Crystallization of cocoa butter with and without polar lipids evaluated by rheometry, calorimetry and polarized light microscopy

In this investigation, we evaluated the crystallization process of triacylglycerols (TAG) in unrefined cocoa butter (CB) and in CB without polar lipids (CB-WPL), under non-isothermal and isothermal conditions. These conditions were obtained by cooling from 80 degrees C at a cooling rate of 1 degrees C/min until achieving particular crystallization temperatures (i.e., 18.5 degrees C, 19.0 degrees C, 19.5 degrees C, and 20 degrees C), at which isothermal crystallization studies were done. Phase shift angle rheograms (delta) showed that the onset of crystallization during the non-isothermal stage, independently of the crystallization temperature (T-Cr) used, was 59.8 +/- 3.7 degrees C in CB and 39.5 +/- 1.5 degrees C in CB-WPL. These results pointed out the nucleating role of phosphatidylcholine (26.53 +/- 0.04%) and phosphatidylethanolamine (57.14 +/- 0.07%), the main phospholipids present in the CB used in this investigation. Under similar crystallization conditions, differential scanning calorimetry (DSC) provided an onset of crystallization during the non-isothermal stage of 15.0 +/- 0.0 degrees C in CB-WPL and of 15.5 +/- 0.1 degrees C in CB. Then, delta rheograms were more sensitive to detect the nucleating role of phospholipids than DSC. Under isothermal conditions, both DSC and delta rheograms showed that shorter times were needed in CB-WPL to complete crystallization than in CB. In the same way, at all T-Cr investigated, a higher crystallization rate was achieved in CB-WPL than in CB, this as measured by the crystallization rate constant (z) of the Avrami equation. These results showed that crystallization of alpha and beta' polymorphs took longer in CB than in CB-WPL, pointing out that polar lipids delayed the alpha-to-beta' polymorphic transition. Additional results confirmed that polar lipids affect the kinetics of TAG crystallization in CB. However, polar lipids do not affect the thermodynamic properties of the crystals [i.e., heat of fusion (Delta H-M), temperature of fusion (T-M')], the storage modulus at the end of the crystallization process (i.e., G' of pseudo-equilibrium), or the mechanism of crystal growth (i.e., the Avrami index, n).