Stress analysis of chemically leavened biscuits: Application of pseudoelastic, bi-layer model

Takashi Nakamura, Purdue University

Abstract

The purpose of this research focus was to develop a procedure for predicting the stress state in a cracker during a simultaneous heat/mass transfer process. The stress analysis incorporated a pseudoelastic, bi-layer model. The bi-layer model is defined as the crust and crumb of the cracker. Temperature and moisture content profiles were predicted via heat/mass transport equations. Predicted temperature and moisture content profiles correlated with profiles obtained experimentally. Four different cooling conditions were selected for simulation: normal, forced, cold, and gradual. A Sintech material testing machine was used to obtain relaxation modulus data for two different crust thicknesses. A generalized relaxation master curve was generated for both 180F and 250F baked crackers, and the relaxation modulus of the crust and crumb were evaluated. The relaxation modulus of the crust and crumb were then incorporated into a pseudoelastic stress expression to determine the stress states from different cooling conditions. The pseudoelastic, bi-layer model predicts failure for all four cooling conditions. For normal cooling, a maximum tensile stress of 20.43(MPa) developed in the top layer of the crust. This tensile stress exceeds the failure stress for this cracker. Gradual cooling (50C, 40%RH,.1m/s) showed the smallest magnitude tensile stress, 4.84MPa. The bending modulus was used to determine the glass transition of crackers. Glass transition at 25C, 50C, and 80C were 11-14(%db), 7-9(%db), and 6.5-9(%db), respectively. The largest stress developed in the crust. Separate theoretical crust and crumb effective diffusivities and thermal conductivities were used in the heat/mass transport equations. The corresponding stress development showed the topmost layer in the crust with the largest magnitude tensile stress. A more detailed investigation into the physical properties of the crust and crumb would provide a more detailed stress analysis.

Degree

Ph.D.

Advisors

Okos, Purdue University.

Subject Area

Agricultural engineering|Food science

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