Abstract
Many food processing techniques such as dehydration, concentration, extrusion, and freezing produce amorphous foods that can be stored below their glass transition temperature (Wungtanagorn and Schmidt 2001). Below glass transition temperature (T g), amorphous food constituents exist in a thermodynamically unstable nonequilibrium and disordered state. Isothermal storage/aging of glassy amorphous food components results in structural relaxations that achieve a more stable equilibrium state over extended time periods (Struik 1978). Since the equilibrium state is a low energy state, some of the energy is lost/relaxed in the nonequilibrium glassy amorphous state during isothermal storage of food components. This energy can be recovered in the form of enthalpy during the reheating of the glassy system by using a differential scanning calorimeter, since physical aging is a reversible process. The enthalpy recovered during reheating of the aged material system is a measure of the system’s molecular mobility at the selected aging temperature (Gupta et al. 2004). Structural relaxation in the glassy state of amorphous food components during isothermal storage/aging is also known as enthalpy relaxation/physical aging. Many macroscopic properties of glassy materials, such as volume, enthalpy, refractive index, electrical conductivity, and viscosity, change during physical aging (Struik 1978). The changes in macroscopic properties may adversely affect the physicochemical stability during the isothermal storage of low water amorphous foods and food constituents (Farahnaky et al. 2008).
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Abbreviations
- C p(glass) :
-
Heat capacity of a material system in the glassy state
- C p(liquid) :
-
Heat capacity of a material system in the rubbery state
- DSC:
-
Differential scanning calorimetry
- KWW:
-
Kohlrausch-Williams-Watts equation
- LSD:
-
Least square difference
- m :
-
Fragility index
- MDSC:
-
Modulated differential scanning calorimeter
- R :
-
Gas constant
- t :
-
Aging time
- T g :
-
Glass transition temperature
- T ge :
-
End point glass transition temperatures
- T gi :
-
Glass transition width approach by identifying the onset
- T gm :
-
Mid temperature between T gi and T ge
- β :
-
Relaxation distribution parameter
- ΔC p :
-
Heat capacity change at the glass transition temperature
- ΔE :
-
Activation energy for structural relaxations at T g
- ΔH ∞ :
-
Total enthalpy available for relaxation during the aging time
- ΔH relax :
-
Enthalpy relaxation during the aging time
- τ (days):
-
Mean molecular relaxation time of the entire amorphous system
- φ(t):
-
Relaxation function
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Syamaladevi, R.M., Barbosa-Cánovas, G.V., Schmidt, S.J., Sablani, S.S. (2015). Molecular Weight Effects on Enthalpy Relaxation and Fragility of Amorphous Carbohydrates. In: Gutiérrez-López, G., Alamilla-Beltrán, L., del Pilar Buera, M., Welti-Chanes, J., Parada-Arias, E., Barbosa-Cánovas, G. (eds) Water Stress in Biological, Chemical, Pharmaceutical and Food Systems. Food Engineering Series. Springer, New York, NY. https://doi.org/10.1007/978-1-4939-2578-0_13
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DOI: https://doi.org/10.1007/978-1-4939-2578-0_13
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