NOVELTY - The method for the production of fat, comprises contacting a liquid phase and/or a residual cell mass or any fraction or fractions of the liquid phase and/or a residual cell, separated before or after a fat recovery or in connection with the fat recovery, from a single-cell mass obtained from a single-cell oil production process, with a microorganism capable of fat production in a cultivation medium, and allowing the organism to produce fat and recovering the resulting fat or passing the microorganism mass to a single cell oil production process.    USE - Method for production of fat useful as a biofuel, biodiesel and/or renewable transportation fuel or as a raw material or component (claimed).    ADVANTAGE - The method improves total fat yield and carbon and energy balance of single-cell fat production processes, reduces environmental burden caused by single-cell fat production, improves manufacturing efficiency of single-cell fat, is simple and very modest in its energy demand, and reduces amount of recycled water.    DETAILED DESCRIPTION - The method for the production of fat, comprises contacting a liquid phase and/or a residual cell mass or any fraction or fractions of the liquid phase and/or a residual cell, separated before or after a fat recovery or in connection with the fat recovery, from a single-cell mass obtained from a single-cell oil production process, with a microorganism capable of fat production in a cultivation medium, and allowing the organism to produce fat, and/or contacting a single-cell suspension or a cell mass obtained from a single cell oil production process, or a liquid phase obtained from the process, or a microorganism cell suspension formed in another purpose, a cell mass or a liquid phase with microorganism capable of fat production, in a cultivation medium, and allowing the organism to produce fat, and recovering the resulting fat or passing the microorganism mass to a single cell oil production process. The method is integrated as part of a single-cell oil production process. The method functions separately of the single-cell oil production process. The cultivation medium comprises some component or components and is supplemented with nutrients. The cultivation medium of the microorganism capable of fat production is supplemented with natural or human activity based fractions, which contain hexose sugar, pentose sugar, carbohydrate polymers, or fractions containing cellulose, starch, non-starch polysaccharide or lignocellulose. The cultivation of the microorganism capable of fat production is carried out by utilization of cell suspension, cell mass, liquid phase or residual cell mass originating from a single-cell oil production process, or a microorganism cell suspension formed in another purpose, cell mass or liquid phase or its any mixture and/or any fractions or utilization of the fractions by a chemical, enzymatic and/or microbiological treatment. A part of cell suspension, cell mass, liquid phase or residual cell mass or its mixture originating from single-cell oil production process, and/or microorganism cell suspension formed in another purpose, cell mass or liquid phase is subjected to chemical and/or enzymatic hydrolysis and the obtained hyrolysate is contacted as such and/or together with unhydrolyzed cell suspension, cell mass, liquid phase or residual cell mass or its mixture with hydrolyzed or unhydrolyzed biomasses originating from other sources, with the microorganism capable of producing fat and the microorganism is allowed to grow and produce fat. The fat produced by the microorganism is recovered or the microorganism mass is passed to a single-cell oil production process, which comprises use of a recovery process for cells and fat or fat, which is the same as that used in the single-cell oil production process. The microorganism capable of producing fat is capable of producing enzymes disrupting cell structures and/or hydrolyzing ingredients of cell structures. The part of the cell suspension, the cell mass, the liquid phase or the residual cell mass originating from the single-cell oil production process, or the microorganism cell suspension formed in another purpose, the cell mass or the liquid phase is contacted with a first microorganism capable of producing enzymes disrupting cell structures and/or hydrolyzing ingredients of cell structures. The treated cell mass is contacted with a second microorganism. The fat provided by the microorganism is recovered or the microorganism mass is passed to a single-cell oil production process, which comprises use of a recovery process for cells and fat or fat, where the recovery process is the same as that used in the single-cell oil production process. The part of the cell suspension, the cell mass, the liquid phase or the residual cell mass originating from a single-cell oil production process, or the microorganism cell suspension formed in another purpose, the cell mass or the liquid phase is contacted with a mixed culture of the first and the second microorganism. The ability of the microorganism capable of producing enzymes disrupting cell structures and/or hydrolyzing ingredients of cell structures, to produce such enzymes or any of such enzymes is inducible. The microorganism capable of fat production is mold, yeast or algae. The microorganism capable of fat production is mold. The microorganism, which produces enzymes disrupting cell structures and/or hydrolyzing ingredients of cell structures, is mold. The microorganism used for fat production is reused after the recovery of fat or as such in the single-cell oil production process as a starting material, such as a component of the cultivation medium. The method comprises treating a cell mass from some other microorganism process or environment such as a microorganism from biotechnical processes, yeast mass from ethanol production, or algal mass from a body of water or treating biomass obtained from other sources such as industrial, community, household waste and/or agricultural products or by-products such as wastes or products of the type containing fat, in a partial replacement of the streams originating from a single-cell oil production process, or a fat-containing cell suspension.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic flow diagram of a method for the production of fat.