RESULTS AND DISCUSSION
Crystal structure and polymorphism
Binary mixtures of dry fractionated pequi oil (source of POP) and solvent fractionated Kpangnan butter (source of SOS) were prepared at different proportions and kept at room temperature to statically crystallize them for 4, 12, and 60 days. In a previous study, we showed that a CBE’s compatibility with CB depends on the main triacylglycerols present in the CBE and their crystal polymorphic structure. This is mostly influenced by interactions between the main TAGs in CBE and CB, namely POP, POS, and SOS (Ghazani & Marangoni, 2018). Small and wide-angle powder X-ray diffraction peaks after 4, 12, and 60 days of storage at room temperature are shown in Tables 1 to 3, respectively. As shown in Table 1 and Fig.1, the most prominent reflections in the WAXS region for 100% solvent fractionated Kpangnan butter were observed at 4.74 Å (s), 4.50 Å (m), 3.9 Å (S), and 3.63 Å (w), suggesting the γ crystal structure of SOS (Sato, Arishima, Wang, Ojima, Sagi & Mori, 1989). In this field, vs, s, m, and w, represent ”very strong”, ”strong”, ”medium” and ”weak” peak intensities. The long-spacing observed for 100% fractionated Kpangnan butter was at 73.7 Å, corresponding to the reflection from the (001) crystalline plane.
Similar short-spacing has been reported by Sato, Arishima, Wang, Ojima, Sagi & Mori (1989) for SOS in γ crystal form at 4.72 Å (s), 4.50 Å (m), 3.88 Å (s), and 3.63 Å (w), while Wang, Sato, Sagi, Izumi & Mori (1987) reported an identical long-spacing pattern at 73.7 Å for SOS in γ crystal form. Similar diffraction patterns in both SAXS and WAXS regions were observed for 10% to 40% mixtures containing fractionated pequi oil. With increasing amounts of fractionated pequi oil, from 50% to 70%, a mixture of γ and pseudo-β’2 polymorphic forms was obtained. This can be deduced from the presence of a reflection at ~42 Å in the SAXS region and WAXS reflections at 4.23 Å and 3.96 Å that indicates the presence of the pseudo-β’2form of POP (Sato, Arishima, Wang, Ojima, Sagi & Mori, 1989). Moreover, the long-spacing value at 42 Å corresponds to the double chain length structure of the POP molecule in the pseudo-β’2 crystal structure (Sato, Arishima, Wang, Ojima, Sagi & Mori, (1989). The binary mixtures containing 80% and 90% fractionated pequi oil showed a mixture of mainly triclinic crystal structures in the β2crystal form plus pseudo-β’2 form. After static crystallization at room temperature for four days, 100% fractionated pequi oil showed a pseudo-β’2 crystal structure.
After 12 days of storage at 22 oC, in all mixtures, the pseudo-β’2 form had transformed to the stable triclinic β2 form, except for 100% fractioned pequi oil and 100% fractionated Kpangnan butter, which displayed the pseudo-β’2 form, and mixtures of pseudo-β’2 and β2crystal forms, respectively. A double chain length structure characteristic of the pseudo-β’2 form was observed in both samples (Table 2 and Fig.1).
After 60 days of storage at 22 oC, all mixtures had transformed to the triclinic β2 form, except for 100% fractionated pequi oil. Two strong reflections in the WAXS region at 4.30 Å and 3.97 Å, and a long spacing of 41.9 Å were observed (Table 3). This suggested the presence of a pseudo-β’1 polymorphic form, similar to the crystal polymorphic form of pure POP in the pseudo-β’1 crystal form (Sato, Ueno & Yano1999). Wang, Sato, Sagi, Izumi & Mori (1987) showed that the polymorphic transition rate for the POP molecule was very slow, and the transition from pseudo-β’ to β2 took about 15 days after storing at 30oC. While for SOS molecule, the transition rate from pseudo-β’ to β2 was fast (16 hours at 35oC).
The addition of fractionated Kpangnan butter to fractionated pequi oil accelerated the crystal polymorphic transition to the triclinic form. The very strong peak at 4.6 Å indicated the β crystal structure for all blends. It is important to notice that after 60-days of storage, the high SOS content in fractionated Kpangnan butter induced a transformation to the β1 polymorphic form for all mixtures. The presence of two strong peaks at 3.73 Å and 3.69 Å indicated the completion of the polymorphic transition to the most stable triclinic form (β1) (Fig. 2). Sato, Arishima, Wang, Ojima, Sagi & Mori (1989) reported a similar diffraction pattern for pure SOS in the β1 crystal form.
Table 1- Small and wide-angle powder X-ray diffraction data for the different crystal polymorphic forms of fractionated pequi oil and fractionated Kpangnan butter binary mixtures after 4 days of storage