Use of Ohmic heating system in peeling tomato and its effect on physicochemical properties of the product

Introduction: Tomato and tomato based product are widely used as fresh, paste, juice, puree, sauce, etc. Canned tomatoes are an important product worldwide. Peeling is one of the important operations in the tomato industry to ensure good final product quality. In tomato peelings, steam and wet-lye are usually used which is associated with high-energy consumption and environmental degradation, respectively (Wongsa-Ngasri and Sastry 2016b). Thermal treatments are known as the most common method to process the food product. Considering the adverse effect of long thermal process on the chemical and physical aspects of the food, novel heating methods are developed to overcome these limitations. Ohmic heating is a quick thermal method which is known as Joule heating, electrical resistance heating, direct electrical resistance heating, electro-heating and electroconductive heating (Aghajanzadeh and Ziaiifar 2018). In batch ohmic heating system, the food is placed between two electrodes and its temperature increases due to electrical current exposure. In fact, it is used to generate heat uniformly throughout food matrices by controlling the electrical conductivities of food components (Wongsa-Ngasri 2004). Considering the ionic concentration of the food, the increase in the temperature could be rapid resulting in better preservation of the nutritional values and physical aspects of the product. Electroporation is the non-thermal phenomenon which occurs during ohmic heating. This produces the pores with different sizes in the cell walls as a result of alteration in the natural dielectric strength of the cells. In general, ohmic heating has been expanding to other applications to the edge of hybrid technologies such as thawing, blanching, extraction and drying. However, to reach its ultimate benefits for new and higher quality products, further insightful research, on other applications, needed. In this study, ohmic heating system was developed to peel the tomatoes and study the effect of this method on different physicochemical properties of the product.
Material and methods: In this study, laboratory scale ohmic peeling system designed and manufactured at Gorgan University of Agricultural Sciences and Natural Resources. The distance between the electrodes (made of stainless steel 314) was 7 cm. In our study, tomatoes ohmically heated in salt solution under various conditions. After initial weighing, the sample placed in salt solution in the ohmic system. Preliminary tests performed to determine the range of operating parameters (salt solution concentration and electric field intensity). The effect of electric field (range from 1500 to 4500 V/m) and concentration of salt solution (range 0.1 to 0.3% w/v) investigated in tomato peeling. After cracking in the tomato skin, the tomato skin easily separated by hand (Wongsa-Ngasri 2004). Hot water peeling at 100° C selected as a control treatment. T-type thermocouple was applied to control the temperature of the cold point of the tomato and the heating medium. The weight loss of the samples was measured considering the difference between the weight of fresh and the peeled tomato vs. the weight of the fresh sample according to Wongsa-Ngasri and Sastry (2016b). Ascorbic acid content of the samples was determined using iodine titration method (Kashyap and Guatam 2012). To study the effect of the peeling methods on the texture, the firmness of tomato was estimated based on the compression percentage as Wongsa-Ngasri and Sastry (2016b) suggested. The optimization of the ohmic peeling was performed using Design Expert software to suggest the best processing condition.
Results and discussion: Tomato skin cracking involves thermal, electrical, chemical, biochemical and physical effects. The increase the fruit temperature caused alteration in different physical and chemical properties. During hot water peeling, the process time lasted for 523.33 s. This method brought 16.08% weight loss, 36.08% degradation of the ascorbic acid and 30.86% alteration in the firmness of the sample. The obtained results showed that peeling using the ohmic heating method resulted in faster skin cracking (82s), the less weight loss (10.7%), less ascorbic acid degradation and more stiffness (19.47% compression). The initial temperature of the tomato was 22°C which increased to 40 and 33.22°C after hot water and ohmic heating peeling methods, respectively. It was found that the electric filed intensity and the salt solution concentration had significant effects on the raise in the cold point temperature of tomato. Considering the thermal sensitivity of the ascorbic acid to thermal treatments, it can be mentioned that the ohmic heating peeling was more effective in preserving this vitamin. As, the least vitamin degradation was observed during the tomato processing at 2500 V/m in 0.2% salt solution due to the lower alteration in the sample temperature.
The change in the weight of the ohmic heated tomato resulted from de-esterification of pectin especially in mesocarp and electroporation effect on the skin destruction. To determine the optimum ohmic peeling conditions was determined based on obtaining the highest level of ascorbic acid content and the least cracking time, weight loss, firmness and alteration in the cold point temperature
Conclusion: It could be concluded that the quick rise in the tomato temperature as well as electroporation phenomena enhanced the peeling process in which the product with high nutritional value and quality was produced. In fact, weight loss and compression of the hot water peeled tomato were respectively 6.08% and 11.39% higher than the ohmic heated sample; however, the ohmic peeling brought a higher (4.1%) ascorbic acid content in comparison to the control method. The optimum ohmic peeling condition was processing the tomato in 0.2 W/V salt solution at 2500 V/m. These conditions have shown the potential to be good for processing because they require a reasonably short time. Ohmic heating method can be introduced as an effective method in peeling process of the fruit and vegetable with thin skin such as tomato to produce the high quality product. However, more studied should be performed to investigate other operational parameter affecting the process performance especially using different solution as heating medium and also combining the ohmic heating with other thermal, non-thermal and even chemical peeling methods based on hurdle concept. Besides more physical and chemical properties of the samples could be studied to have a comprehensive view of the effectiveness this novel method.