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NORTHWESTERN
UNIVERSITY
LIBRARY
EVANSTON
ILLINOIS
AfTtllflt
Contribution on the Chemistry and
Physics of Jelly-Making.
By N. E. Goldthwaitc.
rf':
[Reprinted from the Journal of Industrial and Engineering Chemistry',
Vol. 1, No. 6. June. 1909.]
CONTRIBUTION ON THE CHEMISTRY AND
PHYSICS OF JELLY-MAKING.
By N. E. Goldtbwaitb.
Received February 18, 1909.
The object of these experiments has been to
determine as exactly as possible the conditions
necessary for the formation of fruit jellies. These
conditions are doubtless well known to the manu-
facturers of fruit products, but diligent search of
chemical literature has failed to disclose articles
dealing directly with the subject in hand.
Bigelow's admirable review* of the work done on
the chemistry of pectin bodies, emphasizes the fact
that as yet little has been definitely ascertained
concerning the constitution and function of these
compounds. The experimental results indicate in
general that these pectin bodies are carbohydrates.
Herzfeld^ regarded them as combinations of araban
and galactan, and Tollens' has suggested that they
may be glucosides.
Though the constitution of these bodies is so far
from being determined, yet their fundamental
physical characteristic—that of forming jellies under
certain conditions—has long been well known
practically. Our quest in the present work is to
find the conditions under which this gelatinizing
power best manifests itself, i. e., the conditions
under which pure fruit jellies of the best texture
are formed. Bigelow's arbitrary method for making
jellies* by boiling together equal parts of fruit-juice
(more or less diluted with water) and of cane-sugar,
for 20 minutes, does not, in our experience, neces-
sarily yield jellies of a uniformly good texture.
By a jelly of good texture we mean one obtained
by so treating fruit juice that the resulting mass
will quivir, not flow when shaken—one which can
be cut easily with a spatula, the angles so produced
' Bureau of Chem., Bull. 94* 67-87.
2 Zts. Ver. d. Zuchr. Ind. 41, 295. 667 (1891).
286, 292 (1895).
* Jour. Am. Chem. Soc., 23> 347. Bureau of Chem,, Bull. 002 30.
(I)
preserving their shape when a piece is isolated,
and yet one which is not tough.
I. Method of Procedure.—Since the present
work was begun in September, 1908, the jellies so
far investigated have been made from fall fruits
only. In extracting the juices for these jellies
any unsound portion of the fruit was removed and
the sound portion cut into small pieces if possible.
The fruit so prepared was barely covered with
cold distilled water (about i liter, or less, to 2 kgs.
of prepared fruit) and the whole brought slowly
to the boiling point and kept simmering until the
fruit was thoroughly cooked. The mass was then
removed to a filter made of a double fold of cheese-
cloth and allowed to drain completely. It was
found that the pulp remaining on the filter could
be again boiled up with water (the least amount
possible) and that the resulting juice would make
a good jelly. This was particularly true in the case
of grapes, even the fourth and fifth extractions
making an excellent quality of jelly—a quality
better than that made from the first extraction
because potassium acid tartrate crystals did not
appear in it as is usual in the case of jelly from
grapes.
Naturally, considering the nature of our problem,
we followed no set method for making our jellies,
but varied our methods within wide limits, e. g.,
we varied the acidity of the fruit juice from a neutral
condition to 0.669 cent, of acid; the amount
of sugar used from none to 2 volumes per i volume
of juice; the time of boiling sugar with juice from
the whole time of cooking to the last five minutes
only. The total time of cooking was always limited
to the time wHen we obtained a good jelly test,
i. e., when a portion of the hot liquid would jell
as it was dropped from a cold metal spatula.
II. Necessary Constituents of Jelly, (a) Pectin.—
Braconnot, who first investigated pectin, pointed
out^ that very palatable jellies could be produced
from the alkaline pectates which he isolated from
carrots by treating the latter with alkalies. He
made jellies by dissolving these alkaline pectates
1 Ann. de Chim. et. de pkys. [2], 38, 173 (1825); Ibid. [2], 30. 96.
(2)
in water, heating with sugar and then adding a
small quantity of a mineral acid.
Although the jelly-maker never handles directly
such alkaline pectates or even pectin itself, yet it
has long been recognized that the first essential
constituent of a fruit juice for making jelly is this
latter body—pectin. Its presence in a given juice
is readily ascertained by adding to a portion of the
juice an equal volume of alcohol, shaking and cooling;
a gelatinous precipitate filling a considerable portion
of the total volume of the liquid indicates pectin.
Good jelly-making consists in so controlling con-
ditions as to cause this pectin to be precipitated
evenly throughout the total volume of liquid al-
lowed it. That it can be precipitated by alcohol,
various metallic salts, acids and sugar was dis-
covered by Braconnot.
(h) Add.—The second essential constituent of
fruit juice for making jelly is acid. This fact
miy be proved in a twofold manner. First,
negatively, e. g., when the acid of a primarily good
jelly-producing juice is neutralized, no jelly can
be made from the resulting juice although it con-
tains plenty of pectin; second, positively—if a
juice which contains pectin, but which in its natural
condition will yield no jelly, be acidified properly,
a good jelly may be produced.
Negatively, the necessity of acid in a jelly-making
juice was proved as follows: A given quantity of
crab-apple juice was divided into three equal por-
tions. Of the first portion an excellent grade of
jelly was produced, using an equal volume of sugar,
boiling the juice lo minutes before the addition of
the sugar and 20 minutes thereafter, i. e., till the
jelly-test was obtained. To the second portion of
juice a sufficient quantity of N/io NaOH was added
to just neutralize the acid present, and then an
attempt made to make jelly as above. Result, a
dark cc^ored thick, fruit syrup which never gave
any signs of forming a jelly, but from which cane-
sugar crystals were slowly deposited. To the third
portion of juice sufficient N/io NaOH was added
to neutralize half the acid present, the subsequent
treatment corresponding to the above. Result,
(3)
a thick fruit syrup which never showed any signs
of jelly formation although at the end of four
months pectin could be precipitated abundantly
by the addition of alcohol.
Experiments corresponding to the first and second
of these cases were carried out with grape juice,
with exactly analogous results. That the juices
contained plenty of pectin is evident from the fact
that in their natural state they easily yielded
jellies.
These experimental results are summed up in
Table I:
Table I.
Time of
Proper* boilins:.
Per cent.
tion of
of acid
cane
Before
After
Experi-
in juice
susrar
adding adding
Texture
men! Kind of
sp. gr.
(calc. as
to juice sugar.
sugar.
of
No. fruit.
of juice.
H2SO4).
by vol.
Min.
Min.
jelly.
3 Crab-apple
1.043
0.550
1 : 1
10
10
Excellent
6 Crab*apple
1.043
0.000
1 : 1
10 -
10
Syrup
7 Crab-apple
1.043
0.275
1 : 1
10
10
Syrup
126 Grape
1.025
0.313
1 : 1
30
10
Excellent
127 Grape
1.025
0.000
1 : 1
30
10
Syrup
Nos. 5 and 126 represent respectively crab-apple
and grape juices in their natural condition from
which excellent jellies were easily prepared. Nos.
6 and 127 represent these same juices after neutrali-
zation from neither of which could jelly be made.
Though No. 7 represents only a half-neutralization of
crab-apple juice yet this also refused to yield jelly.
It should be noted that the low sp. gr. and acidity
of the grape juice used in these experiments is due
to the fact that this juice was composed of the
later extractions of the pulp. On account of this
low sp. gr. a considerably longer time for boiling
was necessary, t)ut for reasons shown further on,
this preceded the addition of the sugar.
Positive proof concerning the necessity of acid
for the formation of jelly may be cited as follows:
Sweet apple, pear and peach juices contain an
abundance of pectin as is evident from the alcohol
test, yet under ordinary conditions it was found
impossible to produce jellies from these juices.
However, addition of an organic acid to each juice
so modified conditions that a very fair jelly could
be made as shown in Table II:
(4)
Table II.
Proper-
Add- tion of Time of
ity cane- boiling.
Natural of sugar « * —■
% of
juice
to
Before After
1' ■
acid in
after
juice
add-
add-
Expeti-
Sp. gr.
juice
addition
by
ing
ing
Texture
ment
Kind
of
(calc. as
of
vol-
sugar.
sugar, of
No.
of fruit.
juice.
HjSO.).
acid.
ume.
Min.
Min.
jeUy.
80 Sweet apple
1.042
0.098
0.098
H: 1
10
20
Syrup
110
1.042
0.098
0.308
4 (
20
7
Fair jelly
111
1.042
0.098
0.395
< t
20
7
Good jelly
112
1.042
0.098
0.552
20
7
Very good
117
1.037
0.098
0.675
20
10
Fair jelly
106
Pear
1.047
0.284
0.284
< i
20
10
Syrup
100
1.047
0.284
0.405
1 4
20
10
Syrup
105
1.047
0.284
0.511
••
20
10
Good jelly
150
Peacji
1.033
0.311
0.311
20
10
Syrup
151
1.033
0.311
0.575
< (
20
7
Fair jelly
Examination of this table shows that the texture
of these jellies was best when the acidity cor-
responded to about 0.5 per cent, for juices whose
sp. gr. was about 1.04. It is interesting that this
acidity and sp. gr. correspond in general to those
of crab-apple juice, the best natural juice for jellies
so far investigated in these experiments. Increas-
ing the acidity of these juices beyond about 0.5 per
cent, seemed to impair the texture of the resulting
jelly, as is evident from No. 117.
111. Discussion.—It appears then that through
this addition of an organic acid to a fruit juice
containing naturally little acid, jellies can be
made if these juices contain pectin. However,
the flavor of the fruit is generally changed; the
sweet apple jelly so made tasted much like that
made from sour apples, the tartness of course in-
creasing with increasing percentages of acid; the
flavor of the pears, in pear jelly, was fairly well
preserved, while in peach jelly the peach flavor
was completely destroyed. Repeated experiments,
varying the percentages of acid were made in an
endeavor to preserve the peach flavor, but success
was not attained in this respect, though a jelly
of faii^ texture could be produced. From ex-
periments testing the relative merits of tartaric
and citric acids in connection with the use of one
or the other to acidify a fruit juice, the balance
of favor seemed to be with the former. Both
texture and flavor of jellies made by its use were
(5)
superior to those in which citric acid was used.
Nos. 151 and 152 in Table V represent tests in
which tartaric acid was the acid added, while in
Nos. 153 and 154 citric acid was used. The method
of making Nos. 151 and 153 corresponds, like-
wise Nos. 152 and 154. There was no question
concerning the superiority of Nos. 151 and 152,
i. e., the jellies made by the help of tartaric acid.
That a mineral acid like hydrochloric can be
substituted for an organic acid was demonstrated.
While this is interesting from a chemical stand-
point, it naturally would not be recommended
in the preparation of fruit jellies for food.
In determining the acidity of fruit juice better
end-reactions were obtained when the juice was
titrated into the N/10 alkali (using phenolphthalein
as an indicator) rather than the reverse as is usual.
In the case of grape or plum juices no indicator
was necessary, but the titration was best carried
out titrating the N/10 alkali into a definite volume
of the juice (well diluted) and taking as the end-
reaction that point at which the pink color of
the liquid just merged into green.
That no acid disappears as such in the prepara-
tion of jellies was proved by the following ex-
periments; Crab-apple juice (100 cc. acidity =
0.532 per cent.) was made into jelly using an equal
vol. of sugar. The resulting unskimmed jelly
was dissolved in distilled water, diluted to 500 cc.
and the acidity again determined. The acidity
of this dissolved jelly was found to be 0.107 per
cent., or, if calculated in terms of the original
volume of juice (100 cc.) it equaled 0.535 per cent.
This is identical with the original acidity (0.532
per cent.) within the limits of experimental error.
Experiments substituting lime-water and also
calcium salts as a precipitant of pectin in making
jellies from sweet fruit juices were carried out.
However, good jellies have not so far been obtained
by this method.
It was found that glycerin could be used in place
of sugar in the making of crab-apple jellies. The
jellies so made were of excellent texture and of
fair taste and they had remarkable keeping qualities.
(6)
When glycerin and sugar were used simul-
taneously with peach juice, a fair jelly was pro-
duced, but from this sugar subsequently crys-
tallized out. These crystals were remarkably large
and perfect.
IV. Sugar in Jelly-making.—That so-called jellies
can be produced by merely boiling down fruit-
juices is well known. Naturally the flavor of
such a jelly is decidedly fruity, but in texture it
is very tough. The making of such jellies requires
a large amount of juice, e. g., we found to make
one glass of such apple jelly required a volume
of juice sufficient for six glasses when made with
sugar. Ordinarily, sugar is cheaper than fruit
juice, so from the standpoint both of palatability
and of economy (possibly also of digestibility)
jellies made with sugar are preferable.
(o) Amount of Sugar.—The proportion of sugar
to juice is a somewhat difficult one to decide.
E.xperimental results indicate that it depends
more upon the proportion of pectin present in a
juice than upon any other one factor. It seems prob-
able that in the actual practice of making jellies
poor results are very likely to occur through the
use of too much sugar, and this because of the
relatively large quantities of water that have been
used in extracting the fruit juices. Through
using these relatively large quantities of water
the jelly-maker is deceived regarding the actual
volume of fruit juice, hence the use of an over-
abundance of sugar. Fruit juices which have
required little water for their extraction, apparently
will utilize a larger proportional amount of sugar
than those that necessarily have been much diluted
with water. Some of these statements will be
made clearer from the following records of ex-
periments:
•
(7)
Xablb Hi.
Time of boUmg.
4-»
c
c
E
•C .
Kind of fruit.
Sp. gr. of
juice.
% of acid in
juice (calc as
HrSOt).
Volume of
juice cc.
roportion of
cane-sugar to
juice by vol-
ume.
efore adding
sugar. Min.
fter adding
sugar. Min.
mount of
jelly formed.
Grams.
exture of
jelly.
H
P4
n
<
•<
H
1 Crab-apple* 1.0375 0 .350 300
'A: 1
5
15
237
Tough
2
1.0375
0.350 300
H: 1
5
12
316
Good
3
< € •
1.0375
0.350 300
K ; 1
5
10
474
Excellent
4
1 «
1.0375
0.350 300
1 : 1
5
9
553
Too soft
120 Grape
1.066
0.487 300
H : 1
0
30
266
Tough
121
1.066
0.487 300
»/« : 1
0
30
281
Fair
122
f <
1.066
0.487 300
H : 1
0
23
382
Good
123
( 1
1.066
0.487 300
1 : 1
0
20
473
Very good
Note in each of these two series of experiments
that with an increasing proportional amount of
sugar, the volume of jelly produced constantly
increases. Note also that the texture of the grape
jelly improves with increasing volume of sugar,
while in the case of the crab-apple the best jelly
is that in which proportion of sugar to juice is
as 3/4:1; increasing this proportion of sugar
impairs the texture of the jelly. An explanation
of these facts concerning the texture of the jelly
is suggested when we consider that we are here
dealing with a grape juice of high sp. gr. and of
comparatively high acidity, but with a crab-
apple juice of low sp. gr. and of low acidity, hence
this grape juice is able to utilize a larger propor-
tional amount of sugar than the crab-apple juice can.
It may also be noted here that as the propor-
tion of sugar in grape juice increases (and con-
sequently the volume of jelly as shown) the crys-
tals of acid potassium tartrate naturally deposited
from this jelly decreased. Possibly the increased
volume of jelly from a given amount of juice ac>-
counts for this, inasmuch as this increased volume
of jelly means a less concentration of the potas-
sium acid tartrate contained in it. This behavior
of grape jelly is interesting especially when taken
in connection with what has been previously
remarked concerning making it from other than
the first juice extraction. That too great a pro-
portion of sugar to juice (depending on the sp.
' Both the low sp. gr. and acidity of this crab-apple juice are ex-
plained by the fact that it was the fourth extraction of juice.
(8)
TtmiE'
12^ 8 S Time of .
Ss .3 g boiUng. .05 i
■ss e i '[I ig • ' r
— 4».2?rtL: ^ !
y §S?ll ft Tj
9^o •O^oSSftS®' '
o ei a «i sS 5 "" ^
Z _■ o "a .2 -o S 5f ,. o +
- -5 "3 uJ "S .0 0° " .5 3 £« * .
o 2 Sf! jrc>,-2-5 3« ».i
E f o ^ ° S -l-^ ° S ^ ? S -S ° ^
t -O & S| "S §2 a-g §2 g| s| ^ ^
& -S d «•' fe" £•" "sS fiS Ei!S ^ .
100 Pear 1.047 0.295 0.405 317 ff: 1 190.5 20 10 103.0° 1.28 34«0
101 " 1.047 0.295 0.522 316 K: 1 190.5 20 10 103.0° 1.28 33Jt
102 •• 1.047 0.295 0.669 316 K: 1' 190.5 20 10 103.0° 1.28 344J
103 •' 1.047 0.295 0.669 261 1:1 253.0 20 4 103.0° 1.30 .4Hi
104 '• 1.047 0.295 0.669 263 1:1 253.0 0 20 103.0° 1.285 419S
no
Sweet apple
1.042
0.100
0.308
312
K: 1
190.5
20
7
103.0°
1.27
346*
111
♦♦
1.042
0.100
0.395
314
K: 1
190.5
20
7
103.0°
1.27
345*
112
"
1.042
0.100
0.552
315
K: 1
190.5
20
7
102.5°
1.25
^1561
113
1.042
0.100
0.552
314
K: 1
190.5
0
26
103.0°
1.26
3421
114
*'
1 .042
0.100
0.552
256
y*. 1
150.0
10
13
103.0°
1.27
2631
120
Grape
1.066
0.487
0.487
316
Vi: 1
95.0
0
30
104.0°
1.26
■266^
121
"
1.066
0.487
0.487
318
»/«: 1
127.0
0
30
103.0°
1.26
2(1.0
122
1.066
0.487
0.487
320
K: 1
190.5
0
23
103.0°
1.26
382.1
123
1.066
0.487
0.487
319
1: 1
253.0
0
20
103.0°
1.26
4731
124
••
1.066
0.487
0.487
298
1: 1
239.0
15
4
104.0°
1.30
130
Plum
1.033
0.623
0.623
254
1: 1
215.0
0
27
103.0°
1.26
365.1
131
1.033
0.623
0.623
254
1: 1
215.0
10
10
103.0°
1.26
370)
^ /
140
Medium sour apple
1.041
0.257
0.257
259
1: 1
210.0
0
22
103.0°
1.27
362)
14l
<• <• •«
1.041
0.257
0.257
259
1; 1
210.0
0
18*
103.0°
1.27
3446
142
I .041
0.257
0.428
259
1: 1
210.0
12
13
103.0°
1.28
3MI
150
Peach
1.033
0.311
0.311
259
K: 1
159.0
20
10
103.0°
1.29
A
2531
151
"
1.033
0.311
0.575
259
K: 1
159.0
20
7
103.0°
1.29
2611
152
1.033
0.311
0.575
259
K: 1
159.0
0
29
103.0°
1.28
2711
153
"
1.033
0.311
0.600
259
K: 1
159.0
20
7
103.0°
1.28
?«i
154
■■
1.033
0.311
0.600
259
K: 1
159.0
0
32
103.0°
1.28
* Hard boiling.
I
Syrup
Thin jelly
Poor jelly
Fair jelly
•o
e
I
el
O M
Sg
e P
fc-S
Pn
i's-
S.S
a
S
e
<8 .
O «
H
o "
si
82
Sj-S
a<
56.00
56.59
55.34
60.30
60.30
'5 +
■n
n
a i
«>
s-'
ftg
<8 ^
Is
56.00
56.59
55.34
60.30
60.30
Saccbarim-
eter
readings.
«2
«
19.11
20.00
15.50
23.61
6.04
e
i
<
—13.39
-13.03
—12.78
—13.47
—12.38
s.
•s §
<8 A
(A 4>»
X 4)
p w
•w M
(8 C
E S
{8
20.0®
21.0°
20.0°
21.0°
22.0°
•o
B
B
O
<«•«
S
M
S
■sii
4J >►
to--
04
48.50
49.48
42.20
55.55
27.70
s+
Sa
^ N
O4.S
86.60
87.43
76.25
92.12
45.93
3 +
0°
SJS
04 >
13.40
12.60
23.75
7.88
54.07
a
<8 •
o—»
c ®
.2 «
•w o
« C9
>><8
u
U
Slight
None
Mudi
None
Fair jelly
7.71
55.00
62.71
24.30
—11.20
22.0°
53.30
85 .00
15.00
Good
7.71
55.00
62.71
21.60
—11.20
22.0°
49.30
78.45
21.55
Very good
7.71
53.50
61 .21
20.40
—11 .16
22.0°
47.40
77.43
22.57
Good
7.71
55.70
63.41
2.40
—11 .00
23.0®
20.20
31 .85
68.15
Very good
7.71
57 .00
64.71
14.00
—10.94
24.0°
37.80
58.41
41 .59
Tough
Fair
Good
Very good
Very good
§.|
•
35.00
45.00
49.00
53.00
55.00
35.00
45.00
49.00
53.00
55.00
—0.30
+ 2.46
5.90
9.64
18.38
-10.37
-11.15
-10.64
-11.22
-12.48
24.0°
24.0°
24.0°
23.0°
23.0°
15.25
21.30
25.06
31.56
46.48
43.57
47.33
51.14
59.54
84.70
56.43
52.67
48.86
40.46
15.30
Slight(?)
None
Slight.
Good
0.0
59.00
59.00
12.60
—9.25
23.0°
33.00
56.00
44.00
None
**
0.0
58.10
58.10
23.35
—9.90
23.0°
50.18
85.05
14.95
Slight
Good
0.66
58.00
58.66
20.90
—12.10
21.0°
49.40
84.21
15.80
Good
0.66
61.00
61.66
20.00
—11.77
21.0°
47.50
77.03
23.00
None
Very good
0.66
61.00
61.66
20.00
—12.6
21.0°
48.80
79.11
20.88
Slight
Syrup
2.8
62.41
65.21
31.82
—12.18
19.0°
65.18
100,00
0.00
Some
Fair
2.8
60.30
61.11
26.05
— 11.05
18.0°
55.63
91 .03
9.00
♦♦
2.8
57.22
60.02
12.35
—11.18
19.5°
35.05
58.41
41 ^9
None
Thin jelly
2.8
59.40
60.20
27.30
—11.71
17.5°
56.20
93.35
6,65
Some
**
2.8
60.00
62.80
16.80
—11.60
18.0°
42.07
67.00
43.00
None
gr. and acidity of the latter) impairs the texture
of the jelly, is emphasized in Table IV.
Table IV.
Time of boiling.
Experiment
No.
Kind of fruit.
Sp. gr. of
juice.
% of add in
juice (calc.
as tSOi). J
Proportion of
cane-sugar to
juice by vol.
Before adding
sugar. Min.
After adding
sugar. Min.
Texture of
jelly.
lis
Sweet apple
1.037
0.675
X: 1
20
15
Very good
170
44
1.037
0.675
1 : 1
20
10
Good
171
4 4
1.037
0.675
IM : 1
0
15
Poor
172
(Mof 171)
. . .
....
• • • •
15
Poor
173
(K of 171) + equal vol. of juice
• . . •
,,
20
Very good
174
Sweet apple
1.037
0.675
2 : 1
0
15
Very poor
The texture of No. 115 was veiy good, but in-
creasing the proportion of sugar beyond the pro-
portions used in this experiment resulted in a
deterioration of the texture of the jelly. Nos.
171-172 and 174 are striking examples of the use
of too great a proportion of sugar. Nos. 171 and
174 were a very poor quality of jelly; pectin was
precipitated in lumps throughout the mass, not
in a continuous, semi-firm substance as in a good
jelly. Subsequent boiling of 171 (i. e., 172) served
only to darken the product, but not to improve
its texture. However, upon the addition to 171
of an equal vol. of juice (No. 173) and upon further
boiling, a jelly fully equal in texture and taste
to No. 115 was produced. Evidently then, in
making fruit jellies, close attention must be given
to the proportion of sugar used. Too little renders
the jelly tough, too much {i. e., more than the
proportion of pectin present in the juice will war-
rant) greatly impairs the texture and flavor of
the jelly. This latter fact may account more
than any other for jelly failures.
(6) Inversion of Sugar.—It has been pointed
out by Bigelow* that the amount of inversion
of sugar in jellies and jams varies in general with
the length of time of heating, the content of acid,
and also the nature of the acid. Experimental
data further substantiating these statements are
tabulated in Table V.
A word of explanation concerning the data
in this table should be given here.
' BureoM of Chem., Bull, 66» p. 52.
(9)
The specific gravity of the juice in each instance
was determined by means of a hydrometer. The
acidity of the juice either before or after adding
more acid was determined as already indicated.
The percentage of cane-sugar put into the jelly
was calculated from the weight of sugar added
and the total weight of jelly and skimmings ob-
tained. Unfortunately it was necessary to weigh
the jelly itself hot.
The percentage of cane-sugar found in the jelly was
calculated from the direct and invert readings accord-
ing to Clerget's formula S = 100(0-6)/(144—i/2).
Polarizations were made upon normal weights
of the juices and half-normal weights of the jellies,
all the results being calculated to normal weights.
Solutions were clarified and inverted according
to the method elaborated by Sherman.'
Examination of this table shows, as we should
expect, that the percentage of inversion of sugar
is vastly influenced by the time the sugar and
juice are boiled together, e. g., compare No. 103
with 104; Nos. 112-114 with each other; No. 123
with 124; No. 130 with No. 131; No. 151 with
No. 152; and No. 153 with No. 154. Note in
these comparable cases that the proportional
amounts of sugar and the acidity are identical.
That the percentage of acid in the juice affects
this inversion is evident from a comparison of
Nos. 100-102 (note discrepancy in loi); Nos.
110-112; and No. 150 with No. 151 and No. 153.
In these instances the time is generally identical.
That the nature of the acid also affects some-
what the amount of inversion is illustrated by
comparison of Nos. 151-152 with Nos. 153-154,
the acid added to the two former being tartaric,
and to the two latter citric.
The amount of this inversion that is desirable,
in order to produce jellies of good texture, is a point
more difficult to decide than the proportional
amount of sugar to be used. Examination of
the table shows that crystallization of cane-sugar
occurred more readily generally, from the jellies
in which inversion was least. However, in every
1 Organic Analysis, p. 91.
(lo)
case, this crystallization always occurred at the
surface of the jellies and in none of the cases had
any care been taken to protect these jellies from
the air, as is ordinarily done, by paraffining the
surfaces. Hence, there was every opportunity
for evaporation from the surface of these jellies
and the consequent formation of crystals.* That
the sugar should not be boiled sufficiently long
with the juice to insure practically complete in-
version, is quite probable from the .following ex-
periments, as shown in Table VI.
That jellies can not be made through substi-
tuting fructose for cane-sugar is evident from
Nos. i6i and 143, as only a thin syrup resulted
in these cases. Corresponding experiments (not
tabulated) substituting d-glucose for cane-sugar
resulted apparently in a good firm jelly; how-
ever, d-glucose shortly began crystallizing from
this jelly and soon could be seen throughout the
mass. No. 162 in which equal weights of fructose
and d-glucose were substituted for cane-sugar
(representing a condition of complete inversion)
resulted very similarly to the case in which d-glu-
cose was used alone, i. e., d-glucose crystallized
out. No. 163 in which cane sugar was boiled with
the juice for only two minutes finally showed evi-
dence of cane-sugar crystals on the surface.
From all the foregoing results two things are
apparently evident: First, some inversion of cane-
sugar is desirable; second, an inversion approach-
ing completeness is undesirable, see No. 162. So
far as our researches now extend it seems better
not to boil sugar and juice together from start
to finish in jelly-making, but rather to add the
sugar so that it may be boiled with the juice for
a period not to exceed one-half the total time of
cooking. However, 'we wish to investigate this
point further.
Sojne experiments regarding excessive cook-
^ In other cases not dted in the table but within the writer's pre*
vious practical experience—cases in which such jellies as currant, red
raspberry, and blackberry were paraffined as soon as they had set,
no crystallization of cane sugar occurred even after a lapse of two
years. In these cases a very small amount of inversion had probably
taken place, since the sugar had been boiled with the juice not to ex-
ceed five minutes.
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(12)
ing of an over-diluted juice have been carried out.
Results seem to show that the texture of the jelly
may be greatly impaired in this way. This also
demands further investigation.
V. Physical Constants}—In connection with our
work it seemed desirable to determine whether
or not there is a boiling point or a definite specific
gravity of the boiling jelly indicative of that con-
dition at which a good jelly may be formed on cool-
ing. Although it was impossible to determine
these points with absolute accuracy yet reference
to Tables V and VI shows a remarkable coinci-
dence among these physical constants. Boiling
points were observed by suspending a thermometer
with the bulb in the boiling liquid, and specific
gravities were observed by means of a hydrometer
floated in the hot liquid which had been trans-
ferred with as great rapidity as, possible to a glass
cylinder previously heated with boiling water.
That these boiling points and specific gravities
coincide so nearly, even though we are dealing
with different fruit-juices and very. different pro-
portions of sugar, is worthy of notice.
. VI. Artificial Jelly.—^That the essential sub-
stances for the formation of a jelly of good texture
have been indicated in the foregoing is probable
from the success attending the attempts to make
jelly from an artificial fruit-juice. For this pur-
pose it was necessary to isolate pectin. Pectin
was obtained from crab-apple, grape, sweet apple,
and peach juices The isolation of this pectin
was a difficult matter and the methods indicated
by different experimenters" were tried. The fol-
lowing modification of their methods was finally
adopted as the one giving the best results: a
given volume of the cold, filtered (previously boiled)
fruit-juice was added drop by drop by means
of a separatory funnel to an equal volume of alco-
hoi, %tirring constantly; the mixture was allowed
to stand over night in a cold place and then the
1 We understand that some trork (unpublished) on these points,
also on the relation between the proportion of sugar used and the
amount of jelly produced, has already been done by Miss Jenny Snow,
of the University of Chicago.
2 ChodneWs Ann., 48» 56. Fremy, Ibid., 64> 383.
(13)
precipitated jelly-like mass of pectin was sepa-
rated from the liquid by filtering off through
very fine cotton cloth stretched tightly over the
mouth of a 4-liter jar. The liquid was worked
out of the mass of pectin by means of a spatula.
Repeated re-solution and re-precipitation of the
pectin improved its purity. The pectin so ob-
tained was a very stable, nearly colorless, trans-
parent solid, of insipid taste. Its principal reac-
tions in solution were tried and correspond to those
given by Braconnot, Fremy and others.
Jelly of excellent quality was easily made from
this pectin by preparing a i per cent, solution
of it in a 0.5 per cent, solution of tartaric acid,
heating to boiling, then adding a 3/4 volume of
sugar and boiling until the jelly-test was observed.
The total time of boiling did not exceed 15 minutes.
Jellies made as above were nearly colorless, but
were excellent in texture and taste. When a
few cc. of grape-juice were added to the pectin
solution the color of the jellies was very pleasing.
That these jellies were not made directly from
fruit-juice would not be suspected from taste or
texture. Jellies equally good were made in this
way from the pectin from sweet apples, crab-
apples, and peaches. This last fact would seem
to indicate that the pectin of peaches does not
differ materially, at least from a practical stand-
point, from that of other jelly-making fruits.
It may be of interest to add that pectin was
extracted from jelly (the jelly being previously
dissolved in an equal volume of water) by the
method used for extracting pectin from fruit
juices. This pectin apparently corresponded in
its physical and chemical characteristics with
that extracted directly from fruit-juices. Good
fruit jelly invariably gives the alcohol test for
pectin. These facts would seem to indicate that
pectin in the formation of jelly does not undergo
any deep-seated change, but rather that the phe-
nomenon of jelly-making is more nearly physical
than chemical.
VII. Summary.—i. The essential constituents
(14)
of a jelly-making fruit-juice are, first, pectin;
second, acid.
2. A desirable accessory constituent is cane-
sugar. Too much sugar is likely to be used in
jelly-making with a consequent deterioration of
the quality of the jelly. The amount of inversion
preferably produced in this cane-sugar is yet un-
determined.
3. Over-dilution of fruit-juice should be avoided,
since this leads to the use of too much sugar;
probably the extra boiling thus rendered neces-
sary also impairs the texture of the jelly.
4. The physical constants of hot juice ready
to jell on cooling are, substantially, boiling point
103° C., and specific gravity 1.28.
5. Jelly is readily made through boiling pectin
with acid, water and sugar.
6. Jelly-making seems to consist in so control-
ling conditions by means of aqid and sugar and
boiling as to cause the pectin to be precipitated
in a continuous mass throughout the volume al-
lotted to it.
Research Laboratory of the
Department of Household Science,
University of Illinois.
(15)
3 5556 005 386 081
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