Thermal and structural changes of rapeseed oil during isothermal storage at low temperature

Abstract

Abstract The crystallization behavior of rapeseed oil during isothermal storage was investigated using differential scanning calorimetry (DSC), and wide-angle and small-angle X-ray scattering analyses. Rapeseed oil is composed of many kinds of triacylglycerols. When this oil was cooled to its freezing temperature, a meta-stable phase was immediately formed. After a certain period, exothermal crystallization was induced. DSC analysis showed that the induction period, which is defined as the time from the beginning of isothermal storage until an exothermal change is detected, was affected by the storage temperature but not by cooling rate and sample size. Exothermal behavior after the induction period at −25 °C was different from those stored at −17 and −20 °C. X-ray scattering analysis clarified that this difference was due to differences in the formation of the meta-stable phase during the induction period. At −17 and −20 °C, the triple chain-length (3L) structure transformed to a double chain-length (2L) structure, where the β form was the dominant component. In contrast, at −25 °C, the 2L structure was formed while maintaining the 3L structure, and further structural changes were not observed during storage. It has been suggested that the induction period for the crystallization behavior of rapeseed oil during isothermal storage is governed by the kinetics of the meta-stable phase transition. It is important to understand this meta-stable phase transition for controlling the crystallization of oil in foods because the induction period are related to the subsequent crystallization behaviors that affects food quality, and is responsible for food deterioration.