36D-22 |
|
L. Castro, P. Bouchon, and J. M. AGUILERA. Dept. of Chemical and Bioprocess Engineering, Pontificia Univ. Católica de Chile, Santiago, Chile Although most engineering properties of foods depend on their structure no clear relationships are usually derived, as is the case of textural properties and microstructure of dry cereal products. The objective of this work was to relate microstructural features of a simple dry starch model to mechanical parameters and fracture mechanism as a function of water activity and drying temperature. A pregelatinized potato starch suspension (23% w/w) was dried into strips at temperatures between 65-105ºC to induce different microstructures. Probes were equilibrated to water activities (aw) in the range of 0.00 to 0.75 and analyzed for mechanical properties and glass transition temperature. From the force-deformation curves two textural parameters were calculated: Maximum Force at Breakage (MFB) and Work to Fracture (WF). Images were obtained by light and scanning electron microscopy, and main features (e.g., bubble size, thickness) quantified by image analysis. Probes dried at high temperature were thicker and less dense than those dried at low temperatures, and contained several bubbles of increasing mean size. Mechanical parameters for all samples were similar and low at aw below 0.44-0.65, but increased drastically above this range and depended strongly on the drying temperature. For example, WF and MFB decreased 12 and 5 times, respectively, as drying temperature increased from 65º to 105º C. The fracture behavior changed from brittle to ductile in the mentioned aw interval, as studied in real time by coupling videomicroscopy to the mechanical assay. Glass transition temperatures of samples at the same water activity were similar. It is concluded that: 1) several microstructures, hence, different mechanical behaviors may be induced by drying the starch model; 2) water activity has a dramatic effect on mechanical properties and observed fracture mechanism, and; 3) the glass transition temperature itself is not a good predictor of the mechanical properties of the system.
Session 36D, Food Engineering: Rheology
2005 IFT Annual Meeting, July 15-20 - New Orleans, Louisiana |