Measurement of Central Temperature and Color Changes of Onion Slices During Frying Process

Frying is applied as one of the oldest methods of food preparation both in domestic and industrial scale. Color development during the frying process is a surface phenomenon that depends on the processing temperature and time. The aim of this research is the measurement of central temperature, surface changes and color changes the behavior of onion slices during frying process as a mathematical model at various temperatures.

Onion slices with 1 cm thickness and cylindrical form were fried at the temperature of 150, 175 and 200 ºC and its central temperature was recorded using K type thermocouple with 1 mm thickness by 5-second intervals. The surface temperature of the product was measured using a laser thermometer. Color parameters as brightness (L*), redness (a*), yellowness (b*) and color change intensity (∆E) were recorded during frying time and kinetic model was fitted on their changes versus time and models coefficients were reported.

The temperature of the oil had a negative effect on the brightness of the fried onion and, with increasing process temperature, the brightness parameter decreased at the same time. The results of this experiment showed that most of the color changes occur in the early stages of the process. Higher temperatures increased the yellowness and redness of the onion surface from 14.24 to 29.31 and 5.73 to 17.86, respectively. Fried samples at a temperature of 200°C reduced the size about 40.48 %. The central temperature became closer to the water boiling point at a temperature of 200°C and after a brief period at this temperature, the temperature of the center increased again with full water exhaustion.

The kinetics of the surface color parameters changes and color change intensity (∆E) of the onion followed the incremental exponential function and power function, respectively.