PECTIN STUDIES. I11
GENERAL THEORY OF PECTIX JELLY FORMATIOK AKSEL G. OLSEN General Foods Corporation, Battle Creek, Michigan Received A u g u s t 341 1063
Most of the recent work on the chemistry of the pectin molecule fails to consider that pectins may vary depending upon their source. Even among those who have considered the pectins on the basis of their physicochemical properties, there appears to be a school of thought which favors the view that pectins of whatsoever source are the same and that such differences as are observed are to be considered as due merely to impurities present (6). It has, however, long been recognized commercially that citrus and apple pectins, irrespective of grade, form je!lies of definitely different types, although this difference has not generally been recognized in scientific pectin literature. APPLE AND CITRUS PECTINS
In two earlier papers (7, 11) of the present series the interrelationship of sugar, acid, and pectin was considered for citrus pectin and for apple pectin, respectively. Whatever the grade of pectin used, the type of jeily obtained remains typical of either citrus or apple. The former jellies are comparatively friable, with little elasticity, while the latter jelly is highly elastic. I t is a common observation that apple pectin jellies require-in fact, toleratemuch less acid than citrus pectin jellies similarly prepared. It has also been observed in this laboratory that pectins from other sources, such as cranberry and turnip, differ markedly in their characteristics‘from either of the above. THE “OPTIMUM
ACIDITY”
COXCEPT
A number of explanations of “optimum acidity” as related to pectin jellies have been offered by various investigators since Goldthwaite (3) in 1909 first called attention to the importance of acid. La1 Singh (9) demonstrated that the necessary acid may be reduced by increasing the sugar concentration. However, Tarr (12) and Baker (1) for the first time placed the matter on a definite quantitative basis not only by measuring the acidity in ternis of pH but by devising a means for expressing jelly 919
920
dKSEL G. OLSEN
strength accurately. These authors repeatedly suggested that the optimum acidity indicated a stoichiometric relationship between pectin and acid. Spencer (10) in 1929 insisted that Tarr and Baker’s optimum was related to the hot method only, She presented data to show that with a cold method, that is, the mixing a t room temperature of a sugar syrup, acid, and pectin solutions, there is observed a minimum acid requirement, but no maximum acid limit. She suggested that the turning point in the jelly strength curve, called “optimum acidity” by Tarr et al., “might simply mean that at this hydrogen-ion concentration the decomposition of pectin begins to predominate over the increased strength due to increased acidity.” The present writer has elsewhere suggested that data obtained for citrus pectin do not conform to these interpretations (7), and Cole, Cox, and Joseph (2) have indicated that jelly failures past the optimum acidity may be caused by the increased rate of setting. It appears to the present author that a few simple concepts from colloid chemistry may suffice to explain quite satisfactorily the observed phenomenon. Kruyt (4, 5) has outlined a concept of the stability factors operating in a system containing the emulsoid agar-agar. His views may readily be translated into terms of pectin, sugar, and acid. Granted that pectin is a negatively charged hydrophilic colloid we may assume the following : (1) The sugar functions as the dehydrating agent. (2) The hydrogen-ion concentration functions by reducing the negative charge on the pectin, thereby permitting the pectin to precipitate and coalesce in the form of a network of insoluble . fibers, providing the concentration of the sugar is sufficiently great. (3) The dehydration of the pectin particle by sugar is not instantaneous but requires time to come to an equilibrium. (4) The rate of dehydration and precipitation increases directly as the hydrogen-ion Concentration. ( 5 ) The maximum jelly strength is reached when the system reaches equilibrium, and depends upon the position of that equilibrium. ( 6 ) Any component added to a pectin jelly system, including salts, which causes a change in the ultimate jelly strength of that system may function either (a) by changing the rate ofsgelation, or (b) by affecting the position of the ultimate equilibrium of the system, or (c) by a combination of these two effects. In any given jelly mixture the rate of precipitation and orientation of fibers or micellae will go on at a rate varying directly with both sugar concentration and hydrogen-ion concentration and each pectin-acid-sugar combination will have a time limit, inherent in the procedure, at which the jelly structure is so well formed that any mechanical disturbance will reduce the ultimate perfection and therefore the strength of that struc-
PECTIR’ STUDIES. 111
921
ture. It is to be assumed that temperature mill be an additional factor in these relationships. On this basis for a given time-temperature-pectinsugar combination, that is, for any given empirical jelly-making procedure, there must be a maximum hydrogen-ion concentration which results in a rate of gelation that barely permits the completion of the procedure within the time limit of the system. This would be the optimum acidity for that particular combination. From this viewpoint the changes observed with varying sugar concentrations become logical. The disappearance of an optimum at the lower sugar concentrations simply means that the rate of pectin precipitation has become sufficiently slow a t all acid concentrations so that the time factor does not enter. Likewise the absence of an optimum in Spencer’s jellies does not distinguish a cold method from a hot method, but simply indicates that she operated within the time limit of the system. In other words, the “optimum pH” may be considered purely manipulative in character and is that hydrogen-ion concentration in any given procedure at which premature gelation becomes a measurable factor in the jelly strength of the finished jelly. On the above basis we may postulate that: (1) Other types of pectin will show a behavior similar to that of citrus pectin, although not necessarily at the same proportions of sugar and acid. Using the same shortboil empirical method as previously used there should be for each pectin some sugar level at which the rate of jelly formation becomes sufficiently slow so that the so-called optimum disappears. (2) The optimum pH may be made to pass through these same stages by merely changing the time relationship of the process. APPLE PECTIN
The above postulates were considered in our observations on apple pectins. The experimental procedure was identical with that previously described ( 7 ) . In order to obtain a greater variation in acidity, phosphoric acid was used instead of tartaric acid. The acid was added in the form of a 25 per cent solution, allowance being made for the amount of water thus added. The pH of the finished jelly was determined by the quinhydrone method (11). The apple pectin used, 119F, is an alcohol-precipitate from Certo. Details of procedure and the results obtained appear in table 1 and figure 1. It may be noted that although the characteristics of this pectin are definitely different from those previously shown for citrus pectin, nevertheless a sugar concentration can be found in each case a t which the rate of gelation is such that no definite optimum is shown. For 119F this is at 50 per cent sugar; The 60, 55, and 50 per cent curves for 119F resemble respectively the 70, 65, and 60 per cent curves previously shown for citrus pectin. This is in agreement with postulate 1.
922
AKSEL G. OLSEN
TABLE 1 E f e c t of sugar concentration o n “optimum acidity” and jelly strength of apple pectin jellies (119F alcohol-precipitated pectin) All series of jellies identical except for sugar concentration. 2.5 g. of pectin 119F and the indicated amount of acid was used. Final net weight of jelly 555 g. Total heating time, about 4f minutes. See also figure 1. 26
SUGAR CONCENTRATIONS PER CENT
PHOBPHORIC ACID P E R BATCR OF JELLY
50 per cent
55 per cent
60 per cent
J.S.’
PH
J.S.
PH
0.15
-
3.34
-
-
-
0.25
-
-
-
2.87 2.61 2.53 2.50 2.42 2.39
3.21 3.06 2.92 2.81 2.74
J.S.
PH
70 per cent
J.S
PH
CC.
0.30 0.35 0.40 0.50 0.55 0.60 0.70
5
-
2.67 -
0.80
-
-
0.90 1.00 1.20 1.40 1.50 1.60 1.80 2.00 2.50 3.00 3.50 4.00 5.00 10.00 35.00
18.5 21.5 31
2.41
-
26 26 31 27 27.5 27 33 31
-
2.22 2.12
-
1.98 1.83 1.79 1.72 1.66 1.59 1.37 1.05
11 43
{;; 51 42 34: 27 25.5 21.5 24.7
-
32 36.5 31I
-
2.26 2.10 1.98 1.94
-
2.46
2.34
-
2.16
-
2.02
-
-
1.92 1.62
1.61 1.37 0.86
1.20 0.98
-
1.73
-
-
* Jelly strength. 7 0.42 cc. of acid. 1.3 cc. of acid. $ 4 0 cc. of acid. 7 30 cc. of acid. Figures in brackets indicate “curdled” jellies.
It becomes apparent from a study of these cupes that for most practical purposes jellies containing 60 to 65 per cent sugar should be preferable. I n all cases the further lowering of sugar concentration carries the penalty of
923
PECTIN STUDIES. 111
lower jelly strength and greater acid requirement. This lowering of jelly strength may be assumed to be due to incomplete dehydration and therefore either incomplete precipitation or a more flexible network of pectin. T H E TIME FACTOR CONCEPT
The second postulate made above was first tested by preparing jellies in the manner suggested by Spencer, except that the temperature of the jelly mixture was 55°C. instead of room temperature. This allows a greater time factor without involving any measurable hydrolysis of the pectin. The jellies made were 60 per cent. The pectin used was the 119F used in previous observations and the amount used identical with the prior series.
FIQ.1. EFFECT OF SUGAR COXCEXTRATION ON OPTIMUM pH APPLE PECTIN JELLIES
AND
JELLY STRENGTH OF
Phosphoric acid used
The 2.5 g. of pectin and 30 g. of sugar were dissolved in just enough water to make a total of 140 g. and brought t o 36°C. The sugar, the minimum amount of acid used, and the remaining water were boiled together, cooled to 65"C., and adjusted to correct weight with distilled water. The additional acid was then added and the pectin solution rapidly stirred in while the beaker remained immersed in a water bath a t 55°C. The temperature of the jelly mixture was in each case 55°C. Allowance was made in all weights for the amount of pectin solution remaining in the original beaker. Three series of jellies were made, varying the acid within the same limits used in previous series. For series A the total mixing time from the moment the pectin solution was poured into the warm syrup until the mixT H E J O U R N A L OF PHYSICAL CHEMISTRY, VOL. XXXVIII, X O . ?
924
AXSEL G. OLSEN
ture was poured into the glasses was exactly 8 seconds; for series B the time was 50 seconds; for series G the time was 90 seconds. The results in every way conformed to the postulate made. The magnitude of the jelly strength for the 8-second series was quite remarkable, indicating that under the conditions of the usual hot method even at the optimum point only a fraction of the true jelly strength is obtained. The results are listed in table 4 and are shown graphically in figure 2 along with the corresponding 60 per cent curve from table 2. There is also shown a curve obtained in a manner identical with the latter except that an additional 80 cc. of distilled water was added to each batch of jelly, thereby changing the heating TABLE 2 T h e relation o j the time jactor to o p t i m u m acidity The time indicated is the interval between the pouring of the pectin solution into the sugar syrup and the pouring of the mixture into the glasses. Jellies contain 60 per cent sugar, 2.5 g. apple pectin 119F, phosphoric acid as indicated. Temperature of pectin solution, 36°C.; of sugar solution, 65°C.; of final mixture, 55°C. 25 PER PHOSPHORIC ACID
1
SERIES B
SERIES C
SERIES D
90 SECONDS
50 SECONDS
8 SECONDS
J.S. ce
.
0.26 0.36 0.56 0.86 0.96 1.06 1.26 1.56 1.96 3.96
-
J.S
-
33 69
3.10 2.80
41
2.39
-
-
19 14.5
-
-
-
2.13 2.03
-
0
-
80
-
J.S.
3.27
-
2.50
-
-
-
36.5 67
3.13 2.88
-
2.40
2.13 2.01 1.76
75 61.5
2.37 2.24
-
28.5 -
2.00
120 145 ( 1 ) 135
-
-
-
-
115
-
time from 42 to 10 minutes. The final net weight was the same in all cases. One may consider the data shown in figure 2 as those of five investigators using methods AI, Az, B, C, and D in their individual studies of the same pectin 119F. It is apparent that AI and A2 would conclude that this particular pectin had an optimum pH of either 2.96 or 2.90. B would place the optimum for the same pectin near p H 2.8, and C about p H 2.6, while D would conclude that the maximum jelly strength is not reached without sufficient acid to give a pH of 2.0 or less, and that apparently we cannot have too much acid. It still remains true, however, that with any given procedure the acid relationship of each type of pectin is a definite characteristic which must
925
PECTIh’ STUDIES. I11
be ’ carefully considered in order to obtain best results. Evidently the factor which is most involved is the rate of gelation of the pectin as affected by the acid. The largest or “optimum” amount’of acid to be used will be that point a t which an additional increase of acid will increase the rate of setting to a point where loss in jelly strength due to a disturbance of the jelly in the stirring or pouring exactly balances the strengthening effect of that same increment of acid. Such observations emphasize the necessity for close adherence to a given empirical method of jelly making in any systematic study of the factors involved.
c” 100 W
5$
80
> -I -1
w 60
9
5 $40
ln
rk
U g 20
3.2
3.0
2.8 Zb ACIDITY A 5
24
22
pH.
FIG.2. EFFECT OF METHOD OF MAKING JELLY UPON “OPTIMUM ACIDITY” GO per cent sugar; apple pectin 119F;phosphoric acid THE TEMPERATURE FACTOR
The abnormally high jelly strengths obtained at 55°C. cannot be explained on the basis of the time concept. Apparently the usual high temperature of jelly making either destroys a large proportion of the pectin value, or the character of the gel structure obtained at the lower temperature differs from that formed from a hot mixture. For a number of years it has been a common commercial procedure to place the acid solution in the container into which the non-acidified pectinsugar-water mixture is subsequently poured. This practice was shown by Stuem-er, Beach, and Olsen (11) to eliminate the so-called optimum peak
926
AKSEL G . OLSEN
when the jellies were poured at 96°C. The mixing by pouring is almost instantaneous, but the jelly strengths did not differ greatly from those observed at optimum using the regular method. On the basis of the time factor curves shown in figure 2, it did, however, seem reasonable to expect that this method, which permits of maintaining mixing time at a minimum, should show a rise in jelly strength as the temperature of pouring is lowered from 100°C. to room temperature. Accordingly several double batches of non-acidified pectin jelly mixtures were prepared by the usual hot method. After quickly cooling the mixtures to a desired temperature, with proper adjustment for loss in evaporation, the syrup was poured into a TABLE 3 Effect o n j e l l y strength of preparing j e l l y mixture without acid, cooling to different temperatures, and pouring into glasses containing varying amounts of acid 60 per cent sugar; 2.5 g. of 119F pectin in 555 g. of jelly; acid as indicated JELLY S T R E N G T H AND pH V A L U E S OF J E L L I E S P O U R E D AT T H E FOLLOWING TEMPERATURES
&E!
E% Bn m s
g----
100°C.
90°C.
80°C.
3 E! 1 J.S. pH J.S. pH J.S. pH d s c -- - - __ __ -cc.
cc.
0366 - - - 1 0 i.0 , I 2 0.2c ..80 1 3 3 . 1 1 1 9 3 . 0 9 1 4 3.12 3 0.3C ..70 3 8 2 . 9 4 3 7 2 . 8 5 4 0 2.84 4 0.4C . . 6 0 3 9 2 . 7 4 3 6 2 . 7 0 3 2 ? 2.68 5 0.6C ..40 4 2 2 . 3 8 4 5 . 5 2 . 3 5 4 3 2.48 6 0.8C . . 2 0 3 8 2 . 2 3 4 7 2 . 2 3 4 5 2.23 7 1 . 0 ..O 3 8 2 . 1 4 5 7 2 . 1 1 4 8 . 5 2 . 1 1 8 2.0 ) 3 5 1 , 8 5 4 8 1 . 8 6 4 9 1.89 401-49 -52 8a 2 . 0 I 8b 2 . 0
60°C.
70°C.
5O0CC.
40°C. I
J.S. pH
J.S. pH J.S. pH J.S.pH J.S. pH - - - - -- _ _
-_
__ _ _
--
- 143.1313.53.21 -452.8935 3.00 - - - 4 2 2 . 6 2 4 7 2 . 6 8 6 4 2.73 6 9 2 . 8 5 4 0 2 . 4 0 4 5 2 . 4 2 8 6 2.33 352.5E 4 6 2 . 2 6 5 9 2 . 1 9 8 0 2.31 922.35 5 5 2 . 1 5 6 4 . 5 2 . 1 5 7 6 2.191002.18 6 2 1 . 8 5 6 5 1 . 8 9 9 7 1.88 931.8g
- - -
-
14.5 2.83 17+ 2.68 11.52.52 !5 2 . 3 1
--
series of eight glasses containing varying amounts of 10 per cent phosphoric acid solution plus water to make 2 cc. Rapid pouring results in efficient , mixing of acid, no stirring being necessary; in fact a t the lower temperatures the “set” was so rapid that stirring was not desirable. The results are given in table 3 and figure 3. The figures obtained are rather irregular. It should be noted, however, that each point plotted on the graph represents only one measurement on one glass of jelly, while usually such points represent the average of four jellies. Considerable variation was therefore to be expected. Nevertheless, the trend of these results is quite apparent, and it can be stated that repeated tests with other samples of apple pectin invariably showed similar differences in jelly strength between jellies poured at varying temperatures.
PECTIN STUDIES. 111
927
It remained to determine whether the low jelly strengths obtained a t lOO"C., as compared with the high values obtained at 50"C., were due to acid destruction of the pectin a t the higher temperature or to a tendency of the pectin to set differently from a hot solution than from a cool solution. In this connection it should be borne in mind that the very rapid setting at 5O"C., compared with the somewhat slow set of jellies poured at lOO"C., is unfavorable to uniform mixing of the acid and jelly mixture at the lower
3.3 42 A I
3.0 2.3 2.8 21 & 2 5 26 2.3 2.2 2.1 2.0 1.3 1.6 ACIDITY AS pli.
FIG.3. EFFECT OF TEMPERATURE A N D METHODOF PREPARIKG JELLIES UPOS SO-CALLED OPTIMUM pH AND Ma4XIMUM JELLY STRENGTH OF APPLE PECTIN 33F
temperature. One might, therefore, logically expect a less perfect jelly a t the lower temperature. This question was answered by recovering the pectin from duplicate hatches poured (a) a t 100°C. and (b) at 50°C. and remaking the recovered pectin into jellies, both of which were poured at 50°C. This procedure was repeated. I n each case the pectin from the low-strength jelly when remade into the same amount of jelly at 50°C. gave a jelly strength practically equivalent to that obtained with pectin recovered from the high3trength jelly. This is conclusive proof that the lower strength of the
928
'
AKSEL G. OLSEN
jellies prepared by pouring at 100°C. is not due to hydrolysis of the pectin by the acid. Evidently the rapid set at the lower temperatures results in a different and stronger gel structure. This is contrary to experience with gelatin, where it was found that slow setting at a higher temperature resulted in better jelly formation than quick setting at low temperatures (8). It was thought of interest to determine whether in the course of time jellies identically prepared, except for the temperature of pouring, would tend to approach the same equilibrium in jelly strength. Results indicate the contrary. Observations over a period of four weeks show the highstrength low-temperature jellies to increase in strength at a more rapid
'
TABLE 4 T h e effect of temperature o j contact with acid u p o n jellv strength and u p o n pectin giade TENPERATERE OF POURING
NO
JELLY STRENGTH*
"C.
1 2
3
4 NO.
1
TEMPERATURE oFPoURING
100 50
118 53
100 50
55 132
JELLY STRENGTH R IIH RECL.4INED PECTIN ( P O U R E D AT 50°C ) *
90 98
~
i
112 135 ~-
JELLY STREh'GTHt ~
1st day
I
3rd day
~
8th day
1
%weeks
1
4week3
131
1
122
-
"C.
5 6
100 50
35 87
39
* Each figure the average of four determinations.
t Each figure the average of two determinations. rate than the lowstrength jellies poured at 100°C. These results are tabulated in table 4. The details of the experiments were as follows: Five grams of pectin, 800 g. of sugar, and 545 cc. of water Tyere brought to a boil in the usual manner and cooked to a net weight of 1313 g. From this batch four glasses, each containing 2 cc. of 10 per cent phosphoric acid, were filled at 100°C. (total, 555 g. of jelly). The remaining syrup was cooled rapidly to 50"C., adjusted for loss by evaporation, and four glasses, each containing 2 cc. of 10 per cent phosphoric acid, were filled at that temperature. The extra syrup was discarded. After standing overnight the jelly strength was determined and identical amounts of each batch (as nearly 555 g. as possible) were cut into small pieces and steeped in 60 per cent alcohol. The alcohol was changed three times in the course of that many days. Finally the pectin jelly was drained on a cloth,
PECTIX STCDIES. 111
929
washed with strong alcohol, squeezed dry, and at once dissolved in sufficient water for the preparation of a standard batch (555 9.) of jelly. These jellies were both poured a t 50°C. The entire procedure was subsequently repeated. The results throw an interesting light upon the pectin gel formation. Apparently the structure of the jelly is fundamentally different when slowly set from the hot solution than when rapidly set from the cool syrup. If we assume that pectin exists in two states of hydration, that is, if the amount of water bound by the pectin fibrils differs depending upon the temperature a t which the pectin is precipitated, then a ready explanation is a t hand. Summing up the entire thesis one may say that in the ideal pectin jelly system each increment of acid functions in two ways, although these fundamentally may be counterparts of the same function: first, the rate of gelation is increased towards a maximum; second, the strength of the ultimate gel structure is increased towards a maximum. When in any system the time factor does not interfere with the maximum rate there mill be no optimum acidity (curve D). If, however, at some point the rate of gelation crosses the time boundary of the system, that point will be the optimum for that system because further increases in rate of gelation will be increasingly interfered with until the maximum rate of gelation is reached, at which point further increments of acid will not cause any further change. The latter accounts for the flat portion of the curves in figure 1 of the present paper and in figure 1 of Pectin Studies I (7). It is apparent that anything that may change the rate of gelation will change the position of the optimum in any given system. SUMX4RY
A colloid theory to explain the process of pectin jelly formation is formulated. It is shown that it assists in explaining and to some extent anticipating the behavior of different types of pectin. On the basis of this theory it is postulated that the so-called “optimum hydrogen-ion concentration” of jelly making may be varied by changes in time, the same as it is through changes in sugar concentration. Experimental data supporting this thesis are presented. The effect of any substance added to a pectin jelly system which registers as a change in jelly strength of that system may be due to (a) an effect on the rate of gelation, or (b) an effect on the ultimate jelly structure, or (c) a combination of these two. It is shown that apple pectin jellies rapidly prepared at 65°C. have abnormally high jelly strength as compared with jellies prepared by the short-boil method, indicating that only a fraction of the true jelly strength is obtained by the usual methods of preparing jellies. Data presented show that the original amount of pectin is recoverable
930
ABSEL G . OLSEN
unchanged from jellies prepared either at 50°C. or at 100°C. This proves that the difference in jelly strength is due not to pectin destruction at the higher temperatures but to structural differences in the pectin network forming the jelly. It is suggested that the amount of mater bound by the precipitated pectin may differ depending upon the temperature at which the precipitation occurs. Acknowledgment is due IT7. VanCamp and J. S. Kemp for assistance in the preparation of the jellies and in the determination of pH and jelly strengths. REFERENCES (1) (2) (3) (4)
(5) (6) (7) (8) (9) (10) (11) (12)
BAKER,G. L.: Ind. Eng. Chem. 18, 89-93 (1926). COLE,G. M., Cox, R. E., . ~ N DJOSEPH, G. H.: Food Ind. 2, 219-21 (1930). GOLDTHWAITE, N. E . : Ind. Eng. Chem. 1,333 (1909); 2, 457 (1910). FREUNDLICH, H.: New Conceptions in Colloidal Chemistry, pp. 75-91. E. P. Dutton and Company, New York (1927). KRUYT,H. R.: Colloids, pp. 188, 189. John Wiley and Sons, Inc., New York (1927). MEYERS,P. B., AND BAKER,C. L.: Fruit Jellies, V, p. 39. Univ. Del. Expt. Sta. Bull. No. 149 (1927). OLSEN,A. G. : Ind. Eng. Chem. 26, 699 (1933). OLSEN,A. G.: J. Phys. Chem. 36, 529-33 (1932). SINQH,LAL: Ind. Eng. Chem. 14,710 (1922). SPENCER,GENE: J. Phys. Chem. 33,1987-2011 (1929). STUETVER, R., BEACH,N. M., AND OLSEN,A. G . : Ind. Eng. Chem., Anal Ed. 6, 143-7 (1934). TARR, L. W.: Univ. Del Expt. Sta. Bull. No. 142 (1926).
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