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Title: Movement of a secondary immiscible liquid within a suspension of hydrophilic particles in a continuous hydrophobic phase
Author: Islam, Syed Foredul
ISNI:       0000 0004 6058 591X
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2016
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Numerous food products are suspensions of hydrophilic particles dispersed in a continuous fat phase, examples being peanut butter, chocolate spread, chocolate, praline and various confectionary fillings. The uptake of moisture by such materials and its interaction with the suspension can profoundly affect their qualities of texture, taste, appearance and conservation. The moisture could be transferred between different components of a product or from the environment, provided there is a gradient of water activity between the two regions, or perhaps introduced directly. Therefore, an understanding of the mechanisms (e.g. gravity, capillary and diffusion) responsible for moisture migration is needed to design better products and optimise the distribution chain. The aim of this study is to understand the changes which occur, when model suspensions (sugar and sunflower oil) interact with a secondary immiscible liquid (water, sugar solutions and glycerol), via both direct addition and atmospheric exposure (varying humidity), during both short (seconds to minutes) and long (hours to days) term storage. A novel methodology developed using nondestructive dynamic X-ray computer tomography (CT), proved successful in tracking the secondary liquid and the effects of moisture migration from atmospheric exposure. Upon direct addition of a secondary liquid droplet to a suspension, following gravitational effects as the liquid encounters the surface of the suspension, surface penetration was assumed to be driven by capillary effects, dominated by the viscosity and contact angle of the liquid with the sugar particles. Subsequent penetration of the liquid within the suspension was driven by the capacity of the liquid droplet to absorb solid sugar from the bulk suspension. The movement of particles into the liquid was against gravity as the droplet progressed into the suspension. Final transport of the liquid was believed to be driven by diffusion into the bulk suspension. This was based on microscale observations using high resolution X-ray CT, with the continued movement of the liquid being ascribed to the thermodynamically favoured wetting properties leading to a network formation between solid sucrose particles. A model was developed for the stage in which solid sucrose moves into the secondary liquid based on the flux of sucrose from the bulk suspension. It was found that the penetration of the liquid into the suspension was dominated by the capacity of the liquid to take up sucrose, with density and gravitational effects playing a minimal role in driving the kinetics. For atmospheric exposure of the suspension to water, it was shown that if the humidity level was above the critical deliquescence relative humidity (DRH) point of sucrose, a contraction of the volume occupied by the solids occurred due to liquid bridge formation causing an inward pulling force. The moisture migration travelled as a uniform front through the cross sectional area of the suspension sample. Finally, the findings from this research provide valuable insight useful to optimise product design and processing and realise improved stability over shelf-life for a key class of food materials. They lead to a better understanding of issues which need to be considered during product development. Focus is on the prevailing crystalline solid used – sucrose – but also considers the interactions between ingredients in influencing the overall properties of the final product. This would be particularly useful for assuring product quality in global market places where high humidity prevails, through tailoring the product to resist undesirable changes.
Supervisor: Salman, Agba ; Hounslow, Mike Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available