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INDUSTRIAL AND ENGINEERING CHEMISTRY
prove feasible and convenient to prepare reproducible membranes by such electrodeposition since the recording ammeter plots should tell a great deal about the history of the formation of the membrane, thus providing a check on the assumed similarity of procedures. 5. The initial current varies with the applied voltage in accordance with Ohm’s law and varies with the dilution in the simple manner characteristic of many electrolytes. Since, in addition, the initial current readings may be obtained with ease and rapidity, the taking of such data recommends itself as a control method. Thanks are due to the Revertex Corporation of America for the samples of Revertex used in this work.
VOL. 30, NO. 11
Literature Cited Budiloff, Kautschuk, 9, 1, 20 (1933); Rubber Chem. Tech., 6 , 422 (1933).
Dogadkin and Sandomirsky, Rev. g i n . caoutchouc, 10, No. 97, 17 (1933); Rubber Chem. Tech., 7, 314 (1934). Esterline-Angus Go., Catalog 336, esp. p. 42 (1936). Greenspan, J., private communication, 1937. Hauser, “History of the ‘Revertex’ Process,” Revertex Corp. of America, 1929. Revertex Corp. of Am., “Revertex Concentrated LatexSuggestions for Its Treatment and Use,” 1929. Sandomirsky and Dogadkin, Russian Patent 43,733 (1936). RECEIVED February 11, 1938.
Oxidizability of Roasted Coffee
S
EVERAL attempts have been made to find an objective
method of measuring flavor deterioration during the air storage of roasted coffee. Balart (1) reported that the oxidation-reduction potential of coffee beverage has been studied with the hope of correlating this measurement with changes in coffee flavgr. He also mentions studies on coffee oil made with a similar end in view. Bengis (2) attempted to correlate staling with changes in the fat fraction of roasted coffee, but the work of Elder (4) indicated that the fat of roasted coffee is stable and that Bengis’ measurements were probably concerned with impurities in the fat. No objective method of measuring flavor deterioration has yet been reported. It has been suggested that staling of coffee, or flavor deterioration, which occurs on exposure to air is due to absorption of oxygen by the coffee and reaction of the absorbed oxygen with flavor constituents. Regardless of the complexity of t h e mixture of flavor constituents, it is reasonable that the rate of absorption of oxygen by these substances is a factor in a possible measurement of their concentration and consequently of the freshness of a coffee sample. Since the amount of flavor substances in coffee is small, it has been necessary to resort to a inicromethod in order to study the oxidation of coffee by molecular oxygen. The BarcroftWarburg assembly has been found to be ideally applicable in measuring oxygen absorptions. By means of this assembly it has been possible to measure the rate of absorption of oxygen by a given weight of roasted coffee and to show that the rate of oxygen absorption decreases as the coffee stales. The studies showed that fresh coffee contains an amount of oxidiaable material, dependent on the degree of roast, which decreases slowly during air storage because of reaction with atmospheric oxygen. Consequently, the measured oxidizability, or rate of oxygen absorption, also decreases and may be correlated with the loss in fresh coffee flavor a t any given time.
Apparatus and Procedure The Barcroft-Warburg equipment and technic are described by Dixon ( 3 ) . The equipment was constructed by the American Instrument Company. The flask volumes were approximately 35 cc., and the manometric fluid used was Brodie solution. Preliminary work on the oxygen absorption of coffee was concerned with the measurement of absorption by dry roasted coffee having the usual moisture content of 1 to 3 per cent.
WILLIAM R. JOHNSTON The Fleischmann Laboratories, Standard Brands Incorporated, New York, N. Y.
The Barcroft-Warburg assembly has been applied to the measurement of the rate of oxidation of roasted coffee infusions. Measurements on the rate of absorption of oxygen by roasted coffee have shown that only a small amount of oxygen is necessary to produce staleness. Moisture, p H , temperature, and other factors affecting the oxidation of coffee are discussed.
It was soon found, however, that daiIy variations in humidity caused such large fluctuations in measured absorptions that results were too variable. Measurements of oxygen absorptions of water infusions of coffee were next tried with much more satisfactory results. The infusions were simply suspensions of ground coffee in water and were not filtered beverages. The pH of the infusion influenced the oxidation rate, so that a buffered infusion was later employed. A number of experiments were made to determine a satisfactory rate of shaking, a suitable ratio of coffee to buffer, a proper amount of 30 per cent potassium hydroxide to be used in the inner cup of the Barcroft flask, and a convenient temperature of measurement. The following procedure was finally devised : The temperature is set at 40’ C., the shaking rate at one hundred and ten oscillations per minute, and the amplitude at 2 cm. The shaking rate and amplitude are sufficient to permit a maximum rate of absorption at a given temperature. The measurement of oxidizability is made by weighing a 0.500-gram sample of ground dry roasted coffee into a Barcroft flask, placing 0.2 cc. of 30 per cent potassium hydroxide in the inner cup, adding 5 cc. of Walpole acetate buffer (pH 5.20) at 25’ C., attaching the flask to the ground joint of the manometer, and then equilibrating the system at 40’ C. for 20 minutes. Ten minutes are a1lowed for the preparative period. After equilibration the system is sealed by turning a stopcock, and manometric measurements are started.
INDUSTRIAL AND ENGINEERING CHEMISTRY
NOVEMBER, 1938
The apparatus includes seven manometers and flasks, one set being used as a thermobarometer. The coffee samples are usually measured in duplicate, three different lots being measured simultaneously. During an experiment the pressure is measured at intervals over a 2-3 hour period. A volume calibration permits the calculation of corresponding oxygen absorptions in the usual manner. The following is a sample calculation for a given system: Vol. of flask and capillary = V r = 34.52 cc. Vol. of coffee infusion = V p = 5.37 cc. Vol. of potassium hydroxide = V K = 0.20 cc. Vol. of air space = V T - (Vp V K )= V G= 28.95 cc. Normal pressure (mm. of Brodie solution) = Po = 10,037 Drop in pressure after 2-hour reaction = h = 92.8 mm. Solubility of oxygen at 40" C. = 01 = 0.023 Absolute temp. of bath = T = 313.1" K.
+ h
( V G F
f
VFO1)
(1000)
Vol. of oxygen absorbed = V , = (92.8) k . 9 5
(m) 273.1 + 5.37 (0.023)]
-
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of calcium nitrate a t approximately 50 per cent relative humidity; and the third, C, was stored in a desiccator over Drierite a t 0 per cent relative humidity. The freshly roasted coffee contained 1.25 per cent moisture; but after storage for 18 days samples A , B, and C contained, respectively, 18.45, 6.83, and 0.72 per cent moisture. Unfortunately a complete set of moisture determinations was not made during the experiment. Table I presents a typical set of measurements of three portions of the same coffeeafter the samples had been stored for 10 days. Figure 1illustrates the rate curves plotted from this data. Measurements were made on 0.500-gram samples, and no correction was made for moisture content. A correction would bring the curves together somewhat but would not alter the general picture of the distinct accelerating effect of moisture on coffee deterioration.
P O
-.E--
(1000)
10,037
= 234.6
cu. mm.
Pressure to vol. conversion factor = K = ( V G
273.1
+ VJ'a) (1000) = 2.5276 for above system
3
P
P O
The potassium hydroxide in the central cup removes the small amount of carbon dioxide evolved after the equilibration period; since this amount changes slowly as the coffee ages, it is evident that the simultaneously evolved carbon monoxide which is not absorbed introduces only a small error in the results. The carbon dioxide evolution is usually about twenty times the carbon monoxide evolution. Measurements in a pure nitrogen atmosphere showed no absorption of gas by a coffee infusion and only a small evolution (5 cu. mm. in 2 hours) of carbon monoxide after the equilibration period, thus confirming the conclusion reached from the measurement of carbon dioxide. I n making a measurement, it is necessary that the temperature control be prec'ise (at least 0.05" C.), and it is also necessary that the temperature during the preparative period prior to equilibration be maintained essentially constant. In this procedure the buffer is added at 25" C., and the flasks are kept at this temperature until equilibrated. Since a sample of coffee contains only a small amount of oxidizable material, it is obviously necessary to control temperature and other conditions so that prior to measurement each sample will have undergone oxidation to the same extent. Otherwise the measured rates would not represent changes in amount of oxidizable substances.
Effect of Moisture on Coffee Staling
w OO T I M E INMINUTES
tOWJo40506oil)B090 STORAGE PERlaD IN DAYS
FIGURE1 (Left). RATE OF ABSORPTION OF OXYGENBY ROASTED COFFEEINFUSIONS AFTER ~O-DAY STORAGE FIGURE2 (Right). CHANGEIN OXYGEN ABSORPTION (0x1DIZABILITY) OF ROASTEDCOFFEEON STORAGE IN AIR AT VARIOUS MOISTURE LEVELS A . 100 per cent relative humidity B . 50 per cent relative humidity C. 0 per cent relative humidity
The results of measurements during a rather long storage period are presented in Table I1 and Figure 2. Since the starting point of the curves is the oxidisability level of fresh coffee, it is perhaps confusing that the initial numerical values differ slightly. This results from the fact that the buffer solution is not added simultaneously to all samples, and hence those to which buffer is added last undergo slightly less oxidation prior to measurement and are therefore measured a t a slightly higher level. A series of measurements on two samples of coffee stored a t 30" C. (sample D a t 93 per cent relative humidity and sample E under somewhat variable conditions averaging about 20-25 per cent relative humidity) is reported in Table 111. The coffee used was of the same blend but of a different roast than samples A , B, and C.
Measurement of the oxygen absorption of coffee permits the quantitative estimation of staling under various storage conditions. The report of Prescott (5) that flavor deterioration is very rapid in the presence of moist air has been confirmed by cup tests. I n order to estimate the degree of deterioration, a fresh lot of roasted coffee was prepared. It was screened to obtain a uniform portion free from fine and coarse particles. The standard ABSORPTION OF ROASTED COFFEEINFUSIONS TABLE I. OXYGEN coffee screens were utilized, and the portion reOxygen Absorbed. Cu. Mm. * ---. Time, >-sample A---Sample B--sample -C maining on the 28-mesh screen was used for the 1 2 Mean 1 2 Mean 1 2 Mean oxidisability measurements. It is desirable to Min. 55.4 56.7 56.1 24.0 24 1 24.1 42.8 46.1 44.4 84.8 89.3 87.1 102 101 102 work with a fairly uniform granulation, since the 20 lo 47.8 48.0 47 9 138 137 138 30 68.5 68 1 68 3 115 120 118 rate of solution of flavor constituents is influenced 40 87.6 88.6 88.1 148 152 150 173 170 172 by particle size. This lot of coffee was divided 50 104 105 105 170 174 172 1'47 196 197 60 121 122 122 195 199 197 225 223 224 into three parts to be stored a t different relative 75 140 140 140 221 226 224 255 254 255 90 160 161 161 249 256 253 288 287 288 humidities. One sample, A , was placed over 105 181 183 182 318 315 317 279 286 284 distilled water a t 100 per cent relative humidity; 120 195 197 196 299 306 303 345 341 343 the second, B, was stored over a saturated solution
INDUSTRIAL AND E X G I N E E R I N G C H E M I S T R Y
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IN OXIDIZABILITYOF COFFEE ON AIR TABLE 11. DECREASE STORAGE
Stornee
Period Days
Absorbed i n 2 Hr. per 0.5-G. of CoffeeSample -4 Sample B Sample C Cu. mm. Cu. mm. Cu. mm.
-02
362 349 323 311 303 289 280 276 261 236
350 342 297 249 196
0 1
4 7
10
18 24 31 49 82
111
77 52 21
...
TABLE 111. EFFECTOF SOLIDSCONTENT ON OXIDIZABILITY COFFEE VALUESOF ROASTED
Stora e Moisture Content, % Perioi Sample D Sample E
,--0xidizability Values, Cu. Mm.-.--Sample D---Sample E--Based on Based on Based on 0.500 g . Based on 0.500 8. 0.500 g. coffee 0.500 g . coffee coffee solids coffee solids
Days
0
3 7 11 17 24
2.93 4.55 6.30 7.23 9.83 11.02
2.93 2.67 2.66 2.85 3.37 3.35
305 290 274 258 226 199
314 304 292 278 251 223
310 299 291 287 284 279
319 307 299 295 294 289
The data of Table I11 reported on the bases of 0.500 gram coffee and 0.500 gram dry coffee solids show that correction for moisture content does not alter the general conclusions which may be drawn from a consideration of oxidizability values. However, even though estimation of moisture in coffee is not precise, it is probably desirable to report oxidizability values on a dry basis. This calculation may be made by simple proportion since the oxygen absorption rate is approximately proportional to the weight of coffee used when the deviation from 0.500 gram is not over.25-30 per cent.
Discussion of Results Cup tests made during storage detected a distinct stale flavor in sample A a t 100 per cent relative humidity after 3 to 4 days. Sample B a t 50 per cent relative humidity required about 7 to 8 days before staleness became apparent, and sample C at 0 per cent relative humidity was only slightly stale after storage for a month. Sample A developed a stale taste when the absorption rate had dropped approximately 100 cu. mm. per gram per 2 hours. Sample B reached an approximately similar state when the rate had dropped about 80 cu. mm. per gram, and sample C reached the incipient staleness level when the rate had decreased by 75 cu. mm. per gram. These differences are due to the difficulty in estimating staleness by cup test with any degree of exactitude and also to the difference in moisture content of the samples. Samples C and D staled much slower than A; therefore it is likely that a drop in absorption rate of about 75 cu. mm. per gram corresponds to incipient staleness for the particular coffee studied. This decrease in absorption rate is doubtless due to oxidation, volatilization, and possibly polymerization of the flavor constituents. It is probable that oxidation is the chief factor, but further work is necessary to establish the relative importance of the various factors involved in staling. The drop in absorption corresponds to a rate decrease of about 35 cc. of oxygen per pound of coffee and should be proportional to the amount of oxygen absorbed per pound of coffee in reaching incipient staleness. The measurements indicate that not all the oxidizable material in coffee is concerned with the development of staleness as ordinarily encountered. The complex mixture of flavor constituents probably contains substances of different sus-
VOL. 30, NO. 11
ceptibility to oxidation ; some react only in the presence of sufficient moisture, others are independent of moisture or capable of reacting only when a small amount is present. Coffee stored in air under the usual variable conditions of temperature and humidity is only slightly oxidized when undesirable flavors are detectable and never reaches the degree of oxidation possible a t high humidity. After storage for 31 days a t the 100 per cent humidity level, the coffee sample lost practically all coffee flavor and was scarcely recognizable as coffee by taste alone. The marked accelerating effect of humidity on the staling of roasted coffee is apparent from the cup tests and is confirmed by the oxidizability measurements. Punnett (6),however, emphatically stated that humidity does not affect the flavor changes normally occurring in roasted coffee which is in contact with air. 'In view of Prescott's observations ( 5 ) , as well as those of the writer, some error must have been introduced into the standard cup method used by Punnett in reaching his conclusions. Many factors are involved in measuring the oxidation of a material as complex as coffee, especially by a manometric method, but results indicate that much useful information may be obtained by the method outlined here. Measurements show that changes in the absorption rate are almost directly proportional to the amount of oxidizable material present, and therefore the rate measurements are probably equivalent to determining the total amount. The present method disregards the oxygen absorption occurring during the period of preparation of the infusion and equilibration. Disregarding the initial absorption permits a convenient technic and does not impair the validity of the results, since corresponding absorption rates are always compared. Work is now in progress on a method which permits measurement of absorption from the moment buffer is added. The current work indicates that the sensitivity and precision of the method can also be improved by working a t a lower temperature. The rate of oxygen absorption by fresh coffee increases with the degree of roast, and it is therefore difficult to compare different samples of coffee. Preliminary data suggest that the amount of oxidizable material in coffee increases from a light to a dark roast essentially up to the point the coffee is charred. It seems likely that a measurement of oxygen absorption is a more precise characterization of degree of roast than the estimation of color by eye or instrument. This point is being investigated and will be reported later. Further work is in progress to attempt a correlation between coffee staleness and rate of oxygen absorption. It is difficult to choose an oxidizability level corresponding to staleness because staleness is indefinite, and the degree of roast is a factor. However, it seems reasonable that the degree of deterioration of a given sample can be estimated quantitatively by a suitable combination of objective measurements.
Acknowledgment The author wishes to thank George Kirby and Lawrence Atkin for assistance, and especially for certain preliminary measurements.
Literature Cited (1) Balart, B. D., Tea Coffee Trade J . , 74, No. 1, 12 (1938). (2) Bengis, R. O., IND. E N G . CHEM.,28, 290-3 (1936). (3) Dixon, M., "Manometric Methods," Cambridge University Press, 1934. (4) Elder, L. W., Jr., IND. ENQ. CHEM.,29,267-9 (1937). (5) Prescott, S. C., Emerson, R. L., a n d Peakes, L. V., Jr.. Food
Research, 2, 1-20 (1937). (6) Punnett, P. W., Tea Coffee Trade J . , 74, N o . 4 , 17 (1938).
RECEIVED May 26, 1938.
