Saturation matters in the behavior of fats because double bonds significantly alter the geometry and the regularity of the fatty-acid chain, and so its chemical and physical properties. A saturated fatty acid is very regular and can stretch out completely straight. But because a double bond between carbon atoms distorts the usual bonding angles, it has the effect of adding a kink to the chain. Two or more kinks can make it curl.
A group of identical and regular molecules fits more neatly and closely together than different and irregular molecules. Fats composed of straight-chain saturated fatty acids fall into an ordered solid structure— the process has been described as “zippering”—more readily than do kinked unsaturated fats. Animal fats are about half saturated and half unsaturated, and solid at room temperature, while vegetable fats are about 85% unsaturated, and are liquid oils in the kitchen. Even among the animal fats, beef and lamb fats are noticeably harder than pork or poultry fats, because more of their triglycerides are saturated.