Freezing as a Means of Retarding Bread Staling A t the present time the most effective and practical method of retarding the staling of bread is sharp freezing. Judges found that bread frozen at -22' C. remained good for 20 days. That frozen at -35" C. for approximately 20 days was indistinguishable from fresh bread, according to the judges. At -35" C. bread was salable even after 110 days in the freezer. Disagreement was found between the physical tests for staling, which were used, and the findings of the judges. Methods of handling the bread in storage and during thawing are discussed.
WILLIAM H. CATHCART AND STEVEN V.
LUBE8
American Institute of Baking, Chicago, Ill.
F
outdoors at an average temperature of -24" C. The bread was left outside for 26 hours, and the temperature varied from -20" C. to -33" C. They thought that the bread that had been frozen had a better flavor and taste and was the fresher (by feel only) of the two. Their results indicate that a freezing atmosphere did not in this case have a deteriorating influence on quality. In contrast to the work of Berg and Morison and of Katz, Glabau and Pirrie (9) reported: "If **** the loaf can be maintained under conditions whereby it does not actually freeze**** but merely becomes cold, by being protected from freezing, then it would appear that the qualities of the loaf are enhanced in that it remains softer for a longer period, and the flavor may be improved." Neither Berg and Morison nor Glabau and Pirrie made any measurements of staleness. Marx (20) reported that the Navy was carrying out experiments on frozen bread "for supplying submarine and small vessels on extended cruises." No details of the experiments were reported, but they were said to be fairly satisfactory. Bailey (2) reported that after fresh bread had been packed in solid carbon dioxide and maintained in an atmosphere of carbon dioxide gas for 2 days and then allowed to thaw out, it had very much the appearance and characteristics of fresh bread. The compressibility (measurement of the depression of the crumb) test was used. Bailey stated that "the next nearest approach to fresh bread was obtained by storing bread for 3 days a t a temperature of -9.4" C." However, the bread seemed to be nearly stale by the compressibility test. Onnes (21) froze bread a t -20" C. in an atmosphere of carbon dioxide, generated from the solid form. I n most cases the bread was held in storage for only 30 hours and, after it thawed, was said to be unchanged from its original fresh condition. This same conclusion was reached even after the bread was held in the vaults for 2 weeks. Onnes said that "the staling action appears to be fully stopped as long as the bread is actually frozen." Knight (17)reported that bread stored at about -18" C. retained its freshness better than bread stored a t -6.7" or - 1.1" C. for the same length of time. He stated that when bread was stored in a container with a tight-fitting top a t
REEZING is fast becoming a popular method of storing various foodstuffs. The modern quick-freezing methods enable most foods to be frozen without the formation of large ice crystals which puncture the cell walls. I n this way the original quality and flavor are more or less preserved. Scalding or blanching is an essential treatment (before freezing) for vegetables, because it reduces the microbial content and inactivates the enzymes responsible for the loss of flavor. It should not be necessary for baked products, when we consider the temperatures of the baking process. Favorable reports have often been made of the freezing of bread, cakes, pastry, etc., by persons living in regions where freezing temperatures are available out of doors. Any practical method of preserving baked products fresh is desirable, for it would eliminate losses due to staling. Freezing seems to be the most promising method at present. The most extensive work on the freezing of bread has been carried on by Katz (12, 1.4). He found that the maximum rate of bread staling takes place a t -2" to -3" C., and that the rate decreases as the temperature is raised or lowered. At around 60" C. [see literature citation ( 6 ) ]and the temperature of liquid air (- 185" C.) Katz reported that bread was entirely fresh even after 48 hours. At 60" C. two difficulties are encountered: a softening or drying out of the bread crust, and the development of sporiferous bacteria which give to the bread crumb a penetrating smell in 12 to 24 hours. The first disadvantage was overcome by proper humidity and ventilation; the second can be delayed by the addition of some acid-reacting ingredient. At -8" C. Katz found that bread is only half stale after 48 hours. He states that "moderate cold (0" C.) furthers the staling process, and intense cold, on the other hand, checks the process and makes it evident that cold has an influence of two different types. I n the first case, i t influences the chemical equilibrium; in this respect it appears that the stale state is somewhat more completely developed by cold. I n the second case, cold restricts the velocity with which the staling process is completed; below -3", this latter factor overbalances the first, and at -8", a strong inhibition of staling is apparent." In one set of experiments Berg and Morison (6)compared bread aged indoors at room temperature with bread aged 362
MARCH, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
-18" C. for 49 days and then thawed, it was similar in quality t o bread 1 day old. Alsberg (1) stated that a temperature of -6" C. delays the onset of staleness and a temperature of -10" to -20" C. prevents it altogether. He said that "it is not yet feasible under ordinary commercial conditions t o freeze bread because of the expense and of the difficulties that arise in thawing. This takes much time, and moisture (from the air) condenses on the crust which makes it soft like that of stale bread. To avoid this the bread must be thawed out in dry air." Landaal and Koster (18) considered the loss in moisture of bread during freezing as a n important factor. By chilling their bread t o -28" C. with solid carbon dioxide, they practically prevented the evaporation of moisture. They stated that when a freezing plant is employed, this moisture evaporation can be prevented by watering the bread with special minute sprays before freezing at - 12" to - 14" C.; they do not report the condition of the crust after thawing. Hutchinson (10) said that a t -20" C. the staling rate of bread is very slow, and when stored at this temperature, it retains its fresh qualities, including aroma and flavor, for a long period of time. Neither Onnes, Knight, Alsberg, Landaal and Koster, nor Hutchinson gave any tests for staleness in their reports. According to Frey and his co-workers (8), the compressibility test showed that bread which had been frozen was partially stale; however, the aroma and taste were as good or better than that of freshly baked bread. One test was reported in which it was found that more than 50 per cent of the judges preferred week-old refrigerated bread to fresh bread 24 hours old. The above statements are more or less conflicting; in order to determine just how long bread can be kept fresh in storage at freezing temperatures, the following study was undertaken. Organoleptic as well as physical tests were made.
Experimental Procedure The rate of staling of the bread was followed by a modification of the swelling power method as reported by Katz (IS, l4,16)and by the compressibility test as reported by Platt (22). The swelling power method was modified in that a brass-frame 200-mesh sieve, 5 inches (12.7 cm.) in diameter, was used instead of the bolting cloth as recommended by Katz. The sieve has approximately the same number of openings per centimeter as the bolting cloth which is recommended. The sieve fits snugly on top of a 2-liter Pyrex beaker which serves to catch the washings. The crumb is rubbed through with the forefinger and the
363
second finger. The sieve is very durable and saves considerable time. Trouble was always encountered in passing the crumb through the fine bolting cloth, since the cloth breaks easily with continued rubbing. The readings reported by the swelling power method represent the cubic centimeters of sediment obtained after 24 hours when 10 grams of finely divided bread crumb are diluted to 250 cc. The number of cubic centimeters of sediment is greater with fresh bread than stale bread, because of the greater swelling power of the crumb of fresh bread. In the compressimeter test the chain was removed by means of a string run over a pulley, suspended overhead, and attached to a windlass equipped with a crank. In this way the chain could be removed more uniformly than by hand. The compressibility readings reported represent the magnified reading of compression in centimeters of a 32 X 45 X 45 cm. piece of crumb taken from the center of the loaf. The compressibility of fresh bread is greater than stale bread. Flavor and taste tests were made as described by Cathcart (7) except that the ballot was changed to the improved one given by Platt (25). With Platt's ballot it is possible to tell whether the judges like both samples and prefer one, like neither but prefer one, or like one much better than the other. This gives a truer picture of what is wanted. For instance, the frozen bread might not be as good as the fresh bread; however, the frozen bread might still be very good. It was important to know this. The judges and the number participating were similar to those described by Cathcart ( 7 ) . Besides flavor tests by a group of judges, some samples were evaluated by an expert on bread scoring. In most cases the bread was scored only for aroma and taste; however, in a few cases complete scores were made. Some of these data are reported in the results. All of this scoring was performed under code. The bread used was regular commercial 1.25-pound white loaves, made from a rather rich formula. It was maintained uniform throughout the experiments. The loaves had been out of the oven 8 hours when they were placed in the freezer. When they were to be tested, they were removed from the freezer a t the same time of day they were put in. The loaves required approximately 4 hours to thaw; therefore, after being removed from the freezer, they were kept at room temperature 4 hours before testing. In most cases the bread which had been frozen was tested against fresh bread. The term "fresh bread" refers to bread which had been out of the oven 12 hours and thus corresponded very well with the frozen bread which always had approximately a 12-hour aging a t room temperature. All of the bread was wrapped in conventional commercial wax paper and was sliced, unless otherwise stated. Large commercial freezers were employed for this work. Batches of one hundred loaves of bread could be placed in them without affecting the temperature of the freezer. Although larger batches were never used, the freezers were large enough t o handle thousands of loaves. Curves showing the rate of cooling of the bread at -22" and -35" C. are given in Figure 1. These curves were obtained by inserting thermometers into the center of the loaves and then taking readings at intervals. The flat portion of the curves ( - 5 " C.) represents the latent heat of freezing of the bread.
FRESH BREADAT LEFT; BREADFROZEN FOR 30 DAYSAND THENTHAWED FOR PICTURE AT RIGHT
INDUSTRIAL AND ENGINEERING CHEMISTRY
364
The temperatures used were below the region of maximum crystal formation; and all except -11” C. are classed as sharp freezing temperatures. Sharp freezing methods (freezing in still air) were employed, and free space was left around each loaf. Determinations of pH were made on all of the bread samples reported. Twenty-five grams of bread crumb were mixed well with 250 cc. of carbon-dioxide-free distilled water. The pH of the decanted liquid was measured with a Coleman pH electrometer. 30
20
+
10
$
0
was frozen (except in the experiment on bread 71, Table I) and the difference compares relatively well in most cases with that of the swelling power test. In the swelling power test a variation in room temperature of as much as *3” affects the results only slightly. Steller and Bailey (94) noted a somewhat similar effect of temperature on the swelling power and compressibility tests. They concluded that “increased storage temperature appeared to have a greater effect on the compressibility than on the water-imbibing capacity of the crumb.”
Fresh Bread
L
The many tests on fresh bread samples were made to be sure that it was constant from day to day. The swelling power test of Table I1 shows that the fresh bread samples were fairly constant throughout. All except those data on breads 1, 5, and 18 are within experimental error of the average of all values, and these are only slightly over and under this error. The scores given the fresh bread by the expert (Table IV)
i$-10 -20
-30b
VOL. 31, NO. 3
40
Bb
Id0 MINUTES
($0 IN
Ed0
$0
eb0
3‘h
36b
FREEZER
FIGURE 1. RATEOF COOLING BREAD WRAPPED IN WAX PAPER
Table I gives the results of experiments on bread which had been frozen. Table I1 includes data for fresh white commercial bread. Respective values are given by series number to correspond with Table I. Several tests were made on the rate of staling of bread which had been frozen as compared to fresh bread. Table I11 shows typical results. The results of the flavor tests are given in Table IV. Flavor tests were not made on all the breads shown in Tables I and 11; those that were, are referred to by the code number of the bread. In reporting the flavor tests made by the judges, any preference shown for a certain sample was taken as a vote for that sample. Ballots showing no difference are tabulated separately. The degree of difference in the two breads tested, and shown by the ballots, is summarized under “Remarks.” Scores given by the expert are based on a perfect score of 15 points for aroma (flavor) and 20 points for taste.
Swelling-Power and Compressibility The swelling power test produced much more reliable results than those from the compressibility test. I n most cases the compressibility test gave results that showed the same trends as the swelling power data; but they cannot be relied upon because a constant-temperature room was not available for the period of the experiments. Also, in a few cases it was not possible to bring the frozen bread up to the desired temperature for testing in the 4 hours allotted for the purpose. [Hutchinson (IO)gives other factors which might cause error.] Thus, compressibility tests made on different days are not comparable. However, if we compare the compressibility results of the frozen bread with the results for fresh bread with which it was tested (i. e., tests made on the same day), the compressibility of the fresh bread is always greater than that for the bread which
TABLEI. TESTSON WHITECOMMERCIAL BREADFROZEN FOR DIFFERENT PERIODS Bread Series Code No. No.
Kind of Wrapper
Time in Freezer Days
1
30 31 32 33 34 348 35 36 37
Wax
Wax Wax Transparent cellulose Wax Wax Wax W8X . Transparent cellulose
Freezer
Temp. c.
-22 -22 -22 -22 -22
*1 *1 *1
42 72
-22 -22
6 f
1 1
33 35
137
-22
t
1
34
4.5
4 15 28 41 100
-22 -22 -22 -22 -22
i-1
4.2 3.2 3.6 3.5 2.6
Unwrapped Unwrapped Unwrapped
Unwrapped
1 3 7 90
-22 -22 -22 -22
49 50
Wax Wax
3 13
-22 -22
5
52 53
Wax Wax
6
54 55 56
8
9
10
3.8 3.6
*1 *1
36 34 34 34 34
13 20 25 34 42
45 46 47 48
7
4.1 4.2 3.8
*1
Wax
4
35 34 38
-22 -22 -22
40 41 42 43 44
3
Cm.
3 4 8
Wax
2
Remarks
cc.
O
38 39
Transparent cellulose
S.well- ComZZer
*1
f
1
Bread s h e d
...
4.5
... ... 4.3
i-
1
34 34 33 33 32
i-
f
1 1 1 1
35 34 34 33
2.8 Bread u n s h e d ; first 7 3.4’ dsys,preadfrozen in 3.8 container oantsining 1.7 salid COS
f
1 1
34 33
3.7 3.4
Bread slioed
f
3 13
-11 * 1 -11 * l
34 31
2.8 3.2
Bread sliced
Wax Wax Wax
2 13 39
-22 * 1 -11 * 1 -11 f l
36 32
3.3 3.0
57 58 59 60 61 62 62a
Wax Wax
1 4 8 19 40 60 110
-22 -22 -22 -22 -22 -22 -22
34 34 36 32 33 36 37
4.0 2.1 2.1 2.8 1.7 1.8
Bread u n s h e d
63 64 55 66 67 68
Wax Wax Wax Wax Wax Wax
1 4 19 40 60
-38 -40 -33 -29 -29 -28
40 38 34 33 36 36
4.1 2.5 2.8 2.9 2.1 3.0
Bread unshed. freezer temp. aboui -40C. at start, then raised to about -30’ C.; readings are for respective days
69 70 71 72 73 74 74a 74b
Wax Wax Wax Wax Wax
4 8 19 40
4.0 2.4 3.7 2.8 3.3 4.6
Bread unsliced
Wax Wax
40 37 36 32 36 39 38 38
Wax
75 76
Wax Wax Wax Wax
Wax
Wax Wax Wax Wax
Wax
Wrtx
S
f
1
f
1
*1
f
*
*1 *1 *1 1 *1 *1 f
i-
1
i-
1
f
1
110 240
-35 -35 -35 -35 -35 -35 -35 -35
f
1
25 25
-22 -22
i-
1
f
1
1
60
*1 f
i-
1 1
*1
*1
9 .
34 32
...
Bread slioed
Bread sliced; frozen 2 days at -22’ C., kept at - 1 1 O C. for rest of time
...
... ... ... 2.1
Bread sliced. No. 76 was 32 hi. o u t . of oven before freezing
*
MARCH. 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE 11. TESTS ON FRESH WHITE COMMERCIAL BREAD WRAPPED IN WAXPAPER Series No.
Code No. of Frozen Bread Bread with Which Ewelling Code No. It Was Tested Power
cc
.
Compressibility
cm.
8
38
40
14 15 16
45 46 47
41 40
39
4.4 4.9
4, 5, 6, sliced
17
49, 52, 54 56
40 42
4.9
18
7, 8, 9, unaliced
19 20 21
57, 63,69 58, 64, 70 59, 65, 71
40 40 40
4.4 4.1
10, sliced
22
76, 76
41
4.9
40
4.6
4.7
2, sliced
3, unsliaed
Average
3.1
3.7
3.5
are very constant; that is, all values for aroma and taste of fresh bread are identical. Although tests were not made on the samples before they were frozen, they would be just as constant, since they were made in exactly the same way and under the same conditions.
Bread Frozen at - 22 * 1' C. Table I, series 3 and 7, shows that the swelling power of bread frozen at -22' C. fell from about 40 to about 34 cc. in 24 hours. After this, however, the value did not decrease much more, as is indicated by series 1, etc. According to Katz, values of about 34 cc. indicate stale bread, and values of about 40 cc. show that the bread is about half stale. Therefore the bread was nearly half stale when it was placed in the freezer and was completely stale after 24 hours in the freezer. Table I11 and other similar experiments show that the drop to 34 cc. of bread frozen for 24 hours is about the same as it would be if the bread were kept a t room temperature. Thus, it seems that the freezing of bread (of the type used) at -22" C. after it has been out of the oven for 8 hours does not retard staleness to any extent, as measured by the swelling power test. However, it must be pointed out that the sediment of frozen bread, after it is thawed and allowed to age at room temperature, will drop (below values of 34 cc.) to values of about 30 cc. Values this low were never observed when the bread was being aged in the freezer a t -22' C. In considering the organoleptic tests, it is important to remember that staleness as measured by swelling power or other physical test is not the same as when measured by the baker or consumer. Almost all workers in the field agree on this point. The consumer knows fresh bread as bread which has been out of the oven for a period varying from 8 to 20 hours. According to the swelling power test (or to compressibility measurements), this bread is half stale to stale; however, persons engaged in the art of bread scoring consider this fresh bread. Other factors must be considered in staling besides the changes in the starch which the swelling power test measures. The swelling power test is not worthless, but it does not tell the whole story. Thus, it was necessary to include tests made by human beings. Experiments made on bread frozen a t -22' C. and compared with fresh bread (Table IV, experiments a, b, c, d, e, f, g , i k , I, m, n ) show a somewhat different picture from that of
365
the swelling power tests. Although some inconsistencies are evident in the results of the judges and scorer in these experiments, the results can be summarized as follows: The judges found the frozen bread as good as the fresh bread up to an age of about 15 to 20 days in the freezer. The scorer detected a slight off-aroma in the frozen bread after 8 days in the freezer. The bread remained salable up to about 41 days in the freezer. Tests made at 72 (experiment i), 90, 100, and 137 (experiment j ) days indicate that the bread would not be salable because of the development of a pungent aroma. This undesirable aroma can be removed fairly well by aeration of the crumb. The cause of the development of this aroma has not been determined. However, since Balls and Lineweaver (3), Balls and Tucker (4),and Tressler (25)have shown the so-called fatsplitting and protein-splitting enzymes to be active in firmly frozen materials, one might suspect them as the cause if it were not for the high temperatures employed in baking the bread. Baking temperatures are sufficient to inactivate these enzymes. Of course, it is possible for some microorganisms to survive the baking temperature and grow in the frozen bread (19). Oxidation of proteins and fats might be a factor and is being investigated at the present time. A slight development of this pungent aroma was also noted in bread frozen at -35' C. for 60 days (experiment to).
Bread Frozen at -11+1'
C.
According to the swelling power test, temperatures of
- 11 '
C. serve just as well as -22" C. in keeping bread for the first 3 days; however, after 13 days the sediment of the bread at -11" C. has dropped to 31 cc. (series 5, Table I), while that at -22' C. is still at about 34 cc. Bread that was frozen a t -22' C. for 2 days and then kept at -11" C. (series 6, Table I) keeps better, according to swelling power tests, than that held at - 11 " C. but not as well as that held at -22' C. Katz found that bread frozen at -8" to -10" C. was only half stale (swelling power about 40 cc.) after 48 hours, These tests indicate little retarding of the staling process until 34 cc. is reached, even at temperatures of -22' C.; however, the bread was not placed in the freezer until it was 8 hours old. Although the authors have not been able to find how old Katz's bread was before he placed it in the freezer, it is assumed that this was done as soon as the bread was cool TABLE111. RATEOF STALING OF FRESHBREAD AND FROZEN AT -22 1 1 " C. FOR 24 HOURS Bread
Age of Bread HOUT8
Swelling Power
cc.
OF
BREAD
Compreasibility C?%
enough. The results of series 10, Table I, indicate that the age of the bread before freezing would be a factor; however, the results are nearly within experimental error. Another factor which might cause this difference is the kind of bread. Katz used Dutch water bread, whereas the bread used in these tests was regular commercial bread made from a rather rich formula. Approximately 2 hours are required to cool bread on an open rack to the safe temperature of wrapping. By the time it is wrapped and placed in the freezer (commercial scale), approximately 3.5 hours will have elapsed if the freezer
366
INDUSTRIAL AND ENGINEERIKG CHEMISTRY
VOL. 31, NO. 3
TESTSON BREADFROZEN AT VARIOUSTEMPERATURES TABLEIV. FLAVOR Code No. of Bread
Freezing Temp.
-Score Aroma, 1.5
a
1 30
Time in Freezer Days Fresh 3
b
2 31
Fresh 4
L22.2 1
..
C
3 32
Fresh 8
-22
......
1
14 13.5
d
4 334
Fresh 13
-22
1
e
5 34
Fresh 20
-22
f
6 34a
Fresh 25
-22
Q
7 35
Fresh 34
-22
h
36b 37a
42 42
-22 -22
i
8 38
Fresh 72
-22
*
i
480 44 39Q 9 40
90 100 137 Fresh 4
-22 -22 -22
*1
-22
11 42
Fresh 28
-22
m
12 43
Fresh 41
-22
n
14 45a
Fresh 1
-22
15
Fresh
Expt. NO.
IC I
0
c.
b y ExpertTotal, Taste, 100 20
Judges' Results For each No sample difference
7
From judges
Remarks From scorer
%
%
..
56 22
22
Very good Good
, .
..
82 18
0
Very good Good
19 19
94 88.5
80 20
0
Very good Good
14 13.5
19 18.5
92.5 91
47
3
Very good Very good
14 12
19 19
94 88
47 39
14
Very good Good
Crumb of 34 seemed "tenaoious"
1 1
14 12
19 19
.. ..
83 12
5
Very good Fair
34a had rancid aroma; taste OK
...... *1
14 13
19 19
92.5 91
84 13
3
Very good Good
35 had slightly rancid aroma
..
50 36
14
68 27
5
...... -22 * 1
*
......
= I =
...... *
...... +
*1 *1
......
1
*1 *
1
...... d=
......
..
..
14 12
19 18
13 12 8
19 19 18
...... f
1
...... A 1
46d
3
-22
P
16 47d
Fresh 7
-22
P
52 49
3
T
53
...... *1
..
..
14 13
........
82 18
0
Very good Good
41 50
9
Good Very good
..
37 53
10
, .
..
50 50
0
Very good Very good
..
53 43
4
Very good Good
.. ..
75 13
12
Very good Poor
19 19
.. ..
Very good Fair
..
.. ,.
..
..
32 had a slight acid aroma.
Good Fair (practically as good as 36)
........
..
...... *1
50
........ ..
1
1
+
, .
Aroma of 38 was very pungent Off-aroma Off-aroma, slightly worse t h a n 4 8 Pungent aroma
Good Good (slightly better t h a n 12)
Crumb of 46 dryer t h a n 15; gassy aroma
-11 f 1 -22 * 1
13 14
18.5 19
..
28 72
0
Fair Good
52, tough eating, old aroma
13
-11
f
1
12.5
18.5
4
50
13
1
14
19
Fair (nearly as good as 50) Good
53, old aroma
f
19 63
Fresh 1
14 13.5
19 19
63 32
5
About Very good as good as
Very little difference
-38
.. .. .. ..
44
-22
t
20 64
Fresh 4
......
14 13.5
19 19
53 47
0
Very good Very good
Very little difference
-40
u
21 65
Fresh 8
......
14 13.5
19 19
44 48
8
Good Good
Very little difference
-33
Fresh 19
.....
19 19
22 78
Good Very good
Very little difference
-29
14 13.5
0
66
62 68 74
60 60 60
-22 -28 -35
12 13 13
18 18.5 18.5
s
21
W
a
..
3
......
Transparent cellulose wrapper.
b
f
1
f
1
Wax wrapper.
0
Unwrapped,
, .
.. .. ..
.. .. ..
..
52
-
19
.......
..
...... .......
Off-aroma Aroma fair, better t h a n 62 Aroma better t h a n 68
d Unwrapped and stored in COz.
is a t the bakery. In the present experiments 8 hours had to be allowed because of the great distance between the bakery and freezer. Any means that will decrease this time between baking and freezing will be of advantage. I n experiments p and r, Table IV, bread frozen at -11" was compared with bread frozen at -22" C. After 3 days the judges and the scorer found a definite preference for the bread frozen at -22" C. After 13 days the scorer indicated a definite preference for the bread frozen at -22" C., but the judges did not find so great a difference. The experiments show that much better results are obtained by freezing bread around -20" than around -10" C.
Bread Frozen at -35*1° and - 4 O O t o -28' C. From series 8 and 9, Table I, at temperatures of about -35" t o -40" C., it is evident that the staling process is
slowed down so that the volume of sediment does not change after 24 hours in the freezer and decreases very little after 4 days in the freezer. As the number of days of freezing is increased in these series, the swelling power reaches a minimum and then increases. This is especially true in series 9 where the temperature was held constant a t -35" C. The same trend is shown by the compressibility tests. Although these tests will need verification before definite conclusions can be drawn from them, they show that a t -35" C. bread will stale and then return to the condition of freshness which was prevailing when it was placed in the freezer. This does not necessarily agree with the theory of bread staling. It has been shown, especially by Katz and co-workers (13-16), that the swelling power test measures such alterations as are directly due to basic changes in the starch. According to Katz, the gelatinization of starch is an equilibrium
MARCH, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
state. Warm temperatures prevent the stale state from developing, while cold tends to shift the equilibrium towards the stale state. However, if temperatures are low enough (-So to -10" C.), they restrict the velocity with which the staling process is completed. If this explanation is correct, and there is much evidence to show that it is, there is no reason why prolonged holding of bread a t - 35" C. should cause the starch to revert towards the fresh state, once it is stale. If the starch is not reverting to the fresh state, then the swelling power test is measuring other changes whose magnitude is as great as those of the starch. Whymper (26) reported: "During the process of becoming stale, there is a fall in soluble starch extract (also used as a method of measuring staleness) obtained from the crumb, followed after a time, apparently independent of staleness by a rise (experiment carried a t room temperature to 144 hours). Investigation shows that a similar fall and rise of soluble extract is to be seen in starch pastes." These results are similar and perhaps due to the same changes taking place in the above swelling power results. This refreshening phenomena did not occur with bread frozen at -22" C. except in series 7, Table I, where an indication towards this effect is observed. From experiments s, t , u, and v, Table IV, the scorer and judges found the frozen bread (temperature varying from -40" to -29" C.) to be just as good, if not better, than the fresh bread after 19 days in the freezer. In experiment w, breads frozen for 60 days a t various temperatures were tested together. The bread frozen a t -28" C. was from the same series as that of experiments s, t, u, and v. The bread frozen a t -22" C. had a decided off-aroma; the bread a t -28' C. had a much better aroma; and that a t -35" C. was the best. However, in the latter case an off-aroma was present. The taste did not show as much variation. The bread frozen and kept at -35" C. was tested a t 110 and 240 days (series 9, Table I, but not indicated in Table IV) and found still to be salable.
367
Since human beings are buying the bread, their reaction is of the utmost importance. We can conclude, as Alsberg and Katz have done for other tests, that there are other factors of importance in bread staling (up to the present time no worker has been able to measure them successfully), besides changes in the starch, which are of importance. The tests reported here indicate that for the type of bread being used, the change in the starch is either of minor importance, as far as human beings are concerned, or the decrease in swelling power from 34 to about 30 cc. is of more importance than has ever been expected. RATEOF STALIKG OF FROZEN BREAD. Table I11 represents a set of typical results on the rate of staling of bread that was frozen for 24 hours a t -22' C. There is practically no difference between the rate of staling of fresh and frozen bread; that is, freezing a t -22" C. does not harm the keeping qualities of the bread on thawing. These same conclusions were reached from an examination of the external and internal characteristics by the scorer. This is in contradiction to the statements of Glabau and Pirrie; however, the findings are in agreement, more or less, with the results of other authors (6, 10).
Bread Frozen at -22*1° C. in Carbon Dioxide These experiments were made in order to duplicate the work of Onnes as nearly as possible. The bread was unwrapped and was placed in a box and sealed with a sufficient supply of solid carbon dioxide. The carbon dioxide was replenished every day. The box was stored in the freezer a t -22*1° C. According to the swelling power test, carbon dioxide does not have any effect. There seems to be no reason for adding it. Experiment n, Table IV, showed that bread frozen a t -22" C. in an atmosphere of carbon dioxide for 24 hours was preferred by just as many judges as the fresh bread. After 3 days in the freezer (experiment 0 ) the judges showed a preference for the fresh bread, and the scorer noted a gassy aroma in the frozen sample. After 7 days (experiment p ) the judges showed a decided preference for the fresh sample; they rated the frozen bread as poor. These experiments, as compared to others a t -22' C., indicate that the presence of carbon dioxide in the freezer does not improve the keeping qualities. These findings do not support the work of Onnes.
Other Results of Tests SWELLING POWER us. HUMANBEINGS. It is generally accepted that the swelling power test measures the change that takes place in the starch. Although this is of great importance since it gives a numerical value based on physical methods and nearly eliminates the human element, the results obtained do not agree with those from human beings.
PH MEASUREMENTS. Within the accuracy of the determinations, the pH was found to remain constant as bread aged a t room temperature. This is in agreement with the results reported by Karacsonyi ( I I ) , who found that the pH either remains constant or shows some increase. When bread was aged a t -22" C., the pH remained either constant or, in a few cases, slightly increased. This was also the case a t -35' C. Thus, freezing has little effect on the hydrogenion concentration of the bread. EFFECT OF WRAPPING. In a few preliminary experiments and in series 3, Table I, the bread was frozen unwrapped.
INDUSTRIAL AND ENGINEERING CHEMISTRY
368
After several days in the freezer, there was a slight collection of ice crystals or frost on the bread. When the bread thawed a t room temperature in the ordinary atmosphere, moisture condensed on the crust, as Alsberg reports. The combination of these two factors gave a soft and irregular crust. (These factors affect only the crust and not the crumb of the bread.) This was overcome almost completely by wrapping the bread before it was taken from the freezer. The soft and irregular crust is completely eliminated if the bread is wrapped in the conventional manner in a moisture-proof wrapper and left this way during freezing and until it is thawed, ready for consumption. The irregularities in the crust are undoubtedly due to the loss of moisture from the loaf. Wrapping will decrease the amount of moisture loss even when bread is frozen (6). The amount of moisture lost from a loaf at -22” C. or colder in 24 hours is of little concern; however, if the loaf is to be stored in the freezer for several days, this loss of moisture becomes of great concern. I n experiment 0, Table IV, the crumb of the unwrapped bread stored for 3 days at-22” C. was found to be more dry (by organoleptic tests) than the fresh bread. In experiment h, Table IV, bread wrapped in wax was compared with bread wrapped in transparent cellulose. Both loaves were in the freezer a t -22” C. for 42 days. Although the percentage preference was in favor of the bread wrapped in wax, consideration of the whole ballot shows that there was little difference. Experiment j shows that the off-aroma develops in unwrapped bread, bread wrapped in wax, and bread wrapped in transparent cellulose. Although it is impossible to tell from the experiment whether there is any difference in the rate of development of the off-aroma in unwrapped or wrapped bread, it shows definitely that the wrapper is not necessary for the development and thus supports the view that it is due to changes in the bread itself. SCORER os. JUDQES. I n experiments where both the scorer and judges made tests, the results, in general, are in good agreement. A discrepancy is present in experiment 2, Table IV. EFFECTOF FREEZINQ ON CRUST. Besides the characteristics of bread already mentioned, the only other affected by freezing was the crust. The scorer observed that freezing seemed to toughen the crust of the bread slightly. EFFECTOF SLICING.Comparing series 1 with series 7 of Table I, the results are found to be the same; therefore, slicing does not affect the results. I n the aroma and taste tests, sliced bread was not compared with unsliced samples.
Although the change in the starch, which the swelling power test measures, is important, it does not tell the whole story about staling. The development of an off-aroma was the limiting factor for the time the bread could be kept salable by freezing a t the temperature used. At -22” C., carbon dioxide in the freezing chamber did not improve the keeping qualities. When bread frozen a t -22” C. was removed from the freezer, it staled a t about the same rate as ordinary bread. The pH of bread did not change or slightly increased on prolonged storage in the freezer. During freezing and thawing, condensation of moisture on the crust was eliminated by keeping the bread wrapped in an ordinary moisture-proof commercial wrapper. Although bread does not keep indefinitely a t the temperatures employed, freezing offers an excellent means of keeping commercial bread in a salable condition for 40 days or longer, It must be remembered that all frozen products deteriorate slowly during storage, when comparing the practicability of freezing bread with other food products.
Acknowledgment The authors wish to express their sincere appreciation to Armour and Company and to Swift and Company for the use of their freezers; to William Walmsley for acting as the scorer; to the staff and students who acted as judges; and to members of the baking and allied industries for reviewing manuscript.
Literature Cited
Summary Bread was frozen by sharp freezing methods and at sharp freezing temperatures (as well as a t - 11O C.). Commercial bread was frozen on a commercial scale in a commercial freezer. According to swelling power tests, bread staled rapidly a t temperatures as low as -22” C. This test showed that it was about half stale when placed in the freezer (8 hours out of the oven) and nearly completely stale after 24 hours in the freezer. At about -35” C. the bread was kept in its original fresh condition for a period of about 4 days and required 8 to 10 days to become nearly stale, according to the swelling power test. When the tests were continued for 60 to 70 days, a refreshening of the bread was observed; the sediment returned to values in the neighborhood of those for unfrozen bread 8 hours out of the oven. According t o aroma and taste tests, bread frozen a t -22” C. remained good for 20 days, and salable for approximately 40 days. At -35” C. it had a much better keeping quality and remained in a salable condition for much longer periods of time.
VOL. 31, NO. 3
(21) (22) (23) (24) (25) (26)
Alsberg, C. L.,’Wheat Studies Food Research Inst., 12, 244 (1936). Bailey, L. H., Cereal Chem., 9, 65 (1932). Balls, A. K., and Lineweaver, H., Food Research, 3, 57 (1938). Balls, A. K., and Tucker, I. W., IND.ENQ.CHEM.,30,415 (1938). Bouasingault, J. B., Ann. chim. phys., [3] 36,490 (1853). Berg, I. A,, and Morison, C. B., Baking Tech., 4, 83 (1925). Cathcart, W. H., Cereal Chem., 14, 735 (1937). Frey, C. N., Proc. Am. Soc. Bakery Engrs., 13, 140 (1936). Glabau, C. A., and Pirrie, P. G., Bakers’ Weekly, April 4, 1925, 57. Hutohinson, J. B., Research Assoo. Brit. Flour-Millers, Special Rept. 15 (1936). Karaosonyi, L. P., Cereal Chem., 5, 477 (1928). Katz, J. R., Bakers’ Weekly, 81, No. 3,43 (1934). I b d . , 84, No. 12, 25 (1934). Katz, J. R., “Ouderzoekingen naar het oudbakken -,orden van brood en de middelen van het te voorkommen,” Ministerium fur Ackerbau, Industrie u. Handel, Holland, 1917; “Gelatinization and Retrogradation of Starch in Relation to the Problem of Bread Staling,” in R. P. Walton’a “Comprehensive Survey of Starch Chemistry,” New York, Chemical Catalog Co., 1928. Katz, J. R., 2.Elektrochem., 19,202 (1913); Bakers’ Weekly, 81, No., 2, 34 (1934); Rec. trav. chim., 56, 785 (1937); see also Ostwald and Liiers, Kolloid-Z., 25, 26 (1919). Katz, J. R., 2.physiol. C h m . , 95, 104, 136, 147 (1915). Knight, H. G., U. S. Dept. Agr., Rept. of Chief of Bur. of Chem. and Soils. 1). 11 (1937). Landaal, D.’K., and Koster, M., Proc. Intern. Congr. Refrig., 7th Congr., The Hague-Amsterdam, 4, 12 (19361. Magoon, C. A., h n . ENG.CHEW 24, 669 (1932). Marx, V., Bakers’ Helper, June 4, 1932, 1074. Onnes, H. J., FoodIndustries, 9, 584 (1937). Platt, W., Cereal Chem., 7, 1 (1930). Platt, W., Food Industries, 9, 7 (1937). Steller, W. R., and Bailey, C. H., Cereal Chem., 15, 391 (1938). Tressler, D. K., IND. EXQ.CHEM.., 24, 682 (1932). Whymper, R., “Conditions That Govern Staleness in Bread,” London, Maclaren and Sons, 1919.
R ~ C E I VSeptember ~D 19, 1938. Presented before the Diviaion of Agricultural and Food Chemistry a t the 96th Meeting of the American Chemical Society, Milwaukee, Wia., September 5 t o 9, 1938.
