UBC Theses and Dissertations
Puffing induced in two model systems by microwave assisted drying under vacuum : an experimental and numerical analysis Ressing, Mareike Johanne
Microwave heating under vacuum (VM) removes water at reduced temperatures while minimally altering desirable biochemical characteristics of foods compared to traditional air drying methods. Concurrently, it achieves much greater drying rates than other vacuum drying methods. Puffing has been observed in VM dried products and reportedly contributes to improved rehydration and eating quality of VM processed foods. However, the underlying mechanisms [of] this phenomenon are poorly understood. Wheat dough and gelatinized potato starch were selected as model systems to investigate VM puffing since their respective rheologies have previously been characterized. The thesis objective was to advance our understanding of the mechanism behind VM puffing, especially with regard to the rheological and dielectric properties of the selected model systems, and use these data, namely wheat dough results, to develop and validate a numerical model of VM puffing. One model system consisted of dough balls prepared with different quality flour, water and salt (0.0-1.5%) and dehydrated at 700 and 1300 W microwave power at 28 Torr absolute pressure. Cylinders of gelatinized potato starch (30-50% starch d.b., 0-2% NaCl, respectively) which served as the second model, were dehydrated at 350, 700, and 1300 W. For both model systems, volume, dielectric properties, Young’s modulus and/or fracture strength were determined. A video recorder was used to observe the dough and starch model systems during dehydration. Both model systems exhibited pronounced puffing which was influenced by applied power levels, salt content, flour type or starch concentration and dielectric properties. While dough already expanded upon dropping chamber pressure, starch gels needed prior heating to decrease resistance to expansion. Both systems exhibited salt dependence with respect to their rheology. Dough showed reduced volume increases as salt concentration and elasticity decreased, while salt containing starch gels exhibited increased brittleness and lack of cohesion that decreased the expansion potential. Finite Element (FE) modeling described the dough development during microwave drying under vacuum. It illustrated that the puffing is highly influenced by the rheological properties of the dough more so than microwave interaction. While the accuracy was lower than expected, it revealed a stronger pressure interaction with the rheological properties in addition to steam-induced puffing. Image analysis used to characterize porosity of the dried product was only moderately successful and pointed to the possibilities in the field rather than offering a solution to current questions.