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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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    Enhancement of aroma and flavour volatiles in apple juice : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Physiology at Massey University
    (Massey University, 1999) Dixon, Jonathan
    Aroma typical to apples develops during ripening and is comprised of a large range of volatile chemical compounds from several chemical classes. Previous research has established that exposing apples to hypoxic conditions induces changes in volatile concentrations; acetaldehyde and ethanol accumulate to high concentrations and after return to aerobic conditions ethyl esters increase and non-ethyl esters decrease. The present study investigated the effect of short term-hypoxic treatments on the enhancement of ethyl esters and decrease in non-ethyl esters with respect to: organoleptic changes in apple aroma induced by exposure to hypoxia; the influence of temperature and time at 0 °C before treatment on the magnitude of enhancement of ethyl esters after exposure to hypoxia; the effect of cultivars and ripeness stage on types and quantities of ethyl esters enhanced after exposure to hypoxia. Brief periods of hypoxia at ambient temperatures have potential for disinfestation treatments or as pre-treatments to maintain fruit quality during extended storage. Volatile compounds were extracted from 20 mL aliquots of apple juice with an equal volume of diethyl ether:n-pentane (2:1 v/v), vigorously stirred for 3-5 seconds, frozen at -18 °C to separate solvent and aqueous phases, concentrated with a fast stream of oxygen free nitrogen (200 mL min-1) to 200 μL and analysed by gas chromatography. Apple juice could be held in ice or air up to 256 minutes without loss of volatile compounds. Loss in solvent washes was 76.5% for octyl acetate and recoveries during concentration of the solvent extract ranged from 2.5% for ethyl acetate to 86.4% for trans-2-hexenal. Solvent extraction was simpler, faster, extracted more compounds, and had better reproducibility than dynamic headspace extracts obtained using Tenax® traps. Nine cultivars of apples, Cox's Orange Pippin, Fuji, Golden Delicious, Granny Smith, Pacific Rose, Red Delicious, Royal Gala, Splendour and Southern Snap were exposed to 100% carbon dioxide for 24 h at 20°C. Apples exposed to hypoxia had concentrations of acetaldehdye, ethanol, ethyl acetate and ethyl esters consistently enhanced while concentrations of acetate esters and aldehydes were depressed. Maximum ethyl ester enhancement occurs within 2 to 3 d after removal from hypoxia. Exposure to hypoxia for 24 h at 20 °C did not affect rates of softening or induce physiological damage. Cultivars varied considerably in response to hypoxic treatment with Cox's Orange Pippin and Golden Delicious having the least and Fuji and Red Delicious the greatest enhancement in ethyl esters. Fruit exposed to hypoxia had larger odour unit scores than control fruit suggesting that such changes in volatile concentration may affect the aroma and/or flavour. Fuji and Royal Gala apples were exposed to 100% CO2 for 24 h, at 10, 15, 20 or 25 °C and maintained at treatment temperature for up to 14 d. Carbon dioxide and ethylene production and firmness were proportional to temperature but were unaffected by exposure to hypoxia. Ethyl esters were enhanced at all temperatures at differential rates according to cultivar. Apples treated and maintained at 10 °C had the greatest overall enhancement of ethyl esters and the least decrease in other esters compared to apples at 15, 20 or 25 °C. This enhancement in volatiles persisted for up to 10 d after removal from hypoxia. Best maintenance of apple quality after treatment with hypoxia is at low temperatures suggesting that apples treated with hypoxia and maintained below 15 °C would have enhanced volatile concentration. Noncooled Fuji and Royal Gala apples at preclimacteric to postclimacteric ripeness stages were exposed to 100% CO2 for 24 h at 20 °C for up to 14 d. A batch of the same fruit were placed at 0 °C, removed to 20 °C and exposed to hypoxia at monthly intervals for up to 5 months. Exposure to hypoxia decreased carbon dioxide production in Fuji apples at the preclimacteric and rising climacteric stages and at the climacteric. Respiration rate, ethylene production and volatile concentration of RG apples were not affected by exposure to hypoxia at any stage of ripeness or period at 0 °C. After exposure to hypoxia Fuji apples had enhanced ethyl esters at the preclimacteric and rising climacteric stages and after being at 0 °C for up to 5 months. Volatile concentrations were lower in apples maintained at 0 °C compared to noncooled apples. Apples at 0 °C had the greatest enhancement of ethyl esters after hypoxia suggesting that exposure to low temperatures did not just slow volatile biosynthesis but had an additional effect on volatile biosynthesis. Apple aroma consists of mainly low molecular weight esters produced by esterification of alcohol's by alcohol acyl CoA transferase (AAT) where acyl CoA's are substrates. Increased esterification activity in apples returned to air, following a hypoxic treatment, is due possibly to enhanced AAT activity or to competitive inhibition of other alcohols by ethanol. Concentrations of acetate and ethyl esters from skin disks of Cox's Orange Pippin, Fuji, Golden Delicious, Granny Smith, Pacific Rose, Red Delicious, Royal Gala, Splendour and Southern Snap apples exposed to 100% CO2 for 24 h at 20 °C, were compared to disks from control fruit, after addition of C2 to C6 alcohols, either individually, or as a mixture in equimolar amounts to the disks. Ethanol added as an individual alcohol induced high ethyl acetate concentrations, but when added as part of a mixture, little ethyl acetate was produced indicating substrate preference was for longer chain alcohols. Apple cultivars had four patterns of change in ester production after exposure to hypoxia: increased acetate and ethyl esters; increased acetate esters and decreased or no change in ethyl esters; no change or decreased acetate esters and increased ethyl esters; no change or decreased acetate esters and decreased or no change in ethyl esters, implying that AAT activity is affected differentially by hypoxia. Hypoxia induces changes in capacity to produce esters which last up to 7 d indicating that pre-storage treatments using hypoxia has the potential to change the aroma profile of apples. Juice of Fuji and Royal Gala apples exposed to a brief period of hypoxia (100% CO2 for 24 h at 20 °C) and ripened at 20 °C for up to 8 d, was analysed by taste panels using quantitative descriptive analysis. Hypoxia induced large increases in ethyl esters including ethyl butanoate and ethyl-2-methyl butanoate in Fuji apples but not in Royal Gala apples. There was no difference in average panellist scores for sensory characteristics for Fuji and Royal Gala apples at any sampling time. The lack of difference may have been due to large variation between panellist's assessment of sensory characteristics and/or inability to assess aroma, flavour and sweetness independently. A number of individual volatiles correlated with aroma in juice from apples exposed to hypoxia, including hexan-1-ol, butyl acetate, 2 methyl butyl acetate and propyl butanoate for Fuji; and ethanol, ethyl acetate, propyl acetate and propyl butanoate for Royal Gala. Multivariate analysis indicated that panellists associated increased ethyl esters with off flavour rather than more intense apple aroma. This could have been due to juice from apples exposed to hypoxia having a different apple-like character than control fruit which did not fit the definition of apple aroma used to train panellists. The enhanced ethyl ester concentrations in fruit exposed to hypoxia are probably due to large increases in ethanol concentration that competitively inhibited formation of non-ethyl esters. Golden Delicious and RG did not have enhanced concentration of ethyl esters and/or decreases in acetate ester concentration even though fermentation volatiles were enhanced to high concentrations and ethyl acetate increased to concentrations similar to those found in fruit which had enhanced ethyl esters. The mechanism producing ethyl acetate and ethyl esters in GD and RG was probably different from that in CO, FU, PR, RD, SP, SS cultivars. Therefore, after exposure to hypoxia, additional factors influence changes in volatile concentration other than the increased pool of substrate available for esterification. A possible mechanism by which hypoxia affects ester biosynthesis is that under hypoxic conditions cytoplasmic pH falls below the optimum of 7 to 8, inducing increased ADH activity and synthesis and producing large increases in ethanol concentration. Ester biosynthesis is suppressed during hypoxia leading to increased alcohol and aldehyde concentrations creating a pool of substrates that could be rapidly utilised by AAT on return to aerobic conditions. It is possible that AAT activity or concentration changes are induced by hypoxic conditions. The different capacity of apple cultivars to esterify alcohols from control and hypoxic treated fruit may be due to changes in substrate specificity of either, or both, newly induced ADH and AAT. Exposure to hypoxia consistently caused increases in ethyl esters in several apple cultivars. The practical uses for treatments where apples are exposed to hypoxia for 24 h include: disinfestation treatments, manufacture of apple juice concentrates, enhancement of aroma in apples maintained in long term air or controlled atmosphere storage and as a tool for examining volatile biosynthesis.