|Ph.D Student||Michael Uzzan|
|Subject||Mechanisms of Effect of Microwave Heating on the Mechanical|
Properties of Protein-Starch Foam Systems
|Department||Department of Biotechnology and Food Engineering||Supervisors||Professor Emeritus Mizrahi Shimon|
|Professor Emeritus Kopelman Ishaiah (Deceased)|
Food products may undergo undesired textural changes during microwave heating. The mechanism of that effect was investigated for those products that can be generally characterized as having a structure of protein-starch foams, especially high-density ones with low starch concentration (e.g. shrimp-like products based on surimi), and low-density ones with high starch concentration (e.g. bread). In both types of foams, a significant increase in toughness is taking place only if the product reaches boiling temperature when heated in a microwave oven. Only a small part of that toughening phenomenon may be attributed to the loss of moisture. The major increase in toughness in high-density protein-starch foams was highly correlated with a density increase. Outflow of steam drives away the non-condensable gases from within the product voids and upon cooling these are collapsed due to steam condensation. Low density foams with an open cell structure behave differently. The steam that is formed by local boiling travels through the foam and condenses on colder surfaces. This process affects not only the heat transfer process but mainly the structure and the properties of the starch granules and the foam cell-walls. SEM and light microscopy revealed the following microstructural changes that are caused by the microwave heating at boiling conditions: The cell walls were more compact; water content in the starch granule perimeter seems to be lower; and changes in the amylose distribution were observed within the cell wall region with higher amylose concentration outside of the granules. The effect of these changes on the toughness is becoming significant only at high enough starch content (>37%), while at low starch concentration (<26%) no differences were found between the microwave and conventional heating. DSC measurements revealed that the changes in the state of amylose increase its availability to form complexes with phospholypids. It was also found that lecithin enrichment in a bread formulation could minimize the microwave-induced toughness of bread.