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Title: A study of the phase transitions of sugars found in chocolate
Author: Jawad, Rim
Awarding Body: King's College London (University of London)
Current Institution: King's College London (University of London)
Date of Award: 2013
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The overall aim of the PhD project was to investigate the phase transitions of chocolate sugars. To achieve this, the chemical purity and the re-crystallisation of both amorphous lactose and amorphous sucrose in a number of model systems were monitored. Both amorphous lactose and sucrose were prepared by spray- and freeze- drying of aqueous solutions of lactose and sucrose respectively. The amorphicity of the dried samples was successfully confirmed by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). NMR analysis was performed and peak areas of partially resolved doublets at 6.3 and 6.6 ppm were used to calculate the percentage (%) of α- and β- lactose present. The β/α anomer contents of amorphous lactose measured by 1H-NMR had standard deviations as low as 0.1% w/w (n = 6). Drying a lactose solution 4 h after its preparation led to almost 35% w/w difference in anomer composition within solid amorphous material compared to samples dried after only 30 min e.g. in freeze-dried samples, the β- content was 60 ± 0.1% w/w (4 h) and 25 ± 1.0% p/w (30 min). Polarimetery was used not only to investigate the kinetics of mutarotation for lactose solutions at different temperatures but also to confirm the purity of amorphous sucrose by measuring the content of invert sugars, if any. A design of experiments (DoE) approach was applied to investigate the impact of minerals, water vapour and sugar composition on the crystallisation of both sugars. Hot stage microscopy was utilised to monitor the phase changes during crystallisation upon heating. DoE work showed that a shortage of water during crystallisation of sucrose led to a higher crystallisation temperature Tcrys. This finding supports the empirical observation from pilot scale chocolate crumb making which states that increasing pressure, which will increase H2O in the head space, encourages more crystallisation as crystallisation temperature is predicted to be lower. Minerals also had an impact on recipe by reducing the rate of crystallisation at a concentration of 3% w/w NaCl. Lactose present in the recipe interacted with sucrose inhibiting the crystallisation of both sugars. DoE has also proven to be a very efficient methodology, by saving time and resources, to investigate the correlations that may exist among different variables.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available