<p> The recrystallization behavior of three Al-Mg-Si-Cu alloys with varying iron and manganese additions was studied by differential power scanning calorimetry under nonisothermal annealing conditions. The influence of cold deformation on the precipitation sequence and its interaction with recrystallization was also investigated. The DSC experiments were complemented by hardness measurements and microstructural studies by optical and electron microscopy. The DSC signals, after optimization of the baseline, were used for the calculation of the kinetic parameters of the recrystallization process. Two different modeling approaches based on global JMAK kinetics were implemented. The first approach utilizes the classical isothermal JMAK expression directly, while the second approach introduces a path variable related to the thermal history of the material in the JMAK description. Model-independent estimates of the activation energy were also evaluated using the Flynn-Wall-Ozawa integral isoconversion method. </p> <p> The results show that the initial stages of recrystallization are not affected by the preceding precipitation processes and recrystallization always follows the precipitation of the Q' phase. However, during recrystallization enhanced coarsening of the Q' phase takes place leading to its transformation to the more stable Q phase. The Q phase exerts a Zener pinning pressures on the migrating boundaries preventing the formation of an equilibrium grain structure. Moreover, for high Fe and Mn additions, discontinuous precipitation of Mg2Si overlaps with the end of recrystallization and exerts an additional pinning pressure on the boundaries. Varying the Fe and Mn content significantly affects the recrystallization kinetics. PSN is promoted in alloys with the higher Fe and Mn content and the recrystallization temperature shifts to lower values. The modeling results show that the recrystallization process conforms to the classical JMAK type behavior. The course of the reaction was reproduced successfully by the path variable approach and the evaluated activation energies were in good agreement with the isoconversional model-independent estimates. However, when the classical JMAK expression was applied directly to non-isothermal measurements, a dependency of the recrystallization process on thermal history was observed. </p> / Thesis / Doctor of Philosophy (PhD)

Protein bars have become a popular option among consumers to increase protein content in their diets. Since there is a large market for protein bars, many factors must be considered when creating a protein bar that both satisfies consumers and has a long shelf-life. Hardening and textural changes in protein bars are some of the most common modes of shelf-life failure in this product category. When the typical product creation timeline from formulation to launch can be as short as 3-6 months and with added pressure from executives to quickly launch another new product afterwards, product development scientists simply do not have time to test the full shelf life of their product before release. For this reason, it is imperative that rapid methods for detecting bar hardness and predicting shelf life of bar formulations are developed. The objective of this research is to utilize calorimetric techniques to rapidly detect and identify bar hardening reactions. Six different protein bar formulations were studied, with each containing a combination of either whey protein isolate (WPI), milk protein isolate (MPI), or partially hydrolyzed whey protein isolate (HWPI), reducing-sugar, non-reducing sugar, and vegetable shortening. All bars were stored at 45°C and ambient humidity for 21 d. Isothermal microcalorimetry (IMC) was used to evaluate bar hardening-related reactions and was compared to objective and subjective hardness measurements. Hardness, color, water activity, moisture content, and sensory evaluation were measured at d 1, 7, 14, and 21. The results of this study indicate that isothermal calorimetry may be used to narrow down bar hardening reactions and points to Maillard browning as a main driver of hardening. These techniques may be used to predict bar shelf life, if Maillard browning is used as the basis for hardening. Furthermore, this research highlights the importance of ingredient selection during bar formulation to minimize hardening.

In systems biology, material balances, kinetic models, and thermodynamic boundary conditions are increasingly used for metabolic network analysis. It is remarkable that the reversibility of enzyme-catalyzed reactions and the influence of cytosolic conditions are often neglected in kinetic models. In fact, enzyme-catalyzed reactions in numerous metabolic pathways such as in glycolysis are often reversible, i.e., they only proceed until an equilibrium state is reached and not until the substrate is completely consumed. Here, we propose the use of irreversible thermodynamics to describe the kinetic approximation to the equilibrium state in a consistent way with very few adjustable parameters. Using a flux-force approach allowed describing the influence of cytosolic conditions on the kinetics by only one single parameter. The approach was applied to reaction steps 2 and 9 of glycolysis (i.e., the phosphoglucose isomerase reaction from glucose 6-phosphate to fructose 6-phosphate and the enolase-catalyzed reaction from 2-phosphoglycerate to phosphoenolpyruvate and water). The temperature dependence of the kinetic parameter fulfills the Arrhenius relation and the derived activation energies are plausible. All the data obtained in this work were measured efficiently and accurately by means of isothermal titration calorimetry (ITC). The combination of calorimetric monitoring with simple flux-force relations has the potential for adequate consideration of cytosolic conditions in a simple manner.

For systems biology, it is important to describe the kinetic and thermodynamic properties of enzyme-catalyzed reactions and reaction cascades quantitatively under conditions prevailing in the cytoplasm. While in part I kinetic models based on irreversible thermodynamics were tested, here in part II, the influence of the presumably most important cytosolic factors was investigated using two glycolytic reactions (i.e., the phosphoglucose isomerase reaction (PGI) with a uni-uni-mechanism and the enolase reaction with an uni-bi-mechanism) as examples. Crowding by macromolecules was simulated using polyethylene glycol (PEG) and bovine serum albumin (BSA). The reactions were monitored calorimetrically and the equilibrium concentrations were evaluated using the equation of state ePC-SAFT. The pH and the crowding agents had the greatest influence on the reaction enthalpy change. Two kinetic models based on irreversible thermodynamics (i.e., single parameter flux-force and two-parameter Noor model) were applied to investigate the influence of cytosolic conditions. The flux-force model describes the influence of cytosolic conditions on reaction kinetics best. Concentrations of magnesium ions and crowding agents had the greatest influence, while temperature and pH-value had a medium influence on the kinetic parameters. With this contribution, we show that the interplay of thermodynamic modeling and calorimetric process monitoring allows a fast and reliable quantification of the influence of cytosolic conditions on kinetic and thermodynamic parameters.

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