Abstract/Summary

The presence of amorphous and crystalline forms of the various sugars present in chocolate plays a significant role in the production of, and taste/texture of the final product. A great deal of work has focussed on characterising the physical forms of fats in chocolate, and much less upon the sugars. This thesis focusses mainly upon the use of laboratory-based X-ray diffraction (in particular, powder X-ray diffraction), as a means of identifying and quantifying the various sugar (sucrose and lactose) crystalline forms (“phases”) present in both chocolate, and the chocolate crumb that is a key intermediate in certain types of chocolate manufacturing processes. Key to this work was the development of reliable procedures for sample presentation in the transmission capillary mode of diffraction, and development of a method for reliable quantitative phase analysis of chocolate and chocolate crumb. It was found that by incorporating a carefully selected internal standard (diamond powder) in the samples being analysed, not only could accurate weight percentages be found for the various crystalline components of the chocolate, but also the percentage of sugars that were present in an amorphous form could be calculated. This approach is fully validated, including by the use of known quantities of amorphous lactose generated “in house” and provides a powerful, complementary approach to other techniques such as DSC, and is applicable to raw materials, process intermediates (chocolate crumb) and finished products. Whilst studying the crystallisation of lactose in isolation, a powder X-ray diffraction pattern was obtained which could not be explained by any combination of the five known crystalline forms of lactose. This pattern, which resisted all attempts at powder indexing, was able to be fully characterised when a microcrystal (76 × 24 × 18 μm) was isolated from the recrystallised powder, subjected to single-crystal X-ray diffraction, and found to be a new crystalline form of αβ-D-lactose. The orientations of the numerous -OH groups in this new Z’=2 form were verified by using periodic dispersion-corrected DFT calculations. Armed with this novel crystal structure, the recrystallised powder was shown to be a three-phase mixture of crystalline lactose forms, with quantitative phase analysis showing that the novel αβ-D-lactose was the dominant component. This highlights the power of combining single-crystal X-ray diffraction and powder X-ray diffraction to solve otherwise refractory problems, shedding new light on a sugar that has been studied for decades. Overall, the work shows the importance of careful experimental X-ray diffraction and its associated data analysis techniques in fully characterising complex mixtures of crystal forms. In particular, the quantitative phase analysis method developed herein has gone on to find routine use by Mondelez in the characterisation of many of the samples they generate.