Prediction of the glass transition temperature of fruit juice powders : a thesis presented in partial fulfilment of the requirements for the degree of Doctor in Philosophy in Process Engineering at Massey University, Manawatu, New Zealand

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To overcome the limited shelf life of fresh fruit juice, manufacture aim to reduce the juice to a powdered form that can be available all year around. One technique of powder formulation is the use of spray dryer. However fruit juices are rich in fruit sugars and organic acids, which make them difficult to spray dry. Their low glass transition temperatures lead to stickiness problems in the spray dryer. This work investigates the prediction of the glass transition temperature (Tg) of fruit juice powders and mixtures including high molecular weight components often used as drying aids of their mixtures. The ability to predict glass transition temperatures enables optimization of powder formulations and spray dryer operation to avoid sticking problems. A semi-empirical model is presented to predict the glass transition temperature of fruit juice powders on the basis of the chemical composition and the Tg values of the individual pure components. The Flory-Huggins Free Volume theory is used to describe the relation between the water activity of the powders and the glass transition temperature based on the composition of the powders. These prediction models were tested for different freeze dried multicomponent systems and validated against the glass transition temperatures measured by Differential Scanning Calorimetry (DSC). The model was then applied to predict the Tg of values freeze dried juice and vegetable powders prepared from commercial juice concentrate. The six fruit and vegetable juice powders exhibited higher measured Tg values at zero water activity those that predicted from the model. The 6-14˚C difference between the measured and predicted Tg values cannot be explained purely by the main sugar and organic acid components, but are rather linked to the presence of residual breakdown products of pectins and other polysaccharides from the base fruit. For several powders, a second glass transition temperature was visible in the DSC thermogram, which can be attributed to these additional components. The prediction of the glass transition temperature of the pure juice powders at various water activities showed better agreement with the experimental data the closer the predicted and measured Tg value of the powders was at zero water activity. The Tg values of different low molecular weight components and the polysaccharide maltodextrin DE 9-13 often used as a drying aid was studied. The Tg for the binary and multicomponent systems was well estimated by the prediction model at zero water activity. The addition of higher amounts of maltodextrin DE 9-13 up to a weight fraction of 0.7 in the mixture resulted in a widening of the transition observed in the DSC thermogram, which can be attributed to dynamic heterogeneity of the samples due to the dynamic response times of different domains in the solid matrix. This could be demonstrated by annealing the samples below their glass transition temperatures for 16 hours. The analytical semi-empirical model proved to be a viable method for predicting the glass transition temperature of mixtures of low and high molecular mixtures. Multicomponent systems consisting of more than one high molecular weight component in the form of various soluble polysaccharides and the monosaccharide glucose were investigated for their Tg values at zero water activity. The different mixtures of the low molecular weight components and polysaccharides showed a shift in the glass transition temperature that depended strongly on the Tg value of the pure polysaccharide. However, the overall trend of Tg values for the mixtures of low and high molecular weight components was found to be similar for all mixtures. The Tg values of all systems with various polysaccharides were well predicted. It also highlighted that a weight fraction of above 0.5 for the polysaccharides with a degree of polymerization of sugar units above 5 is necessary in the solid matrix to have a significant impact on the glass transition temperature. Salts were tested as an alternative drying aid to high molecular weight components at various ratios to the monosaccharide glucose. The salts increased the glass transition temperature of the blends at lower concentrations. A weight fraction of 0.2 of salts in the system has the same effect on the Tg values as a 0.5 weight fraction of a higher molecular weight component. The difference in necessary amount to increase Tg can be attributed to the different mechanisms of salts and polysaccharide in the solid matrix. To demonstrate the utility of the model, it was applied to optimize the spray drying of blackcurrant juice concentrate, with the aid of maltodextrin DE 9-13. It has been reported that sticking can be avoided during drying if the operating temperature does not exceed T=Tg + 25˚C. By combining the prediction of the Tg of the solid mixture at various water activities and a mass and energy balance over the spray dryer, reasonable feed composition and operational conditions were found. This allowed the successful conversion of blackcurrant juice into a powder in a single trial without relying on trial-and-error approaches.
Figures are re-used with permission.
Fruit juices, Drying, Glass transition temperature, Spray drying