Refractometry study of polymorphic transformations in animal fats and their blends

The aim of the work is to carrying out a comparative refractometry study of changes in the refractive index (n_D) during consecutive heating-cooling processes for a series of fats obtained from domestic and wild terrestrial animals, birds, marine mammals and fish, as well as a vegetable oil sample of four groups classified by iodine number (IN). Within the framework of the study, an analysis of liquid and solid lipid blends was carried out according to the ratio of components (0-20-50-70-100 %). Optical refractive indices and IN were measured by an Abbemat WR/MW digital refractometer (Austria) at three different wavelengths (436.4, 589.3 and 657.2 nm) in a temperature range from 20 to 70 °C. On the n_D = f (Т, °С) heating dependencies in the range of 30-45 °С for solid animal fats and vegetable oils, sharp bends were observed corresponding to the melting temperatures of α - and β₁ crystalline polymorphic modifications. In all the cases considered, the dependency sections were linear at higher temperatures and completely coincided with the cooling curves in the given temperature range, thus reflecting the structural homogeneity of the liquid crystalline phase. At temperatures below 30 °C, hysteresis was observed, resulting in cooling curves passing below the heating curves and containing bends corresponding to the melting temperatures of their low-melting α -forms. Both dependencies of the repeated melting-cooling cycle coincided with the cooling dependency of the first. The use of rapid cooling technique for fats melted at 40, 70 and 90 °С allowed desaturation processes of fatty acids in their liquid crystalline phase to be recorded. Regardless of the component ratio, a study of beef and pork fat blends demonstrated a decrease in the melting point of the α -forms in all cases to 25 °C, i.e. noticeably lower than the melting point of the original fat. However, at lower temperatures, the cooling curves diverge, indicating a difference in the melting temperatures of their low-temperature modifications depending on the component composition of the system, as well as a corresponding difference in structural, optical and biochemical properties.

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