School of Chemistry - Research Publications
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ItemThe effect of high-intensity ultrasound on cell disruption and lipid extraction from high-solids viscous slurries of Nannochloropsis sp biomass(ELSEVIER SCIENCE BV, 2018-11-01)The effect of ultrasonication on the cell rupture of marine microalgae Nannochloropsis sp. was studied as a function of the slurry solids concentration and treatment time. The concentrated viscous wet-biomass (~12 to 25% solids concentration) was subjected to ultrasonication (20 kHz) at 3.8 W/mL for up to 5 min. Compared to extraction without cell rupture, sonication led to a significant increase in lipid yield from ~11% to about 70% within 5 min of sonication. The extraction yield was found to decrease with increased solids concentration, with a large decrease between 20% to 25% solids. This is attributed to the increase in viscosity and decrease in speed of sound with increase in solids. The ultrasound attenuation coefficient increased 320-fold as the solids increased from 20 to 25%. Such a large attenuation of ultrasound places a limit of 20% solids to be used for cell rupture by ultrasound. The specific energy requirements per unit mass of extracted lipid were lowest at 20% solids. At lower concentrations energy was wasted heating water, at higher concentrations the lipid yields were reduced due to ultrasound attenuation.
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ItemUltrasonic pretreatment of food waste to accelerate enzymatic hydrolysis for glucose production(ELSEVIER SCIENCE BV, 2019-05-01)Recovering valuable materials from food waste by applying the concept of a bio-refinery is attracting considerable interest. To this effect, we investigated the possibility of enhancing the enzymatic hydrolysis of food waste using ultrasonic technology. The effect of pre-treating blended food waste with high-intensity ultrasound (20 kHz) on subsequent hydrolysis by glucoamylase was investigated as a function of sonication time and temperature. Particle sizing by laser diffraction, and imaging via scanning electronic microscopy showed that ultrasonic pre-treatment could reduce the particle size of the blended food waste significantly, resulting in a better interaction with the enzyme. As a consequence, the glucose yield of enzymatic hydrolysis was ∼10% higher for food waste pre-sonicated using the most intensive ultrasonication conditions studied (5 min sonication at a power of 0.8 W/mL at 20 °C) than for the untreated control. In addition, the time required to achieve high yields of glucose could be more than halved using ultrasonic pre-treatment. This could enable the hydrolysis reactor size or the enzyme usage to be reduced by more than 50%. Therefore, an ultrasound-assisted bioconversion process from food waste into a value-added product has been demonstrated.
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ItemEmulsifying properties of ruptured microalgae cells: Barriers to lipid extraction or promising biosurfactants?(ELSEVIER SCIENCE BV, 2018-10-01)A systematic investigation of the emulsifying properties of ruptured algae cells was performed for the first time. The slurry of ruptured algae cells was separated into different biomass fractions, namely the cell debris, the delipidated debris, the serum, and the lipid. The interfacial interactions of these biomass fractions with a nonpolar solvent (e.g. hexane or hexadecane) were characterized using pendant drop tensiometry and interfacial shear rheology. The stability of the different emulsions (formed by the different biomass fractions) was tested using analytical centrifugation. The extracted lipid was an excellent surfactant that reduced the interfacial tension, however, it was not effective at stabilizing the emulsions. The protein-rich serum produced a strong interfacial film that stabilized the emulsions against coalescence during centrifugation. The cell debris stabilized the emulsions to a lesser extent by adsorbing to the droplet surface, presumably via interactions with hydrophobic extracellular polymeric substances (EPS). However, neither the serum nor the cell debris were very effective surfactants, and required the presence of the lipid fraction to produce small emulsion droplets. When present together, the components exhibited competitive interfacial adsorption, which influenced emulsion stability. In particular, the interruption of the protein film by the presence of lipid or cell debris reduced the stability of the emulsions. This study provides a new mechanistic understanding of emulsification during wet lipid extraction from microalgae that will be useful for determining strategies to improve solvent recovery. The results also suggest potential for developing effective bioemulsifiers or biosurfactants from fractionated microalgae biomass for commercial application.
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ItemFunctionalised dairy streams: Tailoring protein functionality using sonication and heating(ELSEVIER SCIENCE BV, 2018-11-01)Ultrasound can be used to modify the functional interactions between casein and whey proteins in dairy systems. This study reports on ongoing developments in understanding the effect of ultrasound and heating on milk proteins in systems with modified casein-whey protein ratios (97:3, 80:20 and 50:50), prepared from milk protein concentrates that were fractionated by microfiltration, based on protein size. Heating of concentrated casein streams (9% w/w) at 80.0 °C for up to 9 min resulted in reduced gelation functionality and increased viscosity, even in the absence of added whey proteins. 20 kHz ultrasonication at 20.8 W calorimetric power for 1 min was able to break protein aggregates formed during heating, resulting in improved gelation and reduced viscosity. Interestingly, when heated whey protein was recombined with unheated casein the gelation properties were similar to unheated controls. In contrast, when heat treated casein streams were recombined with unheated whey protein, the gel forming functionality was reduced. This study therefore shows that using specific combinations of heat and/or ultrasound, fractionated dairy streams can be tailored for specific functional outcomes.
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ItemA study of the effectiveness and energy efficiency of ultrasonic emulsification(ROYAL SOC CHEMISTRY, 2018-01-07)Three essential experimental parameters in the ultrasonic emulsification process, namely sonication time, acoustic amplitude and processing volume, were individually investigated, theoretically and experimentally, and correlated to the emulsion droplet sizes produced. The results showed that with a decrease in droplet size, two kinetic regions can be separately correlated prior to reaching a steady state droplet size: a fast size reduction region and a steady state transition region. In the fast size reduction region, the power input and sonication time could be correlated to the volume-mean diameter by a power-law relationship, with separate power-law indices of −1.4 and −1.1, respectively. A proportional relationship was found between droplet size and processing volume. The effectiveness and energy efficiency of droplet size reduction was compared between ultrasound and high-pressure homogenisation (HPH) based on both the effective power delivered to the emulsion and the total electric power consumed. Sonication could produce emulsions across a broad range of sizes, while high-pressure homogenisation was able to produce emulsions at the smaller end of the range. For ultrasonication, the energy efficiency was higher at increased power inputs due to more effective droplet breakage at high ultrasound intensities. For HPH the consumed energy efficiency was improved by operating at higher pressures for fewer passes. At the laboratory scale, the ultrasound system required less electrical power than HPH to produce an emulsion of comparable droplet size. The energy efficiency of HPH is greatly improved at large scale, which may also be true for larger scale ultrasonic reactors.
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ItemUltrasonic encapsulation - A review(ELSEVIER SCIENCE BV, 2017-03-01)Encapsulation of materials in particles dispersed in water has many applications in nutritional foods, imaging, energy production and therapeutic/diagnostic medicine. Ultrasonic technology has been proven effective at creating encapsulating particles and droplets with specific physical and functional properties. Examples include highly stable emulsions, functional polymeric particles with environmental sensitivity, and microspheres for encapsulating drugs for targeted delivery. This article provides an overview of the primary mechanisms arising from ultrasonics responsible for the formation of these materials, highlighting examples that show promise particularly in the development of foods and bioproducts.
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ItemThe formation of double emulsions in skim milk using minimal food-grade emulsifiers - A comparison between ultrasonic and high pressure homogenisation efficiencies(ELSEVIER SCI LTD, 2018-02-01)Double emulsions of W1/O/W2-type were formed in skim milk. Skim milk (W1) was emulsified within sunflower oil (O) using ultrasonication that was in turn emulsified within an external skim milk phase (W2) using ultrasonication or high pressure homogenisation (HPH). The internalised aqueous phase was stabilised within the oil phase using food-grade surfactants: polyglycerol polyricinoleate (PGPR) and/or lecithin. Encapsulation yields of the W1/O emulsion into the double emulsion were between 30 and 100%, with increased yields achieved with reduced sonication time or HPH pressure, or increased PGPR or lecithin concentration. Ultrasonication was found to form relatively better monodisperse emulsions that showed greater stability to coalescence than those produced by HPH. Ultrasonication and HPH were found to be translatable in the sense that at a similar specific energy density (∼20 J/g) emulsions with similar size distributions in the range 1-10 μm and encapsulation yields (ca 37 wt%) could be achieved.
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ItemThe inhibitory roles of native whey protein on the rennet gelation of bovine milk(ELSEVIER SCI LTD, 2018-04-01)Rennet gelation is used to produce many types of cheese. The effect of native whey protein on rennet gelation kinetics was investigated. Milks with a wide range of whey protein:casein (WP:CN) ratios (with standardised casein concentrations) were made from powders produced by microfiltration. Measurements of casein macro peptide release showed that native whey protein inhibited the enzymatic action of chymosin, which delayed the onset and reduced the subsequent rate of gelation. Experiments in which increased chymosin concentrations compensated for the inhibition, demonstrated that other factors also contributed to the reduced gelation rate. Neither an increase in viscosity nor a reduction in soluble calcium was responsible, leading to the conclusion that in addition to inhibiting chymosin, native whey proteins present a physical barrier to para-casein aggregation. This study demonstrates and explains how casein-enriched retentates from microfiltration gel faster than regular cheese milk that contains higher amounts of native whey protein.
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ItemRheological properties of concentrated slurries of harvested, incubated and ruptured Nannochloropsis sp. cells(Springer Science and Business Media LLC, 2019-12)Biorefining of microalgae biomass requires processing of high-solids (> 10%) slurries. To date there is little knowledge of how processes for weakening and rupturing microalgae cells affect the rheological properties of these materials. To fill this gap in the literature, the rheological properties of concentrated slurries of marine microalgae Nannochloropsis sp. were investigated as a function of processing and solids concentration (12, 20 and 24% w/w). Freshly harvested, incubated (autolysed), and high-pressure homogenised (HPH) slurries were found to be shear thinning up to a shear rate of approximately 200 s− 1. Viscosity increases were far more prominent for partially processed versus unprocessed algal pastes at the higher concentrations. Slurry viscosity as a function of cell volume fraction could not be fitted to the Krieger-Dougherty model due to a network structure resulting from extracellular polymeric substances (EPS) and the intracellular cell components released during incubation and cell rupture. The 24% slurry, which was near the close packing limit, was much more viscous than the less concentrated slurries when comprising whole cells (i.e. harvested and incubated slurries). Cell rupture by HPH completely altered the characteristics of the slurry, increasing the viscosity of even the less concentrated slurries, and producing irreversible shear thinning behaviour. The magnitude of the increases in viscosities and the irreversible shear thinning behaviour observed in this study, have significant implications for processing and optimising the solids concentration of algal slurries.