Study of heat transfer and kinetic modeling of color changes during frying process of eggplant

Frying is a complex unit operation that is widely used in the food industry. During the process, food is immersed in an oil bath at a temperature above the boiling point of water. This results in counter flow of water vapour and oil at the surface of the product (Moyano et al., 2002; Salehi, 2018). The colour of the fried eggplant slice is one of the most significant quality factors of acceptance for fried products. Heat and mass transfer phenomena take place during frying cause physicochemical changes, which affect the colour and surface of the fried products (Choe and Min, 2007; Bingol, 2012). Process variables such as oil temperature and oil type are expected to affect the colour of the fried products. Although, the investigation on colour properties of fried products has started many years ago, it has continued with increasing interest in recent years (Bingol et al., 2012, Salehi, 2019). Color changes measured may be used to predict both chemical and quality changes in a food. The color parameters have previously proved valuable in describing visual color deterioration and providing useful information for quality control in fruits and fruit products (Krokida et al., 2001, Salehi, 2017). Krokida et al. (2001) studied the effect of oil temperature, oil type and sample thickness on colour changes during deep fat frying of french fries. They reported that oil temperature and thickness of potato strips have a significant effect on the colour parameters, which are not affected by the use of hydrogenated oil in the frying medium. Color changes of redness may he associated to Maillard reactions (Krokida et al., 2001; Pedreschi et al., 2005). The aim of this research is to study the effect of changes in frying temperature on the colour parameters, surface changes and heat transfer during fat frying of eggplant slice and to specifically investigate changes in these quality attributes at each frying time, and determine kinetic parameters for these changes. Eggplants (Solanum melongena. L) were purchased from a local supermarket. Slices of eggplant with 1 cm thickness were prepared with the aid of a steel cutter and were immediately placed into the fryer. Refined sunflower oil (Ladan oil, Iran) was used as the frying media. Frying temperatures of eggplant slices were set at 150, 175 and 200°C. All experiments were performed in triplicate and the present results are the average of the obtained individual values. In order to investigate the effect of frying temperature on color changes of fried eggplant, a computer vision system was used. Sample illumination was achieved with HP Scanner (Hp Scanjet 300) (Salehi, 2017). L* (lightness-darkness that ranges from 0 to 100), a*(redness-greenness that ranges from -120 to 120) and b* (yellowness-blueness that ranges from -120 to 120) were measured in this study (Salehi, 2017). In this study, the image analysis of dried gums was performed using Image J software version 1.42e, USA. The estimation of the model parameters for each one of the colour parameters was carried out using a non-linear regression analysis method, separately for each colour parameter during frying of eggplant. In addition, power and quadratic models were chosen to describe the colour changes intensity (ΔE) within the frying process. The experimental data were modeled by using non-linear regression in Curve Expert 1.34 software. A thin thermocouple (Type K) was inserted in the center of the eggplant samples, assuring the position by measure of the sample dimensions. The temperatures at the geometric center of the sample and bulk oil were measured, with sampling time (t) frequency of 5 s by means of the thermocouples (Lutron, TM-916, Taiwan). Color is one of the most important appearance attributes of food materials, since it influences consumer acceptability. Besides of consumer acceptability, color is also used for process controlling. Figure 1-3 shows changes in L*, a* and b* values of eggplant slices for increasing frying times and temperature during frying. L* is a critical parameter in the frying industry as it is usually the first quality attribute evaluated by consumers when determining product acceptance. Low L* values indicate a dark colour and are mainly associated with non-enzymatic browning reactions. The results of the experimental lightness data are shown in figure 1. As shown in figure 1, L* value of fried eggplant decreased during frying. Oil temperature has a negative effect on the lightness of fried eggplants. The change in L* value was less at lower frying temperature. In figure 4 colour change (ΔE) is presented as functions of frying time. The colour change intensity gets more intense at higher temperatures (200°C). The mathematical models were fitted to the color change (ΔE) data. The results of fitting the proposed model to the experimental data are reported in table 2. The high coefficient of determination and small standard error of estimate of fitting showed that the color change (ΔE) during frying of eggplant could be modeled by this model. Surface change % is a common phenomenon during frying. Rapid water loss resulted in significant shrinkage in chips during frying. In figure 5 surface change is presented as functions of frying time and oil temperature. As shown in this figure, the surface change % was increased with the progression of frying time and temperature. As soon as the eggplant slices were introduced in the hot frying medium, the slices shrunk. High surface change was seen in eggplant slices fried at 200 °C which may be due to fast removal of moisture. Similar behavior was also reported in change in shrinkage during vacuum frying of banana chips. The results showed that most of the color changes occur in the early stages of the process. Color changes during frying process were depends on the processing temperature and time. The higher temperatures cause less lightness and yellowness, but redness increases. In sum, the high temperature and short time process can be suitable for controlling color quality and thus product safety. The kinetics of the surface color changes of the eggplant followed an incremental exponential function. Fried samples at a temperature of 200°C were more dropped in surface and had smaller size.