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.
A pulp may be reduced in a vacuum pan in about one-fourth the time
necessary in the open kettle and with a marked conservation of color and
flavor. The vacuum pan may be used for quick reduction and the finish be
made in open kettles in order to apply the heat long enough to spice and
to sterilize. There are possibilities along these lines which have not
been developed.
The time of cooking a batch of ketchup will depend upon the equipment
and the consistency of the finished product. With a good kettle or coil
and ample steam-supply a batch should be completed in from thirty-five
to forty-five minutes. This gives sufficient time to get the most
desirable flavor from the spices and is not so long as to result in
discoloration.