Sorption Behaviour of Selected Dairy Powder Mixtures: A Study of The Effects of Composition and Mixing Methods Abstract As water mediates physico-chemical reactions (i.e. Maillard reactions, phase changes of sugars and minerals, protein conformational changes), water-powder interactions during storage are critical for dairy powder stability. Therefore, the mechanisms of moisture adsorption from the environment and water distribution among components in the dry state need to be investigated, especially in mixed systems. In order to achieve this goal, the research reported in this thesis had two key objectives. The first was to examine the effect of the sorption properties and phase changes of individual components in relation to the sorption properties of the mixed system. This was achieved by adding three level concentrations of lactose and mineral rich dairy powder (10, 20 and 30%) to Milk Protein Concentrate containing 85% of protein (MPC-85). The second objective was to examine the effect of mixing methods on the sorption behaviour of mixed systems, by applying two different mixing methods, solution mixing and mechanical mixing (particulate mixing). In the solution mixing method, the components were mixed in the same water and then spray dried. In the mechanical mixing method, two individual powder components were physically mixed. For both mixed systems, the kinetics of moisture adsorption were determined at 4 different equilibrium relative humidity levels (22.5, 43.2, 65.4 and 84.3%) and the final equilibrium moisture contents were determined at 8 different equilibrium relative humidity levels (11.3, 22.5, 32.8, 43.2, 52.9, 65.4, 75.3 and 84.3%) at 25°C. The effect of lactose addition to the MPC powder tended to retard the moisture sorption of the mixtures. The increase of concentration level of the lactose that was introduced to the MPC system, through either solution or mechanical mixing, resulted in less moisture adsorption when compared to MPC itself (p-value<0.05). This effect tended to be greater with the increase in relative humidity. A similar effect was observed for MPC/mineral rich milk calcium powder (MC) mixtures. The application of different mixing methods modified the equilibrium moisture content of MPC/Lactose mixtures, even though both mixing methods resulted in similar monolayer moisture value. The monolayer moisture values calculated for both mixing methods were significantly lower than their theoretical values (p-value< 0.05). This suggested that a lactose-protein interaction might exist in both powders prepared by solution and mechanical mixing. Even though the interaction itself is hypothetical, the effect of interaction could be reflected by significant different adsorption rate (p-value < 0.05) of powders produced by different mixing method. A similar type of interaction might exist for the solution mixed MPC/MC system, even though different results were obtained for MPC/MC mixtures prepared by mechanical mixing. The addition of lactose to MPC tended to slow the rate of moisture adsorption. This deceleration might have been contributed to by a reduction of the protein hydration sites by the association of lactose molecules to these sites. In contrast to the MPC/lactose system, the addition of mineral rich MC powder to MPC did not significantly change the rate of adsorption (p-value<0.05). Different mixing methods were found to change the rate of moisture adsorption for the MPC/Lactose systems (p-value<0.05). Even though the mechanically mixed powder adsorbed faster than that of solution mixed powder and X-Ray measurement indicated lactose crystal formation, a drop of moisture during the sorption study was not observed. This suggests that water released during crystallisation might be adsorbed by protein. Meanwhile, the application of two different mixing methods did not modify adsorption rate of MPC/MC system, except for samples stored at RH 84.3%. At this environment, mechanically mixed MPC/MC powders were adsorbed more slowly than the solution mixed powder (p-value<0.05). It might reflect powder compaction or a collapse of the porous structure, leading to limited moisture transfer at the interface. Lactose proportions and different mixing methods influenced the glass-rubber transition temperature (Tg-r) of the MPC/Lactose mixtures. An increase in the proportion of lactose tended to depress Tg-r of the mixtures (p-value<0.05). The Tg-r of the mechanically mixed powder was lower than that of solution mixed powder, presumably on account of the Tg-r for mechanically mixed powder being dominated by phase separated sugar components. Meanwhile, the Tg-r of MPC/MC mixtures was not influenced by either MC proportion or mixing method, particularly for samples stored at below RH 65.4%. The XRD-pattern suggested that the crystal formed during storage of MPC/Lactose mixtures was α-lactose monohydrate. As confirmed by XRD, a drop in moisture for the mixture corresponded to the presence of a peak in the XRD pattern, except for mechanically mixed powder. In this type of powder, even though a peak was detected, a moisture drop was not observed. Within the sensitivity limits of XRD, a crystalline form was not observed for MPC/MC mixtures.
| Identifer | oai:union.ndltd.org:ADTP/288363 |
| Creators | Kiki Fibrianto |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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