14B-40

J. H. OH and B. G. Swanson. Food Science and Human Nutrition, Washington State University, P.O. Box 646736, Pullman, WA 99164-6376

The major fat constituents, triacylglycerols (TAGs) exhibit characteristic polymorphic crystalline structures. The general polymorphic forms of TAGs are a, b', and b crystals, each exhibiting characteristic molecular packing in crystal lattice and physical properties such as melting point and heat of fusion (DH). The unstable a or b' crystals are prone to transition to the more stable undesirable b crystal.

Our objective was to characterize the physical properties of polymorphic forms of synthesized tristearin (C18:0) as a model TAG.

Tristearin was synthesized from stearoyl chloride and glycerol in the presence of pyridine, and purified using Silica gel column chromatography. The polymorphic forms of tristearin were induced from a melt by thermal tempering. Powder X-Ray Diffractometry (XRD) and Differential Scanning Calorimetry (DSC) were used to characterize molecular packing structure, and to determine melting points and DH of polymorphic forms.

The synthetic yield of tristearin was 19.8%. The XRD short spacings of a and b crystals exhibited typical hexagonal or triclinic packing structures. The melting points and DH of a and b crystals observed at a scan speed of 5°C/min with a DSC were 56.1°C and 73.6°C, and 120.0 J/g and 189.4 J/g, respectively. The greater the heating rate of the DSC, the higher the melting points and smaller the DH of the b crystal. Two b' crystals of tristearin were identified as b'₂ (62.4°C) and b'₁ (66.5°C). The a to b and b' to b transition of tristearin near a melting point (56°C) occurred within 4 and 2 hrs.

The study of polymorphism using a simplified tristearin model will aid understanding of polymorphism and polymorphic transition of natural fats. The physical properties of polymorphic forms and stability of desirable a or b' crystals are key factors to maintenance of quality in high fat foods.