II. Sugar-A cid-Pectin R elationships and T heir B earings upon R outine E valu atio n of Apple P ectin
R.
St u e w e r,N. M.
Be a c h, a n dA. G.
Ol s e nGeneral Foods Corporation, B attle Creek, Mich., and F airport, N . Y.
The interrelationship of acid concentration, sugar concentration, and pectin concentration in apple pectin jellies is outlined, and il is shown that the opti
mum p H of apple pectin jellies prepared by the usual method varies with the concentrations of acid and of pectin. The changes in the position of the opti
mum p H with variation in pectin concentration change the slope of the logarithmic jelly-strength- peclin-concenlralion curves to such an extent as lo
render exact evaluation difficult.
Apple pectin and citrus pectins of the same com
mercial grade, or of different grades but used in equivalent amounts in standard jellies, do not result
I
N A N E A R L IE R publication (3) Olsen presented a simple routine procedure for th e evaluation of citrus pectin, in which th e acidity was m aintained constant. W ith apple pectins it is, however, necessary to prepare test jellies a t th e so-called optim um pH . Where several samples prepared by th e same m ethod are to be compared, this is usually accomplished by adding predeterm ined standard am ounts of acid. T he acid requirem ents, however, change with the age of th e sample and w ith th e m ethod of preparation, hence when the history of th e sample is unknown, it is necessary to deter
mine th e optim um acidity. T his m ay involve an extensive series of jellies. A m ethod which would obviate such prior determ ination should therefore prove of considerable help in th e evaluation of samples for routine or research purposes.
The well-known d a ta of T a rr (6) and B aker (1) suggest such a high degree of sensitivity of apple pectin towards moderate changes in acidity th a t the routine m ethod proposed for citrus pectins cannot be expected to be applied w ithout modi
fications in the evaluation of apple pectins. T he d a ta of these workers, in so far as they relate to th e optim um pH of apple pectin jellies prepared by th e usual “ h o t” m ethod (5), are in essential agreem ent with unpublished d ata accum ulated in th e authors’ laboratories over a period of years.
in the same gelometer readings. The elasticity of the apple jellies prevents a sharp break and results in high readings on the instrument and fo r this reason the standard curves previously given fo r citrus pectin jellies are not suitable for evaluating apple pectin jellies. A n average curve for 100-grade apple pectin indicates a value of 50 Tarr-Baker units for 2.6 grams of pectin in 555 grams of jelly.
A method of preparing apple pectin jellies which apparently obviates most of the difficulties of m ain
taining optimum acid conditions and which per
mits ready routine comparisons of samples is de
scribed.
The general procedure described by Olsen (3) was used in obtaining the d a ta here presented except for such modifica
tions as are m entioned specifically. Two typical jelly- strength curves for alcohol-precipitated apple pectin jellies containing 60 per cent of sugar are shown in Figure 1. Id en tical series were ru n w ith lactic and ta rta ric acids, using a newly prepared sample of pectin. The two curves coincide closely. Definite pan gelation or curdling occurred in each case a t a p H of ab o u t 2.55. The authors’ observations invari
ably indicate a progressive change in th e optim um p H towards higher acid requirem ent during storage of samples a t ordinary tem peratures. Studies were subsequently undertaken to ob
serve th e interrelationships of acid, pectin, sugar, and tem perature over a range sufficiently wide to perm it drawing general conclusions and if possible provide a basis for the simplification of th e stu d y and evaluation of samples of apple pectin w ith varying histories.
Sü o a b-Ac id Re l a t io n s h ip
Spencer (6) has a t some length reviewed the more im por
ta n t d a ta and current hypotheses concerning th e roles of sugar and acid in pectin jellies. In 1922 Singh (4) showed th a t w ith citrus pectin th e am ount of sugar necessary barely
144 A N A L Y T I C A L E D I T I O N Vol. 6, No. 2
3.3 3 2 3.1 3.0 ¿3 & ¿7 Zb ACIDITY A 5 .pH
Fi g u r e 1. Ef f e c t o f pH o n Je l l y St r e n g t h o f Ap p l e Pe c t i n Je l l i e s
U su al sh o rt-b o il m eth o d
to form a jelly varied inversely with the acid concentration.
Although no attem p t was m ade to determ ine either pH or optim um acidity for any given sugar concentration, one m ight reason from Singh’s d ata th a t the so-called optim um pH would change w ith variation in sugar concentration. In 1924 T arr and Baker (7), on the basis of d ata in their Table I, concluded th a t the greater th e hydrogen-ion concentration th e more sugar m ay be added (pp. 10, 19, and 20). Y et on page 8 of
the same bulletin they stated
3 5 0 3Z5 300 275 Z.50 A C I D I T Y A S p H . Fi g u r e 2 . Ef f e c t o f Ad d i t i o n o f Ac i d o n Su g a r- Ca r r y i n g Ca p a c it y o f Pe c
t i n o, C ap acity increased b, C apacity decreased
th a t “ the sugar concentra
tion could be decreased m a
terially as the hydrogen-ion concentration was increased.”
I t was difficult to reconcile th e s e tw o statem ents, th e first directly opposite to the view of Singh quoted above, and the second in complete accord with th a t view. An explanation of the first state
m ent may, however, be de
r iv e d fro m a s u b s e q u e n t bulletin by T a rr (<?), where an optim um pH curve is given for jellies containing 70 per cent of sugar and acidified with sulfuric acid. These jellies appear essentially to duplicate those described in the earlier publication. An optim um pH of about 3.05 is shown.
The acidities used in T a rr’s earlier work were pH 3.37,3.23, and 3.10. I t is a t once apparent from a stu d y of this optim um curve why an increase in acid from 3.37 to 3.23 and from 3.23 to 3.10 perm itted the pectin to hold more sugar. Each incre
m ent of acid brought th e pectin nearer to its optim um strength. If, on th e other hand, th e acidities selected had been on th e other side of pH 3.05, say from 3.00 to 2.75, the reverse conclusion would have been reached. T a rr’s opti
mum curve and the m anner in which it serves to explain w hat would otherwise appear as an inconsistency of statem ent in his earlier publication are shown in Figure 2 .
The effect of sugar concentration upon the acid require
ment, with pectin concentration m aintained constant, was studied in these laboratories w ithin th e range of 66 to 56 per cent of sugar. Six series of jellies were prepared, using th e general procedure described by Olsen (3), except th a t alco
hol-precipitated apple pectin was used instead of citrus pectin.
For each jelly, 50 grams of a concentrated apple pectin solution (Certo) were used. The normal pH of this solution is close to 3.0.
Lactic acid was added to obtain the lower pH values and calcium
carbonate to obtain the higher values. The effect of the calcium carbonate itself was separately tested and even when added in excessive amounts, followed by proper adjustment of pH, did not appear to be greater than the experimental error of the jelly- strength determinations. The net amount of jelly made in every case was 555 grams and the amount of distilled water varied with varying pectin, acid, and sugar content to give a gross of 585 grams, allowing 30 grams for evaporation in making the jelly. As usual four glasses were poured, and the average of the four determinations on the gelometer recorded as the jelly strength. These are shown in Figure 3.
The pH was determined on the finished jelly following the jelly-strength determinations. The apparatus used for pH de
terminations on jellies consists of the usual Leeds and Northrup type K potentiometer, type R galvanometer, saturated potassium chloridc-calomel half-cell, and gold wire electrode. About 10 grams of jelly are introduced into a small wide-mouth bottle (about 25 cc. capacity) and brought to the same temperature as the calomel half-cell. A small amount of quinhydrone is added, rapidly stirred in with the electrode, and the reading taken at once. The only precautions found necessary are those of adjusting the temperature and adding sufficient quinhydrone to give a permanent brown color.
The readings are corrected for the temperature a t which they are taken and the pH is calculated. A very convenient table of correction factors used a t the Bureau of Dairy Industry was kindly supplied to the authors through the courtesy of E. O.
Whittier.
One of th e authors (Beach) has m ade a com parative study of pH determ inations of the_ pectin-acid solution and of the jellies m ade therefrom . T he determ inations m ade upon the finished jellies were more consistently in accord w ith the variations observed in jelly strength, although differences were slight.
Curves I, II, I I I , and V in Figure 3 show clearly the rela
tionship of sugar concentration to optim um acidity in jelly form ation. All th e curves have th e sam e general shape, rising w ith increasing acid to an optim um and dropping off upon further addition of acid. T he optim um jelly strength is about the same in each case b u t occurs a t a higher pH as th e sugar is increased. T he 66 per cent curve is im perfect;
subsequent observations lead to th e belief th a t one point is in error and th a t th e true curve would follow th a t indicated by a light dotted line. T he curves for series IV and V are less uniform th a n th e others, probably because of th e charac
te r of the jellies obtained. In series V (56 per cent sugar)
Fi g u r e 3 . Ef f e c t o f Su g a r Co n c e n t r a t i o n o n Op t i m u m pH f o r Ap p l e Pe c t i n Je l l i e s
a jelly was obtained which m ay be described as very tender and crystalline,1 w ith a tendency to break a p a rt very easily when removed from th e glass. In series IV (high-sugar) the jellies were salvy to th e touch and when turned from the glass showed a structure which was n o t crystalline and did n ot break a p a rt easily. T he plunger of the gelometer did not
1 B y “ cry stallin e” je lly . is m e a n t a jelly showing a sh a rp cleavage and a b rig h t sm o o th surface where c u t or b ro k en a p a rt. O pposite of " s a lv y .”
I N D U S 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 145 break through the surface of these high-sugar jellies sharply,
b u t a t tim es sank into the jelly so slowly th a t it was difficult to obtain exact readings. There was a gradual change in texture throughout the several series from low- to high-sugar concen
tration, although between series I, II, and I I I the differences were not pronounced. T he curves in Figure 3 suggest the im portance of adjusting the pH separately for each selected sugar concentration.
The results one m ight ob
tain b y m a i n t a i n i n g th e acidity constant b u t varying th e sugar concentration are apparent from th e same data.
Thus, taking th e values a t which th e v a r i o u s c u r v e s cross th e pH 3.0 line, the re
sults are as follows:
Su g a r Je l l y St r e n g t h
%
56 16
5 8 .8 44
60 56
63 28
66 < 2 0
P E C T I N , GMS.
F i g u r e 4 . E v a l u a t i o n o f A p p l e P e c t i n b y S i i o r t - B o i l O p t i m u m pH M e t h o d Sam ple N. B . 8 — 100 X —— —
These figures would indi
c a t e a n o p tim u m s u g a r concentration of 60 per cent, b u t this apparent optim um s u g a r c o n c e n t r a t i o n is
127 g rad e s i m p 1 y a n o t h e r m anifesta
tion of the c h a n g in g o p t i m um pH , sugar and acid w ithin lim its being compensating factors in the form ation of th e jelly. I t is apparent th a t one m ight reach erroneous conclusions from an isolated series like th e above.
Ev a l u a t i o n o f Ap p l e Pe c t i n s
Apple pectins m ay be evaluated exactly as indicated for citrus pectin except th a t it is obvious from a stu d y of the above curves th a t the acidity m ust be closely adjusted to the optim um . The same logarithm ic curve suggested as a standard of comparison for citrus pectin (3) has, however, been found unsuitable for evaluating apple pectin. When citrus and apple pectin jellies are compared on the basis of their commercial values (grade based on “ finger te st”), the gelometer readings for the apple pectin jellies are invariably much higher than the others. This undoubtedly is due to the elastic properties of the apple jelly which perm it it to stretch considerably before the plunger is finally forced through.
On the basis of a c o m p a r is o n o f a n u m b e r o f c o m mercial samples the authors have found i t c o n v e n i e n t to d e f in e 100-g ra d e apple pectin as a pectin of which 2.6 grams in 555 grams of 60 per cent jelly p r e p a r e d b y t h e standard m e th o d and a t the optim um p H w ill r e g i s t e r 50 cm. pressure on t h e T a r r - B a k e r gelometer. (Some d if f e r e n c e s have been o b s e r v e d in readings obtained w ith different in
strum ents; th e s e were largely caused by slight variations in size of th e plunger
and of the plunger cap. E ach instrum ent should be checked against a known sample used as standard.)
A series of five jellies was made with the standard formula and procedure b u t using different am ounts of apple pectin (sample N . B. 8) with the same am ount of acid in each case.
This resulted in jellies varying in pH from 2.91 to 3.10. W hen the weight of the pectin was plotted against gelometer read
ing on a logarithmic scale th e m ean of the points gave a straight line. There was, however, n o t very good correspond
ence in the points. A second series was therefore m ade up varying the am ount of acid added to give closer agreem ent in pH values. Table I gives th e figures obtained in this series.
T a b l e I . E f f e c t o f P e c t i n C o n c e n t r a t i o n o n J e l l y S t r e n g t h
(p H appro x im ately co n sta n t)
Fi g u r e 6 . Ef f e c t o f Sh i f t i n Op t i m u m pH w i t h Va r y in g Pe c t i n Co n c e n t r a t i o n o n Sl o p e o f Lo g a r i t h m i c JELLY - S T R E N G T II-PE C T IN -
Co n c e n t r a t i o n Cu r v e P lo tte d from d a ta in F ig u re 5
Pb c t i np e r 10 P e r C e n t Ge l o m e t e r pH o f Fo r m u l a Ta r t a r i c Ac i d Re a d i n g Je l l y
Grams Cc.
1.4 2 .0 2 2 .3 2 .0 6
1.8 2 .2 3 7 .8 2 .9 7
2 .2 2 .4 5 8 .0 2 .9 9
2 .6 2 .6 8 1 .2 3 .0 8
A straight line is obtained by plotting these points logarith
mically, as shown in Figure 4, indicating th a t th e logarithm of the strength of apple pectin jelly is directly proportional to th e logarithm of th e pectin concentration. T he average curve for 100-grade apple pectin is shown on the same graph for comparison. A t a jelly strength of 50 th e following rela
tionship obtains:
100 x m 1 2 7 g r a d e
F i g u r e 5 . E f f e c t o f P e c t i n C o n c e n t r a t i o n o n O p t i m u m pH f o r A p p l e P e c t i n J e l l i e s a. 2.0; b, 2.4: c, 2.8 gram s of 33 F pectin for 555 gram s
of jelly. C itric acid used
Ef f e c t o f Pe c t i n Co n c e n t r a t i o n o n Op t i m u m Ac i d i t y
T he above m ethod, although workable, is complicated by the necessity of first determ ining th e optim um pH for the par
ticular sample of pectin under observation. A second dif
ficulty became app aren t in th e comparison of logarithmic graphs prepared from several sets of d a ta on one sample, each varying slightly in the acidity of th e jellies. In each case close approxim ation to a straig h t line was obtained b u t the slope of the curve wa3 found to v a ry w ith each small change in acidity. Inasm uch as th e grade is figured on th e basis of am ount of pectin to give a jelly strength of 50 th e slope of the
146 A N A L Y T I C A L E D I T I O N Vol. 6, No. 2 curve becomes im portant when extrapolation from lower val
ues is involved.
Fi o u h e 7 . Co m p a r i s o n o f Sh o r t- Bo i l a n d Ex c e s s- Ac i d Me t h o d s w i t h Ap p l e Pe c t i n
Je l l i e s
The observed variation of slope would be explainable either by th e optim um pH varying with increased pectin concentra
tion, or by a variation, w ith difference in pectin concentra
tion, of the ra te a t which jelly strength falls off on either side of the optim um. Accordingly acidity-jelly-strength d ata were obtained for three concentrations of pectin, w ith all other factors m aintained constant, and are shown in Figure 5.
A pparently the optim um pH for th e highest pectin concentra
tion is about 3.02, while it is close to 2.90 for th e lowest con
B akers’ jellies m ade with commercial apple pectin or pectin concentrates m ay contain am ounts of acid considerably in ex
cess of optim um and y et show excellent jelly strength. Usu
ally these jellies are
2, A lcohol-precipitated (C erto), 5.2 pectin, 108 grade.
scribed for th e testing of citrus pectin.
T w o cubic centim eters of 12.5 per cen t citric acid are pipetted into each of four glasses. Three hundred grams of sugar are weighed into one 400-cc. beaker and 33 grams into another beaker of the same size. T he smaller am ount of sugar is m oistened with a few drops of water. T he desired am ount of pectin is thoroughly mixed w ith the m oist sugar and 236 cc. of water are then added.
T he mixture is stirred to facilitate solution of the pectin. T he solution is heated and the sugar added exactly as suggested for citrus pectin. Final net w eight is 547 grams of a jelly containing 60 per cent of sugar. After skim m ing the temperature should be close to 96° C., at which temperature the sirup is poured prom ptly.
Because of the rapid setting the glasses m ust be skim m ed a t once.
Where a pectin concentrate is used, th e sam e procedure is followed except th at the entire am ount of sugar m ay be weighed in to the to th e logarithm ic evaluation of three apple pectins of very dif
ferent history. T he values obtained by the usual m ethod
O ptim um Excess O ptim um Excess O ptim um Excess
p H acid p H acid pH acid
m ethod m ethod m ethod m ethod m ethod m ethod
Grams Grams Grams sufficiently close to serve for m ost purposes where an estim ate of pectin grade or of to ta l jelly u nits is desired.
tained by this excess-acid m ethod over a wide range of pH.
U n i t e d S t a t e s F o r e i g n T r a d e i n N o n m e t a l l i c M i n e r a l s 330,213,675 in 1932 and §36,078,287 in 1933. The 1933 trade was, however, below th at for 1929. T he 1933 export figure was 42 per cent of 1929, and im ports were only 23 per cen t of th at period. M ost of the individual export item s in 1933 shared in the upward m ovem ent.
A Shearing Disk Plastometer for
be sufficiently simple in operation to be used by an unskilled
plastom eter th a t would be as simple and as quick in operation as it could be and still rem ain accurate. The shearing plastome
te r described below was th e result.
Sh e a r i n g Pl a s t o m e t e r
Obviously, a quick plasticity m easurem ent requires quick heating of th e sample to a standard tem perature, which in tu rn requires th a t the sample m ust be small in a t least one direction. T his obvious fact was taken as th e first guiding principle in designing a more satisfactory plastom eter; and it appeared m ost desirable th a t the sample should have the form of a thin sheet.
T he next point to be settled was: W hat sort of deformation should th e sheeted sample be given in measuring its softness or hardness? Compressing the sample to a still thinner sheet did not appear promising, according to previous ex
perim ents w ith certain modifications of th e compression type of plastom eter. A pparently th e only practical alternative
gested, to produce shearing in a sample of rubber and prevent slipping a t the surfaces: (1) T he tangentially moving surfaces in contact with the rubber m ust be sharply corrugated or rubber by mechanical means which force and hold th e rubber in place and a t th e same tim e perm it air to escape.
In accordance w ith th e above principles two successful
shearing plastom eters have been developed, one in th e cylindrical form and one in w hat is essen form together a closed cylindrical chamber. The rotor is held vertically centered in the stator chamber by means of a shoulder on the rotor shaft and b y the pin, 4, in the center of the chamber roof. T he top and bottom surfaces of both rotor and stator arc covered with a rectangular array of sharp points, produced by cross-hatching the surface with shallow V-shaped grooves. The sides of both rotor and stator
cm. (400 to 800 pounds per square inch). Any deviations from the standard frequency in the alternating current supply to th e synchronous m otor would cause corresponding devia called the shearing viscosity, is proportional to the m ean absolute viscosity o f the sam ple.