Abattoir Practices and Animal Products Technology

RENDERING OF FAT

• Animal fat occurs initially mixed with tissues or cellular structure making the raw fat, a sort of fatty membrane.
  • Rendering refers to the extraction of fat from animal tissues by the action of heat, which causes the cells to burst and the melted fat run.
  • Raw fat should be removed as quickly as possible after slaughter, as delayed cause resolution of fat into the fatty acids and glycerol (rancidity) in the presence of moisture and lipase especially with the aid of high temperature.
  • In case, the fat is to be kept for a period extending a day, it may be chilled and preserved at a low temperature under dry conditions.
  • Normally, fat should not contain more than 1% of free fatty acids.
  • Its higher content of free fatty acids indicates that there has been some delay in rendering the raw fat after the slaughter of animals.

Procedure

• The raw fat is cut into small pieces to promote rapid and uniform rendering.
• The cut raw fat is spread on to the floor for about 2 to 3 hours or for a day to remove the moisture.
• Edible ox fats are rendered down to yield premier jus, which is separated into oleo oil and oleo stearin, but in wartime all edible ox fat is converted into dripping.
• Fat which is unfit for use is suet, dripping or premier jus goes chiefly for soap manufacturing, though some of the very low grades are used for the dressing of leather.
• Rendering is carried out in steam-jacketed containers (double jacketed vats).
• The tissues begin to burst as the temperature of raises and the fat contained in the tissues starts to melt.
• The whole mash has a milky appearance in the beginning but latter when about all the moisture has evaporated fat becomes clear.
• The tissues become brown in colour and settle down.
• At this stage, the heating is stopped and allowed to cool for sometimes.
• It is then sieved through to an iron sieve or strained through muslin cloth to separate the fat from the tissues and other impurities.
• The residual meat fibres left behind after all the fats have been extracted is called Cracklings or Greaves, which is used in poultry feeds.
• The tissues, which are removed after straining, are pressed in a machine to extract the lard or fat.
• During the entire period of heating, the fat is stained to prevent charring of the tissues.
• Heating on an open fire reduces the value of the products due to over heating and discoloration.
• After processing, the liquid fat is rapidly cooled to get a product, which is smooth, white and uniform in appearance.
• Pig fat and mutton fat also can be rendered in the same manner, but generally raw mutton fat is sold along with meat and only rarely rendered. If it is rendered it could also rendered along with beef and pork fat.
• Fat occurs in many regions of the pig carcass, the best quality fat being obtained from the peritoneal lining (leaf fat), the next best from the back fat, mesentery and omentum.
• The surplus fat of pigs is worked up into various qualities of lard.
• A pig of 90 kg live weight yields about 6.3 kg of lard.
• Sheep fat is rendered in the same way as beef fat or lard and, though it is not conve rted into oleo oil or oleo stearin on account of its strong flavour, it may be used as dripping when blended with other fats.
• Mutton fat is firmer and contains more stearin than ox or pig fat, and is used as preservative layer on the top of glass jars of meat paste.
• Subsequent processing of these fats produces commercial glycerine, a valuable commodity used in many commercial processes, including medicinal preparations, nitroglycerine, gunpowder, cordite and dynamite.
• Fat is also obtained from bones.
  • Fresh bones are processed under a steam pressure of 1.75 kg/sq. cm (25 lbs. per square inch) for one hour is sufficient to render the fat which is allow to trickle off.
  • Fat extraction is done earlier to the gelatin extraction.