Effects of microwave on the drying, checking and mechanical strength of baked biscuits

The influence of high-frequency radiation on quality in terms of checking and mechanical strength of low-moisture baked products was investigated. Biscuits baked in a convection, reel oven were compared to biscuits of very similar moisture content and appearance that were baked in the same oven until the falling-rate drying period but then immediately microwaved for 30 s in a 700 W microwave oven. Microwave baking was found to significantly reduce checking to 5% compared to 61% in conventional biscuits. The average breaking stress required for intact microwaved biscuits two weeks after baking was 997 kPa, while that for conventional samples was 610 kPa. The structure of conventional biscuits was generally found to weaken at a faster rate than microwaved biscuits. Microwaved biscuits were also found to be less susceptible to checking upon exposure to high ambient humidity. In addition the finite element software. FIDAP. was used to predict heat and mass transfer with respect to time and position during microwaving under various conditions. A biscuit was modeled as an axisymmetric short cylinder. Simulations confirmed greater drying rates and more uniform internal moisture profiles for microwaved biscuits. A minimum microwaving time of approximately 28 s was predicted to be necessary to produce a uniform moisture profile for a power absorption of 1.7 W/g. An elevation in the biscuit temperature during baking as well as a more uniform moisture distribution found in microwaved biscuits were thought to be critical factors in determining checking behavior and mechanical strength. (C) 2001 Elsevier Science Ltd. All rights reserved.