Browsing by Author "Robertson, Gordon Lindsay"

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    Solubility relationships of limonin and the phenomenon of delayed bitterness in citrus juices : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University
    (Massey University, 1980) Robertson, Gordon Lindsay
    Unsuccessful attempts were made to produce cloud-stable, non-bitter citrus juices by enzymic degradation of juice-soluble pectin. The failure of these attempts demonstrated the necessity for a greater understanding of the solubility relationships of limonin and the phenomenon of delayed bitterness. The solubility of limonin in model solutions was investigated. It proved impossible to prepare aqueous solutions containing high concentrations of limonin without the use of heat. Even though the addition to such solutions of sugar, pectin and polygalacturonic acid at concentrations commonly found in citrus juices had a statistically significant (95% level) effect on the equilibrium limonin concentrations, the magnitude of the effect was quite small. Moreover, the solubility of limonin shaken at 30 C for five days in model solutions containing citrus proteins and lipids was no greater than its solubility in water, leading to the conclusion that neither lipids nor proteins are responsible for high concentrations of limonin in citrus juices. Direct solubilization of limonin cannot therefore account for high limonin concentrations in citrus juices to which no heat has been applied. High concentrations of limonin in model solutions could be achieved by the application of heat. In refluxed aqueous solutions, the presence of pectin, sucrose and sucrose, glucose and fructose in combination increased the solubility of limonin, the greatest increase occurring with sucrose alone. Even more important was the effect of solutes on slowing down the rate at which limonin came out of solution on cooling. Again the effect of solutes was different, sucrose being more effective than pectin in holding limonin in solution, but high concentrations were maintained longer in solutions containing the sugar mixture or sucrose and pectin. Saturated solutions of limonin in acidic model solutions prepared under reflux deposited limonin with approximately equal rapidity, irrespective of other solutes. On the other hand, solutions of similar limonin concentration, prepared by adding components of the model solutions (citric acid, pectin and sucrose at concentrations found in citrus juices) to a hot saturated solution of limonin, retained moderately high concentrations of limonin provided pectin was present. Limonin analysis by extraction at two pH levels demonstrated that the equilibrium operating in the establishment of high concentrations of limonin in model solutions involved neutral hydrolysis of limonin to one or other of its hydroxyacid forms. On cooling such solutions, limonin crystallized out, affecting the hydroxyacid-lactone equilibrium. The effect of solutes is related to their influence on this equilibrium. When similar extraction procedures were applied to studying the phenomenon of delayed bitterness in citrus juices, evidence was obtained for the presence in freshly extracted citrus juices of two limonin precursors which each showed differing stabilities, both to the presence of natural citrus enzymes and to the application of heat. All previous work relating the effects of various treatments to limonoid bitterness in citrus juices and model solutions must be reconsidered in the light of the possible involvement of the two limonin precursors, the associated enzymes, and the hydroxyacid forms of limonin.
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    Studies on the processing of New Zealand grapefruit juice : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Food Technology at Massey University
    (Massey University, 1974) Robertson, Gordon Lindsay
    The likely origin of the New Zealand grapefruit (NZGF) is discussed and present and future trends in its production and utilisation presented. Early and late season samples of NZGF juice were analysed for the presence of the enzymes pectinesterase, polygalacturonase and ascorbic acid oxidase, no trace of the latter two being found. Samples of juice from NZGF harvested at regular intervals from July until December 1973 were analysed for yield, total soluble solids, titratable acidity, pH, pectinesterase activity, and ascorbic acid content. The average yield of juice obtained (35.6% w/w) was significantly lower than that reported from overseas for true grapefruit. The level of total soluble solids remained fairly constant in the range 12.0 to 12.6%, while the pH of the juice increased throughout the season from 2.95 to 3.40. The titratable acidity was within the range 1.0 to 2.0 grams of citric acid per 100 ml of juice, while the Brix : acid ratio varied from 5.02 to 10.03. The level of pectinesterase in the juice (which increased as the season progressed) was comparable with that found in overseas citrus juices, while the level of ascorbic acid in the juice declined over the season from 32.4 to 23.2 mg/100 ml, in agreement with overseas trends. With the exception of yield, the compositional characteristics of NZGP juice reported here do not differ markedly from overseas grapefruit juices. The important role which pectinesterase plays in the destabilisation of citrus juice cloud is outlined and possible methods for inactivating the enzyme are described. As the application of heat is the only method in commercial use, factors affecting and methods for studying the thermal inactivation of enzymes are discussed. As the major objection to most of these methods is the way in which the heating and cooling lags are evaluated, a new method which adequately describes these thermal lags has been developed for determining the thermal resistance of pectinesterase in NZGP juice. A digital computer was programmed to determine (using a trial and error technique) the constants in two expressions which relate the equivalent effect of unsteady state heating and cooling of NZGF juice to the inactivation of pectinesterase. One expression assumed that the rate of inactivation was exponentially related to temperature; in this case the constant was the z value. The other expression assumed that the rate was related to temperature according to the Arrhenius equation, in which case the constant was the activation energy. The two constants were evaluated for both low and high pH juice. It was found that the latter expression using the Arrhenius equation described the change in rate of inactivation with temperature more adequately than the former expression. From these expressions the times required at different temperatures to inactivate pectinesterase in NZGP juice of varying pH were calculated. The application of these results to the industrial processing of NZGP juice is discussed.