Study on dry fractionation behaviour and crystallization kinetics of coconut oil

Abstract

Dry fractionation of oils consists of two stages, namely crystallization to produce solid crystals in a liquid matrices and filtration to separate the crystals formed from a liquid matrix. Changes that occur at the molecular level during the crystallisation process such as nucleation, crystall growth and changes in the phase behavior (folymorphism, solid-solutions). The overall changes at the molecular level is highly influenced by the treatment at the physical level in the form of heat removal. Methods of cooling (crystallization temperature, cooling rate and duration of the crystallization is applied) will largely determine the success of coconut oil fractionation process. This research aims to study the typical fractionation method for coconut oil at pilot plant scale (120 kg) and identify the conditions and essential requirements that must be managed and maintained in a dry fractionation stages of coconut oil; establish the crucial cooling systems in the dry fractionation of coconut oil as a guide in designing practical coconut oil fractionation; establish an effective cooling procedure to produce coconut oil fractions with a high content of MCT; determine the effect of cooling rate and crystallization temperature on the composition and profile of TAG, the kinetics of crystallization and melting properties of coconut oil fractionation products ; determine the relationship (model equations) between the parameters of fractionation with cooling procedure during the crystallization process, to predict the TAG composition, physical and thermal-mechanical properties of fractions produced. Coconut oil is dominant with lauric (C12:O), miristic (C14:0) and caprilic (C8:0)acid which composed the main trilaurin (LaLaLa), caprodilaurin (CaLaLa) and dilauromiristin (LaLaM) TAG. The content of lauric, miristic and caprilic acid in coconut oil are 51.73; 15.57 and 10.61%, respectively; while LaLaLa, CaLaLa and LaLaM TAG are 20.43; 16.23 and 15.38%, respectively. Coconut oil contains medium chain trigliserides (MCT) of 53.71%, trisaturated (St3) of 82.54%, disaturated (St2U) of 14:24%, monosaturated (StU2) of 3:22% and the proportion of high/low-melting TAG (S/L) by 49.25%. Coconut oil has a high SFC at low temperatures and a sharp decline to a temperature of 25 °C and then constant up to a temperature of about 30 ºC. Coconut oil’s SFC with about 32% was measured in temperature interval 21-22 °C, indicating that coconut oil has a good spreadibility at ambient temperature for countries that have 4 seasons. Coconut oil has SMP ranged between 24.5-26.2 °C, the water content of 0.021% and a free fatty acid content of 0.018%. Our study showed that there were three distinct cooling regimes critical to crystallization process, i.e. initial cooling, critical cooling and crystallization regime. Initial cooling was cooling from certain temperature that the oil has been rejuvenated and there was no more crystalized TAG (in this study rejuvenation was done at 70 oC for 10 minutes) to the onzet of oil crystallization temperature. For coconut oil the onzet of crystallization temperature was found at 29 oC. Critical cooling was cooling from the onzet of crystallization temperature (29 °C) to crystallization temperature. We predicted that during the critical cooling regime the crystal nucleation process was intensively occured (propagation). Crystallization regime was cooling to keep the oil temperature constant at predetermined crystallization temperature. It is estimated that crystallization regime was the stage of crystal nuclei merging to form larger crystals (crystal growth). In the first regime, melted coconut oil might be cooled quickly to save time but in the second regime should be done with a cooling rate of less than 0.176 °C/min to produce physically stable crystal. Oil with high MCT content could be obtained from olein fraction of coconut oil. At the crystallization temperature 21.30-21.73 °C for the critical cooling rate between 0.013 to 0.176 °C/min, the higher MCT content of olein fraction were produced by the lower critical cooling rate and the longer crystallization process. Critical cooling rate has positive correlation with the S/L ratio, the content of St3 and SFC profile of olein fraction but has negative correlation with the content of St2U and StU2 TAG. Interval crystallization temperature between 21.30 to 21.73 °C produced the S/L ratio, the content of St3 TAG and SFC profiles of olein fractions lower and the content of St2U and StU2 TAG higher than the temperature interval below or above it. Coconut oil fractionation more effective in hihger crystallization temperature or lower critical cooling rate. In these cooling treatments, St3 TAG which has high melting point would be concentrated at stearin fraction, while St2U and StU2 TAG and MCT would be at olein fraction. Therefore, it will increase melting properties of stearin fraction and decrease olein fraction. Avrami and Gompertz models are able to quantitatively describe coconut oil crystallization kinetics. Lower critical cooling rate decreases Avrami index, crystallization half time and induction time but increases crystallization rate constant and maximum increase rate in crystallization. Crystallization temperature has positive correlation with the crystallization rate contstant and Avrami index but has negative correlation with induction time and maximum increase rate in crystallization. Critical cooling rates and crystallization temperatures only effected on the thermodynamics and crystallization kinetics of coconut oil but not on its polymorphic occurrence. This study had successfully obtained a typical dry fractionation for coconut oil at pilot plant scale (120 kg) and had resulted in an effective cooling procedure to produce oil fractions with physico-chemical properties as expected. Conditions and essential requirements that must be managed and maintained in a dry fractionation stages of coconut oil had been identified and were known, so the fractionation process for specific purposes have been able to be designed practically.