Tertiary processing is the stage in a food manufacturing plant where partly prepared or cooked products undergo their final transformation into finished foods. It follows primary processing (cleaning, sorting, milling) and secondary processing (cooking, mixing into recognisable products): here, cooked components are assembled, filled, baked, coated, or otherwise finished into ready meals, snacks, confectionery, or packaged goods. The stage typically involves high-speed lines, precise portioning, and packaging under hygienic conditions, since these products often go straight to the consumer. Because tertiary processing adds the salt, sugar, fat, and additives that define ultra-processed foods, it is the stage most scrutinised in debates about diet and health.