STORAGE OF DRIED FRUIT Influence of Temperature on Deterioration of .Apricots E. R. STADTMAN, H. A. BARKER, VICTORIA HAAS, AND E. M. MRAK University of Calijornia, Berkeley, Calij. T h e rates of darkening, sulfur dioxide disappearance, carbon dioxide production, and oxygen consumption in dried apricots have been determined as a function of storage temperature. The logarithms of the rates are a linear function of the reciprocal of the absolute temperature and are thus in agreement with the Arrhenius equation. An apparent activation energy of 26 kg.-cal. was calculated for the darkening reaction. This corresponds to a Qlc of
about 3.9. The temperature coefficient is sufficiently independent of moderate changes in sulfur dioxide, moisture, oxygen concentration, and age of the fruit that the storage life at low temperatures can be predicted from data obtained in accelerated storage tests at 49' C. The use of high temperatures (60-70' C.) to dry fruit to moisture levels below 25% may result in heat damage and a reduction in the storage life.
D
CHANGES IN COLOR. The rate of darkening is increased greatly by raising the temperature of storage. For example, the storage life a t 49" C. was only 16.2 days whereas a t 28" it was 312 days. Table I shows the relation between temperature and storage life. Storage life was decreased by a factor of about 4 for every 10' C. rise in temperature. Storage life is defined as the number of days required for the fruit to reach a n arbitrarily chosen degree of darkness (darkening index = unity), determined by comparing the color of a 50% alcoholic extract of the sample with reference solutions ( 1 ) . SULFUR DIOXIDED ~ P P E A R A N C During E . storage, the SUIfur dioxide concentration of the fruit declines (1). I n Figure 1 the logarithm of the sulfur dioxide concentration at the varioue storage temperatures is plotted as a function of time. A linear relation exists over a considerable interval, a n indication that sulfur dioxide disappearance approximates a first-order reaction. The half life of the sulfur dioxide was determined by interpolation, and the reciprocal of the half life was used as a measure of the rate of sulfur dioxide loss. Table I shows the relation between the rate of sulfur dioxide loss and temperature; as for
RIED fruit gradually deteriorates during storage. This
+
deterioration is greatly accelerated as the storage temperature is increased; consequently commercial distributors have made it a policy not to send dried fruit into localities where it may be exposed to temperatures above 80" F. for any considerable time, I n spite of the general observation that the rate of deterioration increases with temperature, little precise information is available concerning the relative rates of deterioration at various temperatures. I n view of military demands to send fruit to regions with tropical climates, it became necessary to investigate the temperature relations in order to be able to predict the life expectancy of fruit kept above 80" F. Previous papers (I,$) discussed the influence of moisture, sulfur dioxide, and oxygen on deterioration. Four lots of fruit (4,5, 8, and 9) were used. Lots 4 and 5 have already been described (1, 9). Lot 8 consisted of blanched Royal apricots, 1944 crop; details of drying will be disoussed later. Lot 9 was commercially blanched and dehydrated cling peaches; samples were dried a t 65" and 71' C. to moisture levels' of about 10, 15, 20, and 25%. The sulfur dioxide levels varied from 3200 to 3600 parts per million. Storage at temperatures below 37' C. was in small nonforce-draft incubators. The temperature fluctuation was of the order of * l o C. For temperatures above 37' C., water baths, having a temperature fluctuation of 10.2" C., were used. All methods, including the determination of storage life, rate of darkening, carbon dioxide production, oxygen uptake, and the determination and adjustment of sulfur dioxide and moisture levels have been described ( I , $ ) . EFFECT O F STORAGE TEMPERATURE
A typical experiment to show the influence of temperature on the rate of deterioration was carried out as follows: Blenheim apricots (lot 5) containing 23% moisture and 5350 p.p.m. sulfurdioxide were canned in No. 2 C-enamel tin cans. Five cans, each containing 250 grams of fruit, were stored a t 49", 41.8", 36.7', 27.8", and 22.2" C. After various periods of time cans were removed from storage, and the color and sulfur dioxide concentration of the fruit and the oxygen 'and carbon dioxide in the can were determined.
L 50
2.9 3.0 0
100
151
3
206
:STORAGE T I M E
250
300
350
IN DAYS
Figure 1. Change in SO2 Concentration with Time at Varioum Temperatures Lot 5 contmining 5350 p.p.m. SOI cannod ir air.
.
541
542
INDUSTRIAL AND ENGINEERING CHEMISTRY
,
I 3 ,
3.2
I 3A
3.3
JT
x 103
Figure 2. Influence of Temperature on Rate of Oxygen Uptake, SOP Disappearance, COP Production, and Darkening Fruit (lot 5) contained 5350 p.p.m. SO? and 23.5% nioiature. Darkening = reciprocal of storage life; CO2 production = initial rate:in mg. per 100 grams dry fruit per day X 10; SO2 disappearance = reciprocal of half life X 108; 0 2 uptake = reoiproeal of half life X loa. Temperature is degrees absolute.
color, the late increases nearIy four times for each 10" C. rise in temperature. OXYQEN UPTAKE. When the logarithm of the residual oxygen, expressed as milligrams per 100 grams of dry fruit, is plotted against time, a linear curve is obtained over a considerable range of oxygen concentrations. The time required for half of the oxygeii initially present in the can to be consumed was determined by interpolation; the reciprocal of this time interval was used as a measure of the rate of oxygen uptake, Table I gives the relative rates of oxygen uptake a t various temperatures. CARBONDIOXIDEPRODUCTION. For the particular lot of fruit used, carbon dioxide fwrmation was almost a linear function of time, and the initial slope of the carbon dioxidetime curve was therefore taken as a measure of the rate of carbon dioxide production. .Table I presents the relative rates of carbon dioxide production. The influence of temperature on this factor is very similar to its effect on oxygen uptake and darkening. TEMPERATCRECOEFFICIENTS. It has been suggested that the darkening of dried fruit may be due to the formation of a number of different colored compounds, resulting from different re-
Vol. 38, No. 5
actions with different tcmpcraturc coefficients. If this \WIY$ true, measurements on the rate of darkening at one tempcrntiirc~ \I-ould not necessarily bear a constant relation to darkvning at, another temperature. T h o relation between darkening a n d temperature n-as therefore tested by plotting the log of r n k against the reciprocal of absolute temperature (Figure 2 ) . A straight line was obtained, showing that the data are in agrecxnertt with the Arrhenius equation relating temperature anti the rat,ri of a chemical reaction. Therefore it must be concluded oithor that the same reaction controls the rate nt nll temperatures, or that, if several reactions are involved, thcy must have ncarly identical kmperature coefficients. The apparent activation energy of the darkening process, calculated from the slope of the rate-temperature curve, is of the order of 26 kg.-cal. The corresponding Ql0 value is: rate at ( T 10' C.) = 3.9 Qlo = rate at T O C .
+
Data for oxygen consumption, sulfur dioxide loss, and carbon dioxide production are also plotted in Figure 2; the slopes of all curves are nearly the same. EFFECT O F OTHER V A R I A R L E S
lifter a constant relation was observed betwecn darkening and temperature under the conditions of the experiment, it seemed
TABLE I. EFFECT O F STORAGE TEMPERATURE ON DETERIORATION O F D R I E D APRICOTS(LOT5) Tzmp., C.
Storage Life, Days
Relative RateaRate of con 01 Darkeninga Darkening production uptake
_?
502 lass
Half Life of so2
Reciprocal of 5 t o r a g ~life, X 103.
0.0 3. !
J7-
3.2
35
*io3
Figure 3. Influence of SOP Concentration, Vacuum and Air Packing, and Temperature on Darkening Rate
Figure 4. Influence of AIoisture and Temperature on Darkening Rate
Fruit (lot 5) contained 23.5% moisture. Broken lines refer to storage i n air. The rate of darkening is expremed as the reciprocal of the storage life X 10s. Temperature is degrees absolute.
Fruit contained 6000 p.p.m. S O z (air pack). The rate of darkening i s expreased a8 the reciprocal of storage life X 10'. Temperature ia degrees absolute.
May, 1946
INDUSTRIAL AND ENGINEERING CHEMISTRY
TIME I N DAYS A T 2 8 'C.
Figure 5. Influence of Storage at 28' C. on Storage Life at 49' C.
desirable to determine if this relation is modified when fruit is stored under other conditions. Therefore, similar experiments were set up to study the influence of sulfur dioxide concentration, moisture content, oxygen, and age of fruit on the temperature relation. EFFECTOF SO2 AND OXYGEN.Samples of Blenheim apricots of lot 5 were adjusted to 9500 and 5350 p.p.m. sulfur dioxide and were canned in both air and vacuum. Storage tests were made at various temperatures as described above. The data plotted in Figure 3 show that the relation between storage life and temperature is independent of sulfur dioxide and oxygen; Le., the slopes of the rate-temperature curves are identical for the various storage conditions. E F F E COF ~ MOISTURE. Tilton apricots of lot 4 were adjusted to 6000 p.p.m. sulfur dioxide and to moisture levels of 24 and 15%, and stored at various temperatures. The relation of storage life to temperature was the same at both moisture level6 (Figure 4). Tilton apricots (lot 2), conEFFECTOF PREVIOUS STORAGE. taining 24% moisture and 2380 p.p.m. sulfur dioxide, were placed i n a friction-lid 5-galJon tin can and stored at 28' C. At the beginning of storage and a t 40-day intervals thereafter, a sample of fruit was removed and its storage life a t 49" C. determined. The experiment was continued until the fruit at 28" C. had reached the limit of edibility ( I ) , 206 days. The data (Figure 5) show that the storage life a t 49" C. is a linear function of that a t 28". The ratio of the residual storage life at 28" to the storage life a t 49" is, therefore, a constant and is equal to 18.2. This i s nearly the same value as was found in the experiments described above where the ratio of storage life a t 28" to that a t 49" C. varied from 19 to 22. Other experifients 'with fruit a t various sulfur dioxide levels and moisture contents have given similar results. COMMERCIAL
STORAGE
PREDICTION OF STORAGE LIFE. Since there is a constant relation between the storage life of dried apricots and the storage temperature, it is possible to predict the storage life a t one temperature from measurements a t another. For example, an accelerated storage test can be made at 49" C. where the storage life is only 1 to 3 weeks, and from the data obtained it is possible to predict how long the fruit will keep a t any other tempekature, a t least over the range 22" to 49" C. During the course of these studies, this method of predicting storage life has been repeatedly tested. By assuming the rate of darkening to increase four times for each 10' C. rise in temperature, predictions can be made with an accuracy of *lo%. This method of prediction should have a number of commercial applications. I n commercial EFFECTOF DRYINGTIMEAND TEMPERATURE. dehydrators, apricots are dried in a stream of hot air at 60-70" C. Dehydration usually requires 8 to 20 hours, depending mainly upon temperature, humidity, and size and final moisture content of the fruit. Although the drying time is relatively short,
543
the temperature is sufficiently high so that appreciable damage may be expected on the basis of the temperature-storage life relations already described. To test this conclusion and to determine the extent of damage under various conditions of dehydration, Royal apricots (lot 8) were blanched, sulfured, and dehydrated in an experimental model dehydrator to moisture contents of 22.4, 19.8, 15.7, and 13.5%. The air temperature of the dehydrator was kept constant a t 71" C.; the drying time varied from 9 hours at the highest moisture to 14 hours a t the lowest moisture content. An identical experiment was carried out a t a drying temperature of 60" C., the fruit being dried to 26.5, 20.7, 16.2, and 9.5% moisture; the drying times varied from 12 to 24 hours. No obvious signs of deterioration were detectable in any of the samples; all had nearly the same color. After dehydration, all samples were finally adjusted to 2300 p.p.m. sulfur dioxide and 24% moisture, and the storage life was determined at 49' C. (Figure 6).
I
3t 21 9
I
II,
13
15 17 19 21 23 MOISTURE IN PERCENT,
25
2
,
DRYING TIME IN HOURS
Figure 6. Influence of Drying Time and Temperature and of Final Moisture Content on the Storage Life of Dried Apricots Broken linea refer t o the moiature content at the end of dehydration, nolid line. refer t o the d r y i n g time. Storage life i. i n day. at 4 9 O C.
The storage life varies with both temperature and drying time. The temperature of dehydration is of no great consequence, provided the fruit is not dried to moisture contents less than 25%. With continued drying, however, heat damage occurs a t both temperatures, being greater a t 71' than a t 60" C. For example, tke storage life of fruit dried to 25% moisture a t 71" C. was only 4% less than when the drying was a t 60" C. However, heat damage caused by drying from 25 to 11% moisture a t 60" C. decreased the storage life 10%; drying a t 71" C. decreased the storage life 24%. Similar results were obtained when blanched cling peaches (lot 9) were dehydrated a t 60" and 71" C. under commercial plant conditions. These results indicate the undesirability of using high temperatures to dry fruit to moisture levels below 25%. When fruit of lower moisture content is required, the final drying should be done at low temperatures (25-40" C,). LITERATURE CITED
(1) Stadtman, E. R., Barker, H. A., Mrak, E. M., and Mackinney,
G., IND.ENG.CHEM.,38,99 (1946). (2) Stadtman, E. R.,Barker, H. A., Haas, Victoria, Mrak, E. M., and Mackinney, G.,Ibid., 38,324 (1946). P R E S E N Ton ~ Dthe program of the Division of Agricultural and Food ChemCHEMICAL SocImr~Y. A report istry of the 1945 Meeting-in-Print, AMERICAN on a joint research project of the Quartermaster General's O5oe, U.8. Army and University of California.
.
