A study of the effect of fat content and preheat treatment of milk has shown their respective effects on the rheological behavior of curd during the coagulation process of plain yogurt. Increase in fat content leads to an increase in viscosity. The highest viscosity was manifested by milk heated at 137 degrees C while the lowest value was exhibited by milk heated at 65 degrees C. The process viscosity curves for three different stages are described by mathematical relationships. Finally, a two-parameter power law model was used to describe the flow behavior of the yogurt during coagulation. (C) 2000 Elsevier Science Ltd. All rights reserved.