After washing, the tomatoes may be reduced to a pulp in one of three
ways: by running the raw tomatoes directly through a grinder and into
the cyclone; by passing the tomatoes through a scalder and into the
cyclone; and by turning the tomatoes into jacketed-kettles or tanks and
cooking them until soft before running through the cyclone. There is a
difference in the product obtained by these methods. The first one gives
a somewhat larger yield, as the hard parts are cut and torn so that more
will be squeezed through the sieve. The color is generally stronger and
inclined to the purple side rather than the yellow. The color, however,
does not hold so well when exposed to light. The pulp inclines to froth
and there is a marked separation of red pigment on the top. A raw pulp
will begin to separate into a clear layer below and solids at the top in
about fifteen to twenty minutes after standing in a tank. This is due to
the air incorporated in the solids and possibly to difference in
specific gravity, and not to fermentation, as frequently alleged.
Changes will take place more rapidly in such pulp than in that made from
scalded fruit.
There is not a great deal of difference between the second and third
methods, the object in both cases being the same. If a long scalder be
used, the skins will be loosened and the tissue softened so that it will
be easily separated from the green parts, hard cores, or black rot.
There will be no acquisition of color from the stems to discolor the
ketchup. The loss is a little heavier in scalder heating than where the
fruit is cooked in the tanks, but there is the compensation that there
is less carrying of hard or objectionable material. A scalder to be
effective should be much longer than that used in canning, or a greater
volume of steam should be used. The tomatoes should be heated to about
180 deg. F. There is little choice in the two methods, but the
preference is with the scalder, both being preferred to the raw ground
fruit. A pulp made in this way separates slowly and there will be no
material increase in organisms for a rather long time (three or four
hours). There is less separation of pigment on cooking and there is a
clean look to the tissue under the microscope.
In making pulp it is important that the paddles in the cyclone be held
back from the screen and the juice driven through by centrifugal force
rather than by hard grinding. When kept well back, the green butts,
cores, and tissues which have been hardened by brown mold are carried
over the end so that there will be fewer black specks in the finished
pulp and it will have a better appearance under the microscope.
The pulp should be conveyed immediately from the cyclone to the cooking
kettle, and the next operation begun at once. A storage tank is
unnecessary when there is large cooking capacity, and in most cases it
is a source of trouble rather than a help. A sample should be taken as
soon as the batch is drawn, and the specific gravity determined so that
the proper quantity may be used to give a finished product of uniform
consistency. Assuming that 500 gallons of pulp will give a normal
finished batch, if the tomatoes are watery, it may require 550 gallons
or more to give the same result when concentrated. This is easily
calculated from the specific gravity so that reasonably uniform results
may be obtained. Samples should also be tested for acidity once or twice
each day so that the addition of vinegar can be governed accordingly.
The concentration of pulp will vary from 40 to 60 per cent depending
upon its condition and the weight of body desired.