Fat bloom in chocolate: Bloom identification and its corresponding effects on chocolate properties

Abstract

Fat bloom is the leading shelf-life issue for chocolate products. In the literature, several types of fat bloom, each with a distinct appearance, have been reported. However, due to diverse formulations, processing, and storage conditions used, different bloom appearances were observed under similar experimental conditions; different theories were also used to explain the bloom formation, there being no consensus. The first aim of this thesis is to categorise different forms of fat bloom, relying mainly on techniques such as microscopic observation, colour analysis and polymorph identification. To that end, the following researches were carried out. 1) Surface evolution of untempered chocolate was studied, and the fat bloom was found highly associated with form IV-to-V polymorphic transformation, where the form V crystals grew at the expense of form IV crystals under liquid fat diffusion. 2) Whereas, fat bloom formation in well-tempered chocolate was highly storage temperature dependent. Well-tempered chocolate exhibited Vto- VI polymorphic transformation, which was liquid-mediated over 22oC; but, lower than this temperature, it was via solid-state transformation. Chocolate was relatively stable when stored at 20oC (<22oC), partially melted when temperature cycled to 29 or 32oC, and fat bloom formation differed at a static 20oC and under cycling temperatures of 20-29oC or 20- 32oC. Solid extruded fat crystals formed under cycling temperatures of 20-29oC, but the crystals on the chocolate surface were not able to hold their structure at 32oC. Therefore, tempering degree and storage temperature are the two key parameters determining the development of fat bloom in chocolate. Secondly, this thesis aims to explain the effect of bloom-related microstructural factors on the changes in chocolate’s physical properties during storage. 1) The microstructure of well tempered chocolate was characterised using image analysis, and it showed that a smaller particle size distribution (PSD) of non-fat solids resulted in higher particle packing density (PPD). Higher PPD increased the tortuosity and decreased the radius of inter-particle channels, which consequently led to a lower rate and extent of bloom formation. PSD had limited effect on the melting behaviour of chocolate. 2) In addition, the fracture properties of chocolate were evaluated during fat bloom formation using a three-point bend test. The Young’s modulus and fracture stress of chocolate increased with a reduction in fat ratio or an increase in PSD. After 60 days’ bloom induction, the Young’s modulus of bloomed chocolate increased due to a decrease in liquid fat content, but the fracture stress decreased as a result of growth of void space during fat bloom development. However, no significant (P<0.05) interactions between storage time and fat ratio (or PSD) were found. Therefore, the changes in fracture properties over time could be influenced by, but not solely due to, fat bloom formation itself. This project provides a fundamental understanding and reference for further studies on fat bloom in chocolate in terms of fat boom mechanisms, and the influence of fat bloom on chocolate properties. This knowledge could also contribute to better abating such bloomrelated quality deterioration in industrial applications.