| Lipids play an important role in the daily life of human and the economic systems.As a kind of common lipid,cocoa butter is widely used in the production of chocolate.However,fatty acid and triglyceride existing in the chocolate are always crystallizing under a temperature fluctuation,and it may present a kind of phenomenon called polymorphic transformation,and it is also called "blooming" in the chocolate industry,which has affect the taste and the sensory quality to a large extent.Statistically,bloom problems accounts for 65%-80%among the a great number of problems in the food qualities of chocolate.This phenomenon has become a vital issue affecting the whole chocolate confectionery industry,and it has not been solved effectively so far.In this study,Trilaurin(C 12:0),Tripalmitin(C16:0)and Tristearin(C18:0),which are all belonging to saturated mono acid even numbered triacylglycerols,have been used to investigate the state of the crystallization,the melting/cooling behaviour and their polymorphic transformation,trying to find out the regular pattern of the crystallization behavior of this series of triglyceride.Therefore,the major results are as follow:1.Differential Scanning Calorimetry(DSC)has been used to measure the thermal property of crystallize and melting behavior of Trilaurin(LaLaLa),Tripalmitin(PPP)and Tristearin(StStSt).The results indicate that the sample of the saturated triglyceride with the even numbered carbon has formed a melting peak before their melting point during the process of heating.The length of acyl chains connecting with glycerol skeleton have been increased,so that the peak temperature of the melt have become higher.This may be due to the extension of the carbon chains lead to increase the intermolecular force between molecules.The more closely linked to each other,the more energy it may be absorbed to destroy the stable structure of the solid crystal in order to transform into liquid completely(i.e.,melting process).In addition,in the same and extremely slow heating rate(1?/min),an endothermic peak which was very weak has been formed before their melting peak.Therefore,we speculated that a crystallized conversion of this series triglyceride was happening,and as a result,an unstable ?' form has been formed.2.Determining the crystal structure of polymorphs of all 3 triglycerides above by using X-Ray Powder Diffraction(XRD).The result suggests that,they form a stable crystalline of? form respectively at the ordinary temperature.3.By using FTIR spectroscopic analysis,we have confirmed the characteristic infrared absorption peak of the stable ? crystalline of saturated mono acid even numbered triacylglycerols.The result shows that all triacylglycerol samples appear a single band around 717cm-1 at a room temperature,which is consistent of the characteristic IR band(717cm-1)very well.Therefore,it proved that this kind of triglyceride may form a stable ?crystalline at the room temperature.Besides,in the controlled temperature determination of FT-IR,a new absorption band(272.01cm-1)of Tripalmitin was existed around 59?,which is different from the FT-IR absorption peak at the normal temperature,and it indicates that a new crystal form of PPP may take shape.This kind of crystal formation may distinguish the stable crystal form existing at room temperature.4.Finally,through Scanning Electron Microscope(SEM),the crystal morphology of this 3 kinds of saturated triglyceride with even carbon at the normal temperature has been studied.The result of the scans show that,although 3 kinds of triglycerides we selected are belonging to the same series,they have showed significant differences in the micro morphology:LaLaLa presents a rectangle,flake shape,and rough surface with sparse needle hole hollow can be observed;PPP is a parallelogram,flake shape,and the superposition between layers is irregular;however,StStSt shows that it was shaped into a crystal ball consisting of different crystal pieces of irregular size. |
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