Experimental et al., 2012; Moreno et al.,

Experimental exergy analysis of ohmic concentration for tomato juice: effect of salt
content, electrode type and voltage gradient

Abstract

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In this study the performance of an ohmic concentration
system was analyzed based on second law of thermodynamic. The influence
of salt content (0-2% w/w), voltage gradient (5-11 V/cm) and electrode type (316L
St, Al and Br) was 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).

Keywords: Efficiency, ohmic concentration, salt,
electrode, voltage gradient

1. Introduction

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 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.

Exergy
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).

The
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
assessed
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.

2.
Materials and methods

2.1. Material

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
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).

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):

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 exergetic improvement potential
of ohmic concentration system (Icier et al., 2010; Cokgezme et al., 2017).

 

2.5.
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

The
specific exergy required for the ohmic concentration of tomato juice is shown in
Fig. 3. For all electrodes, exergy
consumption decreased significantly (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