To winterise an oil is to chill it and remove the solidified fractions — chiefly saturated glycerides (triglycerides) — so that the oil stays clear at low temperatures instead of turning cloudy. Salad oils and frying oils are winterised by cooling them slowly, holding them cold while the higher-melting components crystallise, and filtering the crystals out. The result is an oil that stays brilliant in a cold salad dressing or a refrigerated bottle. The removed fractions find uses elsewhere in food manufacturing. The process changes the oil’s composition slightly — removing some saturated fat — but its purpose is appearance and performance, not health: clear oil simply looks better and pours better cold.