I.VDUSTRIAL A.VD EiVGINEERING CHEMISTRY
December, 1930
greatest divergence being found in the value for 40 per cent by xeight-namely, 4.4" F. The maximum difference shown with the figure published for glycerol in the same volume of the Critical Tables is 3 O F. a t 30 per cent by weight. Still greater differences are shown with the figures published by the Bureau of Standards (3) for denatured alcohol, the maximum being 9.40' F. a t 45 per cent by volume. It is not certain that the formula for denatured alcohol used in the Bureau of Standards test in 1921 or 1925 is the same as that used today. This may account for the large differences.
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Literature Cited -50
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(1) Curme and Young, IID. ENG.CHEM.,17, 1117 (1925).
50
(2) International Critical Tables, Vol. IV, p. 262. (3) Standards, Bur. of, Letter Circ. 28.
Figure 5
Large-Scale Experiments in Sulfuring Apricots'" E. M. Chace, C. G. Church, and D. G. Sorber LABORATORY O F
HE dried-fruit industry of California is an important factor in the agricultural industry of the state. It is estimated that t h e t o n n a g e f o r 1929 a m o u n t e d to 390,000, of which 22,000 tons mere apricots. The districts in which the fruit is dried extend from R i v e r s i d e County on the south to Sacramento on the north; the largest part of the croD, however, comes from the- Santa Clara and San Joaquin Valleys. More than half of the dried apricots are exported.
T
FRUITA K D
VEGETABLE C H E M I S T R Y ,
BUREAU OF
CHEXIISTRY AND SOILS, L O S ASGELES, CALIF
The commercial process used for sulfuring apricots in California is described. Experiments were carried out in sulfuring fruit, with liquid sulfur dioxide in thermally controlled boxes. The concentration of the gas to which the fruit was exposed, the length of the exposure, and the temperature at which the treatment was carried out were varied, and the effect of these variations upon the retention of sulfur dioxide by the fruit and upon its grade have been studied. The system devised for grading the fruit is described. The results show that the appearance of dried fruit is correlated with the sulfur dioxide content, and that the concentration of gas and length of sulfuring period are important factors, good fruit resulting after 2 hours in 3 per cent of gas. Temperature was a minor factor, although the best results were obtained at 100-110"
F.
The Commercial Process
The treatment of apricots and peaches with sulfur dioxide gas before the fruit is dried in the sun is as old as the dried-
fruit industry. The treatment serves several purposes. It plasmolyzes the cells of the fruit, thus promoting rapid drying; it preserves the color by retarding the usual darkening of the fruit by enzyme action; it prevents the growth of mold and fermentation; and it keeps insects away from the exposed material while it is drying. Naturally some variation exists in the construction of the sulfuring houses used by fruit growers, but the type is usually the same. The greater number are built of tongueand-groove lumber, lined or covered, or sometimes both lined and covered, with roofing paper. Others are constructed of cement or tile. A tight framework is built on the front and a groove provided for a vertical, counterbalanced door. When down, this door is made tight with special door fasPresented under the title "Important Fac1 Received July 28, 1930. tors in Sulfuring -4pricots" before the Diiision of Agricultural and Food Chemistry at the 80th Meeting of the American Chemical Society, Cincinnati, Ohio, September S t o 12, 1930. Contribution No. 83 from Food Research Division, Bureau of Chemistry and Soils.
teners or by driving wedges b e t w e e n it and the outer framework of the groove. The houses haven capacity of from 1 to 2 tons of fruit, and are so constructed as to leave about 18inches between the front fruit truck and the door. This provides room for the sulfur burner, which usually consists of a cement pipe or hollow tile 8 to 10 inches in diameter set in the ground so as to afford clearance for the fruit trucks. A weighed or measured amount of sulfur is placed in these stoves and ignited by means of paper or splints. The amount of sulfur used varies greatly, but from 3 to 5 pounds per ton of fruit are used on most dry yards. No mechanical means of distributing the vapors are employed. The fruit is halved and pitted by the workers, after which it is placed cut side up on trays. These trays, usually about 3 by 6 feet, have sides and ends high enough to keep the halved fruits from being crushed by the trays above, when they are stacked on the trucks. From 20 to 24 trays are placed on each truck and stacked so that the alternate ends project about 6 inches beyond the tray beneath (Figure 1). As the sides fit rather closely, this staggering is necessary to permit the gas to have access to the fruit. I n some dry yards each tray of fruit is sprinkled with water before it is covered by the next one. This may be with the idea that is prevents surface evaporation and the formation of a layer of partly dried tissue, but the writers have not been able to detect differences in the absorption of sulfur dioxide or in the appearance of the fruit when thus treated if it is placed in the sulfuring chamber within a reasonable time after cutting. There is other evidence, however, t o the effect that this treatment may retard the retention of sulfur dioxide (3). The trucks loaded with fruit are handled on tracks and
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I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
with the aid of a transverse carrier are readily shunted into eny of the empty sulfuring rooms. The length of the sulfuring time is determined by each dry-yard foreman. It is seldom less than 3 hours and usually not more than 7, but in some localities the fruit remains in the houses all night.
Vol. 22, No. 12
The sulfur dioxide was run into the boxes through special gas meters measuring to 0.01 cubic foot. Thermometers were placed in contact with the fruit, and the gas was not turned on until the fniit had reached the desired temperature. While the gas was being run in, the fan was turned on, but was not run throughout the sulfuring period owing to the cooling effect of the air current blowing across the fruit. Fruit for the experimental work was prepared according to commercial practice. After the sulfuring the fruit was removed to the dry yard and given the usual commercial treatment. It was generally ready for packing in cans a t the end of about 5 days. At this time the moisture content and the sulfur dioxide were determined, the former by the Bidwell-Sterling method ( I ) , the latter by distillation into iodine solution and titration of the excess iodine (2). Effect of Various Factors upon Retention of Sulfur Dioxide
The concentrations of gas used in the experiments were Figure I-Showing Removal of Sulfured Fruit from Transfer Wagon and Method of Spreading
1, 2, 3. 5, 10, and 15 per cent, with temperatures of 80", loo", llOo, 120°, 130°, and 140" F. and time intervals of l/*, 1 , 2 , 3 ,and 5 hours. It had been planned to work through
all the possible combinations of these variations, but the fruit ripened very slowly early in the season and later so rapidly that all the trials were not carried out. I n making the graphs only comparable data were used, and the content of sulfur dioxide was reduced to a moisturefree basis, which accounts for the relatively high amount of that constituent. The data represent the results obtained from 110 samples of apricots. EFFECTO F CONCENTRATION-The effect Of V8lYing the concentration of sulfur dioxide in the atmosphere of the sulfuring compartment is illustrated in Figure 3. It will be noted that as the concentration of sulfur dioxide is increased its retention by the fruit also increases rapidly. At 1 per cent concentration the retention averages about 1400 p. p. m.; a t 2 per cent the amount retained inc,reases to about 2000 p. p. m., at 3 per cent to 2700 p. p. m., at 5 per cent to 3600 Experimental Equipment p. p. m., at 10 per cent to 4700 p. p. m., and at 15 per cent to After some attempt to carry on experimental work with 6000 p. p. m., the averages being from all samples without commercial equipment, it was decided that better results regard to length of the sulfuring period of temperature. EFFECTOF TEMPERATURE-Thisgraph also shows the could be obtained on a semi-commercial scale with equipeffect of varying the temperatures to which the fruit is exposed ment in which conditions could be controlled. Boxes having a capacity of 1000 liters of air space were built of during the sulfuring. It was not possible to secure temperajoined lumber, with an inner compartment of Celotex cemented tures as low as 80" F. in the daytime where the exDeriments were cairied out, at the joints and so that it did not coated three times seem necessary, as with acid-resistant long as t h e compaint. The inner mer c i a1 sulfuring c h a m b e r was 3 was seldom d o n e i n c h e s from t h e at that temperaouter one all ture, to run a full around. Each box set of s a m p l e s . was equipped with Five sets were run a portable rack for in a t m o s p h e r e s h o l d i n g trays, a containing 5 p e r small space heater cent of sulfur dioxt h e r m a l l y conide and five sets t r o l l e d , a rotary in a t m o s p h e r e s fan with m o t o r containing 10 per o u t s i d e the box, cent; the o t h e r s and a registering a t 8 0 " F. were t h e r m o m e ter. omitted. Each box had a Temperature capacity of 20 kg. seems to have less of fresh fruit, and effect on the recould be closed Figure 2-Commercial Dry Yard with Sulfur Dioxide House and Cutting Shed in the tention of sulfur Background reasonably tight. When the sulfuring is finished, the trucks are shunted onto carriers and taken to the dry yard, where the trays are unloaded and placed on the ground. The front row lies flat, but the other rows are placed with the end or side resting on the end or side of another tray, thus aiding in the circulation of air under the trays (Figure 2). After the fruit has dried so that it can be handled without injury, the trays are stacked until the drying is finished. These stacks are also staggered and the ends are placed toward the prevailing wind, an empty tray being used to protect the top tray of fruit. This prevents the fruit from sunburning and affords more space in the yard. When the fruit has reached a moisture content of about 18 per cent, it is scraped into lug boxes and shipped to the packing houses.
. INDUSTRIAL AND ENGINEERING CHEMISTRY
December, 1930
Table I-Relation 112
so2
CONCN. Per cenl 1
Grade
65 70 73 60 71
3
59 70 75 75
5
73 71 71 63 74 69 71 73 81
2040 2221 2109 2270 2343 2076 2823 2073 5354
60 71
10 15 a
1184
2 HOURS
1 HOUR Grade
SOP
Grade
SOP
P. p . m.
3 HOURS Grade
53 63
1343 1175
72 68
P . p . m. 1296 1384
69 65 65 77
1772 1729 1613 1642
76 76 72 69 85 66
2020 2440 2539 1722 2298 2237
58 66 74 80 77
1796 2027 2463 2900 2216
74 74 78 76 78 74 72 79 90
2497 2546 2162 2937 2922 3739 3664 4034 6496
85 71 82 84 87 92 80 88 91 79 82 83 71 85 79
P. 9 . m. 53
1604 1493 1214 1353 1447 1406 1309 1369 1994 1826 2072
2
of Grade a n d SOYR e t e n t i o n to S(k Concentration a n d L e n g t h of Sulfuring Period
HOUR SOsa
1519
SOP
5 HOURS Grade
P. 9 . m. 52 54 51 74 74 85 84 81 78
P . 9. m.
1340 1249 1365 1749 2335 2296 2085 2710 2340
3228 4056 3404 3164 4777 3683
75 76 76 92 75
4138 4383 4029 5256 3745
5624
81
4432 5577 6995
Soia
54 74 69 77 79 79 85 82 83
1358 1523 1121 1379 2407 2553 2467 2806 2318
87 89 77 90 80
4082 3310 2932 3763 3342
75 89 80 70 83 93 72 91 85
4406 4426 3785 4252 5769 4713 5510 5774 8117
Sulfur dioxide data are on a water-free basis.
dioxide than either the concentration of the gas or the length of the sulfuring period. Aside from this, however, the temperature at which the fruit is sulfured does affect the grade of fruit produced as will be noted later. EFFECTOF LENGTH OF SULFURING PERIOD-In Figure 4 the results obtained by varying the length of the sulfuring period are shown. Only a few samples were sulfured in 10 and 15 per cent atmospheres-but two for each time period in 10 per cent and but one in 15 per cent. The striking feature brought out by this graph is the rapidity of the reaction during the first half-hour. The samples sulfured in atmospheres containing but 1 per cent of sulfur dioxide retained nearly 1200 p. p. m. when exposed for l / Z hour, and only 1340 p. p. m. when exposed for 5 hours. This may be due in part to the fact that at the end of 5 hours the concentration in the compartment was relatively low, for with the higher concentrations this would not be the case. Where the fruit was sulfured in atmospheres containing 2 per cent sulfur dioxide, the samples exposed for 1/2 hour retained an average of 1400 p. p. m. while those exposed for 5 hours retained an average of 2500; that is, more than 55 per cent of the sulfur dioxide retention took place in the first half-hour. Where 3 per cent concentration was used, the samples exposed for hour again retained more than 50 per cent of that found in samples exposed for 5 hours. I n 5 per cent concentrations the percentage was still around 50 per cent. The samples at 10 per cent are too few for accurate conclusions; the half-hour samples have less than the usual amount of sulfur dioxide, but the samples exposed for an hour seem to be in line with the others. At 15 per cent the number of samples is even less and the relative percentage retained much higher. The question of the quality of the samples sulfured in low concentrations and for short periods will be discussed later.
lrgood,” “fair,” and “poor,” but have not attempted to give the samples numerical grades. A system of numerical grading, however, has been used by others in dried-fruit investigations, but it did not seem broad enough for this purpose. After several attempts, a method was chosen by which 25 pieces of fruit were divided into three classes depending upon uniformity of color and brightness. The pieces considered uniform in shade and of desirable color were classed as “fancy.” Slight defects in uniformity or dullness of color lowered the pieces thus affected to the “choice” grade, while marked lack of uniformity or poor color lowered the grade to “standard.” The number of
go0’
Relation between Commercial Grade of Dried Apricots and Their Sulfur Dioxide Content
One of the chief reasons for sulfuring fruit is to retain its natural color; therefore, the relationship between the appearance of the finished product and the amount of sulfur dioxide retained by it is important. About fifty representative samples of the dried apricots have been exhibited to the dried-fruit trade, both a t San Jose and San Francisco. They have passed upon them in the commercial way, grading the fruit as “very good,”
,
2
I
I
5 CONCENT~~OIO I F SULFUR 0
10 ^ x 0 P E P CE.IT
15
Figure 3-Relation of Sulfur Dioxide Concentration a n d Temperature at T i m e of Sulfuring t o R e t e n t i o n of Sulfur Dioxide by the Fruit
fancy pieces among the twenty-five selected as samples was multiplied by 4, the choice pieces by 3, and the standard by 2. Thus, samples containing all fancy pieces would grade 100, all choice 75, and all standard 50. Of course most of the samples were mixtures of all three grades. By repeating the sampling and grading several times, an accurate estimate of the quality could be secured.
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