The development of rendering was
primarily responsible for the profitable utilization of meat
industry by-products, which in turn allowed the development
of a massive industrial-scale meat industry that made food
more economical for the consumer. Rendering has been carried
out for many centuries, primarily for
soap and
candle making. The earliest
rendering was done in a
kettle over an open fire.
This type of rendering is still done on farms to make lard
(pork fat) for food purposes. With the development of steam
boilers, it was possible to jacket the kettle to make a
higher grade product and to reduce the danger of fire. A
further development came in the nineteenth century with the
use of the steam "digester" which was simply a tank used as
a pressure cooker in which live steam was injected into the
material being rendered. This process was a wet rendering
process called "tanking" and was used for both edible and
inedible products, although the better grades of edible
products were made using the open kettle process. After the
material was "tanked", the free fat was run off, the
remaining water ("tank water") was run into a separate vat,
and the solids were removed and dried by both pressing and
steam-drying in a jacketed vessel. The tank water was either
run into a
sewer or it was evaporated
to make
glue or protein concentrate
to add to
fertilizer. The solids were
used to make fertilizer.
Technological innovations came
rapidly as the 20th century advanced. Some of these were in
the uses for rendered products and others were in the
rendering methods themselves. In the 1920's, a batch dry
rendering process was invented, in which the material was
cooked in horizontal steam-jacketed cylinders that were
similar to the fertilizer dryers of the day. Advantages
claimed for the dry process were economy in energy use, a
better protein yield, faster processing, and fewer obnoxious
odours attending the process. Gradually, over the years, the
wet "tanking" process was replaced with the dry process, so
that by the end of
World War II, most
rendering installations used the dry process. In the 1960's,
continuous dry processes were introduced by The Dupps
Company, one using a variation of the conventional dry
cooker and the other making use of a mincing and evaporation
process to dry the material and yield the fat. In the 1980's
high energy costs popularized the various "wet" continuous
processes. These processes were more energy efficient and
allowed the re-use of process vapours to pre-heat or dry the
materials during the process.