CONDENSING THE STRAINED OR CRUSHED TOMATOES
Vacuum Pan
Practically all of the tomato products made in this country are cooked
in an open kettle under atmospheric pressure—either a copper-jacketed
kettle, or a tank with a closed coil. The vacuum pan is used, however,
in several plants, and makes a very high grade product—a better product
as a rule than can be made by condensing under atmospheric pressure.
When tomato juice is condensed in vacuum it boils at anywhere from 54
degrees F. to 175 degrees F., depending upon the degree of vacuum
obtained. At sea level, under atmospheric pressure, it boils at 212
degrees F. By employing a low temperature, and condensing rapidly, which
can be done in the vacuum pan, the natural bright red color of the
tomatoes is affected very little by the cooking. High temperatures and
continued boiling are the agents which destroy tomato color as well as
flavor. The vacuum pan produces a pulp of not only fine color, but of
very fine flavor. The reason the vacuum method is not used more
frequently than it is is undoubtedly due to the difficulty in getting
the vacuum equipment, and also to the expense of the equipment when it
can be obtained.
Types of Open Kettles
The open kettle method produces a very satisfactory product when the
proper precautions are taken, and if the cooking and subsequent
processes are carried out intelligently, the finished product compares
very favorably with that which is condensed in vacuum. The types of
kettles in most common use are the copper jacketed kettle, the
glass-lined tank with closed copper coil, and the cypress tank with
closed copper coil. All of these give good results where the jacket or
coil is properly constructed, the steam trap is of the right type and
takes care of all of the condensation without allowing it to back up
into the coil, and the kettle has a good head of steam so that a
vigorous boil can be maintained throughout the cooking.
Steam Pressure
If the steam pressure at the boiler is kept above 75 lbs., and too much
is not lost by radiation from uncovered pipes, and by leaks at poorly
packed flanges, etc., there should be no difficulty in maintaining a
vigorous boil. The evaporation should be so rapid that the vaporizing
steam will burn the hand, even if placed over the kettle for a second.
If you can hold your hand over a kettle of boiling pulp, even for a few
seconds, you can be sure that you are not getting a proper boil. A
pressure of 100 lbs. is, of course, to be preferred to 75, as it will
effect a quicker condensation, however there is not a large percentage
of boilers in tomato pulp plants that are permitted to carry as much as
100 lbs.
The kettles should be close to the boiler room, and the steam pipes
feeding them should be as straight and short as possible, and covered
with magnesia pipe covering to prevent excessive loss of heat by
radiation. It is not an uncommon sight in canning plants to see the main
pipe from the boiler to the cooking kettles pass through an areaway
between two buildings, and have no protective covering whatever. Such
sights, however, are becoming more infrequent, as packers are plugging
up many of the leaks through which dollars have escaped every day in
years gone by. The condensation from the steam traps can, of course, be
used right over again in the boilers.
Copper-Jacketed Kettles
Copper-jacketed kettles used for condensing tomato products range all
the way from 50 gallons capacity to 500 gallons, the larger size making
a batch of finished pulp of about 250 gallons as a rule. A larger batch
than this can be cooked in a 500–gallon kettle, but it is not advisable
to keep pumping fresh tomato juice into the kettle too long, as it gives
too long a cook to the first that goes in. Forty-five minutes should be
the absolute limit for a batch of pulp, no matter what kind of a kettle
it is cooked in, and it is much better to confine the cooking time to 30
minutes or less. The shorter the cook, the better the color and flavor
of the finished product, other things being equal.
Tinning Kettles
Copper kettles are frequently lined with block tin to prevent the acid
of the tomato from dissolving some of the copper. The desirability of
this is argued both from a standpoint of health and flavor, as dissolved
copper, even in small quantity, has a very bitter taste. It should be
remembered, however, that the acid of the tomato, which is generally
thought to be citric acid, is a weak organic acid, and the amount of
copper dissolved in a batch cooked not over an hour is exceedingly
small; certainly so small as to have no ill effect upon health. As to
the question of flavor, it would indeed take a very delicate palate to
detect among a half dozen samples which ones were cooked in an unlined
copper kettle, and which in a kettle lined with block tin or silver. The
latter is said to be used by some packers.
It is a good idea, however, to play safe, and line all kettles with
block tin, as we know that there is a small amount of copper taken up in
solution in the pulp. This is more important in the case of catsup and
chili sauce than where the kettles are used for pulp alone, as vinegar
and salt act on the copper much more severely than tomato acid does. As
to the thickness of the block tin lining, an inspection of the kettles
used by various packers shows this to run all the way from a good
half-inch thick to a coating so thin that in one season’s use it is
almost all scraped off by friction in cleaning the kettle. The kettles
should be plated heavy enough to last several seasons without
re-tinning, however a coating a half-inch thick is unnecessary and
wasteful of steam. The same remarks apply to the tinning of copper
coils.
Advantages of Various Types of Kettles
Although copper-jacketed kettles have the advantage of being very easily
cleaned, and also the advantage of wasting very little of the product
when the finished batch is being discharged, the large tanks of either
cypress or glass-lined steel are coming into use more and more because
of their greater capacity. It is true that they are very slightly more
wasteful than the jacketed kettles, but this really amounts to very
little if the tanks are built with a concave bottom. They take up no
more room than the jacketed kettles, and will hold a much larger volume,
as they can be built so much higher. If one were to buy a series of
copper-jacketed kettles to cook batches as large as are commonly cooked
in tanks equipped with copper coils, the expense would be very heavy,
there would be much exposure to metal, and the results obtained would be
no better, or possibly not as good. In the tank equipped with a coil,
batches of 300 or 400 gallons are often cooked, and the juice can
generally be condensed to a specific gravity of 1.035 in 25 to 30
minutes.
Glass-Lined Tank
During the last few years quite a few packers, when adding to their
cooking capacity, have put in glass-lined steel tanks. The glass-lined
tank is equipped with coil and steam trap in the same manner as the
cypress tank, and it has certain advantages which make it very nice to
work with, even though the first cost is considerably more than that of
a cypress tank of equal capacity. The only objection I have heard to the
glass-lined tank is that it radiates more heat than a wooden tank, and
on very hot days, unless it is insulated, makes the cook room more
uncomfortable than the same capacity in cypress tanks would. However,
when the tank is covered with cork insulation, or asbestos, or even wood
staves, it radiates no more heat than a cypress tank.
The glass-lined tank is perfectly sanitary, and is very easily cleaned.
It is really the most sanitary thing we have to cook in. The surface is
smooth and entirely free from any irregularities such as there are bound
to be between the staves of the cypress tank. It is always ready for
use, and does not have to be swelled and then scrubbed for a half day
after standing idle for a while before it is fit to use again. There are
no places for molds to creep in and multiply. Furthermore, being of
metal and all in one piece, a battery of glass-lined tanks will not
shake like cypress tanks will when you are running heavy and have
several tanks going full blast at the same time. This may seem rather
unimportant, but it is quite a relief to the cook and his helpers, as
they can go about their work without having their nerves rattled. Just
how long a glass-lined tank will last under average conditions in a pulp
or catsup plant is not known, as they have not been in use a great many
years for this purpose; however, one would guess that they would be
serviceable almost indefinitely.
Cypress Tank
Cypress tanks should be constructed of 2–inch material and made with a
sloping or concave bottom, preferably the latter. They will not impart
any foreign taste to the product, as is sometimes thought, even after
becoming charred by long usage, and if kept clean they will remain sweet
as long as they are kept in continuous use. A musty taste is sometimes
imparted to pulp or catsup cooked in a cypress tank if it has been
standing idle for a long time and is not thoroughly cleaned and boiled
out with soda before it is used again. Cypress tanks have the advantage
of being cheap, and also of having large cooking capacity.
Cleaning Kettles
Cooking tanks and kettles should be cleaned thoroughly after every
batch. If the steam trap is working properly the burning on the coils
and sides of the kettle will be so slight that almost all of it can be
washed off with the hose if the water pressure is strong and the hose is
bushed down to one-eighth inch. The best things to use to take off the
material which burns on hard and black is a pot chain for coils, and for
the jacketed kettles a wire brush or very stiff fiber brush does very
well. If any of the baked on pulp is not removed before the next batch
is cooked it will interfere with the cooking by lengthening the time
required to finish the batch, and by causing the coils to burn still
harder next time. The cook should be instructed to get down in the tanks
frequently, and feel underneath his coils to see that they are properly
cleaned. The manager should also make it a point to do this several
times a day. His quality will depend in no small degree on the way his
coils are cleaned.
Coil Leaks
Leaks in coils should be attended to immediately, as they cut down the
available steam pressure, and soon become so large that it becomes
impossible to cook satisfactorily. An ordinary soldering job will only
hold them for a day or two, and it is necessary to take out the coil and
braze it if the leak is to be permanently mended.
Starting the Cooking
We will say that we are now pumping over the cycloned or crushed
tomatoes into the cooking kettle. Some packers pour a cupful of
cottonseed oil in the kettle to assist in breaking the boil, others rub
the sides of the kettle with fat, such as a piece of cocoa butter, while
others use no oil or fat at all, but take care to feed the kettle slowly
and carefully, and cut the foam with a sharp spray of water from the
hose to assist in breaking the boil. The oil or fat does seem to help
some, but it is not necessary if the kettle is fed carefully. As soon as
the coil or jacket is covered the steam can be turned on full; however,
the exhaust should be opened immediately, and not closed until all the
condensation which has collected in the coil or jacket runs out and the
steam comes through perfectly dry. The condensation of the pulp should
then be conducted as rapidly as possible, pumping over more of the
tomato juice as the kettle is able to take care of it, and boiling
continuously. The eye soon learns to judge the approach of the finishing
point, and then the exact finishing point must be determined. There are
many ways of doing this, and a number of ways have been recommended. No
method can succeed which is not simple and quick, and which offers
scarcely any chance for error when used by a cook, whom, it must be
remembered, is a man of very ordinary intelligence.
Methods of Gauging Finishing Point
The simplest method is gauging the finishing point by the eye, but this
method is also the least accurate in the hands of the average cook.
Cooks who have had years of experience with tomato pulp sometimes get to
be very expert in gauging the finishing point by the eye, and can
determine it with surprising accuracy. Such a cook is the exception,
however, and most packers have to use some sort of a mechanical method
by which at least fair accuracy can be counted on. These methods consist
of: first, condensing the pulp to a certain mark on the kettle, a
definite volume of juice having been taken to start with; second,
cooking a certain length of time; third, determining the specific
gravity of the hot pulp by weighing a definite quantity on a small
balance; fourth, estimating the gravity by the use of a hydrometer.
Cooking to a Gauge on Kettle
With the first method the amount of tomato juice pumped into the kettle,
or the number of bushels of tomatoes from which this juice was derived,
must be measured. This is a nuisance, and is not necessary with any of
the other methods except the second. Furthermore, even though every
batch of pulp cooked during the season be made from a measured quantity
of tomatoes or tomato juice, and cooked to the same point on the
measuring stick, the pulp will not all be of the same specific gravity,
because tomatoes are very watery at some seasons of the year and very
firm and solid at other times. The watery tomatoes will make a thin
pulp, and the firm tomatoes will make a heavier pulp.
Cooking a Definite Length of Time
Cooking for a certain length of time is not dependable because the steam
pressure is not always the same, the coils do not always condense the
pulp with the same efficiency, and the character of the tomatoes varies
during the season. With this method it is also necessary to start with a
definite quantity of tomato stock for each batch.
Determining Specific Gravity by Weight
The third method—that of determining the specific gravity by weight—is
used as commonly as any of the others. The specific gravity of pulp
merely means the comparison between the weight of a definite volume of
the pulp and the weight of the same volume of water at the same
temperature. If pulp has a specific gravity of 1.035, that means that a
gallon of it is 1.035 times as heavy as a gallon of water.
The determination, as carried out in the cook room, is made with a small
trip balance, a set of weights, and a copper flask which is tin lined.
The weight of the empty flask is taken. Then the weight of the flask
filled to the top with boiling water is taken. The difference between
these two weights is the weight of water at the boiling point which the
flask contains. When the boiling pulp approaches the finishing point,
the flask is filled to the top with the boiling pulp. This should be
done rapidly, as the pulp cools quickly, and as it cools, decreases in
volume. The weight of the flask filled with pulp at the boiling point is
taken. This weight, minus the weight of the empty flask, equals the
weight of pulp at the boiling point which the flask contains. You now
have the weight of boiling water the flask contains, and the weight of
boiling pulp the flask contains. Divide the former into the latter, and
the result (the quotient) is the specific gravity. For example, if the
water weighs 1,000 grams, and the pulp weighs 1,035 grams, the specific
gravity is 1.035. If the specific gravity of cold pulp is taken, then
cold water of the same temperature must be taken as the comparison. The
weight of cold pulp must not be compared with the weight of hot water,
and vice versa.
Objections to Weighing Method
One of the objections to the use of this method is that there is usually
a certain amount of air bubbles and foam in the boiling pulp and this
causes an error, as the air takes up volume but does not weigh anything.
Another objection is that the determination is often not made rapidly
enough, with the result that while the flask is being filled with
boiling pulp, the pulp is cooling and correspondingly decreasing in
volume, and by the time the flask is full, it contains more pulp than it
should, that is, instead of containing 1,000 cubic centimeters of pulp
at 210 degrees F., it will contain about 1,010 cubic centimeters of pulp
at 200 degrees F., or possibly lower. One reason for this delay is that
the foam caused by filling the flask with boiling pulp rises in the neck
of the flask, and makes it hard to judge when the flask is exactly full.
The main difficulty with the method, however, is that pulp cooks do not
have scientific minds, they do not appreciate the importance of doing
the determination in a strictly scientific way, and the results they get
are not very accurate. I have frequently checked up on pulp cooks using
this method, and sometimes they were very decidedly off.
Testing With Hydrometer
The estimation of the gravity of the boiling pulp by the use of a
hydrometer is not in very general use, and it has been stated by some
scientific men that a hydrometer is no good for tomato pulp or catsup
except where the sample on which the test is made is previously filtered
so as to get a clear liquid. A test which involves filtering, is, in my
opinion, entirely unsuited for everyday use on the cooking platform.
I have used the hydrometer for testing pulp and catsup direct from the
kettle for seven years, and have had more success with this method than
with any other.
The hydrometer was never intended to be used with semi-solid substances
such as tomato pulp, but only with liquids such as brine, sugar syrup,
etc., in which there is no solid matter in suspension. With liquids, the
number of degrees registered on the hydrometer when it is immersed in
the liquid is equal to a definite specific gravity. This is not true
with semi-solids such as tomato pulp, as each packer must work out the
relation between the degrees of the hydrometer and the specific gravity
on pulp under the conditions which obtain in his plant. This is a very
simple thing to do, and once the packer has established this relation,
it will hold good as long as he is in the pulp packing business. The
reason why this relation is not the same for all packers is because
tomato juice is not screened to the same degrees of fineness in all
plants, and because all hydrometers having the same scale will not work
the same on pulp. This is because they have different shapes. The shapes
may only vary slightly, which will not interfere with their accuracy on
clear liquids, but it will make a difference when the hydrometer is used
in tomato pulp. Furthermore, some of the hydrometers will probably have
too much variation in their diameter at different points, which is a
disadvantage when they are used in tomato pulp. I have therefore always
had the hydrometers which I have used for this test made to order and to
conform to certain specifications which I laid down.
[Illustration:
Fig. 3. Special hydrometer for testing tomato pulp and catsup.
(C. J. Tagliabue Mfg. Co.)
]
In Fig. 3 is illustrated a hydrometer which is manufactured by the C. J.
Tagliabue Mfg. Co. of Brooklyn, and which is made to conform to these
specifications, which are: first, perfect balance; second, a minimum
amount of variation in the diameter of the various parts of the spindle
with the slope very gradual; third, a Beaume scale reading from 0 to 50
degrees; fourth, that it can be obtained in exact duplicate in any
quantity.
If the diameter at various points in the spindle is slight, with the
slope very gradual, it seems to be an advantage. It is, of course,
necessary that exact duplicates can be obtained to use when the first
one, on which your calculations are based, is broken.
In order to avoid disappointments which would be very apt to result from
the use of a hydrometer which was not particularly adapted to this test,
and which would very likely be difficult of exact duplication, the type
illustrated above should be adhered to, and it will be found to give
good results when used as directed in this chapter.
Two pulps of the same specific gravity, and of practically the same
degree of fineness, if tested hot, will register the same on the same
hydrometer, or on two hydrometers which are exactly alike in every
respect. (The hydrometer does not work well on cold pulp unless the pulp
is thin.)
Two pulps of the same specific gravity, showing a very marked difference
in their degree of fineness, will not register the same on the same
hydrometer, because, even though a definite volume of each will weigh
the same, the two pulps are of different thicknesses, which will have an
effect on the degree to which the hydrometer will sink when placed in
the pulp. However, if a packer cyclones his tomatoes before condensing
them, and uses the same mesh of wire screen in his cyclone all the time,
his pulp will all be of the same degree of fineness, and this point is
eliminated. Every pulp packer who cyclones his tomatoes, and who uses
the same mesh of screen in his cyclone throughout the season, can use
the hydrometer for testing his pulp and can obtain uniformity without
the use of any complex apparatus.
The relation between the hydrometer reading and the desired specific
gravity is arrived at as follows:
When the boiling pulp approaches the finishing point, pour a sample into
a tall gallon measure and take the hydrometer reading immediately. Save
this sample, put it through the finishing machine along with several
additional gallons dipped from the kettle so as to get a good sized
sample through the finisher, taking care that the brushes are dry so as
not to dilute the sample, and have the specific gravity determined on
this finished sample when it gets cold. Take other samples from the
kettle at five-minute intervals, and save them for the same purpose,
after getting the hydrometer reading. We will say that one of these
samples, when tested for specific gravity, ran 1.035. The packer looks
up the hydrometer reading he got on this sample, when taken hot from the
kettle, and finds the reading was 18 degrees. He then knows that for
pulp of the degree of fineness he gets from his cyclone, 18 degrees on
his hydrometer equals specific gravity 1.035 after finishing. He may
also find that for the sample that was taken from the kettle five
minutes later, specific gravity 1.04 is equivalent to 24 degrees under
the same conditions. He now has established a relation between the
degrees on his hydrometer and the actual specific gravity. (The reason
for putting the pulp through a finisher before determining the specific
gravity is that he wants to make this test under the same conditions
that the buyer would make it.)
Now, if the packer decides that he wants to condense his season’s run of
pulp to 1.035, he simply cooks it all to 18 degrees on his hydrometer,
providing this is the comparison he got. Under his conditions he may
find that 16 or 20 degrees equals specific gravity 1.035. As stated
before, each packer must determine this for himself.
Method of Using Hydrometer
The cook should fill a tall gallon measure with the boiling pulp and
allow the spindle to sink into it slowly. Do not drop it into the pulp
so that it will fall suddenly, as the weight of the mercury or shot in
the bottom will carry it farther than it should go. It will be noticed
that the spindle will sink rapidly for a few seconds and then almost
come to a standstill. At this point—when a standstill is almost
reached—the reading should be taken. It requires quite a little time to
reach an absolute standstill, and this is not necessary, as it is easy,
with a little practice, to catch the other point and take the reading.
After the reading is taken, lift out the spindle, stir the pulp with it,
and allow it to sink again as a check determination, merely for the
satisfaction of being doubly sure. You will find that you get the same
reading unless you wait so long before making the check determination
that the sample has had a chance to cool considerably. As the test only
takes ten seconds, the two tests can be made in a half minute at the
most, which gives the pulp no chance to cool.
The hydrometer method is not absolutely fool proof, but it is as near
fool proof as anything I know of. Neither is it the acme of perfection
in a pulp testing method, but it is, in my opinion, far better than
anything that has been suggested to date. It is simple, and accurate
enough for all practical purposes, and any ordinary laborer can be shown
in a few minutes how to make the test, and the chances of error are very
small indeed.
This method will not work where crushed tomatoes are being condensed. It
is necessary that the tomato juice be previously cycloned. The pulp must
be homogeneous and not lumpy and full of skins to be capable of being
tested by the direct hydrometer method. When determining the finishing
point on crushed tomatoes it is best to use either the eye or the
specific gravity test by weighing.
Finishing the Cooking
The steam should be kept on the kettle continuously until the batch is
done. While testing is being done it is not necessary to shut off the
steam. If salt is used it should be scattered over the batch slowly a
few minutes before it is done. Salt increases the specific gravity, and
also the hydrometer reading. The relation between the hydrometer reading
and the specific gravity can be worked out just as well where salt is
used as when it is not used.
The addition of salt does add to the flavor, and some buyers prefer it,
while others do not. I have seen buyers pick out a poor quality pulp
with salt in it in preference to a much better quality with no salt.
As soon as the steam valve is closed the exhaust should be opened to
discharge the steam from the coil or jacket and allow it to cool as
quickly as possible.
It is often necessary to hold batches in the cooking kettles for some
time after the cooking is finished. This darkens the pulp some, both
because of contact with the air, while hot, and from long contact with
the sides of the kettle or coil, which remain hot. Cooked pulp should
therefore be discharged from the kettle just as soon as possible. If
held over 30 minutes in the kettle it should be given a momentary boil
before letting it down. Be careful to avoid contamination in the kettle
after the steam is shut off, such as cold dirty water dripping in from
rafters overhead, and leaks from pipes over the kettles through which
cold tomato juice is flowing. Such material will not become sterilized
merely by contact with the hot pulp, and may cause swells in the canned
product if there is very much of it.