Image-related parameters including the difference/ratio of luminance features and percentage oil area (POA) were examined under different cooking conditions to investigate if they could be used as oiling off indexes of cheese in a computer vision method. POA was most suitable because, in addition to its high correlation with the fat leakage obtained from the traditional oil ring test, the information regarding the oiling off property of cheese provided by POA was in good accordance with observation. Free oil formation of both Cheddar and Mozzarella cheeses occurred rapidly during the initial stage of cooking, followed by a notable decrease afterwards. For the Cheddar cheese, obvious oiling off was observed at all temperatures from 70 to 200 degreesC, while the maximum oiling off occurred at 160 degreesC. Little free oil was formed for the Mozzarella cheese below 100 degreesC and the highest value of POA was observed at 130 degreesC. The oiling off property of cheese was significantly influenced by sample dimensions. Regardless of sample dimensions, the maximum oiling off of Cheddar cheese occurred at I min of cooking and little free oil was observed after cooking for 4 min. As the side length of cheese slices increased from 25 to 70 min, the increase in POA after I min cooking can be described by a square polynomial. Free oil formation was limited for Mozzarella cheese in spite of a wide range of heating temperature and sample dimensions applied. (C) 2003 Elsevier Ltd. All rights reserved.