Depending on the characteristics of the fatty waste materials, normally three main steps are performed:

• Extraction of the oil or fat from the fatty feedstock.

• Filtration and removal of contaminants.

• Neutralization or esteri fi cation of the FFA.

The extraction process is relevant in the meat and fish processing industry, as most types of fatty waste materials are normally associated with other materials, for example meat residues and bones or fish’s heads and viscera. Thus, it is necessary to separate the oil or fat from the remaining materials. Depending on the fatty residues to separate, the process varies, involving for example heating or solvent extraction.

At the laboratory scale, the waste animal fats (e. g. tallow, lard, or poultry fat) collected from slaughterhouses or food processing companies can be melted and filtered in order to obtain the fat and remove gums, protein residues, and suspended particles [67]. For extracting the fish oil from the fish’s residues, firstly fish’s viscera and heads can be cooked thoroughly in boiling water. The supernatant oil is taken from the top of the boiling vessel and placed in a separatory funnel, where the oil is washed (with distilled water at about 60°C) and separated from the water and solid residues. The fish soapstock is squeezed and as a result the crude fish oil containing some solid impurities is separated from the cake of fish dregs. The resulting crude oil is centrifuged and placed in a separatory funnel where it is washed. Finally, the oil is vacuum-filtered to remove any remaining impurities.

Industrially, the bulk of the material to be rendered consists of the leftover parts of a slaughtered animal (fats, bones, and other parts). The first step in the rendering
process is the milling and grinding of a mixture of materials to generate a mass that is screw-conveyed to a batch digester where it remains for 4-5 h to be cooked with saturated vapour at about 110°C, until it loses about 70% of its moisture content. Then the digester is opened and its contents are discharged into a percolator tank, heated by steam, where the liquid fat separates from solids by percolation and siev­ing. After percolation, the fat is centrifuged and/or filtered and sent to a decanter tank for storage and eventual final separation from the aqueous phase present. The solid material removed from the fat in this operation is added to the solid material from percolation. The solid material is hot-pressed generating more fat that is added to the one percolated for purification. The pressed material is milled in a hammer mill, and then sent for screening to obtain the particle size of flour. The material retained in screening returns to the mill. Passing through the screening the meat/ bone meal is bagged and stored for shipping and using in pet food [25].

The industrial process for extracting oil from fish by-products (e. g. heads, viscera, fish bones, and skin) operates in a continuous mode. Thus, after milling and grinding the fish, by-products are screw-conveyed to a continuous steam cooker with a resi­dence time of about 15 min. After cooking, the coagulated mass is pre-strained in a strainer conveyor before entering a screw press that separates the press cake from the press liquor. The press cake is disintegrated in a tearing machine (a wet mill) and dried in an indirect steam dryer with internal rotating blades. The meal passes through a vibrating screen furnished with a magnet to remove extraneous matter-like pieces of wood and metal (e. g. fish hooks) before entering the hammer mill. The ground meal is automatically weighed out in bags that are closed and stored. The press liquor then passes through a buffer tank before separation into oil, “stick” water, and fine sludge in a centrifuge. The oil passes through a buffer tank before water and sludge impurities are removed (polishing) in the oil separator. After polishing, the oil often passes through an inspection tank before storage in the oil tank [35].

Another possibility for extracting lipids from fatty waste materials is by using an organic solvent, such as я-hexane. For example, Nebel and Mittelbach [72] tested nine solvents for extracting fat from meat and bone meal, obtaining about 15% fat with all solvents, but я-hexane was found to be the most suitable solvent to perform the extrac­tion, because it is relatively cheap and has a low boiling point. The fat was then con­verted to methyl esters via a two-step process, whose quality was according to the European specification for biodiesel (EN 14214) except the cold-temperature behav­iour and the oxidation stability. Oliveira and Bechtel [74] described a solvent extraction procedure using a 2:3 solution of isopropyl alcohol/hexane (99.9% purity) for extract­ing lipids from salmon’s by-products including heads, viscera, frames, and skin.