded at measured time intervals. The length of the induction period is derived graphically from a plot of oxygen absorbed against time. The shaking mechanism consisted of s 0.l-horsepwer induction motor coupled through a worm-gear speed reducer to a seci rocating drive arm into aud out of which the buret frames eoulBbe respectively iuserted and removed iudependentlyof each other. The mechanism was set to give the bulbs a. horizontal reciprocating motion through adisplacement oi about 1em. at a speed of 168 strokes Der minute. Preliminar? tests had shown that considerable variations in volume of sample and in meed of served.
Storage Tests From a uniform lot of f r e s h l y r o a s t e d , oommercial, vacuum-packed coffee a portion was re served in t h e v a c u u m pack and the contents of the remaining cans (12 Funds) were mixed and stored in a 25-pouud s l i p covered can. At each of several sampling periods, approximately I-pound p o r t i o n s e a c h of the vacuum-packed and airFIQDRE 2. OXYGENABSORPTION packed coffees were withUNIT drawn for extraction hy hexane and at the mme time smaller p o r t i o n s filliw were taken for cup tests. E . Capillary tubingF. Pornt st which eample bulbs are T h e f a t extracts were sealed to capillary then submitted to oxi0. Thermostat water level H. Steel frame dation a t 90" C . and 1 I. Pivot e&ging drive-arm bearing J . Pivots m stationary beannga atmosphere of oxygen
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value during a reasonable storage interval, and particularly the modified Kerr-Sorber oxidizability value, are probably not a measure of the state of the fatty acid glycerides. According to Kerr and Sorber, the oxidizability value of a fat tends to increase as the development of rancidity progresses (14), whereas Bengis’ measurements show a decrease in this value for the fat from roasted coffee with increasing age of the coffee. TABLE I. OXIDATION-INDUCTION PERIODS O F THE FATEXTRACTS The oxidation of such aroma constituents as acetaldehyde, FROM ROASTED COFFEE furfural, and their homologs (26) t o the corresponding acids Induction Period, Hr. Induction Period Hr. would lead to just the results found by Bengis-i. e., decrease Days of Coffee. Days of Codee. Storage Vacuum- packed in Storage Vacuum- packed i n in oxidizability value (aldehydes) and increase in ReichertPrior t o packed slip-covered Prior t o packed slip-covered F a t Extn. coffeea canb Fat Extn. coffeea canb Wollny value (volatile acids) with time. The participation 35 30 .. 3 30 37 of the fatty glycerides is therefore unnecessary to account for 56 33 32 14 37 33 the results cited. 91 33 33 21 30 33
in the volumetric apparatus described for measurement of the induction periods. The variation between duplicate runs was usually less than 10 per cent. The results are summarized in Table I. At the latest sampling interval (91 days) no significant difference had yet appeared between the induction periods of the fats from the two lots of coffee.
a Roasted and vacuum-packed 9 days prior t o beginning of storage test. b Same lot of coffee, removed from the vacuum pack and stored as indicated.
“Blind” cup tests run concurrently during the storage period by two or more observers showed no definite differences in cup quality between the two batches until 2 weeks had elapsed. At the end of 2 weeks differences in favor of the vacuum-packed control were apparent to the majority of tasters and remained apparent a t subsequent intervals. If the stale flavor which developed after 2 weeks in air was due to the development of rancidity in the fat fraction, we should expect a marked decrease in length of induction period in the case of the air-packed coffee. The absence of any such change in the induction period indicates that staling cannot be related to the development of ttrue rancidity in the fat fraction.
Discussion There are some interesting relations between these findings and the data reported recently by Bengis (2, 3). Parenthetically, the practical significance of Bengis’ work is impaired through lack of correlation between the condition of the fat fraction and the cup quality of the corresponding coffees. The measurements reported by Bengis on the fat fraction from coffee stored in air, with the single exception of the modified Kerr-Sorber oxidizability value, were confined to a sample which had been held in paper sacks for 12 months after roasting. Regardless of their numerical values such measurements can throw no light on the state of decomposition which existed more than 11 months earlier, when stale flavor was first apparent. With regard to coffee in the vacuum can, tests made here show no indication of stale flavor at the time of writing, 6 months after starting the test, and the extensive observations of Punnett and Eddy (22) show that staling can be inhibited for a period of years when the modern high-vacuum process is employed. Hence, however extensive the changes in the properties of the fat fraction as measured by Bengis may be, there can be no correlation with the incidence of stale flavor. Those properties which do show a significant change in
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Acknowledgment The author is indebted to W. Holmes for assistance in the fat extractions, to M. L. Spealman and R. H. Walters for assistance in constructing and operating the absorption apparatus. and to R. E. Kremers for his stimulating interest adkce. The service of these colleagues as experienced testers was an invaluable aid.
Literature Cited Bailey, H. S., and Ebert, H. C., Cotton Oil Press, 7, 35 (1923). Bengis, R. O., Food Ind., 7, 490 (1935). ENG.CHEM.,28, 290 (1936). Bengis, R. O., IND. Davies, W. L., Ind. Chem., 4 , 269 (1928). Fellenberg, von, Mitt. Lebensm. Hyg., 15, 198 (1924). French, R. B., Olcott, H. S., and Mattill, H. A., IND.ENO. CHEM.,27, 724 (1935). Grettie, D. P., and Newton, R. C., IND. ENG.CHEM.,Anal. Ed., 3, 171 (1931). Holm, G. E., and Greenbank, G. R., IND.ENQ.CHEM.,16, 618 (1924). Ibid., 16, 598 (1924). Ibid., 17, 625 (1925). Ibid., 26, 243 (1934). Ibid., Anal. Ed., 2, 9 (1930). Holm, G. E., Greenbank, G. R., and Deysher, E. F., IND. ENO. CHEM.,19, 156 (1927). Kerr, R. H.. and Sorber, D. G., Ibid., 15, 383 (1923). Kilgore, L. B., Oil & Soap, 10, 66 (1933). Powick, W. C., J . Agr. Research, 26, 323 (1923). Prescott, S. C., in “All About Coffee” by Ukers, W. H., 2nd ed., pp. 294-5, New York, Tea & Coffee Trade J, Co., 1935. Punnett, P W., Ibid., p. 299. Ibid., p. 305. Punnett, P. W., Food Ind., 2, 401 (1930). Ibid., 8, 178 (1936). Punnett and Eddv. Ibid.. 8. 341 (1936). Royce, H. D., IND.’ENG.‘CHEM., Anal: Ed., 5, 244 (1933). Schibsted, H., Ibid., 4 , 204 (1932). Staudinger, H., U. S. Patent 1,696,419 (Dec. 25, 1928). TaiLfel, K., and Muller, J., 2. Untersuch. Lebensm.. 60, 473 (1930). Triebold, H. O., Cereal Chem., 8, 518 (1931). Wendt, G. L., Food Ind., 4, 129 (1932). Wheeler, D. H., Oil & Soap, 9, 89 (1932). RECEIVED September 28, 1936. Presented before the Division of Agricultural and Food Chemistry at the 92nd Meeting of the American Chemioal Society, Pittsburgh, Pa., September 7 t o 11, 1936.
