Effect of Freezing Rate on Vegetables APPEARANCE, PALATABILITY, AND VITAMIN CONTENT OF PEAS AND SNAP BEANS P'. A. LEE, W. A. GORTNER, AND
a
JOANNEWHITCOMBE
New York State Agricultural Experiment Station, Cornell University, Geneua, N. Y., and The School of Nutrition, Cornell University, Zthuca, N. Y .
paragus, lima beans, corn, spinach, caulifiower, and beans, although minor modifications exist for some products." Eickelberg (8) wrote: "The quick-frozen product more closely resembles the fresh product in texture and flavor, while the slow frozen product is a p t to be tough or fibrous as in the case of asparagus or beans." Eickelberg's most rapid freezing was complete in 10 minutes; his slowest took 6 to 10 hours. Plagge (9),as a result of a rkview of the literature, concluded that ''in certain instances, good or better products have been reported by slow or moderate freezing and also slow freezing such as practiced in many locker storages gives satisfactory results." MacArthur (6) stated: "It was established that when freezing was by the slower methods the tissues werebadly torn by internal ice crystals while the more rapid methods caused less damage." Information is lacking in the literature concerning any vitamin changes that may result from the different rates of freezing, followed by a storage period and cooking after storage. This study was undertaken t o determine whether extreme differences in the rate of freezing would show any noticeable texture differences to a group of disinterested, experienced judges in a starchy vegetable (pea) and a nonstarchy vegetable (snap bean), and whether vitamin changes could be detected immediately after freezing, after storage at -6" F. for 6 months, and after cooking following the storage period.
M u c h confusion apRears in the literature concerning the effects of fast and slow freezing on the quality of vegetables. In this study, five different rates of freezing were employed, ranging from very rapid, by means of liquid air, to very slow, in an insulated box. Peas and snap beans were blanched, frozen, and stored at -6' F. for six months. Analyses were made for ascorbic acid, carotene, and thiamine. Riboflavin was run on peas only. Analyses were made on raw, blanched, and frozen samples, and again after six-month storage and after cooking. Significant differencesin vitamin content could not be detected among the samples from the rates of freezing studied. Significant differencesin taste and texture were not observed, with the exception of the texture of those frozen in liquid air. These were somewhat softer, probably because of cracking that took place during freezing. Photomicrographs show that the slower the rate of freezing, the larger the ice crystals; but in the Corresponding thawed samples, these differences disappeared, and damage was not apparent.
....
A
NUMBER of articles have been published relative to the effect of freezing rate on the quality of quick-frozh vegetables. Woodroof ( I S , 19) indicated that slow freezing of nonstarchy vegetables resulted in flabbiness; in the case of starchy vegetables, such as peas and shelled beans, less change was noted, apparently because of the support lent to the cell walls by the starch grains. Joslyn and Marsh (4) "do not find t h a t there is a direct relation between loss in weight of treated fruits and vegetables and change in texture as judged by the degree of retention of original shape, turgidity, and crispiness" as reported by Woodroof. Table 14 of their article shows that steam-blanched asparagus, frozen in still air at 0"F., lost 25.2 % in weight when thawed one year later. The same lot of asparagus, steam-blanched but frozen in dry ice at -110" F., showed a loss of 14.4% when thawed a year later, The figures for corresponding treatments of peas were 6.6 and 0.6%, respectively, and for string beans, 13.8 and 6.8%, respectively. They further stated: "It has been our experience that with the possible exception of asparagus, increasing the rate of freezing by Using solid carbon dioxide did not appreciably improve the texture of fruits and vegetables examined." Diehl and Berry ( I ) reported: '' i t wouldseem thatvery rapid freezing is not ewential t o quality retention in frozen pack peas, and that temperatures near 0" F. are satisfactory, provided reasonably rapid heat transfer. is obtained in cooling the Sroduct and in finally freezing it. Experiments with other frozen vegetables indicate that this is also true in a general way for a:
FREEZING PROCEDURE
...
I^.
A Brown recording potentiometer equipped with thermocouples of iron and constantan was used; its range is -150' t o $200" F. The chart speed can be altered, and was set at 6 inches an hour. The thermocouple leads were covered with Irvolite extruded plastic tubing, type X T E 30, black, size 14. Three cold rooms, one held at +12', another a t -5", and the third at -60 e F., were provided. The temperatures of the latter two were resetforsnap beansat +10",O0,and -40'F. Anl&inchfanwas used in the coldest room; it was supported so that the packages would receive an air blast from above which provided a uniform air velocity for all the boxes. Preliminary runs showed that the contents of these boxes froze in substantially the same time. Plastic-coated, cardboard, pint-size Dacca Locker-Pak boxes were used, measuring 3l/s inches on each side of the cube. Fastest freezing was obtained by immersing the loose, blanched peas in liquid air, f$lowed by packaging in moisture-vapor-proof cartons and storage a t -6" F. I n the other methods of freezing, the blanched peas were packaged in the usual fashion before freezing. One set was frozen in the coldest room where freezing was hastened by the air blast. Another set was frozen in still air at -5" F. A third set was left in the same room, but the cartons
341
342
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 38, No. 3
least one inch of the thermocouple mire is inside the container, only one pea need be placed on the thermocouple wires. Each package was raised from the floor of the cold room so that the cold air affected all surfaces alike. Eight cartons were placed in the insulated box, in which 0.5 inch of wool lininq q a s used, and the eight pint boxes wwre fitted snugly inside. One box of the upper four was fitted with a thermocouple a t the lower inner corner, and another a t the center of the outside surface. One of the lower boxes was fitted with a thermocouple in the upper inner corner, and another in the center of the outer side face. Experiment showed that such placement measured accurately the extremes of temperature change in the entire cube of eight boxes. The temTIME I N HOURS peratures recorded for the corresponding thermocouples in each tier were approximately the Figure 1. Speed of Freezing Green Peas same. The cartons were sealed in the usual manner with a hot iron. The thermocouples were held 1 INNER O'F. 5 CENTER 0 ' F . in place by using wooden clamps grasping the 6 OUTER )NOT INSULATED i: OUTER) INSULATED 5- 60 insulated lead-in wires. With this device little W 7 CENTER] 4 0 ' ~ . 3 CENTER + I D O F . n: was encountered in actuai use. After difficulty 8 OUTER NOT INSULATED A CIUTFR it407 INSULATED I the start of the run, the cold rooms were not 2 50disturbed until the readings on the potentiomv) W eter showed that 0 ' F. had been reached, except W 40. [I for those in the f12"room. After the freezing w W was complete, the boxes fitted with the ther0 30mocouples were discarded. The arrangement f for snap beans was similar. w a: 20Peas of the Thomas Laxton variety were 3 used for this study. They were harvested and W vined mechanically. The tenderometer (6) a 10. reading was 88; this stage of maturity is used I w commercially in the production of fancy frozen lO+ 15 20 25 30 35 40 45 peas. They were blanched in rapidly boilIO 0 5 ing water for 60 seconds, cooled in cold water, TIME IN H O U R S and drained before packing in the cartons. Figure 2. Speed of Freezing Green S n a p Beans The latter were sealed by a hot iron. The snap beans were Tendergreen variety. They were snipped, cut in pieces 3/4 inch long, blanched in boilwere put into corrugated, insulated boxes, A final set was kept ing water for 2 minutes, and immersed in cold water for rapid in still air at $12" F. Results are shown in Figure 1. cooling. The boxes were filled and sealed as described for peas. The same conditions were used for snap beans except that the Table I shows the rates of freezing recorded by the potentiomtemperatures of the cold rooms were Oo, +lo", and -40" F. ineter. The freezing zone was taken as 32" to 21.2' F. (8). After steadof -5", +12',and -60°F. (Figure2). freezing was complete, one set of samples was taken to the laboraOne box, in which two thermocouples were inserted was used tory for immediate analysis, and another set for cooking folfor measuring the temperature in each set except for those of the lowed by analysis. The others were held in storage a t - 6" F. for insulated package; in that case two boxes were used, one in the 6 months to determine quality changes taking place during this upper tier and the other in the lower. The trends shown in the period. The samples frozen in liquid air showed considerable two tiers were similar. Preliminary experiment showed that the cracking and breaking. h peculiar odor was noticed in those temperatures of these boxes were the same as all the others in a frozen in liquid air, which persist'ed for several days after the given group. One thermocouple was placed in the center of the samples were removed to the -6' F. storage. package and the other on the bottom. Each thermocouple was pushed into one medium-sized pea, leaving the plastic insulation flush with the outer surface FREEZING RATESFOR PEAS AND BEANS TABLE I. RECORDED -Peas-----Snap Beansof the pea. M. R. Sfat had Time in Time from Time from previously showa that the priLocation of freesing starting temp. Time in starting temp. freezing range t o 0' F. Freezing Conditions Thermocouples range t o 0' F. mary method of heat transfer 30 min. 1 hr. 20 min. 25 min. 1 hr. 45 min. Near outer side is conduction t h o u g h the peas. Air blast (not insulated) ; p e a s 45 min. 2 hr. 10 min. 40 min. 1 hr. 54 min. Center a t -60° beans a t -40 F. 7 hr. 50 min. 8 hr. 3 hr. 10 min. Near outer side 2 hr. Also, the ambient temperaStill air (n'ot insulated) ; peas 4 hr. 20 min. 8 hr. 15 min. 4 hr. 8 hr. 20 min. -5q, beans 0' F. Center ture is about the same 8s Still air (insulated box): peas Near outer part 48 hr. 110 hr. 21 hr. 4.5 hr. 30 min. - 5 O beans O D F. Inner 30 h r . 46 hr. 20 min. 50 hr. 113 hr. the pea temperature; hence Still a& (not insulated); peas Near outer part 5 hr. 19 hr. to reach 9 hr. 18 hr. t o reach 12' F. 10' F. f l 2 " . beans + l o o F. the probability of errora due Center ... , , . , .. . . . 10 hr. 20 min. 21 hr. t o reach 10' F. to conduction along the wire is remote. Therefore, a8 long as a t
'A
5
I N D'US 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
March, 1946
Although there appears t o be a considerable difference between the rates of freezing of peas and of snap beans in the insulated box, no special attempt was made in the laboratory t o freeze the peas under exactly the same conditions as the beans. We were interested solely in obtaining a very slow rate of freezing rather than in making any comparison as to the relative speed of freezing of the two vegetables. PREPARATION OF PHOTOMICROGRAPHS
All work was carried out in a refrigerated room a t 0" F. The sections for photographing were prepared as follows: PEAS. Only whole peas were used. Precautions were taken to prevent even partial thawing of the vegetable during sectioning and photographing. The peas were cut with a new fazor blade (cold), approximately midway through the seed to give a transverse cross section. I n all cases a n attempt was made to cut through the hypocotyl which ran along the cotyledon for more than half the length of the pea. Since the first cut was usually uneven, due to some breaking away of the two halves, a slice about l / , 6 inch thick was then removed from one of the halves. This usually left a smooth surface to be photographed. The other half of the pea was discarded. Six peas were prepared for each set of freezing conditions. The only selection requirement of the individual peas was that they be intact and present smboth cross sections. All peas,. except those frozen in liquid air, turned dark green when the section was exposed to air (5' F.) for 1to 2 hours. The cut surfaces of peas from liquid air remained pale green. All sections stood a t least 1hour on a shelf in the freezing room before photomicrographs were taken in order to intensify the difference in appearance of the ice veins and the tissues of the pea. The frozen pea section was then placed on a small paraffin block with the cut surface facing the lens of the camera. The photographs %ere made with a 35-mm. Leica copying camera on a usual type of panchromatic film. Lighting was obtained from a Leitz spotlight directed at the specimen from a relatively low angle. A low power specimen stage was used for focusing; the
343
stage could be raised or lowered with the specimen and, thus, keep the bellows distance intact and ensure similar magnification throughout. SNAP BEANS. The procedure was much the same as for the peas except in the following points: the transverse cross section through the bean was also approximately,midway through the seed, and was such @at the hilum usually showed in the section above the cotyledons. I n the bean the hypocotyl did not extend over the top of the cotyledon, so that i t did not appear in any of the sections as in the case of the pea. After a smooth section was prepared for photographing, the bean was cut approximately inch back, this crude cut being parallel to the smooth surface. I n this way the bean could be placed directly on the specimen stage with the smooth' section facing the camera, and no paraffin block was necessary. At the end of the 0-month storage period, both the beans and the peas were again photographed in the frozen condition. They were also photographed after thawing to show any damage caused by ice. The sections were made as described above but were allowed to thaw on a porcelain plate at room temperature and then photographed. In the case of the thawed peas, the photographs were taken after 1 hour a t room temperature, whereas the beans were photographed promptly after the thawed, cut sections were available. ANALYTICAL METHODS
Each sample was ground in a food chopper in the 0 O F. room and mixed carefully to ensure uniformity. Ascorbic acid was determined by the indophenol-xylene extraction method (IO),carotene by the diphasic method ( I d ) , and thiamine by the thiochrome method (7). The microbiological procedure (11) was used for riboflavin. Total solids were determined in the vacuum oven at 70 O C. for 48 hours. The peas and snap beans were analyzed immediately after freezing. At the same time a set of the peas was cooked in a minimum of water (3), frozen, ground, and analyzed. The differences were insignificant, and this analysis was not run for snap
OF THOMAS LAXTONPEAS AFTER DIFFERENT SPEEDS OF FREEZJNG (WET BASIS) TABLE 11. ANALYSES
Sample
A
Raw Blanched Liquid air -60' F. with air blast -5' F., not insulated, still air $12' F.,,still air -5' F., insulated, still air
0.26 0.17
.. ..
.... ..
Ascorbic Acido, Mg./G. B C D
..
.. o:i5 0.17 0.16 0.14 0.16
o:ii o:i3 0.14 0.15 0.14 0.14 0.12 0.14 0.14 0.14
Riboflavino, B C
A
1.3 1.1
. .. .. ... ...
...
M
Gram/G. D
E
.. o:o5
0-06 0.08 0.06 0.08 E
. . . . . . . . . . . . ...
1.3 1.0 1 .o 1.0 1.0
...
1.2 1.2 1 .o 1.1 1 .o
.
.
I
1.2 1.2 1.3 1.3 1.3
...
0.6 0.7 0.9 0.8 0.8
A 4.6 4.4
Carotene", p Gram/G. B C D
E
... ...
... , . .
.., ..,
... ... ...
...
... ...
4.7 .4.4 4.3 4.7 4.4
A
Total Solids", Per Cent B C D
...
22.97 20.49
.... .. ... ... ...
4.3 4.6 5.6 4.9 5.1
...
...
19:30 21.82 21.66 20.09 21.95
5.5 6.2 5.6 5.3 5.4
Sample Raw Blanched Liquid air -40' F. with air blast Oo F.,notinsulated,stillair 10' F., still air Oo F.,insulated, still air
+
(1
--
0.23 0.18
. . . . . .
. . o:is o:ii 0109 .0.12 0.08 .... 0.17 0.18 0.12 0.09 0.12 0.08 .. 0.18 0.13 0.07 .. 0.17
Carotene", p Gram/G. A B C D
3.80 3.60
.. .. ..
....
. . . . . .
3:30 3.28 3.37 3.41 3.24
3:98 4.00 3.68 4.13 4.13
A raw and blenched B analyzed immediately after freezing C = analyzed after storage for 6 mo. at - 6 O F. D analyzed after cookiRg in relatively large volume of water, after storage
-
4:oo 4.88 4.68 4.78 4.65
E
...
...
20:94 10:57 21.81 19.08 22.27 J8.82 23.23 19.31 23.36 18.93
SNAP BEANSAFTER DIFFERENTSPEEDS TABLE 111. ANALYSESOF TENDERQREEN Ascorbic Acid", Mg./G. A B C D'
5.1 6.3 5,9 5.8 5.5
Thiamine', A B
p
14151 16.86 18.56 18.70 18.19
T
. . . . . . . . . . . . .: .. . . , ,
...
...
... ...
...
2.6 2.8
,
2.4
2.7
2.6,2.6
2.5 2.6
2.3 2.7
,
,
2.8 2.8
2.9
3.0 3.1
,
1.8 2.3 2.5 2.3 2.3
A = raw and blanched B = analyzed immeditaly after freezing C analyzed immediately after freezing after cooking in minimum amount of water D = analyzed after storage for 6 mo. a t -6O F. E = analyzed after cooking in relatively large volume of water, after storage
-
OF FREEZING.(WET BASIS)
Gram/G. C D
. . . . . . .. o:is 0171 o:io .. 0.73 0.54 .. 0.81 0.87 0.69 0.70 .. 0.88 0.74 0.54 .. 0.87 0.69 0.63
0.96 0.93
ThiamineQ,p Gram/G. B C D
A 3.6 3.1
Total Solidsa, Per Cent A B C D
11.34 10.17
... ,.. ... .., ...
. . . . . . . . . 1o:io 1o:ig 10.52 10.55 11.28 10.18 10.78 10.45 10.40. 10.53
9:is 10.39 10.85 10.24 10.14
0
344
INDUSTRIAL AND ENGINEERING CHEMISTRY
(Above) frozen i n liquid air, stored 6 months at -6' F.; ( b e l o w ) same after thawing
-
(Above) frozen i n still air at 5' F . , stored 6 months at -6'; (below) same after thawing
Vol. 38, No. 3
-
(Abooe) frozen in insulated box at 5' F., stored 6 months at - 6 ' ; (below) same after thawing
Figure 3. Photoniicrographs of Cross Sections of Grleri Peas Frozen under Various Conditions
Commercially frozen peas, brand A
Commercially frozen peas, brand B
March, 1946
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Above) frozen in liquid air, otored 6 months at -6O F.; (below) same after thawing
Commercially froeen beano, brand r?$G?&and B,(right) ,
Figure 4. Photomicrographs of Cross Sections of Snap Beans Frozen under Various Conditions . .
(Abooe) frozen i n still air at OD F., stored 6 month6 at -6'; (below) same after thawing
345
*
(Abooe) frozen in insulated box at 0' F., stored 6 months at -6'; (below) same after thawing
346
INDUSTRIAL AND ENGINEERING CHEMISTRY
beans. All cooking tests were carried out, however, after the storage period, as originally planned. After &month storage a t -6 O F. a set of each lot was removed for chemical analysis. At the same time a duplicate set was removed for cooking tests. Both peas and snap beans were cooked after storage by a common method in which a relatively large volume of cooking water was used, as would be done in many homes. According to this method, two cupfuls of boiling water were used for each carton of vegetable. The peas were cooked for 9 minutes after immersion in the boiling water; the snap beans were cooked for 10 minutes. Then the samples were frozen and prepared for analysis as before. TASTE AND VITAMIN CONTENT
The tasters for the cooked samples were a group of six disinterested, qualified judges who were uninformed as to the treatments until the tests were concluded. Two of the judges felt that the samples of peas frozen in the insulated boxes were slightly offflavor, but agreed that it would not have been detected if the other lots had not been available for comparison. However, the textures of all the samples were judged to be the same, with the exception of the one frozen in liquid air which was considered a little more tender. This was probably due to the fact that tlie units were cracked and broken by this treatment, and the cooked product as served had the appearance of being broken. Little difference was detected in any of the bean samples. As far as taste tests are concerned, the results from these two vegetables suggest that both starchy and nonstarchy vegetables are little affected by rate of freezing, even when that rate is extremely slow. Data for both peas and snap beans (Tables I1 and 111) show that even with the very slow rate of freezing (resulting when the cartons were put into the freezer inside an insulated box) there were only slight vitamin differences, even after cooking’. These similarities are especially noteworthy in the case of ascorbic acid. Only after cooking in large volumes of water did large losses result, but this happened in all cases, regardless of the freezing rate of the vegetable analyzed. Before this study was completed, it was suspected that ice crystals might cause enough tissue damage, especially in slow-frozen samples, to result in excessive leaching loss of vitamins if the vegetable was cooked in a relatively large volume of water, in comparison with rapid-frozen samples. This has been shown to be contrary to fact. We do not wish to create the impression that we recommend for use the slowest freezing conditions described in this study because, under some conditions, difficulties of a bacteriological nature might be encountered. We used the very slow rate solely for comparative purposes. Also, we do not mean to confuse rate of freezing with over-all time of handling. The work described in this paper deals only with a comparison of the several rates of freezing described. This should not be confused with an overall time of handling with which this paper is not concerned. The photomicrographs (Figures 3 and 4) disclose that in each successive case the more slowly frozen vegetable shows larger ice crystals and apparently greater damage to the tissues; in the corresponding thawed samples these differences disappear, and all samples look alike, regardless of the conditions under which they were originally frozen, This is true for both peas and snap beans. Because of space limitations photomicrographs are presented only for those vegetables frozen a t one intermediate and the two extremes in freezing rates after six-month storags. Those taken immediately after freezing could not be distinguished from the pictures shown here. Pictures of the cross sections of the vegetables from two boxes of commercial frozen peas and snap beans are shown for comparison. 1 The only exception was carotene which showed slight progressive increase. These apparent increases in carotene have been noted in other laboratories (personal communication from Walter L. Nelson).
Vol. 38, No. 3
SUMMARY
Peas and snap beans show few differences in vitamin content or palatability whether frozen very slowly, very rapidly, or a t intermediate rates. Photomicrographs of cross sections show the formation of large veins of ice in the slow-frozen products, but when thawed no injury is evident. Analyses indicated no significant differences in the vitamin contents of the peas or snap beans, frozen a t greatly different speeds, after freezing, after storage, or after cooking. No significant organoleptic differences could be distinguished by a group of experienced judges. ACKNOWLEDGMENT
This report is based on one of a series of studies supported largely by the Consolidated Edison Company of New York, Inc. The studies are being carried out under the direction of the Research Committee on Food Processing and Storage appointed from the staff of Cornell University. The vegetables used in this study were supplied by W. T. Tapley, of the Division of Vegetable Crops of this station. We wish to acknowledge the technical assistance of Michael R. Sfat. LITERATURE CITED
Diehl, H. C., and Berry, J. A., Proc. Am. SOC.Hort. Sci., 30 496500 (1933).
Eickelberg, E. n7.,Cunning Age, 19,498-9,512 (1938). Fenton, Faith, Cornell Eztenaion Bull. 628 (Dec., 1943). Joslyn, M. A., and Marsh, G. L., Fruit Products J., 12, 203-5, 220 (1933).
MacArthur, Mary, Ibid., 24, 238-40 (1945). Martin, W. McK., Canning Trade, 59, No. 29, 7-14 (1937). Moyer, J. C., and Tressler, D. K., IND.ENQ.CHEM.,ANAL.ED., 14, 788-90 (1942).
Pickett, T. A., and Brown, W. L., Fruit Products J . ,
12, 134
(1933).
Plagge, H. H., Ice and Refrig., 94, 220-3 (1938). Robinson, W. B., and Stotz, E. H., J. Bid. Chem., 160, 217-25 (1945).
Snell, E. E., and Strong, F. M., IND.ENQ.CHBM.,ANAL.ED.,11, 346-50 (1939). (13) (14)
Woodroof. J. G.. Ga. Agr. - Expt. . Sta., Bull. 168 (1931). Zbid., 201 (1938). Zimmerman, W. I., Tressler, D. K . , and Maynard, L. A, Food Research, 6,57-68 (1941).
APPROVED by the Director of the New York State Agricultural Experiment Station for publication as Joulnal Paper No. 644.
Thermodynamic. Properties of _Wethane at Low Temperature-Correc tion An error of statement has been found on page 827 of this article in the September, 1945, issue. This inaccuracy was found as a result of discussion of the results by C. S. Matthews and C. 0. Hurd. The sentence beginning in the tenth line, “Enthalpies of saturated gas . . . , given reference point”, should be replaced by the following sentences: Enthalpies of saturated gas were obtained from the values given by the latter authors by coordinating their value for - 163’ F. n ith the values obtained for the single-phase region and then utilizing their enthalpy differences along the saturation curve. The enthalpies of the saturated liquid were obtained from those of the saturated gas and the enthalpy changes for vaporization given by Keyes, Taylor, and Smith ( 5 ) and also tabulated by Wiebe and Brevoort (11). Some mbdification of the data of Keyes et al. was made near tlie critical state in tlic interest of internal consistenry in the final values. W. H. CORCORAN CALIFORNIA INSTITUTE OF TECHNOLOGY PABADENA, C A L I F .
