The recycling of waste cooking oil (aka restaurant grease, used cooking oil or yellow grease) is a process known as “rendering”. During the rendering process fatty acid is separated from the moisture, the solids and any impurities that are present in the waste cooking oil. The rendering of waste cooking oil produces one usable element and several waste elements.

Fat is the usable element derived from waste cooking oil through the rendering process. Fat is a commodity product traded as Yellow Grease. Yellow Grease is primarily used in the animal feed and ingredient industry as a source of dietary energy but also contributes to overall body function. Yellow grease may also be utilized in some alternative energy processes as a feedstock for the production of biodiesel.

Moisture is the most common waste element contained in waste cooking oil. Moisture must be separated from the usable fat element through a combination of heat and centrifugal processes and then treated in the wastewater process. Transporting and processing the moisture found in waste cooking oil is a significant contributor to cost of processing waste cooking oil. Moisture is detrimental in fats as it accelerates corrosion of equipment and contributes to rancidity of the fat element.

Insoluble impurities are another waste element contained in waste cooking oil. Impurities usually consist of small food particles or fibers that must be separated from the usable fat element through a combination of screening and centrifugal processes and then dried for disposal in a solid waste facility or land application.

Unsaponifiable fats are another element contained in waste cooking oil that detracts from the value of the usable fat. Unsaponifiables have unknown dietary contributions and can reduce the overall energy value of the fat. The detection of pesticides, toxins or other chemical compounds in waste cooking oil can significantly reduce the value of the batch or make it worthless.