Why is beef tallow generally solid at room temperature?

Beef tallow is generally solid at room temperature because it consists primarily of saturated fatty acids, specifically stearic acid and palmitic acid.12

In the chemistry of fats and oils, the physical state of a triglyceride is largely determined by its fatty acid composition. Fats that are composed mostly of saturated fatty acids—which lack double bonds—tend to be solid at room temperature because these straight-chain molecules can pack together efficiently. Conversely, fats containing a higher proportion of unsaturated fatty acids, which contain double bonds that create kinks in the molecular structure, are typically liquid at room temperature.234

Because animal fats like tallow are primarily saturated, they remain solid under standard ambient conditions, whereas many vegetable oils remain liquid due to their higher concentration of unsaturated fatty acids.23