E. R. STADTMAN, H. A. BARKER, E. M . M R AK , A N D G. MACKINNEY
University o f California, Berkeley, Calif.
T h e edible storage life o f dried fruit can be defined as the time required for the fruit to darken to such an extent that it is no longer generally acceptable. A colorimetric pro
cedure, involving a visual comparison o f 50% alcoholic extracts with standardized reference solutions, has been developed for determining the relative degree o f darkness of dried fruit samples. By this method the storage life o f dried apricots has been determined as a. function o f the sulfur dioxide level and the moisture content. The storage life is inversely proportional to the initial sulfur dioxide concentration. Sulfur dioxide disappears on
stor-D
RIED fruits gradually darken and otherwise deteriorate with time, especially when the storage temperature is high.However, relatively little information is available (1, 8) concern
ing the factors influencing the rate of deterioration. Since such information is essential for the development of more satisfactory methods of processing and storing dried fruits, a broad study of this problem was undertaken with dried apricots as the experi
mental material.
Among the factors that affect the rate of deterioration, and consequently the storage life, of dried fruits are temperature, moisture, sulfur dioxide, oxygen, and the previous history of the fruit. The present paper is concerned with the development of quantitative methods for studying these factors and with the influence of moisture and sulfur dioxide on storage life. Three lots of commercially dried and packed Blenheim apricots and one lot of Tilton apricots were used.
Lot 1, sun-dried Blenheim apricots containing 23% moisture, was resulfured to 5800 p.p.m. sulfur dioxide and dehydrated in a tunnel drying to 13% moisture. Part of this fruit was further dried in a vacuum desiccator to a moisture level of 7.1%. None of this fruit darkened appreciably during dehydration. The final sulfur dioxide contents were 4600, 5200 and 5800 p.p.m.
at 7.1, 13.0, and 23.5% moisture, respectively.
Lot 2 was made up of sun-dried Blenheim apricots containing 21.5% moisture and 1500 p.p.m. sulfur dioxide.
Lot 3, steam-blanched' commercially dehydrated Blenheim apricots, contained 5350 p.p.m. sulfur dioxide and 21-23% mois
ture. Some of this fruit was redried in a laboratory dehydrator at 65 0 C. to 10% moisture, and contained 2800 p.p.m. sulfur di
The moisture content of fruit containing less than 14% mois
ture was generally determined by drying ground samples for 16 hours in a vacuum oven at 65-70 ° C. and a pressure of 25-40 mm.
of mercury. Moistures higher than 14% were determined by measuring the conductivity of samples with a dried-fruit mois
ture tester developed by the Dried Fruit Association of California.
Moisture adjustments were made by adding the calculated
quan-age at a rate which is roughly proportional to the log
arithm o f the sulfur dioxide concentration. Approxi
mately 65% o f the sulfur dioxide initially present is lost during storage life. Under anaerobic storage conditions, the rate o f darkening is accelerated by decreasing the moisture content over a range o f 40 to 10%, a maximum being reached somewhere between 5 and 10% moisture.
In the presence o f oxygen, the rate o f darkening is in
creased at high relative to low moisture contents by amounts which vary wfith the quantity o f oxygen available to the fruit.
tit.y of water and allowing the fruit to stand in a sealed container with occasional mixing until the moisture was evenly distributed (3 to 4 days).
Sulfur dioxide was determined by the method of Nichols and Reed (2). Because of sampling errors, duplicate determinations varied by ¿=5%. All sulfur dioxide contents are expressed as parts per million in moisture-free fruit unless otherwise stated.
The term “ initial SO2 content” refers to the content at the be
ginning of storage of the fruit. The SOj content of fruit was ad
justed when necessary either by volatilizing the required amount of liquid sulfur dioxide from a glass vial or by adding charcoal previously saturated with sulfur dioxide at a low temperature.
In the former method sulfur dioxide gas was introduced into long, narrow' tubes which were cooled in liquid air. The sulfur dioxide was thus condensed in the tubes until the proper amount wras obtained (determined by weighing). The tubes were then quickly placed in cans containing the. fruit, and the cans were immediately sealed. After only one day at room temperature, the sulfur dioxide was uniformly distributed in the fruit.
For the latter method activated carbon (Columbia grade F, 20- 48 mesh) was found to be most satisfactory because of its rela
tively high density and adsorptive capacity. When saturated at 5-10° C., the charcoal contained 0.47-0.56 gram of available sulfur dioxide per gram of SCh-free charcoal. Charcoal saturated with sulfur dioxide was weighed into small cans or cardboard boxes and placed in sealed containers with the fruit. At room temperature sulfur dioxide is almost quantitatively transferred to the fruit within 4 days, provided the moisture content of the fruit is above 12%. At lower moisture levels the transfer takes several days longer.
Charcoal has the property of catalyzing the oxidation of sulfur dioxide by oxygen. When charcoal is used in adjusting the SO2 level, it is therefore necessary to add an extra amount of sulfur which obviously provide only a rough measure of quality. For the present studies a more precise determination of quality seemed desirable. Tests showed rather conclusively that most indi-99
100 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 38, No. 1
viduals are more critical of changes in color than of other changes occurring in fruit during storage. As darkening increases, ac
ceptability decreases. While there is no fixed point beyond which fruit is inedible regardless of circumstance, it is possible to select fruit which has darkened to such an extent that it is barely toler
able to the average person. Properly described, this degree of darkness becomes a fixed point of reference. This arbitrary point is defined as the limit of edibility in apricots; the time, from be
ginning of storage, required for fruit to darken to this extent there
fore represents the storage life. The problem then becomes one of unambiguous description of this reference point.
It has been preferable to work with extracts since they are more readily compared than the original fruit, however carefully it may have been blended. The darkening involves the formation of colored compounds of rather uniform character from relatively colorless precursors; since the extracts obey Beer’s law in the region of measurement, it seems logical to characterize the ex
tracts by their photoelectric or optical densities under specified conditions. Unfortunately this is possible only when fruit of the same initial SO2 level is under examination, since the absorp
tion spectrum of an extract varies with S02 level. Figure 1 il
lustrates this change of absorption spectrum with S02 level.
While it would be feasible to calibrate optical densities as a function of S02 level when the previous history of the fruit was known, in unknown cases this would not be possible without more complete spectroscopic data. This does not as yet seem practical under routine conditions. A method involving simple visual comparisons was therefore evolved.
An inorganic colored salt solution served as a st andard solution.
For working purposes an extract of dark apricots was prepared and diluted to give a graded series of reference solutions. The degree of darkening was then determined by preparing an ex
tract of the fruit in a standard manner, and comparing it visually under specified conditions with the reference series, which thus provided a convenient numerical scale. Since it does not involve color in the accepted sense, the value obtained is referred to as an index of darkening.
P R E P A R A T I O N O F S O L U T IO N S
St a n d a r d So l u t i o n. A standard solution representing the color of an extract of fruit at the
“ limit of edibility” was prepared by mixing solu
tions of copper sulfate, potassium dichromate, and cobaltous sulfate. These salt solutions were made up as follows:
A. 50 grams of cobaltous sulfate diluted to 1 liter with 4% sulfuric acid
B. 25 grams of potassium dichromate diluted to 1 liter with 4% sulfuric acid the standard solution when compared under the conditions described in the determination of the darkening index. This reference solution was assigned a numerical value of unity.
Other reference solutions were prepared by suitable dilution of the alcoholic extract so as to give a series of solutions 0.2, 0.4,
lowed to stand at room temperature for about 24 hours with oc
casional shaking. Approximately 15 ml. of the clear extract were placed in a small glass vial (5 X 2.2 cm., inside diameter), the depth of solution was adjusted to exactly 4 cm., and the darken
ing index was determined by visual comparison with the reference solutions. In making the comparisons, the small glass vials containing the extracts and the reference solutions were placed on a white background and viewed from the top looking down through the solution. The bottom of the vial was in direct con
tact with the white background. The only light source was a 500-watt Mazda bulb suspended about 4 feet above the table on which the comparisons' were made. Since the quality of the color produced on darkening is different for fruit stored at different S02 levels, it was found very important that the comparisons be made under these exact conditions. The method involves a comparison of luminosity as well as color density. Only by this method is it possible to obtain a darkening index that is always consistent with the visual appearance of the fruit.
D E T E R M I N A T I O N O F S T O R A G E L I F E
To determine the storage life of a given lot of fruit, the follow
ing procedure was used. The fruit was stored under the desired conditions, and at least four samples were taken after various
pe-January, 1946 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 101 intensity of the extracted color varies greatly with alcohol con
centration.
Reference solutions stored at 0° C. do not change significantly during a year.'
E D I B I L I T Y
To show the relation of sulfur dioxide concentration and color to taste, edibility tests were run by the following procedure: 50 grams of dried apricots were placed in a 400-ml. beaker, covered with 200 ml. of distilled water, and allowed to stand overnight (only 2 hours for steam-blanched fruit). The soaked fruit was heated to boiling on a gas plate in 20 minutes and boiled moder
ately for 10 minutes. Thirty-seven grams of sucrose were then added, and the mixture was simmered for 5 more minutes and cooled. The samples were tasted while the fruit was still warm by at least ten individuals who, without knowledge of the SO2 contents, arranged the samples in order of preference.
Small air incubators give a sufficiently constant temperature ( ± 1 ° C.) only below 32 0 C. For temperatures from 32 to 50 0 C., water baths were used. The temperature fluctuation in these baths was of the order of ±0.2° C.
E F F E C T O F M O I S T U R E
_____1_____ I_____ I_____ 1_____ 1_____
O 2 4 6 8 10 12 14 16 IB
T IM E IN D A Y S
Figure 2. Change in Darkening Index as Function o f Incubation Tim e at 49° C.
F ig u re s o n cu rves refer to in it ia l s u lfu r d io x id e lev el.
nods of time. The first sample was taken as soon as visible signs of darkening were observed; others were taken at intervals spaced so that some were lighter and some darker than unity.
The darkening index of each sample was determined and plotted as a function of time, and the number of days required for the fruit to reach “ the limit of edibility” (darkening index = 1) was determined by interpolation. This time interval therefore repre
sents the storage life of the fruit. If darkening is followed as de
scribed, duplicate determinations of the storage life never differ by more than 5%.
The reciprocal of the storage life was used as a measure of the rate of deterioration. This method of expressing rate was ne
cessitated by the fact that the darkening-time curves are not linear but are convex to the time axis (Figure 2).
Ob s e r v a t i o n s o n t h e Me t h o d. Photoelectric density of the extracts may bo used instead of the darkening index as a measure of darkening when all fruit tested has the same initial SOs con
tent and the same history with respect to sulfur dioxide treat
ment since the color quality of extracts is largely independent of other variables such as time, temperature, moisture, and oxygen.
When a photoelectric method was used, the alcoholic extracts had to be diluted 1 to 20 with distilled water. The transmission of the resulting extract was determined using a blue filter No.
440. The photoelectric density (2 — log <?), determined with an Evelyn colorimeter, is a linear function of the concentration of an extract over the density range 0.0 to 0.4 (galvanometer read
ings from 100 to 40).
When samples of various moisture contents were compared, corrections were made so that the darkening index always refers to fruit containing 24% moisture.
The ratio of fruit to solvent influences the degree of extraction to some extent. The quantity of moisture-free fruit per 100 ml. of solvent should never exceed the range of 5-10 grams.
Increasing the extraction time from 24 to 30 hours increases extraction of colored material only slightly.
The pH of the extract is unimportant over the range 2.5 to 5.0.
Untreated extracts have a pH between 3.5 and 4.0.
Fifty per cent alcohol is more satisfactory than water as a sol
vent because it gives perfectly clear, extracts that need not be filtered, and it also eliminates the possibility of microbial activity. With less than 40% alcohol, the extracts tend to be somewhat turbid; with more than 60% alcohol the quality and
Moisture content has a considerable influence on the deteriora
tion of apricots. This influence, however, is greatly modified, both in magnitude and direction, by the quantity of oxygen to which the fruit is exposed.
10 l_
10
PERCEN T M O ISTU RE
Figure 3. Influence o f Moisture and Oxygen on Storage Life at 49° C. o f Blenheim Apricots Containing 2800
P.P.M. S 0 2 (Dry Weight)
T h e q u a n t i t y o f fr u it (d ry w e ig h t) p e r N o . 2 c a n ( 5 8 5 - m i . ca p a c ity ) waa varied as fo llo w s : s q u a r e s , 200 g ra m a in N s; t r ia n g le s , 308 g r a m s in a ir ; c lo se d c ir c le s , 20 0 g r a m s in air* o p e n c ir c le s, 75 g r a m s in air.
Q u a n titie s o f o x y g e n re fe r to th e a m o u n t o r ig in a lly p r e se n t per 100 g r a m s o f d ry f r u i t .
V
An a e r o b i c St o r a g e. The effect of moisture on storage life under anaerobic conditions may be illustrated by a typical ex
periment. Blenheim apricots (lot 3) were dehydrated to 10%
moisture and then rcwctted to moisture levels of 10, 15, 20, and 25%. The preliminary dehydration of all the fruit equalized any possible heat damage. The fruit was packed in cans, and oxygen was removed by evacuating to 3 cm. mercury and replacing the air with nitrogen. Storage was at 49° C. The upper curve of Figure 3 shows that the storage life increased with increasing moisture content over the range of 15-25%. About a 55%
in-102 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 38, No. 1
PPM SO2 « 1 0 '3
F ig u re 4. In flu e n ce o f In itia l SO- Level o n S tora g e L ife o f B le n h e im A p r ic o ts at 49° C.
crease in storage life was observed over this range. In compar
able experiments with fruit from lots 1 and 4, similar effects were noted over the range of 7 to 25% moisture.
Ae r o b i c St o r a g e. The beneficial effect of high moisture ob
served under anaerobic storage conditions is diminished when the fruit is exposed to oxygen. If less than 10 mg. of oxygen are present per 100 grams dry fruit, the effect is scarcely detectable;
but if the quantity of oxygen is large (more than 100 mg. per 100 gram) an increase in moisture will cause little if any increase in storage life. With some lots of fruit the storage life in air is ac- tually decreased by raising the moisture content.
The interdependence of oxygen and moisture on storage life is illustrated in Figure 3. The conditions were identical with those of anaerobic storage except that the samples were canned in air.
The ratio of oxygen to fruit in the air-filled cans was varied by changing the quantity of fruit per can. Figure 3 shows that the beneficial effect of high moisture decreases progressively as avail
able oxygen increases.
The reasons for this relation will be considered in some detail in another paper. Here it will only be pointed out that oxygen accelerates deterioration in direct proportion to the amount con
sumed by the fruit, and the rate of oxygen consumption increases greatly with the moisture content. At high moisture levels the harmful effect of oxygen largely or entirely neutralizes the bene
ficial effect of moisture that is observable under anaerobic stor
age conditions.
_ When apricots (lot 2) were stored in the presence of a small amount of oxygen (47 mg. per 100 grams), the percentage in
crease in storage life resulting from an increase in moisture con
tent from 10 to 24% was nearly independent of the initial S02 level (temperature being constant), provided the latter is ex
pressed on a dry weight basis; data for two temperatures and seven sulfur dioxide levels are given in Table I. The moisture effect is smaller (10%) at 36.6° than at 49° C. (40%). When sulfur dioxide is expressed on a wet weight basis, as is usual in commercial practice, the percentage increase in storage life ob
tained by an increment in moisture becomes greater with increas
ing S02 level.
In view of the beneficial effect of increasing the moisture con
tent from 10 to 24%, it was of interest to see whether still higher moisture levels would further prolong the storage life of the fruit.
For this purpose, apricots (lot 1) containing 23.5% moisture were rewetted to 40% moisture. A few crystals of thymol were added to prevent microbial activity. At 49° C. the storage life of the 40% moisture fruit was 40% greater than that of the 23.5%
moisture fruit. It is evident that the rate of darkening decreases continuously with increasing dilution of the soluble constituents of the fruit over a wide range. At very high moisture contents fruit would be expected to keep more or less indefinitely. Tins prediction is verified by the fact that ordinary canned fruit, packed in sirup, does not deteriorate at a significant rate.
Most of the experimental samples contained 7% or more mois
ture. It is difficult to attain lower moisture levels without scorch
ing the fruit. However, several exploratory experiments with low-moisture fruit were carried out to see whether more or less complete removal of moisture is beneficial. In one experiment, for example, fresh Tilton apricots were sulfured and dehydrated to 60-70% moisture in a commercial dehydrator. The fruit was then resulfured to approximately 20,000 p.p.m. and dried to 0.7% moisture in vacuo at 65-70° C. The S02 content after drying was 1600 p.p.m. This fruit was normal in appearance.
When stored at 49° C., the rate of darkening was about half that of 24% moisture fruit. It may be tentatively concluded that ex
tremely low moisture levels, like high moisture levels, retard darkening.
E F F E C T O F S U L F U R D I O X I D E
The most effective method of prolonging the storage life of dried apricots is to add sulfur dioxide. In commercial practice, 1000 to 3000 p.p.m. sulfur dioxide are usually added. However, if the incubation temperature is high (37° C. or above) even 3000 p.p.m. will not maintain apricots in an edible condition for more than a few weeks. Therefore, the effect of even higher SO- levels on storage life was studied here.
Figure 4 shows the influence of SO- contents ranging from 1500 to 8000 p.p.m. on the darkening of Blenheim apricots (lot 2), containing 14 and 21% moisture and stored at 49° C. The life increases linearly with the initial S02 content at both moisture levels, but the beneficial effect of sulfur dioxide is somewhat greater at the higher moisture level. Other experiments indicate that the storage life increases less rapidly than the S02 content at levels greater than 13,000 p.p.m. The same general relations
F ig u re 5. L oss o f S u lfu r D io x id e (49° C. a n d 14.2%
M o istu r e ) as a F u n c tio n o f T im e a n d In itia l S u lfu r D ioxid e Level
C ro sse s a n d d o tte d lin e i n d ic a te w h e n fr u i t is a t l i m i t o f e d ib ility . A b o u t 4 4 m l . o x y g e n w ere p r e se n t p e r c a n .
January, 1946 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 103 was nearly doubled by increasing the S02 level
January, 1946 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 103 was nearly doubled by increasing the S02 level