Phase Transitions, Solubility, and Crystallization Kinetics of Phytosterols and Phytosterol−Oil Blends

Abstract

The thermal properties, solubility characteristics, and crystallization kinetics of four commercial phytosterol preparations (soy and wood sterols and stanols) and their blends with corn oil were examined. Differential scanning calorimetry (DSC) revealed narrow melting peaks between 138 and 145 degrees C for all phytosterol samples, reversible on rescan. Broader and less symmetrical melting transitions at lower temperatures with increasing oil content were observed for two samples of phytosterol-oil admixtures. The estimated, from the solubility law, deltaH values (34.7 and 70.7 mJ/mg for wood sterols and stanols, respectively), were similar to the DSC experimental data. Fatty acid esters of soy stanols differing in the chain length of the acyl groups (C2-C12) exhibited suppression of the melting point and increase of the fusion enthalpy with increasing chain length of the acyl group; the propionate ester exhibited the highest melting point (Tm: 151 degrees C) among all stanol-fatty acid esters. Solubility of phytosterols in corn oil was low (2-3% w/w at 25 degrees C) and increased slightly with a temperature rise. Plant sterols appeared more soluble than stanols with higher critical concentrations at saturation. The induction time for recrystallization of sterol-oil liquid blends, as determined by spectrophotometry, depended on the supersaturation ratio. The calculated interfacial free energies between crystalline sediments and oil were smaller for sterol samples (3.80 and 3.85 mJ/m2) than stanol mixtures (5.95 and 6.07 mJ/m2), in accord with the higher solubility of the sterol crystals in corn oil. The XRD patterns and light microscopy revealed some differences in the characteristics among the native and recrystallized in oil phytosterol preparations.

Tangible Health Benefits of Phytosterol Functional Foods

“…Yet, sterols and mixtures of sterols/stanols have higher solubility than stanols alone. Among these, wood stanols were slightly better solubilised in one study than their soya counterparts (Vaikousi et al 2007). The addition of water to oil tends to decrease phytosterol solubility by twofold (Christiansen et al 2002).…”

“…Enova oil marketed by ADM). The partially polar nature of phytosterols, due to the presence of hydroxyl group, may account for this (Vaikousi et al 2007).…”

“…The high melting point of phytosterols, which is in the range of 138-145 • C (Vaikousi et al 2007), affects their solubility. An inverse relationship between log solubility and melting points for many aromatic compounds has been shown before by Yalkowsky (1981).…”

Dispersed free phytosterols as structuring agents in lipid systems with reduced saturated fat

“…Observing all other blends, the presence of FP, in general terms, did not modify their crystallization profile. The low effect on crystallization profile of these blends is due to the high melting point of FP, about 140 °C for FP, and their low solubility in fatty systems, 2 -5% (25 -50 °C) in corn oil, according to data reported by Vaikousi et al, 2007. Once the solubilized FP co-crystallize with triacylglycerols, and they are mainly responsible for the modification of the crystallization behavior of the systems. In these blends, a great fraction of these compounds was probably not solubilized, thus causing little effect on the thermal behavior of the fatty mixtures.…”

“…It was also observed for these blends a X-ray diffraction peak at 2θ of about 15.02° representing the crystal arrangement of the FP molecules. Vaikousi et al, 2007 reported X-ray diffraction lines related to FP at 2θ of 4. 6, 6.9, 9.35, 11.7, 14.85, 16.5, 18.4 and 21.9°.…”