New Triclinic Polymorph of Tristearin

Here we report on the discovery of a new triclinic crystal polymorphic form (β1) of 1,2,3-stearoyl glycerol, or tristearin (SSS), CAS No. 555-43-1. This most stable polymorph of SSS has a melting point of 75.7 ± 0.5 °C and an enthalpy of fusion of 243.9 ± 10.5 J/g. The most stable polymorphic form for this well-known trisaturated monoacid triacylglycerol was obtained by static crystallization from a dilute solution of SSS in acetone at 4 °C over a period of 2 years. In contrast to the well-known β2 form of tristearin, which displays wide-angle X-ray scattering reflection positions (relative intensities) at 4.61 Å (very strong), 3.86 Å (medium), and 3.70 Å (medium), the new β1 tristearin displayed short-spacing positions (relative intensities) at 4.57 Å (weak), 3.84 Å (weak), and 3.68 Å (very strong). The small-angle X-ray scattering long-spacings corresponding to the lamellar sizes from the (001) plane were 44.91 Å, in contrast to 45.21 Å for the β2 polymorph. This work clarifies controversies and uncertainties related to the number of triclinic polymorphs in trisaturated monoacid triaclyglycerols.