Evaluation of different techniques for microstructural characterization of deep-fat fried foods

This study investigated the use of different techniques in evaluating the microstructural characteristics of deep-fat fried chicken nuggets, and considered the effect of frying conditions and batter formulation.;Porosity of preformed and lab formulated deep-fat fried chicken nuggets batter coatings were investigated using helium pycnometry. There was significant (P < 0.05) effect of frying temperature and batter formulation on porosity. Porosity at 170°C was significantly different from those at 180 and 190°C, while porosity of batter with 100% wheat flour was significantly lower than the other formulated batters. However, porosity increased with frying time and temperature, and it ranged from 2.15--47.9% and 9.96--54.8% for the preformed and the formulated batter, respectively. Porosity demonstrated significant (P < 0.05) positive and negative correlation with fat uptake and moisture loss, respectively for all batter coatings.;Images (3-D) of deep-fat fried chicken nuggets batter coating and core were obtained by X-ray micro computed tomography (CT). Porosity for the batter coating and the chicken core varied from 7--14.1% and 4.4--7.7%, respectively. Frying time significantly (P < 0.05) influenced porosity, fragmentation index (degree of interconnectivity) and pore count. Porosity and pore count increased with frying time while pore interconnectivity decreased. Pore shapes were between cylindrical and spherical structure for the coating, while the core pores showed cylindrical shape. Pore size distribution showed that micropores (< 100 mum) increased in volume and number with frying time.;X-ray microCT was also used to investigate the effect of batter formulation on pore characteristics. There was significant (P < 0.05) effect of batter formulation on porosity, fragmentation index and pore count. Pore shape of all the formulations remained relatively the same with structure model index from 2--3, indicating cylindrical shape. Porosity ranged between 18.2--32.1%. The addition of carboxymethyl cellulose (CMC) caused about 25% decrease in porosity, though not significantly. The addition of rice flour into the batter formulation resulted in significant increase in porosity when compared to batter with 100% wheat flour without CMC inclusion. The addition of CMC resulted in decreased interconnectivity between the pores while rice flour addition caused increase pore connectivity. Other properties of the batter such as viscosity, batter pick-up, fried batter color, texture, moisture and fat content were significantly (P < 0.05) influenced by batter formulation. Some correlations were shown between the physico-chemical properties of the batter system and porosity.;Mercury intrusion porosimetry was used to obtain porosity, pore size distribution and pore shape for freeze dried deep-fat fried chicken nuggets batter coating. Porosity was found to range between 40--69%. Frying time and temperature significantly (P < 0.05) influenced porosity. Porosity showed a high positive and negative correlation with moisture and fat contents, and the correlation coefficients was between 0.88--0.96 and 0.78--0.8, respectively. Pore volume, pore area and mean pore diameter varied between 0.54--1.5 cm3/g, 2.53--15.5 m2/g and 0.25--8.32 mum, respectively. Hysteresis phenomenon showed that pore shapes other than cylindrical structure were present within the samples.;Fluorescence and reflective images of the deep fat-fried chicken nuggets batter coating were obtained in 2-D using confocal laser scanning microscopy (CLSM). There was a decrease in fat intensity from the surface towards the core of the sample. Porosity and fat distribution were significantly influenced by frying temperature and time. Porosity ranged between 5--33%, while pore size varied from 1.20--523 mum. There was formation of smaller (pores ≤ 42 mum) and bigger (pores ≥ 270 mum) pores, while medium size (191--270 mum) pores diminished with frying time. There was a strong correlation (r = 0.79) between fat distribution obtained from CLSM imaging and fat content obtained by the conventional method at 180 and 190°C, indicating a reasonable capacity to predict one from the other.;All the techniques used effectively evaluated the microstructural properties of fried foods studied within their limit of measurement. None of the techniques could provide full microstructural properties of the fried foods studied. It is therefore suggested that various techniques be combined in order to obtain a complete data of the pore characteristics of fried foods.