Experimental study on transport phenomenon during deep fat frying of chicken nuggets
MetadataShow full item record
Two important factors affecting oil uptake of food products during deep fat frying (DFF) are water content and pressure development. In the past frying studies, pressure has not been measured physically but was calculated using computer models, which has resulted in some disagreements in the literature about its magnitude. The present study tries to explain the complex mass transfer mechanisms (fat uptake and moisture loss) taking place during DFF with respect to real time pressure variations inside chicken nuggets. The first objective of this study was to measure the pressure changes inside nuggets during frying by using a fiber optics sensor (FISO Technologies Inc, Québec, Canada). The second objective was to perform fat and moisture analysis for methylcellulose (MC) and control nuggets at different frying times and temperatures. The third objective was to visualize the microstructure in the core, crust and cross-section of chicken nuggets using scanning electron microscopy. The fourth objective was to visualize the oil uptake by the nuggets by frying them for various times in oil containing a fat-soluble dye (Sudan Red) and then observing them under light microscope. Breaded chicken nuggets were made with and without 5% MC added to predust. All the frying experiments were performed at two temperatures (175ºC and 190ºF) for 0, 30, 60, 120 and 240 sec. The gauge pressure increased rapidly above the atmospheric pressure immediately after the nuggets were introduced into hot oil. This was expected due to sudden moisture flash-off. As the temperature of the nugget increased, the pressure inside the nugget decreased to negative values (suction). As the nugget was removed from the fryer after 240 sec (post-frying cooling phase), the pressure decreased further for another 2 to 3 min. The negative pressure values caused rapid absorption of surface oil. During the post-frying cooling phase, the pressure tends to reach an equilibrium negative value and then starts rising back to 0 bars (ambient pressure) in 2 to 3 hour. The highest value of pressure was 0.0018 bars and the lowest was -0.19 bars. The MC-coated nuggets had lower fat uptake and higher moisture retention when compared to control nuggets in the core and crust regions for both frying temperatures. From the scanning microscopic analysis, control nuggets had higher levels of randomness in the crust, core and meat layers in terms of microstructure development, surface texture, rigidity and pore sizes when compared to MC-coated nuggets. With an increase in frying temperature, the nuggets had more surface damage and increased complexity of microstructure for both treatment and control nuggets. The nuggets fried in dyed oil showed oil penetration only from 1 mm to 4 mm into the meat layer from the crust. This implied that the oil uptake in the frying process was a surface phenomenon when observed under the light microscope. The present results provided scarce of real time pressure variation data during DFF with respect to the simultaneous mass transfer processes taking place. This will aid in understanding and elucidating of the oil uptake mechanisms, oil distribution, microstructure development and other parameters needed for optimizing the frying process to obtain healthier, low fat fried foods.