Effect showed that increasing of salt content and

Effect of various parameters on
performance of ohmic tomato pasta production

Darvishia*; Naser

a Department of Biosystems
Engineering; Faculty of Agriculture; University of Kurdistan, Sanandaj,
Kurdistan, Iran

*Corresponding Email:
[email protected]

(+98) 87 336277724;

Fax: (+98) 87 33624249;

 P.O. Box: 416


Tomato samples was concentrated about 75% in ohmic heating
system in order to produce tomato paste. The effects of voltage gradient, material of electrode
(316L St, Br, Al) and salt content (0, 1, 2% w/w) on ohmic concentration performances were experimentally studied. Results
showed that increasing of salt content and voltage gradient decreased the
specific exergy consumption and increased
the exergy efficiency (p<0.05). Also, highest exergy consumption obtained when using Al electrode. No significantly difference observed between the exergy parameters of 316L St and Br electrodes (p>0.05). The improvement potential of control samples is 16-35% higher than
the samples with 2% w/w salt content.

Keywords: tomato pasta; exergy; ohmic concentration;
salt, electrode

1. Introduction

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 alternative 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 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.

analysis is useful tool for evaluate 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. Exergy 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 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 suitable electrode in ohmic heating
systems is one of 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 suitable material (Adetunji et al., 2016; Alvarez et
al., 2012; Assiry, 2003; Zell et al., 2009).

generated heat and efficiency values of ohmic heating system are dependent on
the conductive nature of 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 solution.

A lot
of researchers investigated
the effect of electrodes type and salt content on 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 exergetic
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 exergetic
performance of the ohmic concentration system as the first work.


Al                    aluminum

Br                    brass

Cp                    specific heat capacity
(J/kg. K)

EX                   exergy

ex                    specific exergy
(J/kg water)

exevp                 specific exergy of water evaporation (J/kg water)

exin                  specific exergy of
fresh sample (J/kg water)

EXloss               exergy loss (J)

exp                   specific exergy of product (J/kg water)

hfg                    latent heat of sample (J/kg)

I                       current
intensity (A)

IP                     improvement potential (J)

m                     mass

m­in­                   mass of fresh sample (kg)

mp                    mass of condensed sample

mw                   total mass of water
evaporated (kg)

R2                    coefficient of determination

Sc                     salt content (%, w/w)

St                     stainless

T                      temperature

t                       time

To                     ambient temperature (K)

Tp                     product temperature (K)

V                     voltage

?ex                    exergy efficiency (%)

?V                   voltage
gradient (V/cm)

Materials and methods

2.1. Material

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
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.,

2.2. Ohmic process

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 mass sample recorded by a digital balance (A 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 to 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 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.

2.4. Exergy analysis

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):

exergy efficiency was calculated using Eq. (4) stated as follows (Darvishi et
al., 2015):

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 exergetic 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 analyses 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.

3. Results
and discussion

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 the 316L St and Br electrodes under different concentration processes (p<0.05). The results also showed that there was no significant difference between the exergy consumption of 316L St and Br electrodes (p>0.05) 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 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 exergetic efficiency of the system. In order to estimate the mean amount of exergy efficiency at the desired level of the variables, 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 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 electrode for ohmic concentration/evaporation processes due to the higher exergy consumption and lower exergy efficiency as compared with the 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, 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. Conclusion The effect of salt content, electrode type and voltage gradient evaluted on exergy aspets of ohmic tomto paste prdocution, and found as: -          Exergy efficincy increased with increasing salt content and voltage gradient. -          Applied of Al electrode incresed 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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