Abstract

The ohmic heating process is one of the novel electrical heating methods that are considered as fast, homogeneous and efficient heating. In this study, the details of the relationship between electrical conductivity (EC)-Temperature (Temp) and EC-pH-Temp at different Total Soluble Solid Content (TSSC) and different pH values were examined. Especially, the study explains details of the electrical conductivity value due to pH change (2, 2.5, 3, 3.5) and expressing EC change depending on the pH value of fruit and vegetable juices. At constant pH, EC values decrease with increasing TSSC values (10%, 20%, 30%, 40%, 50%, 60%) and decrease with increasing pH at constant TSSC value. When the relationship between Temp-EC is analyzed that the EC value increases as Temp increases in all cases and this increase is linear. It was found that the Temp-EC relationship was in high compatibility with an R² value above 0.97. Also, in the Temp-EC-pH relationship, the R² value was found to be an acceptable value above 0.95.

Key words
Ohmic heating; electrical conductivity; pH; TSSC; mathematical model

INTRODUCTİON

In the ohmic heating process (OHP), also known as Joule heating and electrical resistance heating, alternating current is passed through the food material (solid/liquid) between the two electrodes. OHP is based on the principle of heating by the generation of heat in the food material due to its resistance to the flow of electric current. In addition to being expressed as homogeneous heating due to its homogeneous current flow especially in liquid applications, it has been reported as fast heating with high energy efficiency (Icier 2012İCIER F. 2012. Ohmic Heating of Fluid Foods. In: Novel Thermal And Non-Thermal Technologies For Fluid Foods. Elsevier Inc., p. 305-367., Kaur & Singh 2016KAUR N & SINGH AK. 2016. Ohmic Heating: Concept and Applications—A Review. Crit Rev Food Sci Nutr 56: 2338-2351. https://doi.org/10.1080/10408398.2013.835303.
https://doi.org/10.1080/10408398.2013.83... ).

Although OHP was first used for the pasteurization of dairy products in the 19th century, it was not used for a long time due to the technology was not fully developed, and electricity prices were high in those years. However, it seems that its usage has increased gradually since the second half of the 20th century, due to technological developments and decreasing electricity costs (Icier 2012İCIER F. 2012. Ohmic Heating of Fluid Foods. In: Novel Thermal And Non-Thermal Technologies For Fluid Foods. Elsevier Inc., p. 305-367.).

In the literature reviewed on OHP, there is a tendency on system development/design, and electrical conductivity at the beginning (de Alwis & Fryer 1992DE ALWIS AAP & FRYER PJ. 1992. Operability of the ohmic heating process: Electrical conductivity effects. J Food Eng 15: 21-48. https://doi.org/10.1016/0260-8774(92)90038-8.
https://doi.org/10.1016/0260-8774(92)900... , Qihua et al. 1993QIHUA T, JINDAL VK & VAN WINDEN J. 1993. Design and performance evaluation of an ohmic heating unit for liquid foods. Comput Electron Agric 9: 243-253. https://doi.org/10.1016/0168-1699(93)90042-Y.
https://doi.org/10.1016/0168-1699(93)900... , Roberts et al. 1998ROBERTS JS, BALABAN MO, ZIMMERMAN R & LUZURIAGA D. 1998. Design and testing of a prototype ohmic thawing unit. Comput Electron Agric 19: 211-222. https://doi.org/10.1016/S0168-1699(97)00044-6.
https://doi.org/10.1016/S0168-1699(97)00... ). However, after 2000, the studies were on the examination of EC value especially on liquid food applications (Icier & Ilicali 2004ICIER F & ILICALI C. 2004. Electrical conductivity of apple and sourcherry juice concentrates during ohmic heating. J Food Process Eng 27(3): 159-180. https://doi.org/10.1111/j.1745-4530.2004.tb00628.x.
https://doi.org/10.1111/j.1745-4530.2004... , 2005a, b). In the following years, food engineering applications increased and the applications were distillation (Tunç & Koca 2019TUNÇ MT & KOCA İ. 2019. Ohmic heating assisted hydrodistillation of clove essential oil. Ind Crops Prod 141: 111763. https://doi.org/10.1016/j.indcrop.2019.111763.
https://doi.org/10.1016/j.indcrop.2019.1... ), drying (Lebovka et al. 2006LEBOVKA NI, SHYNKARYK MV & VOROBIEV E. 2006. Drying of Potato Tissue Pretreated by Ohmic Heating. Dry Technol 24: 601-608. https://doi.org/10.1080/07373930600626677.
https://doi.org/10.1080/0737393060062667... ), extraction (Aamir & Jittanit 2017AAMIR M & JITTANIT W. 2017. Ohmic heating treatment for Gac aril oil extraction: Effects on extraction efficiency, physical properties and some bioactive compounds. Innov Food Sci Emerg Technol 224-234. https://doi.org/10.1016/j.ifset.2017.03.013.
https://doi.org/10.1016/j.ifset.2017.03.... ), thawing (Cevik & Icier 2018CEVIK M & ICIER F. 2018. Effects of voltage gradient and fat content on changes of electrical conductivity of frozen minced beef meat during ohmic thawing. J Food Process Eng 41:1-13. https://doi.org/10.1111/jfpe.12675.
https://doi.org/10.1111/jfpe.12675... ) dairy products (Kuriya et al. 2020KURIYA SP ET AL. 2020. Impact assessment of different electric fields on the quality parameters of blueberry flavored dairy desserts processed by Ohmic Heating. Food Res Int 109235. https://doi.org/10.1016/j.foodres.2020.109235.
https://doi.org/10.1016/j.foodres.2020.1... , Rocha et al. 2020aROCHA RS ET AL. 2020a. Possibilities for using ohmic heating in Minas Frescal cheese production. Food Res Int 109027. https://doi.org/10.1016/j.foodres.2020.109027.
https://doi.org/10.1016/j.foodres.2020.1... ), food safety (Barros et al. 2020BARROS CP ET AL. 2020. Paraprobiotic obtained by ohmic heating added in whey-grape juice drink is effective to control postprandial glycemia in healthy adults. Food Res Int 140: 109905. https://doi.org/10.1016/j.foodres.2020.109905.
https://doi.org/10.1016/j.foodres.2020.1... , Rocha et al. 2020bROCHA RS ET AL. 2020b. Ohmic heating does not influence the biochemical properties of Minas Frescal cheese but decreases uric acid levels in healthy Wistar rats. J Dairy Sci 103: 4929-4934. https://doi.org/10.3168/jds.2019-17712.
https://doi.org/10.3168/jds.2019-17712... , Pires et al. 2021PIRES RPS ET AL. 2021. Ohmic heating increases inactivation and morphological changes of Salmonella sp. and the formation of bioactive compounds in infant formula. Food Microbiol 97: 103737. https://doi.org/10.1016/j.fm.2021.103737.
https://doi.org/10.1016/j.fm.2021.103737... ) and evaporation (Icier et al. 2017ICIER F, YILDIZ H, SABANCI S, CEVIKC M & COKGEZMEC OF. 2017. Ohmic heating assisted vacuum evaporation of pomegranate juice: Electrical conductivity changes. Innov Food Sci Emerg Technol 39: 241-246. https://doi.org/10.1016/j.ifset.2016.12.014.
https://doi.org/10.1016/j.ifset.2016.12.... ). In this sense, not only the studies carried out are mostly on electrical conductivity, but also quality properties and performance analyses attract attention.

It is known that one of the most important parameters for OHP is electrical conductivity change during the process (Icier 2012İCIER F. 2012. Ohmic Heating of Fluid Foods. In: Novel Thermal And Non-Thermal Technologies For Fluid Foods. Elsevier Inc., p. 305-367., Kaur & Singh 2016KAUR N & SINGH AK. 2016. Ohmic Heating: Concept and Applications—A Review. Crit Rev Food Sci Nutr 56: 2338-2351. https://doi.org/10.1080/10408398.2013.835303.
https://doi.org/10.1080/10408398.2013.83... ). In the literature review, the electrical conductivity change was mostly depending on the relationship between voltage gradient and temperature. It has been reported in many studies that the electrical conductivity value increases due to the increase in voltage gradient. The molecular mobility increases due to the increase in temperature and the relation usually are linear (Castro et al. 2004CASTRO I, TEIXEIRA JA, SALENGKE S & VICENTE AA. 2004. Ohmic heating of strawberry products: Electrical conductivity measurements and ascorbic acid degradation kinetics. Innov Food Sci Emerg Technol 27-36. https://doi.org/10.1016/j.ifset.2003.11.001.
https://doi.org/10.1016/j.ifset.2003.11.... , Halden et al. 2007HALDEN K, DE ALWIS AAP & FRYER PJ. 2007. Changes in the electrical conductivity of foods during ohmic heating. Int J Food Sci Technol 25: 9-25. https://doi.org/10.1111/j.1365-2621.1990.tb01055.x.
https://doi.org/10.1111/j.1365-2621.1990... , Assawarachan 2010ASSAWARACHAN R. 2010. Estimation model for electrical conductivity of red grape juice. Int J Agric Biol Eng 3: 52-57. https://doi.org/10.25165/ijabe.v3i2.96.
https://doi.org/10.25165/ijabe.v3i2.96... , Gomathy et al. 2015GOMATHY K, THANGAVEL K, BALAKRISHNAN M & KASTHURI R. 2015. Effect of Ohmic Heating on the Electrical Conductivity, Biochemical and Rheological Properties of Papaya Pulp. J Food Process Eng 38: 405-413. https://doi.org/10.1111/jfpe.12172.
https://doi.org/10.1111/jfpe.12172... , Sabanci & Icier 2017SABANCI S & ICIER F. 2017. Applicability of ohmic heating assisted vacuum evaporation for concentration of sour cherry juice. J Food Eng 262-270. https://doi.org/10.1016/j.jfoodeng.2017.06.004.
https://doi.org/10.1016/j.jfoodeng.2017.... ).

There are several studies on the relationship between EC and increased TSSC values. In this sense, studies for different juices and concentrates have been reported that electrical conductivity increases up to certain content of TSSC and then tends to decrease (Castro et al. 2003aCASTRO I, TEIXEIRA JA, SALENGKE S & VICENTE AA. 2003. The influence of field strength, sugar and solid content on electrical conductivity of strawberry products. J Food Process Eng 17-29. https://doi.org/10.1111/j.1745-4530.2003.tb00587.x.
https://doi.org/10.1111/j.1745-4530.2003... , Icier & Ilicali 2004ICIER F & ILICALI C. 2004. Electrical conductivity of apple and sourcherry juice concentrates during ohmic heating. J Food Process Eng 27(3): 159-180. https://doi.org/10.1111/j.1745-4530.2004.tb00628.x.
https://doi.org/10.1111/j.1745-4530.2004... , Icier & Ilicali 2005bICIER F & ILICALI C. 2005a. Temperature dependent electrical conductivities of fruit purees during ohmic heating. Food Res Int 1135-1142. https://doi.org/10.1016/j.foodres.2005.04.003.
https://doi.org/10.1016/j.foodres.2005.0... ). Besides, in ohmic evaporation studies (under vacuum and atmospheric conditions), the electrical conductivity value for the different liquid applications (tomato, seawater, pomegranate juice, sour cherry water, etc.) increases up to the critical TSSC value and then decreases after the evaporation begins (Assiry et al. 2010ASSIRY AM, GAILY MH, ALSAMEE M & SARIFUDIN A. 2010. Electrical conductivity of seawater during ohmic heating. Desalination 260: 9-17. https://doi.org/10.1016/j.desal.2010.05.015.
https://doi.org/10.1016/j.desal.2010.05.... , Hosainpour et al. 2014HOSAINPOUR A, DARVISHI H, NARGESI F & FADAVI A. 2014. Ohmic pre-drying of tomato paste. Food Sci Technol Int 20: 193-204. https://doi.org/10.1177/1082013213480360.
https://doi.org/10.1177/1082013213480360... , Icier et al. 2017ICIER F, YILDIZ H, SABANCI S, CEVIKC M & COKGEZMEC OF. 2017. Ohmic heating assisted vacuum evaporation of pomegranate juice: Electrical conductivity changes. Innov Food Sci Emerg Technol 39: 241-246. https://doi.org/10.1016/j.ifset.2016.12.014.
https://doi.org/10.1016/j.ifset.2016.12.... , Sabanci & Icier 2017SABANCI S & ICIER F. 2017. Applicability of ohmic heating assisted vacuum evaporation for concentration of sour cherry juice. J Food Eng 262-270. https://doi.org/10.1016/j.jfoodeng.2017.06.004.
https://doi.org/10.1016/j.jfoodeng.2017.... ).

The current study aims to heat the model solutions, which are at different pH (2-3.5) and TSSC (10-60%), by using the ohmic system from 15 °C to 95 °C. The study focuses on the effect of some influencing parameters (TSS, pH, and Temperature) on the EC of the model solution.

Abbreviations

EC → Electrical Conductivity

Temp → Temperature

TSSC → Total Soluble Solid Content

OHP → Ohmic Heating Process

MATERIALS AND METHODS

Materials

Glucose syrup was prepared as the model solution for OHP. In the study, glucose syrup with 80% TSSC content was adjusted to 10%, 20%, 30%, 40%, 50%, 60% and 0.15% (w/v) NaCl solution was added to provide sufficient ionic mobility. The TSSC adjustments were made by a Digital refractometer (Hanna, Romania). The pH (2, 2.5, 3, 3.5) values of the model solution were adjusted with citric acid, which is the dominant acid in fruit juices. After that, the solutions were placed in the refrigerator for preservation.

Methods

Ohmic heating

In OHP, a laboratory-scale special design system is consisting of a power supply (360V-50 Hz), isolated transformer, microprocessor unit, computer connection, and test cell (6×7×8 cm) (Figure 1). The test cell used in ohmic heating is made of polyoxymethylene and the electrodes (60×150×1 mm) are made of stainless steel. The applied voltage gradient was 20 V/cm and the solution was subjected to heating from 15 °C to the target temperature of 95 °C. Temperature, current, and voltage values are recorded every second by using a specially designed microprocessor. In the extraction process, the solution temperature data was recorded by an isolated T-type thermocouple (Cole- Parmer, UK).

Electrical conductivity

Electrical conductivity values are calculated by using equation (1) in the unit of S/m (Icier & Ilicali 2004ICIER F & ILICALI C. 2004. Electrical conductivity of apple and sourcherry juice concentrates during ohmic heating. J Food Process Eng 27(3): 159-180. https://doi.org/10.1111/j.1745-4530.2004.tb00628.x.
https://doi.org/10.1111/j.1745-4530.2004... ).

Electrical conductivity value (S/m)

where L represents the distance (m) between two electrodes, V voltage (volts), I current (amps), and A represents the liquid contact area (m²) of the electrode.

Electrical conductivity model

Temp-EC relation between 15-95 °C was determined with the help of equation (2). The relationship was examined by non-linear regression analysis, and electrical conductivity was taken as a dependent, and the temperature was taken as an independent variable in the model expression.

Here, σ refers to electrical conductivity (S/m), T (°C) temperature value, and a (S/m×°C) and b (S/m) equation constants.

Equation 3 was determined as the most suitable model by nonlinear regression analysis by testing pH and temperature in different combinations to express the relationship between EC, Temperature, and pH. The relationship was examined by non-linear regression analysis, and electrical conductivity was taken as a dependent, and temperature and pH values were taken as independent variables in the model expression.

Here, σ represents the electrical conductivity (S/m), T (°C) temperature value, a (S/m×°C), b (S/m) and n value represent the equation constants.

Statistical evaluation

SPSS 16.0 (IBM, USA) package program was used in the statistical evaluation of the results (heating time, EC). Completely determined by one-way variance (Post-Hoc Duncan test) analyses. The effect of TSSC and pH on the heating time was examined with the Duncan test and a one-way analysis of variance Confidence level was taken as 95% (p = 0.05). If p values are less than 0.05, it means that there is a statistically significant difference. Also, non-linear regression analysis was used with SPSS 16.0 (IBM, USA) for model experiments.

RESULTS AND DISCUSSION

For this reason, in the present study, the prepared model solution at different TSSC (10-60%) and different pH values (2-3.5) was heated from 15 °C to 95 °C, and the details of EC-TEMP and EC-TEMP-pH relationships were examined in following parts.

The heating time of the model solution

The processing time required for the model solution having different TSSC and different pH values to reach from 15 °C to 95 °C with ohmic heating is given in Table I. It has been determined that the concentration and ionic mobility of the model solution affected the heating time during OHP. In this sense, especially molecular mobility affects the electrical conductivity, and accordingly, the processing time is longer as the pH value increases at the same TSSC value. Increasing the pH value means a decrease in the molecular concentration of [H⁺] in the solution. For this reason, the higher pH value of the solution has negatively affected the warm-up time when it reaches 95 °C in the process.

Similarly, at the same pH values, it was observed that the processing time increased due to the increase in TSSC value. For instance, the warm-up of pH 2 solution with 10% TSSC value to reach 95 ° C is 176s but at the same pH (pH 2), 1831s was measured for 60% TSSC and this value was found to increase approximately 10 times than 10% TSSC (p<0.05). Similarly, it was figured out that the time to reach 95 °C increased between 8-10 times due to the increase of TSSC at different pH values (p<0.05). The warm-up time varies depending on the content of the product used in the ohmic heating process. In particular, ionic mobility and molecular mobility are important variables on electrical conductivity. In the study conducted with sour cherry juice and apple juice, the warm-up time is shorter at the sample that has a lower pH value and higher ionic activity (Icier & Ilicali 2004ICIER F & ILICALI C. 2004. Electrical conductivity of apple and sourcherry juice concentrates during ohmic heating. J Food Process Eng 27(3): 159-180. https://doi.org/10.1111/j.1745-4530.2004.tb00628.x.
https://doi.org/10.1111/j.1745-4530.2004... ). Similarly, in a study conducted with strawberries and products, it was found that the gel structure, which significantly restricts molecular mobility, extends the processing time (Castro et al. 2004CASTRO I, TEIXEIRA JA, SALENGKE S & VICENTE AA. 2004. Ohmic heating of strawberry products: Electrical conductivity measurements and ascorbic acid degradation kinetics. Innov Food Sci Emerg Technol 27-36. https://doi.org/10.1016/j.ifset.2003.11.001.
https://doi.org/10.1016/j.ifset.2003.11.... ). As a result, it has been determined that the decreasing pH value affects the mobility in the product and causes the processing time to be shortened. Therefore, the pH value has an effect on the processing time in the current study. Besides, the change in the TSSC value affected the processing time and the processing time was negatively affected due to the increased TSSC value.

Change of electrical conductivity (EC) of the model solution

The electrical conductivity change during the heating of the model solution having different TSSC and pH values from 15 °C to 95 °C is given in Figure 2. It was determined that increasing TSSC value negatively affects EC values at the same pH value (Figure 2) (p<0.05). The main reason for this is the increasing TSSC value is thought to limit molecular mobility. Intramolecular forces and water-soluble substances limit molecular motion and therefore electrical conductivity is affected. These forces depend on intermolecular spaces and the resistance of the hydrogen bond. Therefore, with the increase of TSSC, an increase in a hydrogen bond with hydroxyl groups can occur and is thought to increase viscosity value and restrict mobility due to a decrease in molecules in the solute liquid state (Constenla et al. 1989CONSTENLA DT, LOZANO JE & CRAPISTE GH. 1989. Thermophysical Properties of Clarified Apple Juice as a Function of Concentration and Temperature. J Food Sci 54: 663-668. https://doi.org/10.1111/j.1365-2621.1989.tb04677.x.
https://doi.org/10.1111/j.1365-2621.1989... ). Similarly, Giangiacomo, (2006)GIANGIACOMO R. 2006. Study of water-sugar interactions at increasing sugar concentration by NIR spectroscopy. In: Food Chemistry. Elsevier, p. 371-379. examined the water-sugar interactions in different concentrations of sugar solutions (glucose, fructose, and sucrose) using the NIR spectroscopy method. In the study, the low concentrations of sugars acted like a breaker of the water cluster, and that they acted as structure makers at higher concentrations. As a result, he reported that this structure restricted molecular mobility. In the same TSSC value, the increasing pH value negatively affects ionic mobility this time and accordingly negatively affects its electrical conductivity (p<0.05). In their study, Icier & Ilicali (2004)ICIER F & ILICALI C. 2004. Electrical conductivity of apple and sourcherry juice concentrates during ohmic heating. J Food Process Eng 27(3): 159-180. https://doi.org/10.1111/j.1745-4530.2004.tb00628.x.
https://doi.org/10.1111/j.1745-4530.2004... heated cherry and apple juice at different concentrations reported that cherry juice with a lower pH value warms faster and the EC value is higher compared to all other concentrations (Icier & Ilicali 2004ICIER F & ILICALI C. 2004. Electrical conductivity of apple and sourcherry juice concentrates during ohmic heating. J Food Process Eng 27(3): 159-180. https://doi.org/10.1111/j.1745-4530.2004.tb00628.x.
https://doi.org/10.1111/j.1745-4530.2004... ). In general, the electrical conductivity value was affected depending on the temperature, pH, and TSSC value and the EC value increased as the temperature value increased. Also, at the same TSSC value, the EC value decreased as the pH value decreased. Also, when the pH value was kept constant, the EC values increased with the decrease of the TSSC values. Therefore, EC values were positively affected by decreasing pH and TSSC values and increasing temperature.

The electrical conductivity change of the model solution depending on the temperature change at different pH and TSSC values is given in Figure 2. Depending on the increasing temperature value, it was found that the electrical conductivity value increased for all TSSC values. From the literature review, a similar trend was found for different juices. For example, the electrical conductivity value is 0.8-1.6 S/m for cherry juice (20-60V/cm; 20-60 % TSSC; 30-75 °C) (Icier & Ilicali 2004ICIER F & ILICALI C. 2004. Electrical conductivity of apple and sourcherry juice concentrates during ohmic heating. J Food Process Eng 27(3): 159-180. https://doi.org/10.1111/j.1745-4530.2004.tb00628.x.
https://doi.org/10.1111/j.1745-4530.2004... ), 0.35-1.00 S/m for apple juice (20-60V/cm; 20-60% TSSC; 30-75 °C) (Icier & Ilicali 2004ICIER F & ILICALI C. 2004. Electrical conductivity of apple and sourcherry juice concentrates during ohmic heating. J Food Process Eng 27(3): 159-180. https://doi.org/10.1111/j.1745-4530.2004.tb00628.x.
https://doi.org/10.1111/j.1745-4530.2004... ), 0.16-1.16 S/m for orange juice (20-60V/cm; 20-60% TSSC; 30-75 °C) (Icier & Ilicali 2005b), 0.2-0.5 S/m for strawberry pulp (32-90V/cm; 14-59.5% TSSC; 20-85 °C)(Castro et al. 2003b), 0.41-1.02 S/m for lemon juice (30-55V/cm; 20-74 °C) (Darvishi et al. 2011DARVISHI H, HOSAINPOUR A, NARGESI F, KHOSHTAGHAZA MH & TORANG H. 2011. Ohmic processing: Temperature dependent electrical conductivities of lemon juice. Mod Appl Sci 209-216. https://doi.org/10.5539/mas.v5n1p209.
https://doi.org/10.5539/mas.v5n1p209... ), 3.1-8.5 S/m for tomato juice (6-14 V/cm; 9.1%; 25-100 °C) (Torkian Boldaji et al. 2015TORKIAN BOLDAJI M, BORGHEI AM, BEHESHTI B & HOSSEINI SE. 2015. The process of producing tomato paste by ohmic heating method. J Food Sci Technol 52: 3598-3606. https://doi.org/10.1007/s13197-014-1424-5.
https://doi.org/10.1007/s13197-014-1424-... ), 0.40-1.11 S/m for red grape juice (10-15 V/cm; 10.5-14.5% TSSC; 25-80 °C) (Assawarachan 2010ASSAWARACHAN R. 2010. Estimation model for electrical conductivity of red grape juice. Int J Agric Biol Eng 3: 52-57. https://doi.org/10.25165/ijabe.v3i2.96.
https://doi.org/10.25165/ijabe.v3i2.96... ), 0.40-0.76 S/m for grape juice (20-40 V/cm; 30-80 °C) (Icier et al. 2008ICIER F, YILDIZ H & BAYSAL T. 2008. Polyphenoloxidase deactivation kinetics during ohmic heating of grape juice. J Food Eng 410-417. https://doi.org/10.1016/j.jfoodeng.2007.08.002.
https://doi.org/10.1016/j.jfoodeng.2007.... ) and 0.21-1.10 S/m for pomegranate juice (30-55 V/cm; 20-85 °C) (Darvishi et al. 2013DARVISHI H, KHOSTAGHAZA MH & NAJAFI G. 2013. Ohmic heating of pomegranate juice: Electrical conductivity and pH change. J Saudi Soc Agric Sci 12: 101-108. https://doi.org/10.1016/j.jssas.2012.08.003.
https://doi.org/10.1016/j.jssas.2012.08.... ). It has been reported that molecular mobility increases due to the increasing temperature value and the electrical conductivity value increase accordingly. It has been reported that molecular mobility increases due to the increasing temperature value and the electrical conductivity value increase accordingly.

The EC-Temperature relationship (Figure 2) may be fitted by a linear expression. The equations and the regression coefficients of the linear relationship obtained at different pH and TSSC values are given in Table II. It has been generally demonstrated that model constants decrease with the increase in TSSC at the same pH values. In the same way, the model constants decreased as the pH value increased with the same TSSC value. Regarding the model compatibility that the regression coefficient is over 0.97. It has been reported in many studies that the temperature-electrical conductivity relationship is linear for juices. Besides, some different equations have been reported for other fruit pulp such as apricot, peach, and papaya pulp (Icier & Ilicali 2005aİCIER F & ILICALI C. 2005b. The effects of concentration on electrical conductivity of orange juice concentrates during ohmic heating. Eur Food Res Technol 220: 406-414. https://doi.org/10.1007/s00217-004-1043-x.
https://doi.org/10.1007/s00217-004-1043-... , Gomathy et al. 2015GOMATHY K, THANGAVEL K, BALAKRISHNAN M & KASTHURI R. 2015. Effect of Ohmic Heating on the Electrical Conductivity, Biochemical and Rheological Properties of Papaya Pulp. J Food Process Eng 38: 405-413. https://doi.org/10.1111/jfpe.12172.
https://doi.org/10.1111/jfpe.12172... ).

Temperature-pH-Electrical Conductivity Relationship

In this study, it has been found that there is a relationship between temperature-pH-electrical conductivity. Model equations and regression coefficients belonging to different TSSC values are given in Table III. It was found that the effect of pH on the electrical conductivity is not linear and has a negative effect in contrast to the effect of temperature.

The EC value obtained for the 40% TSSC and pH 2.5 model solution heated from 15 to 95 °C by OHP and the EC values obtained from both EC-Temperature and EC-pH-Temperature models are given in Figure 3. In the literature review, no model equation expressing the electrical conductivity change due to pH value was found in model solutions for different TSSC values. Different models were tried to express the relationship between EC-Temperature-TSSC and EC-Temperature-pH-TSSC, but it was determined that the R² value ranged between 0.90-0.93 and it was found insufficient to express these relationships. In particular, electrical conductivity may differ depending on the content of the product. Therefore, it is recommended to examine pH changes with different sugar types.

CONCLUSION

The model solutions with different TSSC and different pH values ​​were heated from 15 °C to 95 °C and it was determined that the increased TSSC and pH value on the total time during warming had a negative effect on the EC of the solution. The EC value increased in all heated model solutions due to the increasing temperature value. Also, since the increase in pH value decreases ion mobility, it negatively affects the EC value and the restriction of molecular mobility in the increased TSSC value. Besides, the electrical conductivity temperature relationship was found to be linear. Also, there is a relationship between EC-pH-Temp, and equations expressing this relation are acceptable over 95% of model compatibility. Especially, it is envisaged that the determination of EC-pH-Temp relationship may be data and source for model ohmic designs. However, it is recommended to determine the effect of ohmic heating in different model sugar solutions and to determine different relationships.

ACKNOWLEDGMENTS

This study was financially propped by TUBITAK (2209/A Program).

REFERENCES

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