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Part 1: Invited Speakers and Oral Presentations
Session 1
Water Mobility/Dynamics and Its Application in Food and Pharmaceutical Systems
Invited Speakers
1
Complementary Aspects of Thermodynamics, Nonequilibrium Criteria, and Water Dynamics in the Development of Foods and Ingredients
M. P. Buera
Abstract
Temperature and water content of the systems have been the variables most widely employed to define and predict the kinetic coefficients of desirable and undesirable changes in foods. Supplemented temperature vs composition phase diagrams have been demonstrated to be helpful in determining the feasibility of occurrence of phase or state transitions. These diagrams include the glass transition temperature (Tg) curve and the equilibrium liquidus curves. The inclusion of the nonequilibrium curves enables relationships with the time coordinate and, thus, with the dynamic behavior of the systems to be established and helps to predict whether the systems are under thermodynamic or kinetic control for given composition vs temperature conditions, provided that the thermal history of the samples is known. The present work analyzed how complementary aspects of thermodynamics and nonequilibrium criteria and water dynamics can be assembled in order to demonstrate the formulation and processing strategies to optimize the stability of food products and ingredients, especially in dry systems.
A wide variety of kinetic data from several chemical reactions (in vegetable and animal tissues, dairy products, ingredients, and pharmaceutical formulations or model systems) from the published literature on the results from specifically designed experiments were distributed in supplemented phase diagrams. The results indicated that both solid-water interactions and structural characteristics of the systems governed the dependence of reaction rates on relative humidity. In addition to the supplemented phase diagrams, structural aspects of the matrices where the reaction takes place, water-sorption properties, and water mobility itself were key aspects for a complete interpretation to describe the dynamics of the chemical reactions.
Introduction
The kinetic control of desirable and undesirable aspects of chemical reactions represents a challenge in basic and applied areas of chemistry and food sciences. Therefore, the impact of the variables and mechanisms that determine reaction rates has been given much attention. Temperature and water content of the systems have been the variables most widely employed to define and predict the kinetic coefficients. The significance of state and phase transitions in the stability of amorphous food materials and also their impact on chemical and enzymatic reactions have been evaluated since the 1980s (Slade and others 1989; Roos and Karel 1991; Karmas and others 1992; Levine and Slade 1992; Bell and Hageman 1994; Bell 1995; Buera and Karel 1995; Lievonen and others 1998; Bell and White 2000; Kouasi and Roos 2000). Since an equilibrium state does not exist in these systems, the conservation of desirable properties in foods and ingredients is governed by conditions of metastability, often based on the maintenance of the systems in an amorphous state (Levine and Slade 1992): although the system components are not thermodynamically stable, they are kinetically stabilized.
Supplemented temperature composition phase diagrams have proven helpful in determining the potential of phase or state transitions (Slade and others 1989; Levine and Slade 1992). These diagrams include the nonequilibrium glass transition temperature (Tg) curve and the equilibrium liquidus curves. The inclusion of the nonequilibrium curves enables the relationships with the time coordinate and, thus, with the dynamic behavior of the systems to be established and helps to predict whether the systems are under thermodynamic or kinetic control for given temperature/composition conditions, providing that the thermal history of the samples is known. Structural aspects of the matrices where the reactions occur, sorption, and water mobility itself were also detected as key aspects in describing the dynamic of this reaction. The present work analyzed how complementary aspects of thermodynamics and nonequilibrium criteria and water dynamics can be assembled in order to point out formulations or processing strategies to optimize the stability of food products and ingredients, especially in dry systems.
Reactions, Materials, and Methods
The stability of selected dehydrated systems toward the Maillard reaction, the loss of enzymatic activity, or carotene degradation was analyzed. Data obtained in freeze-dried vegetable tissues, dairy products, ingredients, and pharmaceutical formulations or model systems as the result of specifically designed experiments (Mazzobre and others 2001; Longinotti and others 2002; Prado and others 2006; Acevedo and others 2006, 2008b; Sutter and others 2007) were plotted in supplemented phase diagrams. Additional information on water dynamics and water-sorption properties was obtained.
Differential scanning calorimetry (DSC) was helpful in analyzing thermal transitions and generating temperature- vs composition-supplemented state diagrams. Time-resolved proton nuclear magnetic resonance (1H-NMR) was used to complement DSC with the aim of obtaining a better understanding of the mobility of water and food solids in the systems (Schmidt and Lai 1991; Kou and others 2000; Tang and others 2000; Chatakanonda and others 2003). X-ray diffraction and microscopy provided information on molecular and microscopic structural changes.
Maillard Reaction
The Maillard reaction may have a positive or a negative contribution to the quality of food products, and its complexity necessitates a deep kinetic analysis of reaction media and the variables involved to direct the reaction with the desirable outcome. In dehydrated foods, the reaction is the cause of off-flavors, off-colors, and loss of nutritional value. The kinetics of the Maillard reaction must be controlled also as a requirement of new food technologies: in the development of natural flavors, pigments, emulsifiers, antimicrobials, and antioxidants; in the formulation of protective media for biological systems and ingredients; and for controlled modification of biomolecular functionality or structure.
The reaction rate is strongly dependent on the concentration, ratio, and chemical nature of reactants, temperature, water content, pH, and water activity (aw) (Labuza and Baisier 1992), but it is also influenced by the physical properties of the media.
In liquid systems, the Maillard reaction rate diminishes continuously as relative humidity (RH) increases, mainly because water is a product of the reaction (Hodge 1953; Eichner and Karel 1972; Labuza and Saltmarch 1981). However, in solid or quasi-solid systems, in which reactants are constrained by mobility restrictions, a maximum rate of nonenzymatic browning (NEB) is observed at a given intermediate RH value. Thus, the presence of a maximum in the plot of rate versus water content (or aw) is a consequence of the low reaction rates due to mobility limitations of the reactants (at low water content) and inhibition by the product (at high water content) (Buera and Karel 1995; Van Boekel 2001). Systems with different structural characteristics were analyzed to elucidate the relationship among browning rate, water-solid interaction, and water mobility and the incidence of structural changes accompanying phase or state transitions: highly collapsible polymeric (polyvinylpyrrolidone [PVP-40]) matrices, crystallizing lactose and milk systems, and vegetable tissues exhibiting an intermediate degree of collapse because of the presence of water-insoluble polymers capable of providing residual structure. The shaded areas in Figures 1.1 and 1.2 represent the temperature vs composition conditions at which the Maillard reaction was analyzed, and the circled regions show the conditions at which the maximum rates were observed.
The corresponding Tg curves are shown in Figures 1.1 and 1.2 also as a function of water content. Since sugars are the main soluble components determining Tg values in vegetable and dairy systems, solubility curves for the main sugars present were included as a reference.
In PVP systems (Figure 1.1) at a given temperature, the Maillard reaction rate increased as water content increased, and the maximum rate occurred at a water content at which Tg was close to the storage temperature. Above this point, the samples presented a fully collapsed fluid aspect, and the Maillard rate decreased when water content increased (behavior similar to that of liquid systems). In the crystallizable lactose- or trehalose-containing systems (Figure 1.1), the maximum rate occurred at conditions in which a considerable degree of sugar crystallization had occurred, but when the samples were totally crystalline the rate decreased. Figure 1.3 shows the rate of the Maillard reaction as a function of water content for lactose systems compared with the rates with milk and lactose-starch.
It should be noted that, in milk or lactose-starch systems, the presence of proteins or biopolymers retarded lactose crystallization. Consequently, the maximum rate of browning occurred with a higher water content, and the maximum rate was maintained in higher water contents than in the sugar-only matrices (complete crystallization occurred at higher water content values in those samples). In vegetable tissues containing structure-maintaining water-insoluble biopolymers and presenting an intermediate degree of collapse, the maximum rate of NEB occurred at RHs in a range of 50%–80% when the samples were well above Tg at the storage temperature (Figure 1.2).
In all cases analyzed, the Maillard reaction occu...
