June, 1924
INDUSTRIAL A N D ENGINEERING CHEMISTRY
595
DIMBNSIONS OF TEST PIECE AND TORSION INSTRUMENTDortionalitv of results was obtained. The chief obiection to the Smith “tester appears to be the imperfect reprohucibility The fact that test pieces held by adhesion a t the ends and the gradual of the rubber membrane. showed influence of the ends indicated that this effect was JELLY STRENQTH AND PH likely to be found with the torsion instrument. Therefore, molds of lesser and greater length than the standard (1 x The authors are not yet satisfied with the relation of 1inch) were made and the same jelly was used for comparison. jelly strength (elasticity) to hydrogen-ion concentrations. The results are shown in Table IV. It is to be noticed that there is still some end effect at 1 inch They consider that any critical values (maxima, etc.) are more likely to be smoothed out at high than a t low concen(2.5 cm.), but that it is very small. (Fig. 3) trations. A recent series using the plunger - t -w- e instrument C O M P A R I ~ O N O F PLUNGER TESTER AND S ~ ~ I TJELLY H TESTER on a 2 per cent ash-free gelatin jell;. gave results shown in Fig. 5. According to this, both of Wilson’s points of miniThe fjmith jelly strength tester and modifications6 are mum swelling show maxima of jelly strength-although a t widely used in gelatin and glue control. Its operation has p H 4.7 to 4.8 a minimum of viscosity is found.’ The authors been compared on the same jelly with the plunger type in- have been able to check repeatedly a maximum a t p H 7 to 9, strument, and, as will be seen from Fig. 4, satisfactory pro- but not with certainty a t gH 4 to 5. ’ THISJOURNAL, 16, 71 (1923). 0 THISJOURNAI,, ia, 355 (1920).
Effect of Calcium Chloride on Acid-Sugar-Pectin Gels1 By Evelyn G. Halliday and Grace R. Bailey DEPARTMENT OF HOME ECONOMICS, UNIVERSITY OF CHICAGO, CHICAGO, ILL
T
The percentages of pectin, acid, and sugar used for a standard in Part to the salt content HE purpose of this jelly were each in turn lowered to the point where an acceptable of the respective solutions investigation Was to learn what effect, if jelly failed to form in a given time. To these nonjellying mixtures used. Perhaps the widest calcium chloride was then added, and in each case gelation occurred. divergence in the results of any, calcium chloride has on Above a certain maximum amount furfher additions of calcium any two workers is found the concentration of pectin, chloride had no appreciable effect on the stiffness of the jelly, but in Some research done in acid, and sugar required for this laboratory and some tended to cause syneresis. the gelation O f fruit jellies. The writers have considwork reported by Singh.6 erable evidence that certain Using the analyzed hot salts may increase the gelation efficiency of a mixture. Tarr2 water extract of Ben Davis and Baldwin apples as a found that jelly formed a t a slightly lower hydrogen-ion source of pectin and acid and calculating the composition concentration in an apple pomace pectin solution than it did of the jelly from the weight of juice and sugar taken and in a water solution of the same pectin concentration. This of the jelly obtained, it was found that the pectin content difference, it appears, can only be attributed to the favoring of satisfactory jellies ran anywhere from 0.424 to 0.7 per action of the salts present in the apple pomace extract. That cent. At the lower pectin value, 0.424 per cent, the acid such extracts contain an appreciable amount of salts can be calculated as malic was 0.165 per cent, the sugar 63 per cent. inferred from the ash content of apples, which Sherman* Singh, however, who used lemon pectin and citric acid, places a t about 0.2 per cent of the edible portion. More- reports that it was impossible to obtain a jelly a t any concenover, the apple pomace extract used for this investigation tration of sugar a t a pectin content below 0.9 per cent with contained 3.3 per cent of ash, of which 15.8 per cent was the acid kept constant at 1.5per cent. calcium. The fact that Singh’s jellies were made with citric acid Haynes4 observed that all the alkaline-earth hydroxides while those of the writers contained chiefly malic6 could and, to a lesser degree, the alkali hydroxide induced the gela- hardly account for his high pectin requirement, because, tion of pectin solutions without the aid of either sugar or acid, although Tarr estimates citric acid to be less efficient than and that the speed of gelatin was much accelerated by the malic, the larger amount of citric used by Singh, 1.5 per various salts of the respective hydroxides. Furthermore, cent as against the writers’ 0.165 per cent, ought to more Haynes found that calcium hydroxide and its salts were more than compensate for the lower efficiency of the citric acid. effective than any of the other alkaline earths. The fact A possible difference in the jellying value of the two pectins that the weaker alkali had the greater effect, and that this may be partly responsible for the difference in results. As effect was increased by salts which would depress the ion- Denton? has observed, the alcohol precipitation method iaation of the hydroxide and thus lower the concentration of estimating pectin cannot be relied upon to give products of the hydroxyl ion, make it appear that the positive rather of uniform jellying properties. than the negative ion is the reactive agent. It is highly probable, however, that the writers’ low pectin Since, then, certain positive ions seem to favor the gelation and acid requirement was a t least partly due to the favoring of pectin, it is probable that the apparent lack of agreement action of certain salts, more especially certain positive ions, among investigators regarding the relative concentrations which were present in their apple juices. of pectin, acid, and sugar required for gelation may be due The experimental work which follows was done for the * Received January 5, 1924. 6 THISJOURNAL, 14, 710 (1922). * Delaware Agr. Expt. Sta., Bull. 184 (1923). fl Bigelow and Dunbnr, Ibid., 9, 762 (1917); Franzen and Helwert, C. 4
Sherman, “Chemistry of Food and Nutrition.” Biochem J., 8, 553 (1914).
A , 17, 290A (1923). 1
THISJOURNAL, 16, 778 (1923).
596
INDUSTRIAL AND ENGINEERING CHEMISTRY
purpose of determining the possible effect of calcium on the pectin, acid, and sugar requirement for gelation. The choice of calcium as the ion to be used was influenced by Haynes’s discovery of its efficiency in causing pectin solutions to gelatinize and also by the fact that it formed a high percentage of the ash of the fruit extract and pectins studied in this laboratory. As already stated, the ash of the apple pomace extract used for this work contained 15.8 per cent of calcium, while the ash of the pectin precipitated from this extract had a calcium content of 12.8 per cent. Of two varieties each of orange, grapefruit, and lemon extract ash, the calcium ranged from 5.7 per cent in one variety of oranges to 11.4 per cent in one variety of lemons, while the pectin ash showed a calcium content of 16.2 per cent a t its lowest in one variety of oranges, and 23.1 per cent at its highest in one varietyof lemons. EXPERIMENTAL
The general plan of procedure was to find some proportion of the given pectin, acid, and sugar which would make a desirable jelly. Such a jelly may be described as one which holds its shape, makes well-defined angles when cut, and is tender. One component after another was then lowered until a point was reached in each case where an acceptable jelly failed to form in a given time. This point having been determined for pectin, acid, and sugar, the next step was to determine what amount, if any, of the calcium salt would cause these nonjellying mixtures to form acceptable jellies.
PECTIN The pectin, or pectic substance, used for these experiments, was precipitated from a commercial preparation made from apple pomace. Different bottles of this preparation were found by quantitative analyses to contain anywhere from 3.35 to 4.64 per cent pectin. Precipitation of the pectin was carried out by adding one volume of the commercial preparation to eight volumes of 95 per cent ethyl alcohol with constant stirring throughout the operation. The precipitate thus obtained was broken up and washed with two portions of 95 per cent alcohol, after which it was dried in an electric oven a t 50’ C.
JELLY The method employed for making the jellies was uniform throughout the entire problem. The citric acid was made into a solution of such a concentration that 10 CC. of the solution contained 1 gram of the acid, calculated on the crystallized basis. The pectin, weighed from a weighing bottle kept in a desiccator over calcium chloride, was dissolved in distilled water over a water bath, after which the sugar and acid were added in quick succession, and the weight was brought up to 100 grams with distilled water. The mixture was heated over a low flame just long enough to dissolve the sugar, then brought back to weight (100 grams) by adding distilled water. The first part of the problem was to make a standard jelly using the particular pectin and citric acid that had been selected for the whole experiment. Because of the tendency observed by Singh6 for one jelly component to substitute for another, in making this standard jelly the concentration of two of the components, the pectin and acid, was fixed a t 1 per cent and the concentration of the sugar determined by experiment. The results given in Table I show that for 1 per cent of this pectin and 1 per cent of citric acid, 50 per cent of sugar was required to form an acceptable jelly in the given time, which for convenience was placed a t 18 hours. I n an occasional experiment, specifically Experiments
Vol. 16, No. 6
1 (a), 5 (a), and 6 (a),the pectin, after having been dissolved, was thrown out of solution and settled throughout the mixture in small lumps. Why this happened in some cases but not in others we are unable to say, but probably because of some unintentional difference in manipulation. Every effort was made to standardize the process throughout, but it is possible that the heating was not always uniform nor the rate of stirring the same. Such differences in manipulation might in some instances have resulted in undue agitation a t a critical point which, according to The Svedberg, is sufficient to cause the precipitation of colloidal particles. It will be observed that the same tendency is shown in some of the jellies of Tables IV and VI. TARLF:I-SUGAR REQTIIREMENT FOR 1 PERCBNT PECTIN AND 1 PERCBNT CITRIC ACID Sugar CONDITION O F JELLY AT END OF Expt. Per cent 18 HOURS 1 (a) 45 Pectin precipitated (bl Thick sirup (C) 2 (a) Minimum jellying proprrties 3 (( ba )) (hl 47 4 (a) Jelly, hut too tender to cnt well (bJ 49 Pectin precipitated 5 (a) (b) Jelly, but too tender to rut well (C) Pectin precipitated 6 (a) 50 Good jelly
2; ]
2; ]
Itl
gg ]
EFFECT OF CALCIUM CHLORIDE WHENPERCENTAGE OF SUGAR Is LOWERED It was the purpose of this part of the experiment to find out whether a solution too low in sugar to form jelly could be made to do so by the addition of calcium chloride. TABLE 11-EFFECT
OF CALCIUM CHLORIDE WHENPERCENTAGE OF S ~ ~ G A R Is LOWERED (1 per cent pectin, 1 per cent citric acid, and 26 per cent sugar) OF JELLY AT ENDOF Jelly CaCln CONDITION Number Per cent 18 HOURS 0 1 (Q) o ,. 0 Minimum jellying properties (h) 2 (a) Good jelly, best of series
(b)
:$
3 (a) (h) 4 (a) 5
;:[
(b)
2.0 2.0
]
1
All of practically the same consistency and all slightly stiffer than’ Jelly 2
The percentage of sugar chosen was 46 per cent, which, as Table I shows, was insufficient to form jelly with the standard proportions of acid and sugar in the time allotted. I n order to check the results of Table I and to make sure that the conditions of this experiment as a whole were as nearly constant as it was possible to make them, two more jellies were made without calcium chloride, using the same proportions of ingredients as in Experiment 2, Table I. Immediately afterwards other jellies were made, similar in all respects except for the addition of calcium chloride. Chemically pure, fused calcium chloride was used- throughout the experiment, and from i t a solution was made of such a concentration that 10 cc. of solution contained 1 gram of the salt. This solution was added in such amounts that the calcium chloride content of the final solutions ranged from 0.5 to 2.0 per cent, with intervals of 0.5 per cent between successive jellies in the series. Table I1 shows that the calcium salt was effective in bringing about gelation a t a concentration of sugar too low for jelly a t the given pectin and acid content. This table also shows that above a certain optimum amount calcium chloride has no decided effect on gelation, since the jellies containing 2 per cent of the salt were but slightly stiffer than those containing but 0.5 per cent.
EFFECT OF
597
INDUSTRIAL A N D ENGINEERING CHEMlSTRY
June, 1924
CdLCIUM CHLORIDE W H E N PERCENTAGE O F ACID
Is LOWERED Here the object of the experiment was similar to that of the preceding one, except that calcium chloride was substituted for a portion of the acid instead of a portion of the sugar. A preliminary experiment was made to determine the quantity of acid required for minimum jellying properties with 1 per cent pectin and 50 per cent sugar. As before, the standard jelly with 1 per cent pectin, 1 per cent acid, and 50 per cent sugar was used as a check. TABLE 111-EFFECT
O F LOWERING PERCENTAGE O F Acm WITH 1 PER CENT PECTINAND 60 PER CENT SUGARAS CONSTANTS Jelly Acid CONDITION OF JELLYAT ENDO F Number Per cent 18 HOURS 1 1.0 Good jelly 0.5 2 (a) Jelly, but too tender to cut well 3 (b) (a, Soft jelly, collapsed (b) 0.4
::
E‘;
11
TABLE VI-EFFECT
CALCIUM CHLORIDE WHEN PERCENTAGE OF PECTIN Is LOWERED (0.5 per cent pectin, 50 per cent sugar, 1 per cent citric acid) Jelly CaCh CONDITION OF JBLLYAT ENDO F 18 HOURS Number Per cent 1 0.0 Very soft jelly, failed to stand Pectin precipitated 2 (a) 0.5 Soft jelly, collapsed 1.0 Good jelly, the best of the series 1.0 Pectin precipitated
Minimum jellying properties
Table I11 shows that a minimum of jellying properties was obtained with 0.3 per cent acid. This concentration of acid was therefore chosen for the experiment with the calcium salt, the results of which are given in Table IV. From the table it can be seen that calcium chloride can be used as a substitute for a part of the acid, and also that in this experiment, as well as that in which the sugar was lowered, optimum results were obtained with 0.5 per cent of this salt. T A B L E IV-EFFECT
OF CALCIUM CHLORIDEWHEN PERCENTAGE O F ACID I s LOWERED (1 per cent pectin, 50 per cent sugar, 0.3 per cent citric acid) Jelly CaCln CONDITION O F JELLYAT ENDOF Number Per cent 18 HOURS 1 0.0 Minimum jellying properties 2 3 4 0.3 ) 5 0.4 Jelly, but too tender t o cut well 6 (a) 0.5 Soft jelly, heavy pectin precipitate Good jelly, best of the series 7 (a) 1.0 Good jelly 1.0 Good jelly (b) White froth on top, otherwise 1.0 (6) good jelly
8
It is interesting to note (Table V) that as the pectin decreased a point was reached where syneresis, or “weeping,” began. This is in accord with Tarr’s results, for he found that syneresis was caused by excess hydrogen ion, and he also found that pectin lowers the hydrogen-ion concentration of a solution, or, in other words, acts as a “buffer.” Here, then, apparently a decrease of the pectin content allowed the hydrogen-ion concentration to increase to a point high enough to cause syneresis. The dividing line seemed to lie between 0.5 and 0.4 per cent pectin; hence the former quantity was chosen for the experiments with calcium chloride. This quantity, it will be observed, is too low for jelly yet high enough to prevent syneresis.
g: E }
OF
g:;
}
’
\
“
1 .. 50 2
Fair jeIfy, syneresis
Table VI shows that calcium chloride can be used as a substitute for part of the pectin as well as for a portion of the sugar and acid. It is also to be observed from this table that the optimum effect is here produced by 1.0 per cent of the calcium chloride, in place of 0.5 per cent asin the preceding experiments. Another point of interest shown by Table VI is that when excess calcium chloride, 1.5 per cent and over, was used there was a marked tendency for syneresis. If syneresis is always accompanied by high hydrogen-ion concentration, the calcium chloride has evidently acted to increase the hydrogen-ion concentration of the solution. Such a n action, according to Haynes,* is the normal result of adding a neutral salt to a buffer mixture. SUMMARY AND CONCLUSION
The concentration of pectin, acid, and sugar required for gelation is somewhat lowered by the presence of calcium 2.0 chloride. This leads to the conclusion that certain inorganic elements present in fruit extracts may be responsible for the 2.0 (b) 2.0 ] (6) fact that gelation occurs a t a lower pectin content in such extracts than it does in a water solution of pectin. EFFECT OF CALCIUM CHLORIDEWHEN THE PERCENTAGEAbove a certain maximum, which varies between 0.5 and OF PECTIN I s LOWERED 1.0 per cent, further additions of calcium chloride have no appreciable effect on the stiffness of the jelly, but may cause The purpose of the last experiment in the series was to syneresis. determine the effect of substituting calcium chloride for a Two possible theories are offered to account for the action part of the pectin. The method of procedure was the same of calcium chloride on gelation. One is that the calcium as that in the preceding seriesrthe pectin was first lowered forms a salt with the pectin and the salt thus formed has a to the point of minimum gelation, and then, using this concen- greater tendency to gelatinize than does the original pectin. tration of pectin and keeping the acid and sugar constant, In favor of this theory is the fact that the pectin used for at 1 per cent and 50 per cent, respectively, calcium chloride these experiments showed acid properties, a 2 per cent soluwas added in increasing amounts up to a maximum of 2 per tion being found to have a hydrogen-ion concentration of cent. Table V shows the results of lowering the pectin, 3.5, expressed as a p H value, by both the electrometric and Table VI the effect of adding calcium chloride. indicator methods of determination. A second possibility is that the calcium chloride acts TABLE V-EFFECT OF LOWERING PERCENTAGE OF PECTIN WITH 1 PER CENT ACIDAND 50 PER CENT SWGAR AS CONSTANTS simply to increase the hydrogen-ion concentration of the soluJelly Pectin CONDITION OF JELLYAT ENDO F tion by lowering the buffering action of the pectin. In 18 HOURS Number Per cent support of this theory is the fact that when the pectin con1 0.8 Jelly, but too tender t o cut well 0.6 Soft jelly, collapsed 2 tent was low, excess calcium chloride caused syneresis, 2 (a) : E \ Very soft jelly, failed to stand (.b,) which Tarr found normally occurred when the hydrogen 3 Minimum jellying properties, ion was increased beyond a certain value. 4 (a) E syneresis
]
8 (( ba)) 9 ia)
5
(b)
0.3 0.2
1
Good jelly Unstable jelly, heavy pectin precipitate Good jelly
Sirup
8
Biochem. J . , 15, 443 (1921).
