The mechanical action of rendering is created by reducing the size of the scraps with a grinder, while the thermal action is created by a cooker. The thermal input boils the water contained in the scraps being rendered. Animal scraps are usually about 50 percent water, all of which must be removed.
Some plants today employ a batch process, where scraps are dumped into
receiving bins, screw-conveyed through an electrically powered grinder, where they are
reduced in size. The next step is the cooker, which is usually a cylindrical
vessel with an outside steam jacket used for heating purposes. Since this is a
batch process, the cooker must go through the "load-cook-unload" cycle for
each charge of scraps processed. When the cooking cycle is complete, the entire
feed has been reduced to a slurry. Free fat is skimmed off the top, and the
residue is pumped to a fat press to make meal.
With the rising cost of energy, there has been a trend toward continuous rendering. In addition to being more energy efficient, continuous rendering reduces labor requirements, improves product quality, requires less space, and provides easier clean-up.
The process in continuous rendering is essentially the same as in batch rendering, except that a continuous cooker receives a steady charge of reduced scraps from the grinder and discharges a steady flow of products.
In a typical continuous rendering process, raw materials are prepared, sent to the cooker, and heated by steam jackets to evaporate the water. The evaporated water (contaminated steam) contains some foams and may contain ammonia, fat aerosol, ketones, food particles, and air. The cooking operation may last from 20 minutes to several hours, depending on the nature of the raw materials being processed. After most of the moisture is removed, the cooked materials are finish processed.
Removing moisture in the manner described is an inefficient process and typically takes approximately 1.5 to 1.9 pounds of clean boiler steam for each pound of water removed. The bulk of this energy is carried away as contaminated cooker vapor steam, which can be recovered to heat water for cleanup or recycled using mechanical vapor recompression.
