STORAGE OF DRIED FRUIT Influence of Moisiure and Sulfur Dioxide on Deterioration of Apricots E. R. STAD@MAN, H. A. BARKER, E. M. MRAK, AND G. MACKINNEY University of California, Berkeley, Calij. T h e edible storage life of 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 procedure, involving a visual comparison of 50% alcoholic extracts with standardized reference solutions, has been developed for determining the relative degree of darkness of dried fruit samples. By this method the storage life of dried apricots has been determined as a function of 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-
age at a rate which is roughly proportional to the logarithm of the sulfur dioxide concentration. Approximately 65% of the sulfur dioxide initially present is lost during storage life. Under anaerobic storage conditions, the rate of darkening is accelerated by decreasing the moisture content over a range of 40 to lo%, a maximum being reached somewhere between 5 and 10% ‘moisture. In the presence of oxygen, the rate of darkening is increased at high relative to low moisture contents by amounts which vary with the quantity of oxygen available to the fruit.
D
tity of water and allowing the fruit to stand i n a sealed container with occasional mixing until t h e moisture was evenly distributed (3 t o 4 days). Sulfur dioxide was determined by the method of Nichols and Reed ( g ) . 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 t o the content at the beginning of storage of the fruit. The SO, content of fruit was adjusted when necessary either by volatilizing t h e required amount of liquid sulfur dioxide from a glass vial or by adding charcoal previously saturated with sulfur dioxide at a low temperature. I n 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 was 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, 2048 mesh) was found t o be most satisfactory because of its relatively high density and adsorptive capacity. When saturated at 5-10’ C., the charcoal kontained 0.47-0.56 gram of available sulfur dioxide per gram of SOz-free charcoal. Charcoal saturated with sulfur dioxide was weighed into small cans or cardboard boxes and placed in sealed containers with t h e fruit. At room temperature sulfur dioxide is almost quantitatively transferred t o the fruit within 4 days, provided the moisture.content of t h e fruit is above 127& 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 SO1 level, it is therefore necessary t o add a n extra amount of sulfur dioxide equivalent to the oxygen in t h e container. Oxygen is rapidly removed from a sealed container when charcoal and a n excess of sulfur dioxide are present.
RIED fruits gradually darken and otherwise deteriorate
with time, especially when the storage temperature is high. However, relatively little information is available (1, 8 ) concerning 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 a s the experimental 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 i n a tunnel drying t o 13% moisture. P a r t of this fruit was further dried i n a vacuum desiccator t o 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. a t 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 21-23y0 moisture. Some of this fruit was redried i n a laboratory dehydrator a t 65 O C. to 10% moisture, and contained 2800 p.p.m. s’ulfur dioxide. Lot 4 was made up of both steam-blanched and unblanched samples of Tilton apricots which had been sulfured and dehydrated. The fruit contained 8-9% moisture and 1000-2000 p.p.m. sulfur dioxide. MOISTURE AND SULFUR DIOXIDE
The moisture content of fruit containing less than 14% moisture was generally determined by drying ground samples for 16 hours in a vacuum oven a t 65-70 ’C. and a pressure of 25-40 mm. of mercury. Moistures higher than 14% were determined by measuring the conducti.bity of samples with a dried-fruit moisture tester developed by t h e Dried Fruit Association of California. Moisture adjustments were made by adding the calculated quan-
EVALUATION OF QUALITY
I n previous investigations (1, 3 ) the quality of the fruit was described by such terms as good, fair, poor, dark, black, etc., 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 t h a t most indi-
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PREPARATION OF SOLUTIONS
STANDARD SOLUTION.il standard solution representing the color of an extract of fruit a t t,he “limit of edibility” was prepared by mixing solutions of copper sulfate, potassium dichromatc:, and cobaltous sulfate. These salt solutions were made up as follows: A4. 50 grams of cobaltous sulfate diluted to 1 liter with 4% sulfuric acid B. 25 grams of potassium dichromate diluted t o 1 liter with 4y0 sulfuric acid C. 25 grams of cupric sulfate (CuSO4.5HzO) diluted to 250 mi. with 4 2 sulfuric acid The standard color \%asprepared by mixing 53 ml. of solution A, 33 ml. of B, 6 ml. of C, and 100 ml. of 4Ye sulfuric acid. The color of this solution was similar to that of a 50% alcoholic extract of apricots which were COIIsidcred to be a t the “limit of edibility”. Tlie darkening index of this standard solution wa3 arbitrarily taken t o be unity. REFEREKCE SOLUTIOKR. The reference soluFigure 1. Change in Light Abmrptioii as Fun-tion of Wave Length tions were prepared by dilution from a 50% a t Different SO2 Levels alcoholic extract Of dark apricots’ The exOpen circles: fruit containing 800 p.p.m. Son, aampled at increasing time intervals; a = o days, b = 4 days, c = 8 days, d = 19 days at 49” C. Closed and partly closed tract was obtained by suspending 100 grams circles: fruit containing 20.000 p.p.m. samples at e = 29 days, f 36 days, g = 50 days. A t 20,000 p.p.m. Son there is no significant change in absorption as incubation of ground dark fruit in 1 liter Of 50% time is increased; alight but regular changes occur at 800 p.p.m. 411 curves were alcohol and letting it stand a t room temrelated to unity at 510 m p (log 1 = 0). perature for 24 hours with occasional shaking. Part of the clear extract was diluted with 50Ye alcohol until a color was obtained which matchcd viduals are more critical of changes in color than of other changes the st,andard solution when compared under the conditions occurring in fruit during storage. AS darkening increases, acdescribed in the determination of the darkening index. Thii ceptability decreases. While there is no fixed point beyond which reference solution was assigned a numerical value of unity. fruit is inedible regardless of circumstance, i t is possible t o select Other reference solutions were prepared by suitable dilution of fruit which has darkened to such a n extent t h a t it is barely tolerthe alcoholic extract so as to give a series of solutions 0.2, 0.4, able t o the average person. Properly described, this degree of 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0 times as concentrated as darkness becomes a fixed point of reference. This arbitrary point the reference solution of unit concentration. Dilution of the is defined as the limit of edibility in apricots; the time, from hestandard solution does not give the same result as dilution of ginning of storage, required for fruit to darken to this extent therethe fruit extract. fore represents the storage life. The problem then becomes one of unambiguous description of this reference point. DETERMIN4TION OF DARKENIRG INDEX It has been preferable t o work with extracts since they are more readily compared than the original fruit, however carefully it To determine the darkening index, 20 grams (wet weight) of may have been blended. The darkening involves the formation of ground fruit were suspended in 200 ml. of 50Ye alcohol and alcolored compounds of rather uniform character from relatively lowed to stand a t room temperature for about 24 hours with occolorless precursors; since the extracts obey Beer’s law in the casional shaking. A4pproximately15 ml. of the clear extract were' region of measurement, it seems logibal t o characterize the explaced in a small glass vial ( 5 X 2.2 em., inside diameter), the tracts by their photoelectric or optical densities under specified depth of solution was adjusted to exactly 4 cm., and the darkenconditions. Unfortunately this is possible only when fruit of ing index was determined by visual comparison with the reference the same initial SO, level is under examination, since the absorpsolutions. I n making the comparisons, the small glass vials tion spectrum of an extract varies with SO, level. Figure 1 ilcontaining the extracts and the refereiice solutions v ere p1act.d lustrates this change of absorption spectrum with SO? level. on a white background and viewed from the top looking donm While it would be feasible t o calibrate optical densities as a through the solution. The bottom of the vial was in direct confunction of SO2 level TI hen the previous history of the fruit was tact mith the white background. The only light source mas a known, in unknown cases this would not be possible without more 500-x-att Lfazda bulb suspended about 4 feet above the table on complete spectroscopic data. This does not as yet seem practical which the comparisons were made. Since the quality of the under routine conditions. A method involving simple visual color produced on darkening is different foi fruit stored a t differcnt comparisons was therefore evolved. SO, levels, it was found very important that the comparisons be made under these exact conditions. The method involves a An inorganic colored salt solution served as a standard solution. comparison of luminosity as ne11 as color density. Only by thii For working purposes a n extract of dark apricots was prepared method is it possible t o obtain a darkening index that is alwagi and diluted to give a graded series of reference solutions. The consistcnt with the visual appearance of the fruit. degree of darkening was then determined by preparing an extract of the fruit in a standard manner, and comparing it visually DETERMINATION O F STORAGE LIFE under specified conditions with the reference series, which thus provided a convenient numerical scale. Since it does not involve To determine the storage life of a given lot of fruit, the folloacolor in the accepted sense, the value obtained is referred to as ing procedure was used. The fruit was stored under the desired an index of darkening. conditions, and a t least four samples were taken after various pe-
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intansity of the extracted color varies areatlv with alcohol concentration. Reference solutions stored a t 0" C. do not change significantly during a year.
r
EDIBILITY
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 t o boiling on a gas plate in 20 minutes and boiled moderately for 10 minutks. 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 ( =t1O C.) only below 32" C. For temperatures from 32 to 50" C., water baths were used. The temperature fluctuation in these baths was of the order of t 0 . 2 " C. EFFECT OF MOISTURE
I
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Figure 2.
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I
6 8 1 0 1 TIME I N D A Y S
2
1
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Moisture content has a considerable influence on the deterioration of apricots. This influence, however, is greatly modified, both in magnitude and direction, by the quantity of oxygen t o which the fruit is exposed.
Change in Darkening Index as Function of Incubation Time at 4 9 O C.
I 17
Figures on curve8 refer t o initial sulfur dioxide level.
riods of time. The first sample was taken as soon as visible signs of darkening were observed; others were taken a t 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 represents the storage life of the fruit. If darkening is followed as described, 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 necessitated by the fact that the darkening-time curves are not linear but are convex to the time axis (Figure 2). OBBERVATIONS O N THE METHOD. Photoelectric density Of the extracts may be used instead of the darkening index as a measure of darkening when all fruit tested has the same initial SO2 content and the same history with respect t o sulfur dioxide treatment 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 t o be diluted 1 t o 20 with distilled water. The transmission of the resulting extract was determined using a blue filter No. 440. The photoelectric density (2 - log G ) , determined with a n Evelyn colorimeter, is a linear function of the concentration of a n extract over the density range 0.0 t o 0.4 (galvanometer readings from 100 t o 40). When samples of various moisture contents were compared, corrections were made so t h a t the darkening index always refers t o fruit containing 24% moisture. The ratio of fruit to solvent influences the degree of extraction t o 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 t o 30 hours increases extraction of colored material only slightly. The p H of the extract is unimportant over the range 2.5 to 5.0. Untreated extracts have a p H between 3.5 and 4.0. Fifty per cent alcohol is more satisfactory than water as a solvent because i t gives perfectly clear extracts t h a t need not be filtered, and i t also eliminates the possibility of microbial activity. With less t h a n 40% alcohol, the extracts tend t o be somewhat turbid; with more than 60% alcohol the quality and
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/
cn 16
2
n
z 15 w
14
II
15 PERCENT
20 MOISTURE
25
Figure 3. Influence of Moisture and Oxygen on Storage Life at 4 9 O C. of Blenheim Apricots Containing 2800 P.P.M. SOz (Dry Weight) The quantity of fruit (dry weight) per No. 2 can (585-m1. capacity) was varied as follows: squares, 200 grams in Nz; triangles, 300 grams in air) closed circles 200 grams i n air; open circles, 75 g r a m s i n air. Quantities of oxygeLrefer to the amount originally present per 100 grams of dry fruit.
ANAEROBIC STORAGE. The effect of moisture on storage life under anaerobic conditions may be illustrated by a typical experiment. ' Blenheim apricots (lot 3) were dehydrated t o 10% moisture and then rewetted t o 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 t o 3 cm. mercury and replacing the air with nitrogen. Storage was a t 49" C. The upper curvc of Figure 3 shows that the storage life increased with increasing moisture content over the range of 15-25%. About a 55% in-
INDUSTRIAL AND ENGINEERING CHEMISTRY
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2
3
4
PPM SO2 Figure 4.
5
0
7
8
x
Influence of Initial SO2 Level on Storage Life of Blenheim Apricots at 49. C.
crease in storage life was observed over this range. I n comparable experiments with fruit from lots 1 and 4,similar effects were noted over the range of 7 t o 25% moisture. AEROBICSTORAGE. The beneficial effect of high moisture observed under anaerobic storage conditions is diminished when the fruit is exposed t o 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 actually 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 t o fruit in the air-filled cans was varied by changing the quantity of fruit per can. Figure 3 shows t h a t the beneficial effect of high moisture decreases progressively as available oxygen increases. The reasons for this relation will be considered in some detailin another paper. Here it will only be pointed out t h a t oxygen accelerates deterioration in direct proportion to the amount consumed 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 beneficial effect of moisture that is observable under anaerobic storage conditions. When apricots (lot 2) were stored in the presence of a small amount of oxygen (47 mg. per 100 grams), the percentage increase in storage life resulting from a n increase in moisture content from 10 to 24y0 was nearly independent of the initial SO2 level (temperature being constant), provided the latter is expressed on a dry Myeight basis; data for two temperatures and seven sulfur dioxide levels are given in Table I. The moisture effect is smaller (10%) a t 36.8' 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 obtained by a n increment in moisture becomes greater with increasing SOz level. I n view of the beneficial effect of increasing the moisture content from 10 t o 24%, it was of interest t o see whether still higher moisture levels would further prolong the storage life of the fruit.
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For this purpose, apricots (lot 1) containing 23.5y0 moisture were rewetted t o 40% moisture. A few crystals of thymol were added t o prevent microbial activity. At 49" C. the storage life of the 40% moisture fruit was 40% greater than that of the 23.57, moisture fruit. It is evident that the rate of darkening decreases continuously with inxeasing dilution of the soluble constituents of the fruit over a wide range. At very high moisture contents fruit would be expected t o keep more or less indefinitely. This prediction is verified by the fact that ordinary canned fruit, packed in sirup, does not deteriorate a t a significant rate. Most of the experimental samples contained 7% or more moisture. It is difficult t o attain lower moisture levels without scorching the fruit. However, several exploratory experiments with low-moisture fruit were carried out t o see whether more or less complete removal of moisture is beneficial. I n one experiment, for example, fresh Tilton apricots were sulfured and dehydrated t o 60-70% moisture in a commercial dehydrator. The fruit was then resulfured t o approximately 20,000 p.p.m. and dried t o 0.770 moisture i n vacuo a t 65-70" C. The SOe content after drying was 1800 p.p.m. This fruit was normal in appearance. When stored a t 49" C., the rate of darkening was about half t h a t of 24% moisture fruit. It may be tentatively concluded that extremely low moisture levels, like high moisture levels, retard darkening. EFFECT OF SULFCR DIOXIDE
The most effective method of prolonging the storage life of dried apricots is t o add sulfur dioxide. I n commercial practice, 1000 t o 3000 p.p.m. sulfur dioxide are usually added. However, if the incubation temperature is high (37 O C. or above) even 3000 p.p.m. will not maintain apricots in a n edible condition for more than a few weeks. Therefore, the effect of even higher SO2 levels on storage life was studied here. Figure 4 shows the influence of SO2 contents ranging from 1500 t o SO00 p.p.m. on the darkening of Blenheim apricots (lot 2), containing 14 and 21% moisture and stored a t 49" C. The life increases linearly with the initial SO2 content a t both moisture levels, but the beneficial effect of sulfur dioxide is somewhat greater a t the higher moisture level. Other experiments indicate that the storage life increases less rapidly than the SO2 content at levels greater than 13,000 p.p.m. The same general relations
k
7
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8
IO
I2
14
I6
1
TIME IN DAYS
Figure 5. Loss of Sulfur Dioxide ( 4 9 O C. and 14.2% Moisture) as a Function of Time and Initial Sulfur Dioxide Level Crosses and dotted line indicate when fruit i s nt limit of edibility. About 44 ml. oxygen were present per can.
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also apply a t 36.7" C. although the absolute life of the fruit is four to five times as long a t the lower temperature. I n all experiments a t 49' and 36.7" C. the storage life of the fruit wm nearly doubled by increasing the SO2 level from 2000 to about 7000 p.p.m. Having established the relation of storage life t o the initial SO2 content of the fruit, i t seemed desirable t o determine whether a given quantity of sulfur dioxide is more effective when applied a t one time or in several parts during the period of storage. Therefore, 4800 p.p.m. of sulfur dioxide were added to apricots of lot 2 in the following three ways: (a) All of the sulfur dioxide was added before storage; (b) 2400 p.p.m. were added before storage and a n equal amount after 10 days of storage; (c) 1600 p.p.m. were added a t the beginning, and a n equal quantity after 5 and again after 15 days. All samples were incubated at 43" C., and the color and SO2 levels were determined I N I T I A L so2 L E V E L ( P P M . ~IO-j) at intervals to the limit of edibility (52 days). Figure 6. Loss of Sulfur Dioxide as a Function of Initial Sulfur Dioxide The results showed no significant differences Content in the rates of darkening as a result of the difCiroles. 36.7 Co.; trianglem, 49 Co.; open symbols, 14.2% moisture; ferent treataents, and the 501 levels a t thelimit solid symbols, 21.5% moisture. of edibility were the same for all three samples. I t m a y be concluded that a given quantity of sulfur dioxide is equally effective whether applied all at lightly sulfured fruit had darkened appreciably. It was then once or gradually during about the first third of the storage life. found that highly sulfured fruit is generally preferred t o lightly During incubation a t moderate Loss DURING INCUBATION. sulfured fruit. This is illustrated by a test conducted in cooperaor high temperatures, the SO2 content of apricots declines steadily tion with the Army's School for Cooks and Bakers a t the San (Figure 5). This decline is due partly to the oxidation of sulfur Franciscd Presidio. Blenheim apricots containing 7500 and 12,-dioxide t o sulfuric acid and partly to the formation of unidenti000 p.p.m. of sulfur dioxide were incubated at 49" C. until the fied sulfur compounds (4). The rate of loss of sulfur dioxide ap7500 p.p.m. fruit had darkened appreciably (darkening index = proximates a first-order reaction, although in some experiments 0.8), while the more highly sulfured fruit was still normal in color. During incubation the SO2 contents declined from 12,000 significant deviations from this relation were observed. Approximately 65% of the initial sulfur dioxide is lbst by the time to 9000 p.p.m. and from 7500 t o 3500 p.p.m. the fruit reaches the limit of edibility (Figure 6). This effect is Each lot of fruit was cooked on separate days and placed on independent of moisture (14 to 24%), SO2 content (1000-9000 the dinner menu served to 270-300 soldiers. All servings were p.p.m.), and temperature (37-50' C.). taken voluntarily. The number of men taking helpings, the The results show that apricots darken even though the SO2 amounts of fruit consumed, and the amounts taken but dislevel is relatively high. This is confrary t o the conclusion of carded in the garbage were recorded. The results indicate that Nichols and Reed @)'that a definite relation exists between color the highly sulfured fruit which had retained its normal color and SO2 content; but their conclusion was based on limited'obduring incubation was preferred. Thirty per cent of the men servations, since they worked with apricots containing only served took helpings of the normally colored fruit, all of which 1000-2000 p.p.m. sulfur dioxide. Under the conditions of their experiments the apparent relation between color and SOz content was more or less fortuitous. TABLEI. EFFECTOF INCREASING MOISTURE CONTENTO N EDIBLE LIFE OF APRICOTS AT VARIOUS SO2 LEVELS The rate of sulfur dioxide loss varies with temperature, but the (47 ml. of oxygen per 100 grams, lot 2) general form of the sulfur dioxide-time curve is the same a t all yo Increase in Storage Life by Increasing Moisture temperatures studied (22-49" C.). The loss of sulfur dioxide Content from l0-24% Initial during storage is important from a practical point of view since SOz, wet basis SOz, dry basis so2 P.P.M. 49' CI 36.7' C. 49" C. 36.7' C. it means that a relatively high initial SO2 level is consistent with a 38.5 36.0 more moderate level a t the time the fruit is eaten. 50.8 .... 35.5 .... EFFECT ON PALATABILITY. If high SO2 levels are to be used 19.6 66.7 10.4 40.0 20.2 40.3 68.8 9.9 t o prolong the storage life of dried apricots, i t is essential to 71.8 20.8 40.6 9.6 23.0 74.5 10.0 know what sulfur dioxide concentrations are acceptable from the 40.0 24.0 77.2 10.4 42.0 standpoint of palatability. Fruit from lot 4, with 502 concentrations ranging from 2000 to 15,000 p.p.m., was used for palatTABLE 11. EFFECT OF SULFURDIOXIDE CONCENTRATION ON ability tests. The results are shown in Table 11. EDIBILITY OF DRIEDAPRICOTS The data show that there are no consistent trends relating the Order of -SOB Level before Cooking, P.P.M.order of preference to SO2 concentration, nor is there any consistPreference 2000-3000 5000 9500 12,000-15,000 ent order of preference for fruit with a given 502 content. It 29a 20Q '6 3 155 13 32 15 42 may be concluded that the group of individuals tested were, on 32 24 31 12 26 24 19 31 the average, indifferent to the SO2 level. With one exception, The figures give the percentage of the total number of samples of a all tasters considered every sample of fruit to be palatable. given SO1 content that were placed in each preference group by the 28 T o evaluate palatability under more practical conditions,tests tasters. were made after a period of incubation such that unsulfured or
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was consumed. Only 10% of the men took helpings of the darkened fruit, and 25% of that taken was discarded in the garbage. Similar laboratory tests were carried out on light and dark fruit of various sulfur levels. Here again normal appearing fruit xi-ith a moderately high SO2 level was preferred to lightly sulfured fruit that had darkened considerably during storage a t 49" C. From the edibility tests it may be concluded that apricots containing as much as 6500 p.p.m. of sulfur dioxide prior t o cooking are in no way objectionable to most persons. CONCLUSIONS
1. The influence of moisture on the rate of deterioration of apricots a t moisture contents greater than 10% is dependent upon the quantity of oxygen available to the fruit. Under anaerobic conditions the rate is decreased by increasing the moisture content. As the quantity of oxygen is increased, the beneficial effect of high moisture becomes progressively smaller; in the presence of very large amounts of oxygen the rate of deterioration may actually increase with moisture. 2 . Increasing the moisture content from 10 t o 25y0 causes a 15-30% increase in the storage life of apricots kept a t 49 ' C. i n an oxygen-free atmosphere. The effect of moisture a t 36.7" C. is somewhat smaller. 3. The influence of moisture is nearly independent of sulfur dioxide concentration when the latter is expressed on a dry weight basis. 4. Deterioration appears to be slower a t moisture contents below lyOthan a t 25%, but such low moisture contents are difficult to obtain without scorching the fruit. 5 . The storage life of apricots is directly proportional to the
WATE
initial SO2 level, a t least over the range 1500 to 8000 p.p.ni. suifur dioxide. Higher SO2 levels (up t o 25,000 p.p.m.) cause a further increase in storage life, but the rate of increase gradually declines. The effect of sulfur dioxide on the percentage increase ~ I L storage life of apricots stored a t eit,her 36.7" or 49' C. is almost the same; the life is approximately doubled by increasing the SO,level from 2000 t o 7000 p.p.m. 6. Although sulfur dioxide retards deterioration, it, does not prevent it. Apricots can deteriorat,e to the point of inedibllitj even though the SO2 level never falls below 5000 p.p.m. 7. During storage at moderate or high temperatures, the SO2 content of apricots steadily declines until approximatelg 657, of t,he initial sulfur dioxide is lost by the time the fruit has reached the limit of edibility. The effect is independent of [E&ture and temperature between 36.7' and 49' 6. 8. Palatability test's indicate t h a t apricots containing as much as 6500 p.p.m. sulfur dioxide prior to cooking are not, ohjectionable to most people. LITERATURE CITED
(1) Nichols, P. F., Mrak, E. M.,a i d Bethel,
R.,Food
Research, 4 ,
67-74 (1939).
(2) Nichols, P. F., and Reed, H. M.,
IXD. ENG.CHEM.,ANAL.Ea
,,
4, 79-84 (1932).
(3) Xichols, P. F., and Reed, H. M., Western Canner & Packer, 23, 5 , 11-13 (1931): Fruit Products J., 22, 206-8, 247-9 (1943) (4) Sorber, D. G., Ibid., 23, 8, 234-7 (1914). PRESEXTED on the program of the Division of Agricultural and Food Chemistry of the 1945 Meeting-in-Print, AXERIC.ASCHEMICAL SOCIETY.A report on a joint research project of the Qusrtermast,er General's Office, L , B .4rmy and University of California.
SISTANCE
Improvement through Chemical Modification HAROLD S. OLCOTT AND HEINZ FRAENICEL-CQNRAT Western Regional Research Laboratory, U . S. Department of Agriculture, Albany, CaliJ. Proteins were treated with a number of organic reagents and the products examined for water resistance by measurement of the uptake of water by pressed disks. Aryl and long-chain alkyl isocyanates and also aromatic acid anhydrides and chlorides proved most effective. A number of proteins yielded phenyl isocyanate derivatives showing 24-hour water absorption of 1 to 2Yo. Phthalic anhydride gave products of low w-ater absorption w-ith egg white and cattle hoof. Protein derivatives of low water absorption showed a tendency to plastic flow without the addition of water a s plasticizer.
T
HE use of proteins in plastics is seriously restricted by their
poor n-ater resistance ( 2 ) . Obviously the first step to render these materials more valuable for such use would be a modification to improve this property. If this could be accomplished, the next step would be modification t o render them compatible with water-repellent plasticizers. Possibly both ends could be attained by one reaction. The present paper, however, is concerned primarily with experiments designed t o produce protein derivatives with reduced affinity for water, through chemical reaction. Proteins n.ere treated with a number of substances known or believed to react with the various typeS of polar groups. I n many cases the extent of chemical modification was ascertained analytically. The modified proteins were then examined for evidence of a successful reaction by a water absorption technique. Disks pressed from the products were immersed in water for 24 hours, and the increase in weight was used as a quantitative
measure of their affinity for water. This method of approarlt m EL^ patterned after t h a t described by Brother and McKinney ,3j. Of a considerable number of reagents tested, organic m c j a nates and aromatic anhydrides produced. the most pronourised changes in the nature of several proteins. The best p r o d w t s flowed to translucent disks in the absence of water, and t h e 24hour water absorption of the disks was only 1 to 37,. Th'. q plicability of these derivatives in plastics is receiving f t i tliec study.
THE proteins used for most of the experiments were te.:ttril& products similar to those t,hat could be made available for iriiiustrial utilization. Hoof powder was prepared from catt'le hoof' which had been dried at 70" C., by grinding in a hammer niiil.. Feathers and hog hair were ext'racted with benzene and theri. ground in a lt7iley mill to pass a 60-mesh screen. The technical. egg white, gluten, acid casein, and zein were commercial sarnpies Peanut protein was supplied by the Southern Regional Resear 9;. Laboratory. Plastic disks were prepared by the following met'hod: The proteins or protein derivatives were conditioned in a n ~7::e~; ai; 50 C. for one day, a t which time the water contents ranged from 0.6 t o 2.6y0 for different preparations. Ten per cent water wa~i added to those that required it as a plasticizer immediataiy b e fore the pressing operation. A circular positive die 1 id^ Ir; diameter was held in the press until the temperature rearheu. 300" F. (149' C.). Two grams of sample were quickiy iiitroduced and pressed at 5 tons of total pressure for 6 minutes, at which t,ime the platens were usually at about 320" F. (160' C.) O
