The effect of coating with xanthan gum on the colour indexes and surface change of fried eggplant slice

Introduction:
Frying is a complex unit operation that is widely used in the food industry. Frying temperatures can range from 140–190°C, but the most common temperatures are 170–190°C. These conditions lead to high heat transfer rates, rapid cooking, browning, and texture and flavor development. 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 (bubbles) and oil at the surface of the product. Colour changes measured may be used to predict both chemical and quality changes in a food. The colour parameters have previously proved valuable in describing visual colour deterioration and providing useful information for quality control in fruits and fruit products (Salehi 2019b). The oil uptake problem associated with fried products can be decreased by using hydrocolloids as edible coating. Also, the sensorial examination results showed that the coated products with gums have total acceptability better than the uncoated samples (Salehi 2020). Using of hydrocolloids (gums) to decrease the oil uptake during deep-fat frying is one of the easy and most convenient way which does not needs changes in frying devises. The term “gum” is used to explain a group of naturally occurring polysaccharides. Gums have a good functional characteristic such as emulsifying, coating agents, packaging films, gelling, stabilizers, solubility and textural improvement. Gums influence on the gelatinization and retrogradation of starch and decreased the retrogradation of starch (Salehi 2019b; Salehi 2020). Kim et al. (2011) studied the influence of the Guar gum coating on the oil uptake and heat transfer of the potato strips during frying process. Their results showed that the addition of guar and gellan gums as hydrocolloid coatings decreased the heat transfer coefficient and oil uptake in potato during the frying process. They reported that the use of 0.9% Guar gum for coating decreased the oil content of the fried potato up to 8.9%, that was 41.0% lower than the control sample. Potential of almond gum (0-20 g/L) application as a coating agent to improving the quality and sensorial properties of fried potato chips was investigated by Bouaziz et al. (2016). Their results showed that the raise of almond gum levels up to 20 g/L reduced the 34% oil uptake of chips and raised the 29.5% moisture content. Optimum conditions for coating and frying establish with response surface methodology (RSM) were thus reached with 20 g/L almond gum at 160°C frying temperature for 75 s. The sensorial examination results showed that the coated potato chips with almond gum have total acceptability better than the uncoated samples. The coated chips have a notably lighter colour (higher L* index) and were considerably yellower than the uncoated samples. The lightness (L* value) was 58.2 for the uncoated samples, while 88.3 and 87.5 were found for fried samples coated with almond and arabic gums, respectively. Application of edible hydrocolloid coatings is one of the best ways to improve appearance properties of fried foods. The aim of this study was to investigate the changes in colour parameters (L*, a*, b* and ΔE) and the area of eggplant slices coated with different concentrations of xanthan gum during deep fat frying.
Material and methods:
In this study, eggplant slices in a cylindrical shape with a thickness of 1 cm were coated using xanthan gum in three concentrations of 0.5, 0.5 and 1.5%. Then they were placed in the fryer and the effects of frying temperature at three levels of 150, 175 and 200 °C on the appearance characteristics of the samples were investigated. To examine the changes in colour indexes including lightness (L*), redness (a*), yellowness (b*) and colour changes (ΔE) as well as changes in the area of the samples, images were taken continuously during the frying time. In order to investigate the effect of frying temperature and xanthan gum concentration on the colour changes of fried eggplant, a computer vision system was used. Sample illumination was achieved with HP Scanner (Hp Scanjet 300). 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. The Power, Quadratic, Gompertz, Logistic, Richards, Weibull, and MMF kinetic models were chosen to describe the colour changes index within the frying process of eggplant slices. The estimation of the models parameters for each one of the colour parameters was carried out using a non-linear regression analysis method, separately for each treatment during frying of eggplant slices. The experimental data were modeled by using non-linear regression in Curve Expert 1.34 software.
Results and discussion:
The L* index calculated for the frying process of eggplant samples showed that the coated samples were brighter in terms of lightness and eggplants coated with xanthan gum with a concentration of 1.0% had higher L* values. Since lightness is a very important colour quality parameter, lower frying temperatures with lower boiling point of water are preferable to preserve the lightness and hence the attractiveness of fried products. The reduction in L* may be attributed to intense browning reaction and increase crust formation due to exposure to high temperature. With increasing the concentration of xanthan gum from 0.5 to 1.5%, the redness of the samples increased from 2.82 to 14.58. In terms of colour change index (ΔE), eggplants coated with xanthan gum showed the least colour changes during frying. The mean ΔE values for the control sample, 0.5%, 1% and 1.5% of xanthan gum were 30.97, 29.13, 30.57 and 28.12, respectively (at 150°C). For modeling the colour change index, the MMF model had less error compared to the Power, Quadratic, Gompertz, Logistic, Richards, and Weibull models and it fitted well with the experimental data. Surface change% (shrinkage) is a common phenomenon during frying. Rapid water loss resulted in significant shrinkage in products during frying. The average area changes calculated for the control sample, 0.5%, 1% and 1.5% of xanthan gum were 49.41, 48.57, 42.54 and 40.77, respectively. The surface change of eggplant slices was increased with the progression of frying time and temperature. High surface change was seen in eggplant slices fried at 200°C which may be due to fast removal of moisture.
Conclusion:
Heat and mass transfer phenomena take place during frying cause physicochemical changes, which affect the colour and surface of the fried products. Xanthan gum concentration, oil temperature and frying time are the process parameters which affect the colour parameters of eggplant slices during frying. The results showed that coating of eggplant slices with 1.5% of xanthan gum preserves the appearance of the fried product and also the least area changes were observed in these samples in the frying time at different temperatures. Different kinetic models were used to fit the experimental data and the results revealed that the MMF model was the most suitable to describe the colour change intensity (∆E).