Biological Systems Engineering, Department of


Date of this Version

Summer 6-2015

Document Type



Portions of this dissertation were published as the following:

J. Chen, K. Pitchai, S. Birla, R. Gonzalez, D. Jones, J. Subbiah. 2013. Temperature-dependent dielectric and thermal properties of whey protein gel and mashed potato. Transactions of the ASABE. 56(6): 1457-1467.

J. Chen, K. Pitchai, S. Birla, M. Negahban, D. Jones, J. Subbiah. 2014. Heat and mass transport during microwave heating of mashed potato in domestic oven – model development, validation, and sensitivity analysis. Journal of Food Science. 79(10):E1991-E2004.

J. Chen, K. Ptichai, D. Jones, J. Subbiah. 2015. Effect of decoupling electromagnetics from heat transfer analysis on prediction accuracy and computation time in modeling microwave heating of frozen and fresh mashed potato. Journal of Food Engineering. 144, 45-57.


A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Engineering (Agricultural and Biological Systems Engineering), Under the Supervision of Professor Jeyamkondan Subbiah. Lincoln, Nebraska: June, 2015

Copyright (c) 2015 Jiajia Chen


Nonuniform heating is the biggest issue in the microwave heating of prepared meals. Multiphysics based models are promising tools to improve microwave heating uniformity by properly designing the food product. However, limited availability of accurate temperature-dependent material properties, inadequate model prediction accuracy, and high computational power and complexity in model development are three gaps that greatly limited the application of these models in the food industry.

To fill in the gaps, firstly, we developed a multitemperature calibration protocol to measure temperature-dependent dielectric properties (dielectric constant and loss factor). The temperature-dependent dielectric and thermal (thermal conductivity and specific heat capacity) properties of mashed potato and whey protein gel were measured from -20 to 100 ˚C and were provided as input to the models.

Secondly, a three-dimensional (3-D) finite element model coupling electromagnetic and heat and mass transfer was developed to fully understand the interactions between the microwaves and fresh and frozen mashed potato. Transient point temperatures, spatial surface temperatures, and total moisture loss predicted by the models matched well with the experimental validation. A sensitivity analysis of the effect of input parameters on the model prediction was evaluated in the fresh mashed potato model and found that the gas diffusion coefficient, intrinsic water permeability, and the evaporation rate constant are sensitive parameters that need to be determined accurately. Frequency of updating dielectric properties were evaluated in the frozen mashed potato model and found that dielectric properties can be updated for every rotational cycle without affecting the accuracy.

Finally, these models were further simplified to improve their utility in the microwaveable food development. The simplification of decoupling electromagnetic from heat transfer analysis (use a constant heat source term based on dielectric properties at room temperature) did not affect the predicted temperatures considerably, while reducing the computation time by 93%. A simple 1-D analytical model based on planar wave assumption was developed to determine the thicknesses of multicompartment meals based on the dielectric, thermal, and physical properties, so that two compartments could achieve same heating rate.

These models with different complexity could be used in different stages of microwaveable foods design.

Adviser: Jeyamkondan Subbiah