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Title: Polymorphic transitions in cocoa butter studied by time-resolved small-angle and wide-angle x-ray scattering using synchrotron radiation
Author: Wye , Steven Matthew
Awarding Body: Keele University
Current Institution: Keele University
Date of Award: 2011
Availability of Full Text:
Full text unavailable from EThOS. Thesis embargoed until 05 Jan 2020
Abstract:
X-ray diffraction is a non-destructive technique which may be used to reveal information about the structure of materials, and is already a widely used tool in the study and determination of the polymorphic nature of cocoa butter and associated triacylglycerols which comprise cocoa butter. Simultaneous wide-angle x-ray scattering (WAXS) and small-angle scattering (SAXS) using synchrotron sources has been carried out on cocoa butter and cocoa butter containing selected additives. Conventional WAXS in a SAXS/\VAXS set up gives information on the cocoa butter atomic distances in the range of 3 - 8 angstroms (A), i.c. packing of cocoa butter chains, and is a familial' and powerful technique in enabling the identification of the polymorphic form of cocoa butter. Whereas SAXS provides information on length scales of 50 - 500 A, i.e. the layout of whole molecules in the form of layers. By using synchrotron sources and the associated high brilliance x-rays, it is possible to study the crystallisation in real-time. Whereas WAXS patterns can take a long time to develop, the associated SAXS patterns evolve before any peaks are observed in the WAXS and are concerned with the onset of crystallisation as well as changes in the pre-crystalline structure and pre-cursors prior to the onset of crystallisation which can determine the final characteristics and properties of the product. Similarly, during melting, the SAXS is the last to survive, in some cases long after the WAXS patterns have disappeared. By recording the SAXS patterns in real-time and using a suitably fast time resolution it is possible to observe the formation of the different polymorphic forms of cocoa butter. As well as transitions between polymorphs via intermediate phases which were observed, including the transition from form I to form III via an intermediate phase, and the formation of form V from the melt phase via an intermediate phase. SAXS studies also provided the ability to study cocoa butter systems containing additives such as sucrose, which itself produces a very strong pattern, making it impossible to identify cocoa butter using the conventional WAXS. It was observed that the addition of sucrose to cocoa butter at the onset resulted in a faster crystallisation rate and a shorter induction time. Other additives were used including silica particles of size 7 and 14 nm which provided a large surface area and also resulted in an increased crystallisation rate and a shorter induction time, seen to depend on surface area. The effect of stearic acid, which is one of the fatty acids present in the many triacylglyccrols of cocoa butter, was also observed in the following two situations: i) stearic acid present and already largely crystallised ii) stearic acid itself crystallising rapidly. It was found that cocoa butter crystallised at higher temperatures with stearic acid present, which promoted nucleation in particular.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.590678  DOI: Not available
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