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Effects of Production by Microwave Heating After Conventional Baking on Moisture Gradient and Product Quality of Biscuits (Cookies)¹

¹Presented in part at the AACC 79th Annual Meeting, Nashville, TN, October 1994. ²Danone S.A., Campinas, SP, Brazil. ³Engenharia de Alimentos, Universidade Estadual de Campinas CP 6121 CEP 13081-970, Campinas, SP, Brazil. Corresponding author. E-mail: yokic@fea.unicamp.br ⁴Laboratorio de Investigación en Materiales Centro de Investigación y de Estudios Avanzados del IPN-Universidad Autónoma de Queretaro. Facultad de Química- CINVESTAV, Centro Universitario, Cerro de las Campanas, CP 76010, Querétaro, Qro. México.

After conventional (forced-convection heating) baking at 240°C for 4 min, biscuits (cookies) were baked further in a microwave oven at medium and high settings (617.3 and 745.5 W, respectively) to study the effects of microwave baking on the moisture gradient and overall quality of the cookies. Microwave baking significantly (P < 0.05) reduced the moisture gradient and total moisture content of the cookies. Initially, a complete factorial design at baking times of 15, 20, 30, and 40 sec with microwave ovens at high and medium power settings was used to evaluate the moisture gradient and total cookie moisture content. Applying high and medium microwave settings for 30 or 40 sec, respectively, avoided cracking, although the products were slightly darker. Treatment at a high power setting for 20 sec resulted in a moisture gradient of 1.11%/cm and 2.8% cracking. Gradients >1.5%/cm produced significant levels of cracking. Cookies postbaked at a medium microwave power setting for 29 sec produced the same moisture gradient as a high microwave power setting for 23 sec, which was significantly (P < 0.05) lower than the control (cookies baked using the traditional process). The cookies were softer and the color did not differ significantly from that of the control. The expansion ratio of the control sample (11.3) was significantly higher than the combined process sample (10.7), showing a shrinkage effect attributed to the microwave treatment. The removal of the residual moisture during microwaving also increased product weight losses (from 0.912 to 0.956 g).