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.
The cooking is done in copper-jacketed kettles, in glass-lined metal, or in wooden tanks, the tanks being heated with coils. The glass-lined tank has the advantage of very little metal coming in contact with the pulp and can be kept cleaner than wood. A question has been raised regarding the suitability of copper for a cooking utensil, though no positive objection has been made. The vacuum pan is coming into use for concentrating pulp, but has been little used in making the finished ketchup. The jacketed-kettle is used by most manufacturers, though the tank and coil is being adopted by those who wish to make large batches, as it is the more economical. Agitators are no longer used, as by proper handling of the steam and automatic traps, little burning occurs on either kettles or coils. The efficiency of the open tank or kettle is increased by providing a strong exhaust or suction for the air at the back and just above the top of the kettle. A swiftly moving current of air across the top of the kettle will carry off the steam and shorten the time of heating from ten to twenty per cent.