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    Studies on the concentration of apple juice by reverse osmosis : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Food Technology at Massey University
    (Massey University, 1978) Le Van, Dan
    The Reverse Osmosis (RO) process and its food industry applications were reviewed. Because most of the work published in the literature on the concentration of fruit juices by Ro was empirical, it was decided to select one fruit juice (apple juice) and study the retention of certain components (namely sugars and acids) when that juice was concentrated by RO. A method was developed for the analysis of sugars and non-volatile organic acids in apple juice. In this method, acids wen precipitated as their lead salts from fruit ethanolic extracts, and the sugars in the remaining supernatant and washings were participated into aqueous methanol. These preparations with internal standards were then dried and converted to their trimethylsilyl derivatives for analysis by gas-liquid chromatography. The method provided a rapid and simple procedure for the concurrent separation, identification and quantitative snalysis of sugars and non-volatile acids in apple juice. A pilot-plant scale Abcor TM5-14 RO module was used in this study so that the results obtained could be applied to industrial processing. Preliminary experiments were conducted with dilute salt solutions to ensure that the membrane performed satisfactorily, and to monitor any changes in the operating characteristics of the membrane as the experimental work progressed. These data provide the common means for comparing different RO systems. The results obtained established that the membrane performed satisfactorily, and that the membrane characteristics (Permeate flux and % Rejection) responded as expected to changes in the operating parameters of pressure, temperature, flow rate, concentration and operating time. The membrane characteristics did not alter significantly over the time during which the experiments reported here were carried out. A current theory (the Kimura-Sourirajan analysis) was used in an attempt to predict the membrane performance of the RO module when the system sodium chloride-water was used as test solution. The Kimura-Sourirajan analysis had previously led to the development of a sot of basic transport equations which, together with the correlations of the RO experimental data, enabled the prediction of membrane performance from a minimum of experimental data. The application of this analysis to the RO system under study did not establish any significant correlations between the solute transport parameter (DAM/k5), and feed concentration and operating pressure; neither were the average mass-transfer coefficient values (k) significantly correlated with feed flow rate. Experimental results obtained suggested a more complex relationship between these parameter, and the narrow range of feed flow rates under which the RO system was able to be operated meant that tho Kimura-Sourirajan analysis could not be used to meaningfully predict the performance of the membrane. A further attempt was made to predict membrane performance from a knowledge of the Taft numbers of the sugars and acids present in the juice. Experiments carried out on model solutions of sugars and acids present as single components or as complex mixtures confirmed the Taft number as a criterion for predicting the organic rejection of the RO membrane. It was also established that molecular weight was indicative of solute rejection, higher molecular weights gave higher rejection by the membrane. Results obtained further confirmed the fact that the mechanism of solute rejections by RO cellulose acetate membranes involved both preferential absorption and capillary flow of solutes through the membranes. Finally, actual apple juice was concentrated by RO and tho results obtained on permeate flux and soluto rejection confirmed those found previously with model solutions of sugars and acids. It was established that apple juice (initial concentration 11°Brix) could be concentrated to 35° Brix at 7C and 99 atm pressure without any significant loss of sugars and organic acids. Experiments were also carried out to assess the advantage of operating at a higher temperature (20 C), since any increase in flux would be desirable from a commercial point of view. The end-to-end flux of the TM5-14 module was found to be 16.4 l/m2hr at 20 C compared to 11.7 l/m2hr at 7 C when single strength apple juice was concentrated to 35° Brix under maximum pressure (99 atm), an increase in flux of 40%. The pilot plant data thus obtained for the RO module were applied to a study of the feasibility of using RO as a pre-concentration step prior to evaporation. An RO plant comprising of 296 modules (membrane area 308 m2) with a permeate flux of 20.7 l/m2hr was found to be feasible for concentrating the juice from 11°Brix to 20°Brix in 7.5 hours. The economy of such a process was also assessed, and compared with that obtained by using a triple effect APV plate evaporator. A comparison of the concentration costs ($ /tonne of water removed) of the two systems revealed that RO was more than twice as expensive than evaporation ($122 compared to $51) for 900 operational hours per year, thus confirming results from the published literature, which also suggested that the cost of RO was competitive with plate evaporation when operated year round (6,300 hours/year). The results found in this study indicated that the annual operating costs for RO ($142,200) were almost twice as high as the equivalent for plate evaporation ($71,500). As well, the capital investment for RO was substantially higher than that for the equivalent plate evaporator ($700,000 compared to $282,300), thus making RO very unattractive for short seasonal operation. Thus it is concluded that the use of RO as a preconcentration technique in apple juice processing will never be realised unless capital costs are reduced considerably and operating hours are increased substantially. On the basis of this study, it is not financially economical for the Apple and Pear Board in Hastings to consider RO for the preconcentration of apple juice when the capacity of their present evaporator is no longer adequate.
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    Properties of milks concentrated by reverse osmosis : a thesis presented in partial fulfilment of the requirements for the degreee [i.e. degree] of Doctor of Philosophy in Food Technology
    (Massey University, 1998) Kaw, Anil Kumar
    Reverse osmosis (RO) is an energy efficient way of concentrating milk that can be operated at ambient temperatures, avoiding the product damage associated with thermal processes, and resulting in concentrates with better functional and nutritional properties. The objectives of this study were to examine in detail the effects of RO concentration on the rheological properties of milks, the states of proteins and fat globules in milk, the stability of RO concentrates towards ultra high temperature (UHT) processing, and changes in the UHT sterilized product during storage. Whole milk, homogenized milk and skim milk were concentrated to 1.5X, 2.0X, 2.5X and 3.0X by RO, and rheological properties were measured at 5, 15, 25, 40 and 60 °C before and after 48 h storage at 5 °C, using a Bohlin VOR rheometer. The values of k (the consistency index) and n (the flow behaviour index) were obtained by analysis of the flow curves using the power law model. These values indicated that the samples were generally very slightly shear thinning (i.e. pseudoplastic). k increased with decreasing temperature and increasing concentration. A shift factor approach was used to develop a relationship between k, temperature and total solids concentration (as n remained virtually constant). The equation can be used to predict the flow behaviour of RO concentrated milk at various total solids contents and temperatures. The data was also analysed using the Fernandez-Martin approach (1972); this was found to be more successful at modelling the effects of concentration and temperature on k. The viscosity data was also analysed using a hydrodynamic approach by means of Eilers' equation. A closer relationship was found between experimental values of basic viscosity and those predicted values by Eilers' equation when lactose was included in the equation as a component contributing to the volume fraction of the dispersed phase, rather than as a component contributing to the continuous phase viscosity. Concentration by RO results in inevitable homogenization of whole milk, by the back pressure device needed to maintain the pressure driving force for concentration in the RO plant. Analysis of RO milks by particle size distribution measurement and electron microscopy revealed that the original fat globules were broken up into new globules in the size range 0.02-0.3 μm; these globules were smaller overall than in milks homogenized with conventional valve homogenizers. The microstructures of protein membranes around the fat globules in RO concentrated milks were very different from those in milks homogenized conventionally. There were very few intact or semi-intact casein micelles at the surfaces of fat globules or in the serum. There were numbers of small fat globules grouped together, apparently held together by quantities of protein. Some unique particles were observed, which appeared to be similar to casein micelles, but had a number of very small fat globules embedded within their structure. Particle size measurements and electron micrographs of samples clearly showed that UHT treatment of RO concentrated milks resulted in the formation of large aggregates of intact fat globules. This aggregation was protein-mediated and no evidence of fat globule coalescence was observed. When milks containing native fat globule membrane (non-homogenized reblended concentrated milks) were UHT treated, the large aggregates formed consisted of protein alone with no inclusion of fat globules. In RO concentrated milks, the extent of formation of aggregates, as well the aggregate size, decreased with increase in milk pH from 6.3 to 7.2 or with the addition of disodium phosphate (DSP) prior to UHT treatment. UHT treated KU concentrated milks were examined for physico-chemical and structural changes during storage at 5, 20 and 37 °C. The effects of various processing variables (forewarming prior to concentration, post-UHT homogenization and the addition of phosphates prior to UHT treatment) on the storage-induced changes were also investigated. The results showed that irrespective of processing treatment, there was a decrease in pH with storage time, the extent of pH decrease being greater at higher storage temperatures. Gel formation, determined by a penetration test, correlated with the particle size distribution determined by light scattering. Samples stored at higher temperature showed greater quantities of large particles and gelled more quickly than those stored at lower temperature. The viscosity (measured at storage temperature) increased with storage time, the greater rate of increase being observed at the lowest storage temperature of 5 °C. The non-protein nitrogen content of all samples increased with storage time (indicating protein breakdown), the greatest rate of increase being observed at the highest storage temperature of 37 °C. Homogenization of concentrates after UHT treatment had no appreciable effect on gel formation during storage for pasteurized milk concentrates, but it decreased the rate of gel formation for concentrates that had been made from milk forewarmed prior to concentration. While forewarming decreased the rate of gelation for homogenized milks, an opposite trend was observed for concentrates that had not been homogenized. Results from electrophoresis analysis and electron microscopy showed minimum changes in the samples stored at 5 °C, whereas storage of samples at 37 °C for 25 weeks resulted in an increase in non-disulphide covalent cross links, and increases in fibre-like material and proteinacious particles in the serum. The addition of sodium hexametaphosphate (SHMP) resulted in the formation of smaller aggregates during storage, whereas the addition of DSP resulted in the formation of larger aggregates, suggesting that SHMP provides some stabilization of the concentrates. Electron microscopy and electrophoresis results did not show any significant differences between the samples with added SHMP or added DSP. Based on these results, a gelation mechanism in RO concentrated milks is proposed. RO concentrated milks have unique structures; recommendation are made for further work aimed at a better understanding of such milks, and at the development of new or improved dairy products that utilize their unique functional properties.