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In this study, the performance of an ohmic concentration system was analyzed based on the second law of thermodynamics. The influence of salt content (0-2% w/w), voltage gradient (5-11 V/cm) and electrode type (316L St, Al, and Br) were evaluated on the exergy aspects. Results showed that increasing of salt content and voltage gradient decreased the specific exergy consumption and increased the exergy efficiency (p0.05). Nowadays the demand for newest technologies in the area of food thermal processing with low energy consumption, high energy efficiency, and preservation the foodstuffs quality is growing. Ohmic heating is one of the alternatives and latest technologies in food thermal processing whereby the electrical resistance of the food itself generates heat as electrical current passes through it (Sakr and Liu, 2014). The advantages of the ohmic heating method are the rapid and uniform heating process, improving product quality, decreasing of energy consumption and saving the cost of the process (Sakr and Liu, 2014; Farahnaky et al., 2012; Moreno et al., 2012).
The previous study also expressed that Ohmic heating could be a promising method in the fruit juice industry especially in the evaporation/concentration of fruit juice process. The process of producing juice concentrate by conventional vacuum heating needs high energy and capital (Nargesi, 2011). Most of the thermal processes and heating equipment have low energy efficiency. Therefore, it is vital for researchers and engineers to increase the thermal efficiency of heating systems using engineering analyses. Exergy analysis is a useful tool for evaluating the energetic performance of an ohmic concentration system for the production of tomato paste. The use of the exergy analysis can overcome the limitations of energy analysis which focuses only on the quantity of energy, and thereby becomes more meaningful.
Exergy analysis determined of the energy quality disintegration during energy transfer and conversion (Prommas et al., 2012). Also, Exergy is a more easily understood thermodynamic property than entropy to represent irreversibilities in complex systems (Nanaki and Koroneos, 2017; Hammond and Winnett, 2009). From the second law of thermodynamics, exergy can help identify the irreversibilities associated with the energy flow and its conversion. Energy is defined as the maximum possible useful work that a system can deliver when it undergoes a reversible process from the initial state to the state of its environment, the dead state (Akbulut and Durmu, 2010; Prommas et al., 2012). The exergy method is a particularly useful tool in handling energy planning and decision-making for sustainable development. Exergy analysis of the ohmic heating system of liquid food presents a novel approach to performance evaluation of ohmic systems, which could be especially used in the industrial implementation of these systems. Bozkurt and Icier (2010) performed the exergy analysis of ohmic cooking of ground beef in an ohmic heater, and reported that the energy and exergy efficiency values for ohmic cooking process at the voltage gradients between 20 and 40 V/cm were in the range of 0.69–0.91% and 63.2–89.2%, respectively. Darvishi et al. (2015) studied only voltage gradient effect on thermodynamic aspects of ohmic tomato juice concentration and their results revealed the values of energy and exergy efficiencies increased with increasing voltage gradient.
Choice of the suitable electrode in ohmic heating systems is one of the important parameters that need to be considered. Undesirable electrochemical reactions at the interface the electrode and solution, and corrosion may affect the efficiency of the ohmic heating system and this can be avoided by selecting electrodes with a suitable material (Adetunji et al., 2016; Alvarez et al., 2012; Assiry, 2003; Zell et al., 2009). The generated heat and efficiency values of the ohmic heating system are dependent on the conductive nature of the material to be processed and the electrical field strength. Many researchers by adding salt to products increased the electrical conductivity and improved the heating performance and quality of the final product (Icier and Ilicali, 2005; Assiry et al. 2003; Zell et al., 2009; Marra et al., 2009; Icier et al., 2006). Assiry et al. (2010) reported that the electrical conductivity increased with increasing dissolved ionic in solution because the electrical current is passed by the ions in the solution. A lot of researchers investigated assessed the effect of electrodes type and salt content regarding corrosion of electrodes, heating rate, electrical conductivity, and quality of final product. But, ohmic heating systems have not been studied from the point of view of the second law of thermodynamics (exergy analysis).
On the other hand, the studies such as Darvishi et al. (2015), Cokgezme et al. (2017) and Bozkurt and Icier (2010) have only examined the effect of voltage gradient on exergy aspects. In the literature review, it isn’t found any studies about the effect of electrode type and salt content on the energetic performance of the ohmic concentration system. Thus, the specific aim of this study was to study the effect of salt content, type of metal electrode and voltage gradient on the energetic performance of the ohmic concentration system as the first work. Tomato fruits (Early Urbana111 Var.) were purchased from a local market, in Sanandaj, Kurdistan, Iran. After washing of tomato samples, the skin of tomatoes peeled using a hot-cold water method. Peeled tomatoes were processed in a plain mixer/juicer to produce freshly tomato juice. Tomato juice was filtered using a vacuum filter for the separation of seeds. Juice samples were stored at 2±0.5 °C during experiments in order to slow down the respiration, physiological and chemical changes. The average moisture content of the tomato samples was as 9.53 ± 0.15 (dry basis), as determined by the oven at 103±1 °C for 24 h (Hosainpour et al., 2014). Fig. 1 shows the static ohmic heating system. The ohmic heating unit consisted of a cylindrical Teflon cell (50 mm internal diameter; 10 mm wall thickness; 150 mm length), two removable electrodes (three types: 316L St, Al and Br) with a 100 mm gap between them and 2 mm thickness, a power analyzer (DW-6090, Lutron, Taiwan), two k- type thermocouples with Teflon coated (connected to digital thermometers), a voltage regulating transformer (1 kW, 0–320 V, 50 Hz, MST – 3, Toyo, Japan), and a computer. Type of metal electrode (316L St, Br, and AL) selected based on studies of Torkian et al. (2017); Adetunji et al. (2016); Alvarez et al. (2012), Zell et al., (2011).
Properties of electrodes and ohmic cell are presented in Table 1. Three holes with diameters of 1 mm and 10 mm were created on the surface of the cell for insert of thermocouples and exit of vapor on the cell, respectively. To prevent the flow out of the juice from cell due to rapid juice boiling (from 10 mm hole), we used a column trap on the top surface of the ohmic cell (Torkian et al., 2015) as shown in Fig. 1. Variation of the mass sample recorded by a digital balance (A&D GF 600, Japan) with ±0.01 g accuracy which is placed under the ohmic cell as shown in Fig. 1. About 100 g (± 0.5) of fresh tomato juice with 20 °C initial temperature was poured through the column trap into the ohmic cell (cell is completely filled). Heating process was carried out until the final moisture content reached 2.43%±0.02 (dry basis) by using different voltages 50, 70, 90 and 110 V (as 5, 7, 9 and 11 V/cm voltage gradient) at 50 Hz frequency (Torkian et al., 2017; Hosainpour et al., 2014). The salt content of the tomato paste samples varied in the range of 0.6 to 2.5% (w/w) for various production companies (Sobowale et al., 2012).
According to the Food and Drug Administration, the maximum salt content of tomato paste is 2% (w/w). Two levels of salt concentration 1:100 g/g (ratio of salt/tomato) and 2:100 g/g (as 1 and 2% w/w) were provided by the salt (NaCl) and results compared without salt sample as a control sample. Salt added to tomato samples during the process by mixer/juicer in order to be uniformly distributed throughout the tomato juice. After each test, the electrodes were rinsed using a brush and distilled water. Voltage, current, mass and temperature data were measured during heating and passed this information to the computer with a data logger.
According to the heating control volume (Fig. 2), the exergy balance for the ohmic system was expressed as follows (Darvishi et al., 2015): The rate of exergy transfer due to evaporation in the heating control volume was (Nanaki and Koroneos, 2017; Sarker et al., 2015): The specific exergy of the input or final product was calculated using Eq. (3) stated as follows (Prommas et al., 2010): The exergy efficiency was calculated using Eq. (4) stated as follows (Darvishi et al., 2015): Exergy loss is determined by Eq. (5): The specific exergy consumption was determined using the following equation: Furthermore, the following equation was applied to find the energetic improvement potential of ohmic concentration system (Icier et al., 2010; Cokgezme et al., 2017).
Statistical method All of the data are expressed as mean and standard deviation values from three replicate measurements for different heating conditions. The ANOVA and Duncan test were used to analyze the effect of salt content, voltage gradient and electrode type on selected properties at the 5% significance level (p=0.05). The statistical evaluation was performed by using software SPSS V.18. Also, the software Table Curve 3D, V4 was used to plotting 3D view of the relationship of parameters and extraction of regression equations. Results and discussion The specific exergy required for the ohmic concentration of tomato juice is shown in Fig. 3.
For all electrodes, exergy consumption decreased significantly (p<0.05) as the voltage gradient and salt content increased. This was because of the dramatic reduction in the concentration-time with an increase in voltage gradient and salt content. The electrolytic content increases with the salt concentration which increases the electrical conductivity. Therefore the heat generating rate increased inside the sample (Duguay et al., 2016; Icier and Ilicali, 2005; Sarkis et al., 2013; Darvishi et al., 2015). However, exergy consumption of Al electrode is higher than 316L St and Br electrodes under different concentration processes (p0.05) at the same heating condition. The minimum specific exergy consumption of 316L St and Br electrodes was obtained 2.73 (MJ/kg water evp) and 2.85 (MJ/kg water evp), respectively, at high voltage gradient (11 V/cm). Fig. 4 demonstrated that the exergy efficiency increased with increasing of voltage gradient and salt content (p<0.05). This consequence indicates that heating and water evaporation rates within the sample were quicker with higher salt content and voltage gradient. Because the passing current through the sample was higher and this increased the heat generation rate in the sample and consequently exergy efficiency increased significantly (p<0.05). As can see in Fig. 4, the exergy efficiency of 316L St (10.12-17.63%) and Br (9.84-16.73%) electrodes is higher than the exergy efficiency of the Al electrode (8.41-15.17%). A similar trend has been observed by Bozkurt and Icier (2010) in the ohmic cooking process of beef, and Darvishi et al. (2015) in the ohmic concentration of tomato juice. They reported that the lower processing time and higher homogeneous heating reduced the exergy losses or equivalently entropy generation, which meant the increase in the energetic efficiency of the system. In order to estimate the mean amount of exergy efficiency at the desired level of the variables, a variation of exergy efficiency was correlated as follows: Values of exergy loss for different heating conditions are presented in Table 2.
The specific exergy loss values varied between 2.25 and 4.42 (MJ/kg water evp) for 316L St electrode, 2.39 and 4.04 (MJ/kg water evp) for Br electrode, 2.75 and 5.11 (MJ/kg water evp) for Al electrode, and significantly decreased as the voltage gradient and salt content increased (p<0.05). The treatment time was longer under low salt content and voltage gradient levels hence entering exergy to the heating cell was increased. For this reason, exergy loss increased with decreasing salt content and voltage gradient. From a thermodynamic point of view, the exergy loss increased when the temperature boundary of the heating system is higher than the ambient temperature (Darvishi et al., 2015; Corzo et al., 2008). Thus, prevention of heat transfer across the boundary of the system could reduce the exergy loss. It is not recommended using of the aluminum metal as an electrode for ohmic concentration/evaporation processes due to the higher exergy consumption and lower exergy efficiency as compared with 316L St and Br electrodes at the same heating conditions. Figure (5) shows that the IP increased with increasing of voltage gradient and salt content. In fact, the IP is the maximum useful exergy which can be absorbed from the exergy loss and increased the exergy efficiency of process by applying some changes in the initial system such as isolation of cell wall, selection of suitable electrode, and applied the energy out of cell by water vapor for preheating of fresh product.
The IP of control samples varied between 2.37 – 3.64 (MJ/ kg water evp) for Br electrode, 2.89 – 3.70 (MJ/kg water evp) for 316L St electrode, and 2.94 – 4.68 (MJ/kg water evp) for Al electrode. While these values at 2% w/w salt content varied between 1.99 – 2.81 (MJ/kg water evp), 1.86 – 2.68 (MJ/kg water evp), and 2.39 – 3.98 (MJ/kg water evp) for Br, 316L St, and Al electrodes, respectively. Also, the IP values of 316L St and Br electrodes are lower than that found for Al electrode at the same heating conditions. Maximum improvement potential can be assessed and structural inefficiencies become apparent, which might trigger interests in process innovations.
The effect of salt content, electrode type, and voltage gradient evaluated on exergy aspects of ohmic tomato paste production, and found as: – Energy efficiency increased with increasing salt content and voltage gradient. – Applied of Al electrode increased the exergy consumption than Br and 316L St electrodes. – There is no significant difference between exegy aspects of Br and 316L St electrodes. – Exergy loss significantly decreased with increasing voltage gradient and salt content (p<0.05). – The minimum improvement potential was obtained 1.86 MJ/kg water in 2% (w/w) and 11 V/cm for 316L St electrode.
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