Fig. 1 shows the stop-and-return curves recorded during isothermal
crystallization of palm oil at different temperatures. The crystallization
mechanism at 15, 18 and 20 °C was clearly different from that
observed at 25 and 28 °C. At the lower temperatures, crystals were
present from the start of the isothermal period onwards. After a certain
time, the melting peak that was initially present seemed to diminish
while another one with a higher peak maximum appeared.
The shift in peak maximum indicates the occurrence of a polymorphic
transition. The crystallization thus occurred in two steps at a lower
temperature. At the higher temperatures, crystals only started to appear
a certain time after the isothermal temperature was reached and
the melting peak steadily growed further as the isothermal period
progressed. In this case, crystallization thus occurred in one step.