[15] All specific gravities given in this bulletin are on a 20°C/20°C
basis.
The specific gravity of tomato pulp is used as one criterion for
establishing the value of pulp that is offered for sale and is also
used in connection with the manufacture of pulp to determine the point
at which evaporation should be stopped.
In the former case there is ample time for making the examination,
and conditions may be established which permit a reasonable degree of
accuracy in the work.
In the determination of the specific gravity of hot pulp during the
process of its evaporation speed is essential, and the conditions of
a manufacturing plant do not always permit a high degree of accuracy.
It becomes necessary, therefore, to consider what methods may give the
highest degree of accuracy obtainable under the conditions of the work
and at the same time afford quick results.
Tomato pulp, owing to its high viscosity, retains a large quantity of
air bubbles which increase the volume of the pulp and hence interfere
with the accuracy of the determination of specific gravity. In working
with cold pulp this air may be eliminated by whirling in a centrifuge.
With hot pulp that operation is impossible, and the specific gravity
must be determined in the presence of the air bubbles mentioned.
Moreover, in working with cold pulp the temperature can be more
accurately controlled, and the error caused by variation in temperature
can be corrected. With hot pulp these conditions cannot be obtained
nearly so well. The determination of specific gravity of hot pulp is
therefore only roughly approximate at best. Where time permits it is
strongly advisable to cool the pulp under conditions that prevent
evaporation before determining specific gravity.
The importance of accuracy in the determination of specific gravity in
tomato pulp is discussed on page 50.
Methods are given below for the determination of specific gravity in
both hot and cold pulp.
When salt has been added, the amount should be determined and a
correction applied by deducting .007 from the specific gravity for each
per cent of salt present.
(a) <sc>COLD PULP AFTER CENTRIFUGING TO ELIMINATE AIR BUBBLES</sc>
This method may be employed for pulp of any degree of concentration
or for unconcentrated cyclone juice. A specific gravity flask such as
is shown in Figure 1 is used together with a “2-bottle” Babcock milk
tester (the centrifuge referred to below). The flask may be obtained of
Eimer & Amend, Third Avenue, 18th to 19th Streets, New York City, or of
Emil Greiner & Co., 55 Fulton Street, New York City, and in ordering
it should be designated as “specific gravity flask for tomato pulp of
Pyrex glass with a capacity of about 125 cc.” The “2-bottle” Babcock
milk tester may be obtained of any dairy supply house. It may also be
obtained of any dealer in chemical apparatus by designating it as E. &
A. No. 1833.
The specific gravity flask may be calibrated as follows:
Obtain the weight of the flask after thoroughly cleaning and drying,
fill to overflowing with water (preferably boiled and cooled distilled
water) and remove the excess water from the mouth of the flask by means
of a straight edge. Wipe dry and weigh immediately. If the flask full
of water is weighed at any other temperature than 20° C. (68° F.) a
correction must be made to obtain the weight at that temperature. These
corrections are as follows:
=============================================
Temperature |Correction to be added
|with flasks having a
| volume of—
----------+----------+------------+----------
Fahrenheit|Centigrade| 125 cc. | 400 cc.
----------+----------+------------+----------
| | _Grams_ | _Grams_
69 | 20.6 | .02 | .05
70 | 21.1 | .03 | .09
71 | 21.7 | .05 | .14
72 | 22.2 | .06 | .20
73 | 22.8 | .08 | .25
| | |
74 | 23.3 | .09 | .30
75 | 23.9 | .11 | .35
76 | 24.4 | .13 | .41
77 | 25.0 | .15 | .47
78 | 25.6 | .17 | .53
| | |
79 | 26.1 | .18 | .59
80 | 26.7 | .20 | .65
81 | 27.2 | .22 | .71
82 | 27.8 | .24 | .77
| | |
83 | 28.3 | .26 | .83
84 | 28.9 | .28 | .89
85 | 29.4 | .30 | .96
86 | 30.0 | .32 | 1.02
----------+----------+------------+----------
If it is desired to use somewhat larger samples and thus secure
correspondingly more accurate results, a similar flask, but made
somewhat larger (capacity approximately 400 cc.), may be employed. Such
a flask, with a diameter of a little over 3 inches, is illustrated in
Figure 2. This larger flask will not fit into the Babcock tester, and
when it is used a special head for the Babcock tester must be made.
Such a head is illustrated in Figure 3, and can be made by any good
tinner.
The larger flask shown in Figure 2 holds a heavier weight than the
Babcock machine is intended to carry, and the advisability of its use
is perhaps questionable. In any case, whatever size flask is used, it
is important to build a guard around the centrifuge in order to protect
the operator when the apparatus gives way, as it eventually will.
[Illustration:
Fig. 1. Small Specific Gravity Flask.
Fig. 2. Large Specific Gravity Flask.
Dimensions given are outside measurements. Thickness of walls is about
3/64 in.]
The details of the method of determining specific gravity by the use of
this apparatus are as follows:
Fill the flask shown in Figure 1 with the sample of pulp and place
in the centrifuge (the Babcock milk tester mentioned above). Place a
suitable counterpoise[16] in the other receptacle of the centrifuge.
Whirl for from one-half to one minute at a speed of about 1,000
revolutions per minute, that is with the handle turning about 100
revolutions per minute. Because of the air bubbles removed by
whirling, the surface of the pulp will now be considerably below the
top of the flask. Fill the flask and whirl in the centrifuge again.
Repeat this filling and whirling until the flask is practically full
of pulp after whirling. Ordinarily two or three separate whirlings are
sufficient. Then add a few more drops of pulp so that the pulp comes
above the top of the flask, and strike off flush with the top of the
flask with a straight edge. Wash the outside of the flask, wipe dry,
and weigh. Then read the specific gravity of the pulp from a table
prepared, giving the weight of the flask full of pulp and the specific
gravity of the pulp in parallel columns, or calculate the specific
gravity as described below.
[16] This counterpoise may be prepared with a can or bottle weighted
with some heavy material such as solder or shot, until it balances
approximately the weight of the can full of pulp. The counterpoise,
once prepared, may be left in the centrifuge.
[Illustration: Fig. 3. Special Head and Flask Receptacles for Babcock
Milk Tester.]
While the weight is being taken a thermometer may be placed in the
pulp remaining in the can or dipper from which the flask was filled.
If the temperature varies from 68° F. the specific gravity may be
corrected by Table 8. In order to use this method the temperature of
the pulp should not be below 50° F., or above 86° F.; otherwise, it
should be warmed or cooled, as described above.
The method is accurate, simple, easily operated and fairly rapid. To
calculate the specific gravity from the weights obtained, the weight
of the clean, dry flask and of the water it contains at 68° F. are
necessary. The weight of the clean, dry flask is then subtracted from
the weight of the flask full of pulp to obtain the net weight of the
pulp. This divided by the weight of the water the flask will contain at
68° F., gives the specific gravity.
A table can be constructed readily for each flask, which will
give in parallel columns the weight of the flask full of pulp and
the corresponding specific gravity. This greatly simplifies the
determination, as it eliminates all calculation. When such a table is
employed a balance giving actual weights is practically as convenient
as one reading specific gravity directly. It has the very important
advantage that the balance, weights, and flask may be tested from time
to time.
In preparing such a table it is convenient first to draw a curve
representing specific gravities of the pulp and corresponding weights
of the flask full of pulp of various degrees of specific gravity. The
table may then be constructed from the curve.
For instance, let us suppose that the flask weighs 56.00 grams and
that when full of water at 68° F. (20° C.) it weighs 176.63 grams.
The water contained at the temperature mentioned then weighs (176.63
- 56.00) 120.63 grams. Now the specific gravity of the pulp is its
weight compared with the weight of an equal volume of water. Having the
figures given above we can easily calculate the weight of the flask
filled with pulp of any desired specific gravity. We may therefore
calculate the weight of the flask plus pulp of two different specific
gravities; mark those points on a sheet of coordinate paper with
specific gravity of the pulp entered at the bottom and the weight of
flask plus pulp at the side, and a straight line drawn through the two
points mentioned gives us the weight of the flask when filled with pulp
of any specific gravity.
For instance, if the flask mentioned above be filled with a pulp of the
specific gravity of 1.03, the weight of the pulp is (120.63 × 1.03)
124.25 grams. This added to the weight of the flask (56.00 grams) gives
us 180.25 grams. Similarly, if the flask be filled with pulp of 1.04
specific gravity the weight of the contents at 68° F. will be (120.63 ×
1.04) 125.46 grams. This added to the weight of the flask (56.00 grams)
gives 181.46 grams as the weight of flask plus pulp. Now if a sheet
of coordinate paper be prepared with specific gravities entered at the
bottom and weight of flask plus pulp at the side, these points may be
entered. This is done in Figure 4 and the two points mentioned are each
indicated by a circle and are connected by a straight line. A table may
be constructed from this line, giving the weights of flask plus pulp in
one column and the corresponding specific gravity in another.
[Illustration: Fig. 4. Weight and Specific Gravity of Tomato Pulp.]
As an illustration of this there are given below a series of figures
illustrating the beginning of the table that could be constructed from
Figure 4. If a large sheet of coordinate paper be taken the line shown
in Figure 4 may be extended so that a table may be constructed for pulp
of all concentrations.
---------+--------
Weight of|Specific
flask and|gravity
pulp |
---------+--------
180.25 | 1.0300
180.31 | 1.0305
180.37 | 1.0310
180.43 | 1.0315
180.49 | 1.0320
180.55 | 1.0325
180.61 | 1.0330
180.67 | 1.0335
---------+--------
The highest degree of accuracy can be secured by filling the flask and
making the weighing at exactly 68° F. This is obviously not practicable
under factory conditions, however, and satisfactory results can be
secured by taking the temperature of the pulp at the time of weighing
and correcting for temperature by the use of Table 8. This table gives
the correction to be added to the specific gravity when the pulp is
taken at temperatures between 68° and 86° F., and the correction to be
deducted from the specific gravity for temperatures, between 55° and
68° F. As a matter of principle, correction factors should be avoided
as far as practicable, and the smaller the correction factor the more
accurate the results will be. This table will be found especially
useful in determining the specific gravity of the partly concentrated
pulp, as is directed on page 50.
As stated above, in determining specific gravity by this method it is
advisable that the reading be made to the second place of decimals. For
this purpose an assay pulp balance is suggested. An assay pulp balance
carrying a maximum load of 300 grams is listed by dealers in chemical
apparatus at $52.50. This balance may be obtained from dealers in
chemical apparatus by designating it as “Assay pulp balance E. & A. No.
292, capacity 300 grams.” The same balance, more heavily built, and
preferable for that reason, carrying a maximum capacity of 600 grams,
is listed at $63.
A satisfactory set of weights, suitable for weighing a cup similar to
that shown in Figure 1, may be obtained from any dealer in chemical
apparatus by designating it as E. & A. No. 516, “Metric brass weights
in wooden box, 200 grams to 1 centigram.” This is listed at $5.50.
For convenience, all of the apparatus necessary for using this method
of determining specific gravity is listed below. With the exception
of the specific gravity flask this apparatus may be purchased of
any dealer in chemical supplies. The specific gravity flasks have
not heretofore been available except through this laboratory, which
purchased a considerable quantity of them and supplied them to
manufacturers of pulp as long as this supply lasted. At the urgent
request of the writer, Eimer & Amend and Emil Greiner & Co., both of
New York City, have finally stocked this item and stand ready to supply
it to those wishing to secure it.
Specific gravity flask for tomato pulp, of Pyrex glass, 1½ × 6¼ inches
(outside measurements), capacity about 125 cc.
Two-bottle Babcock milk tester, with 2 brass holders, E. & A. No. 1883.
Assay pulp balance, maximum load 300 grams, E. & A. No. 292.
Metric brass weights in wooden box, 200 grams to 1 centigram, E. & A.
No. 516.