Chemical and Biomolecular Engineering - Theses
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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.