School of Chemistry - Theses
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ItemUltrasonic formation of stable food emulsions for the delivery of nutrients( 2014)Growing market trend for the consumption of healthy food has led to production of foods rich in bioactives. This thesis reports on the outcomes of an investigation into the formation of stable emulsions of a bioactive, flaxseed oil, in two complex liquid food matrices, namely, skim milk and carrot juice by employing low frequency high intensity ultrasound, particularly in the absence of external food additives, pre-emulsification procedures and external heat and/or pressure. Initially, the study focussed at analysing the process-induced changes to skim milk at sono-emulsification experimental conditions. Sonication of skim milk for up to 8 min (maximum emulsification time) caused minor modifications to about less than 20% of total whey proteins. The modifications to whey proteins is also an advantage since they can act as emulsifiers to stabilise ultrasonically generated flaxseed oil emulsion droplets. The minimal change noticed at short sonication time indicates the safety of the technique. The changes to whey proteins resulted in a decrease in the turbidity of milk but did not affect the viscosity of milk. The casein micelles showed stability against disruption even until 60 min of sonication, except for the very minor changes in their sizes. This work is published in Innovative Food Science and Emerging Technologies and presented in Chapter 3. The feasibility of making finer and stable 7% flaxseed oil-skim milk emulsions was explored at different power levels ranging from 88 to 176 W and processing time from 1 to 8 min. Highly stable emulsions were obtained at 3 min & 176 W and 5 min & 132 W in the absence of external food additives. The emulsion stability test reported a shelf life of 9 days at 4±2ºC against phase separation of emulsions. The efficiency of sono-emulsification was checked up to oil loading of 21%. The factors responsible for emulsion stability were identified to be partially denatured whey proteins and the optimum power-process time combinations employed in the process. The study also drew comparisons between conventional and sono-emulsification processes. High shear mixing and pressure homogenization processes were compared against ultrasound at equivalent energy densities. On comparison, it was observed that at similar energy densities, high shear process of 12.5 min at 70 W & 17500 rpm and pressure homogenization at 200 bar (5 pass) & 655 W did not produce finer and stable emulsions as observed with 5 min of ultrasound process at 176 W. This work is published in Food Hydrocolloids and presented in Chapter 4. After characterization of emulsions, the functionality studies of 7% flaxseed oil-skim milk emulsions were conducted primarily to understand the nature/characteristics of downstreamed products, viz., beverage emulsions (ready-to-drink) and yoghurts (lactic acid gel). Results showed that sonication process did not induce oxidation in emulsions and did not change their viscosity until 9 days of storage at 4±2ºC. Also, the acid gels obtained from milk emulsions showed improved functionalities such as higher gel strength, reduced syneresis, higher storage modulus and reduced gelation time against controls. The evidences obtained suggested that the newly formed fat globular surface (by sonication) stabilized by denatured whey proteins played a major role in functionality, mainly by interacting with other whey proteins and caseins during gelation and formation of network. All the observations helped at arriving at a complete mechanism of gelation of those sono-emulsions. This work is published in Ultrasonics Sonochemistry and presented in Chapter 5. Finally, the feasibility of sonication to disperse flaxseed oil in carrot juice was explored. Stable emulsions of 1% bioactives were obtained at 4 min & 176 W and the size of emulsions were 0.6 μm until 8 days of storage at 4±2ºC. The pectins and/or proteins of carrot juice were responsible for emulsion stability, either by electrostatic repulsion or steric stabilization. The emulsions showed high negative zetapotential values of above -30 mV until 9 days of storage at 4±2ºC. The results of oxidative stability of emulsions study suggested the need for inert atmospheric conditions while processing. It showed that carotenoids exhibited pro-oxidant behaviour mainly because of its concentration, presence of oxygen rich atmosphere and formation of few process-induced free radicals during sonication. The study also looked at delivering flaxseed oil in an aqueous mixture of milk and carrot juice system. The sonication of carrot juice under proposed emulsification conditions also led to many advantages in the juices, such as improvement in cloud stability, increased carotenoid content, reduction in pectin methyl esterase activity, absence of non-enzymatic browning, etc. This work is submitted in Food and Bioprocess Technology and presented in Chapter 6.
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ItemUltrasonic synthesis and characterization of multifunctional nano/microcapsules( 2013)Liquid and air-filled nano/microcapsules have potential applications in diagnostic and therapeutic medicine. Air and liquid cores are generally coated by a protein, lipid or polymer shell. Air-filled nano- and microcapsules, commonly referred to as nano- and microbubbles (NBs and MBs), are ultrasound responsive colloidal particles with a strong potential to become theranostic agents, combining the contrast and therapeutic functionalities. In the last decades, they have played a significant role as ultrasound contrast agents in diagnostic imaging. Similarly, liquid and solid encapsulated nano/ microcapsules have potential applications in targeted drug delivery in medicine. This thesis investigates the use of high energy ultrasound processing (batch and flow-through) to synthesise nano- and microcapsules using proteins and biocompatible polymers (e.g., polymethacrylic acid) in aqueous solutions. The ultrasound-assisted self-assembly and cross-linking of lysozyme at water–air and water–perfluorohexane interfaces are shown to produce lysozyme-shelled hollow microbubbles (LSMBs) and microcapsules (LSMCs), respectively. The arrangement of lysozyme at the air–liquid or oil–liquid interfaces is accompanied by changes in the bioactivity and conformational state of the protein. A comprehensive study of the microspheres (LSMBs and LSMCs) size distribution, degradability, mechanical and surface properties, acoustic response and biofunctional properties has been performed. A novel flow-through sonication technique for synthesizing stable and monodispersed nano- and micrometer-sized bubbles has been developed. It has been demonstrated that the size and size distribution of the bubbles are controlled by the active cavitation zone generated by ultrasound. The cytoxicity of LSMBs and LSMCs on adherent cell line derived from a human breast adenocarcinoma cells (SKBR3) has been evaluated. SKBR3 shows capability to phagocyte LSMBs and LSMCs in vitro. In addition, a one-pot ultrasonic procedure has been developed as a versatile route for synthesizing polymerv coated microspheres, PMASH MB, that have potential application as drug delivery vehicles. The use of biocompatible thiolated poly(methacrylic acid) as the shell material offers control over size, morphology and functionality of the microspheres. LSMBs and PMASH MBs are shown to possess echogenic properties and can be used for loading oligonucleotides. Finally, LSMBs were engineered as a support for the immobilization of gold nanoparticles and an enzyme, alkaline phosphatase, in order to develop micro-antimicrobial and biosensing devices.
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ItemInterfacial effects on aqueous sonochemistry and sonoluminescence( 1999-06)The dissolution of quantum sized CdS and MnO2 particles in water was conducted using 20 kHz ultrasound. CdS particles were found to dissolve chemically via an oxidation process while MnO2 particles dissolved via a reductive process. It was found that the dissolution of the colloids could be controlled via the addition of surface active chemicals to solution and by varying the saturation gas type. In the presence of Na2S or propan-2-ol and argon gas, the dissolution of CdS was inhibited, whereas the addition of alcohols (methanol, ethanol, propan-2-ol, butan-1-ol and pentan-1-ol) to the MnO2 system led to an increase in the amount of dissolution for a given time of sonication. This increase in dissolution was found to be dependent on the ability of the surface active radical scavenger to accumulate around the bubble interface during the cavitation process. Eventually, at higher alcohol concentration there was a plateau or a limiting value reached for the efficiency of colloid dissolution which was common for each alcohol. (For complete abstract open document)
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ItemUltrasonics as a new platform technology in dairy processing( 2013)It has been shown recently that the use of ultrasound (US) on dairy whey systems can lead to a reduction in the size of whey protein aggregates and a consequent reduction in viscosity. Further, application of ultrasound after heat treatment can lead to a heat stable dairy product. However, a number of unanswered questions remain regarding this approach and this thesis attempts to answer some of these questions. Prior to commercialisation, a comparison with other physical shear and high pressure processes is required to determine the viability of US in the dairy industry. The thesis shows that at identical energy density of 153 J/ml, both sonication at 20 kHz and homogenisation at 80 bar of a 5 wt% whey protein concentrate (WPC80) solution provided similar reductions in whey protein aggregate size and viscosity. Smaller reductions were observed in samples subjected to high shear mixing at the same energy density, which may be the result of excessive foaming. The work shows that free radicals are absent in both the high shear mixing and homogenisation processes, implying that high shear forces is responsible for the observed changes, rather than acoustic collapse events. In addition, heat stability was achieved in all systems, with the best results again obtained for both homogenisation and sonication. Hence, the combination of a heat treatment followed by any suitable high shear process is capable of producing a low viscosity, heat stable product. A further concern was whether ultrasound impacted only the aggregate size or whether there were more subtle changes to the secondary structure of the protein. In this thesis, the changes in aggregation and secondary structure of the individual whey proteins and their mixtures upon sonication and the combination of heat and sonication were studied. No structural changes were observed in any native protein solution upon sonication at 31 J/mL using a 20 kHz sonicator. Prolonged sonication led to minor structural changes in pure β-lactoglobulin (β-LG) and α-lactalbumin (α-LA) solutions, as shown using fourier transform infrared spectroscopy (FTIR) analysis, but these proteins remained predominantly in their native β-sheet and α-helical structure respectively. None of the heated and sonicated protein samples showed any large increases in β-sheet content. Hence, with the conditions performed for heat and US treatments in this thesis, no drastic alteration in the secondary structure of the protein were observed in US of dairy whey solutions. In the dairy industry, membrane ultrafiltration (UF) is used for the concentration of whey to produce whey protein concentrate. A further aim of the thesis was to determine the impact of ultrasound if applied upstream of the UF unit to reduce membrane fouling and increase productivity. In addition, the combination of heat and US pre treatment is investigated as it is a promising approach to produce heat stable powders while alleviating membrane fouling. The use of sonication on 5 to 10 wt% WPC80 solutions had a small but significant effect on membrane fouling – for the 10 wt% solution, the cake growth factor fell from 0.66 to 0.44 × 10 11 m/kg and the steady state flux increased from 16.8 to 17.7 L/m2.h. Similar subtle effects were observed with fresh whey, with the cake growth factor falling from 3.7 to 3.0 × 10 11 m/kg. This may reflect the low solids concentration used in these experiments and the use of more concentrated protein solutions might lead to more positive results. Conversely, a pronounced effect was observed in the heat-treated feeds: with increasing solids concentration, both pore blockage and cake growth grew for all heat-treated feeds but these two parameters remained low for the pre-heated and sonicated feed. Sonication was also found to delay the ‘gelling’ of proteins as indicated by the higher gel concentration obtained in the pre heated and sonicated feed (27 wt%) relative to a solution exposed only to heat (21 wt%). However, 100% flux recoveries upon cleaning were not achievable in heat treated feeds and surface charge measurements indicated that protein deposit remained attached to the membrane surface. This may be due to the inability of the chemical cleaning agents to break down large, denatured protein aggregates formed during heating. microfiltration (MF) of skim milk on its selectivity and productivity was also investigated. In skim milk MF, the best selectivity was obtained at the lowest TMP of 55 kPa. A pre heated and sonicated feed provided the lowest whey protein and highest casein rejections, with values of 85.8 and 97.9 % respectively, and the greatest absolute flux of approximately 10 L/m2.h. However, the selectivity and flux obtained were considerably lower than that generally observed with the use of ceramic membranes.
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ItemEffect of sonication treatment on the physico-chemical properties of starch-based suspensions( 2011)Starch, one of the most abundant polysaccharides in nature, is used in a wide range of applications, particularly in Food Industry. Ultrasound treatment has been identified as a potential Food Processing Technology. The aim of my thesis is to investigate how the application of ultrasound affects the physico-chemical properties of starch-based suspensions. Acoustic cavitation-induced structural modifications were achieved by varying ultrasonication parameters (temperature, time, sonication power and frequency) and the characteristics of the starch dispersion (botanical origins, amylose/amylopectin ratio, granular or macromolecular states and granule surface properties).