Chemical and enzymatic interesterification of tristearin/triolein-rich blends: Microstructure and polymorphism

Mixtures of fully hydrogenated canola oil and high-oleic sunflower oil were interesterified chemically and enzymatically with sodium methoxide and Candida antarctica lipase, respectively. Polymorphic behavior and crystal morphology of the 10-100% blends before and after interesterification were studied using powder X-ray diffraction spectroscopy and polarized light microscopy. X-ray analysis revealed a predominance of the beta' form in the IE-blends (interesterified, both chemical and enzymatic) and the 0 form crystals in the NI-blends (non-interesterified). Dilution and crystallization at 20, 30, 40 and 50 degrees C did not change the polymorphism of the NI-blends, but did in the IE-blends. Small changes in the long spacings of the blends after interesterification were observed. Small-angle reflections centered at 45 angstrom suggested the presence of a 2L lamellar crystalline structure. Needle-like crystals were predominant in the IE-blends while large symmetrical spherulites were observed in the NI-blends crystallized at 30 degrees C for 24 h. Over 75% of the structural elements had areas between 1 and 20 mu m(2) in the NI- and IE-blends, and the NI-blends showed a higher percentage of crystals in this range compared to the IE-samples. The box counting dimension (D-b) increased as a function of crystalline area fraction in polarized light micrographs of the samples. Moreover, a higher Db in the chemically interesterified sample relative to the enzymatically interesterified sample at an equivalent area fraction, was attributed to the homogeneity in the distribution of crystalline mass.