Electrical conductivity of particle–fluid mixtures in ohmic heating: Measurement and simulation

The electrical conductivity (EC) of food systems is a key parameter of the ohmic heating process. Ohmic heating experiments were conducted using a Teflon cylindrical cell. Electrical conductivity values were evaluated for the ohmic processing (25–125 °C) of blanched, 2-cm cubic particles (carrot, potato, radish, beef muscle, pork muscle and commercial ham) dispersed in carrier fluid (5% w/w starch–water solution with 0.15–1.5% w/w salt concentrations). Assuming an analogy between thermal conductivity (TC) and EC five existing TC theoretical models (series, parallel, two forms of Maxwell–Eucken models and effective medium theory) were applied and evaluated for EC of various solid–fluid structural systems. Parallel and Series models are not practically applicable in real food system, however they can indicate the useful limit values of highest and lowest EC among all two-phase structures. The first Maxwell–Eucken model, which describes solid particles dispersed in continuous liquid, showed the best agreement between predicted EC values and experimental data. This model provided a useful and relatively simple new approach to predict the effective EC for mixtures of different types of solid food particles immersed in liquid food.     

胡萝卜、油菜的通电加热烫漂研究

研究了通电加热烫漂对胡萝卜、油菜中维生素含量的影响,并与普通电炉加热进行了比较;研究了胡萝卜、油菜通电加热烫漂的最佳工艺条件。实验证明,随着烫漂时间的延长,胡萝卜中的β-胡萝卜素含量明显降低,油菜中VC的含量也明显降低;通电加热烫漂后的维生素含量明显高于电炉加热;而胡萝卜通电加热的最佳烫漂工艺为,烫漂时间3 min,颗粒尺寸10 mm,料液比(g:mL)1∶4,电压220 V;油菜通电加热烫漂的适宜工艺条件为料液比(g:mL)1∶4,烫漂时间0.5 min,电压220 V。     

Sensitivity analysis of continuous ohmic heating process for multiphase foods

The process sensitivity analysis of continuous Ohmic heating process for soup products containing large particulates was performed by use of a validated computer modeling package to determine the critical control factors and their acceptable control ranges under given control targets. The major processing variables investigated included electrical conductivities of carrier fluid and particles, particle thermal diffusivity, surface heat transfer coefficient, particle size, particle concentration, flow rate, and initial product temperature. The response variables as the indicators of effects from processing variable deviations were the process temperatures and accumulated lethality values for both carrier fluid and particles at the end of holding tube. The results demonstrated that under giving base processing conditions with a control target process temperature of 133 °C (the carrier fluid temperature at the entrance of the holding tube or the end of heating tube), increasing carrier fluid electrical conductivity from 1.42 to 2.13 S/m at a reference temperature of 70 °C, particle size from 16 to 24 mm, and flow rate from 1600 to 2400 L/h could result in lower particle center temperature at the end of holding tube, while increasing particle electrical conductivity from 0.96 to 1.44 S/m at a reference temperature of 70 °C, thermal diffusivity from 1.16 × 10⁻⁷ to 1.74 × 10⁻⁷ m²/s, surface heat transfer coefficient from 112 to 168 W/°C/m², particle concentration from 52% to78%, and initial product temperature from 56 to 84 °C could generate higher particle center temperature. Within the variable deviation ranges investigated, the importance order of effects of major processing variables were determined with respect to the particle center temperatures, carrier fluid temperatures and accumulated lethality values. The acceptable deviation ranges were built up for six major processing variables under meeting the required target Fo (at the particle center) values. The information and knowledge obtained from this study can be useful for understanding how major processing variables of continuous Ohmic heating processing system affect the required process target variables and the methodology developed in this study can be further used as a protocol for both process establishment and optimization purposes.     

ELECTRICAL CONDUCTIVITIES OF HYDROCOLLOID SOLUTIONS

Ohmic heating was applied to solutions of 5 hydrocolloids (carrageenan, 1–3% w/w; xanthan, 1–3% w/w; pectin, 1–5% w/w; gelatin, 2–4% w/w and starch, 4–6% w/w) in a static cell to study the effect of concentration and temperature on electrical conductivities. For each experiment, the sample was poured into the cell and heated at a constant voltage gradient of 7.24 V/cm. Voltage, current, time and temperature were logged at time intervals and used to calculate electrical conductivities as a function of temperature. Of the thickening agents examined, carrageenan gave the highest value for electrical conductivity (0.2 S/m at 25C and 1% and 1.4 S/m at 100C and 3%) followed by xanthan (0.14 S/m at 25C and 1% to 1.1 S/m at 100C and 3%). Pectin and gelatin samples were found to exhibit lower, but similar electrical conductivities (0.06 S/m at 25C at the lowest concentration to 0.37 S/m at 100C at the highest concentration). Starch samples had the lowest electrical conductivity varying from 0.05 S/m at 25C and 4% to 0.21 S/m at 100C and 6%. Regression equations were established for each type of hydrocolloid solutions revealing a strong interaction between the concentration and the temperature and the electrical conductivity values. Major differences were observed between hydrocolloid types on electrical conductivities that were mainly attributed to their ash content.     

Residence time distribution of carrot cubes in starch solutions in a pilot scale aseptic processing system

Residence time distribution (RTD) of carrot cubes flowing in starch solutions was evaluated in a commercial pilot-scale aseptic processing system using a full factorial design of experiments employing flow rate (15 and 20 kg/min), temperature (80 and 100°C), holding tube length (0, 60 and 100%), particle size (6 and 13 mm) and starch concentration (3 and 5% w/w) as factors. All factors were found to be significant (p<0·05) in influencing the fastest particle residence time (FPRT), mean particle residence time (MPRT), particle residence time variance (PRTV) and their normalized versions. While particle size, holding tube length and starch concentration increased the FPRT, fluid flow rate and temperature had the opposite effect. The FPRT was lower than the average fluid retention time under all testing conditions.     

The geometry of shadows: effects of inhomogeneities in electrical field processing

Ohmic heating uses the inherent electrical resistance of a material to generate heat. To produce safe, high quality, food, it is important to investigate any inhomogeneities which may occur in the process. The presence of any inclusions of significantly lower electrical conductivity in a solid–liquid mixture can cause a cold shadow within the liquid; internal heating differences can arise in a particle caught in this region. Experimental measurements and computational modelling of these problems have been carried out. When the particle was an electrical insulator, differences in heating rate due to thermal conduction gave significant temperature differences, convective mixing within the liquid had a marked effect on thermal response. For electrically conducting particles, heating rates are due to internal resistance heating as well as thermal conduction effects. In both cases, the size of the shadow is of the order of magnitude of the inclusion size. Simulations predicted the type and magnitude of the effects seen: it is difficult to model temperatures without accurate data on convective mixing.     

MATHEMATICAL MODELING and EXPERIMENTAL STUDIES ON OHMIC HEATING of LIQUID-PARTICLE MIXTURES IN A STATIC HEATER1

Understanding of the ohmic heating of liquid-particle mixtures requires preliminary study and model development within a static heater. A mathematical (3D finite element) model was developed for prediction of temperatures of mixtures of liquids and multiple particles within a static heater. Experiments were conducted using cubic potato particles within sodium phosphate solutions, for various particle sizes, orientations, concentrations, and liquid conductivities. the mathematical model was found to yield satisfactory prediction of experimental trends. Critical parameters affecting the relative heating rates of particles and liquid were the conductivities of the two phases, and the volume fraction of each phase. Ohmic heating appears most promising with high-solids concentration mixtures.     

Identification of Fastest Velocity Particle in Tube Flow: Single vs Multi-particle Approach

Velocities of particles (carrot, parsnip, potato, and Nylon) of different sizes (d_p/D=0.25-0.5) and shapes (spheres, cylinders and cubes) were evaluated in a transparent holding tube (L=3.8m and D=0.05m) with water and water/pectin (up to 1.2%) solutions as carrier liquids (T=20–80°C). Particles were introduced individually or in groups of 2–135, and evaluated with respect to a given type, shape, size and concentration. Results showed that, when densities of particle and liquid were different, the fastest traveling particle in a multi-particle system was the largest sphere traveling alone in the carrier liquid of highest velocity and viscosity. Conservative values of critical particle velocities were obtained by using the single particle approach.     

机械加工方式及油脂对胡萝卜中β-胡萝卜素生物接近度的影响

研究不同机械加工方式(1 mm×1 mm×1 mm切丁、2 mm×2 mm×2 mm切丁、打浆)和油脂(添加量0%、3%、5%、10%)对β-胡萝卜素生物接近度的影响。采用高效液相色谱法(high performance liquid chromatography,HPLC)测定β-胡萝卜素浓度,采用静态体外消化法、以释放率和胶束化率为指标评估β-胡萝卜素生物接近度。结果表明:3种机械加工处理的胡萝卜在相同油脂添加量条件下,β-胡萝卜素的释放率和胶束化率由大至小的排序均为:打浆处理(释放率为2.069%~32.565%,胶束化率为0.324%~1.999%)、切丁1 mm×1 mm×1 mm(释放率为1.088%~6.162%,胶束化率为0.226%~0.911%)和2 mm×2 mm×2 mm(释放率为0.335%~4.102%,胶束化率为0.109%~0.242%);不同油脂添加量均提高了胡萝卜中β-胡萝卜素的释放率和胶束化率,且油脂添加量分别与释放率和胶束化率均呈线性关系,但是提高幅度因机械加工方式不同而异:10%油脂添加量与无油脂添加相比,打浆处理的释放率提高了约15倍,胶束化率提高了约5倍,1 mm×1 mm×1 mm胡萝卜丁的释放率和胶束化率分别提高了约5倍、3倍,2 mm×2 mm×2 mm胡萝卜丁的释放率和胶束化率则分别提高了11倍、1倍。     

Ohmic heating assisted vacuum evaporation of pomegranate juice: Electrical conductivity changes

A novel electrical heating method, named as ohmic heating, was successfully integrated to vacuum evaporation system, and pomegranate juice was concentrated until its total soluble dry matter content reached to 40% by applying three different voltage gradients (7.5, 10, and 12.5 V/cm) at 180 mm Hg absolute pressure in this system. Total evaporation times were determined as 152, 78, and 53 min at the voltage gradients of 7.5, 10, and 12.5 V/cm, respectively. The concentration time of pomegranate juice was shortened about 56% by ohmic heating relative to conventional evaporation. The electrical conductivity values were increased up to reach 25% TSDM for 7.5 V/cm and at 35% TSDM for 10 V/cm, then showed a decreasing pattern. However, it was constant (0.55 ± 0.01 S/m) during evaporation process at 12.5 V/cm. It is recommended that the ohmic heating method could be successfully integrated to vacuum evaporation process to shorten the processing time significantly.Industrial relevanceOhmic heating has been utilized as alternative method for the purpose of heating, pasteurization, cooking etc., and relatively better products can be obtained by ohmic heating. Nowadays, energy efficient systems are needed in concentrated juice production. The integration of ohmic heating to the conventional vacuum systems could serve the production high quality juice concentrates with efficient use of energy. In present study, ohmic heating assisted vacuum evaporation of pomegranate juice was conducted, successfully. This method decreased the total process time for juice concentration, which is critically important for industrial scale productions. This novel method can be implemented to the fruit juice production lines by taking into account of design characteristics of ohmic systems such as electrical conductivity changes depending on both temperature and total soluble solids concentration. The effects of main process parameter, named as voltage gradient, on electrical conductivity changes during electrical heating assisted evaporation process has been also reported in the present study.     

FLUID-TO-PARTICLE CONVECTIVE HEAT TRANSFER COEFFICIENT AS EVALUATED IN an ASEPTIC PROCESSING HOLDING TUBE SIMULATOR

A pilot scale aseptic processing holding tube simulator was fabricated for evaluating fluid-to-particle convective heat transfer coefficients at temperatures up to 110C. the simulator was calibrated to give carrier fluid flow rate as a function of CMC concentration, temperature, pump rpm and pipe diameter. Fluid-to-particle heat transfer coefficients (h_fp) were estimated with model and real food particles held stationary in a moving liquid. Data were gathered under various conditions: CMC concentration (0- 1.0% w/w), flow rate (1.0 - 1.9 × 10⁻⁴m³/s) and particle size (diameter: 21 and 25.4 mm; length: 24 and 25.4 mm). Depending on operating conditions, average heat transfer coefficients (h_fp) ranged from 100 to 700 W/m²C with corresponding Biot numbers (Bi) ranging from 10 to 50. CMC concentration, fluid temperature and flow rate, as well as their interactions, had significant effect (p < 0.05) on h_fp for both Teflon and potato particles. Some differences were observed with respect to the associated h_fp for Teflon and potatoes due probably to differences in their structural/textural characteristics. Heat transfer coefficient associated with cooling were significant ly lower (p < 0.05) than those associated with heating.     

点燃和加热条件下不同滤嘴通风率卷烟的气溶胶释放特征

为研究滤嘴通风率对不同类型卷烟烟气气溶胶释放的影响,制备了不同滤嘴通风率的两用型烟支（可燃吸、配套加热烟具加热后抽吸）,采用气相色谱仪（GC）和DMS500快速粒径谱仪,研究了点燃和加热条件下烟气气溶胶中的主要化学成分和粒径分布。结果表明：(1)在点燃和加热条件下,烟气出口温度均随滤嘴通风率的增加逐渐降低。(2)随着滤嘴通风率从0增加至64.08%,在点燃条件下,总粒相物及粒相物中烟碱、丙二醇、丙三醇和水分的释放量减少;在加热条件下,总粒相物、粒相物中烟碱和丙三醇的释放量先增加后略微减少,粒相物中丙二醇的释放量增加,粒相物和气相物中水分的释放量减少。(3)随着滤嘴通风率的增加,在点燃条件下,烟气气溶胶粒径均分布在60～2 000 nm范围内,粒数浓度减小,粒数中值粒径（CMD）增大;在加热条件下,滤嘴通风率为0时,烟气气溶胶粒径主要分布在40～500 nm;滤嘴通风率为47.86%和64.08%时,粒径主要分布在40～1 000 nm,粒数浓度与粒子体积浓度先增加后减小,CMD和体积中值粒径（VMD）增大。该研究结果可为卷烟通风打孔工艺参数的选择优化提供科学依据。     

Modeling of ohmic heating patterns of multiphase food products using computational fluid dynamics codes

Modeling of the thermal behavior of multiphase food products with various electrical conductivities under ohmic heating has been a challenge. Distortion of electric field due to heterogeneous food properties and electrical conductivity distribution should be taken into consideration for accurate prediction of the thermal performance of ohmic heaters. The objective of this study was to model ohmic heating pattern of solid–liquid food complex that contain three different solid particles with substantially different electrical conductivities and 3% NaCl solution. The solid food samples used in this experiment were potato, meat, and carrot, which are less conductive than carrier medium. The transient heating patterns of each solid food and carrier medium were simulated using computational fluid dynamics (CFD) codes with user defined functions (UDFs) for electric field equations. The predicted temperature values were in good agreement with the experimental data with the maximum prediction error of 6 °C. Hot spots existed on the continuous phase in zones perpendicular to the solid cubes and cold spots were in between the particles where the current density lacks. CFD model prediction of detailed thermal profiles of multiphase food mixtures under the static ohmic heating will assist in designing of a continuous flow ohmic heater with pursuit of heating uniformity, furthermore, ensuring food safety and quality.     

The effect of addition of flaxseed gum on the emulsion properties of soybean protein isolate (SPI)

The effect of addition of flaxseed gum on the emulsion properties of soybean protein isolate (SPI) were investigated in this study. Flaxseed gum with 0.05-0.5% (w/v) concentration was used together with 1% (w/v) SPI to emulsify 10% (v/v) soybean oil. The emulsion was analyzed for emulsion activity (turbidity), stability, particle size, surface charge, and rheological properties. The turbidity and absolute zeta-potential values decreased initially by the addition of flaxseed gum and subsequently increased with further increase in the gum concentration to reach their peak around 0.35% (w/v) gum. The particle size of the emulsion decreased and reached a minimum value at 0.1% (w/v) gum concentration. Any increase in gum concentration beyond this value resulted into increase in the particle size. This study would help to widen the application of SPI and flaxseed gum mixture, and also contribute to the understanding of protein-gum interaction in emulsion.     

ELECTRICAL CONDUCTIVITY of APPLE and SOURCHERRY JUICE CONCENTRATES DURING OHMIC HEATING

Ohmic heating is based on the passage of electrical current through a food product that serves as an electrical resistance. In this study, apple and sourcherry concentrates having 20–60% soluble solids were ohmically heated by applying five different voltage gradients (20–60 V/cm). the electrical conductivity relations depending on temperature, voltage gradient and concentration were obtained. It was observed that the electrical conductivities of apple and sourcherry juices were significantly affected by temperature and concentration (P < 0.05). the ohmic heating system performance coefficients (SPCs) were defined by using the energies given to the system and taken up by the juice samples. the SPCs were in the range of 0.47–0.92. the unsteady‐state heat conduction equation for negligible internal resistance was solved with an ohmic heating generation term by the finite difference technique. the mathematical model results considering system performance coefficients were compared with experimental ones. the predictions of the mathematical model using obtained electrical conductivity equations were found to be very accurate.     

PARTICLE RESIDENCE TIME DISTRIBUTIONS IN A MODEL HORIZONTAL SCRAPED SURFACE HEAT EXCHANGER1

Residence times and their distribution characteristics of potato cubes with aqueous solutions of sodium carboxymethyl cellulose (CMC), simulating non-Newtonian fluid foods, in a horizontal scraped surface heat exchanger (SSHE) were investigated. Minimum and maximum normalized particle residence times (NPRTs) and standard deviations of mean values were not significantly affected by the particle concentration while mean NPRTs of up to 10% particle concentration were signficantly lower than those of 20–40% particle concentrations (P < 0.05). Mean NPRTs were significantly influenced by process parameters including concentration of carrier, viz., viscosity, mutator speed and particle size (P< 0.001) as well as 2-way interactions among flow rate, mutator speed and particle size (P < 0.05 or 0.001). Furthermore, most of the individual particle residence time distributions in a horizontal SSHE flow could be described by either normal or gamma distribution models.     

Advantages of ohmic cooking in the kilohertz range – Part II: Impact on textural changes and approaches to improve heating uniformity

The impact of ohmic heating (OH) in the low (12 kHz) and high (300 kHz) kilohertz-range on textural changes and texture uniformity of potato cubes and whole potato tubers was investigated. Potatoes were heated in aqueous NaCl solution with a conductivity of 2.5 mS/cm. Texture was characterized by cutting and puncture tests and the energy (mJ) necessary to cut or penetrate the samples was found to be a suitable indicator. The impact of the initial temperature of the surrounding liquid on the heating uniformity was analyzed by starting the cooking treatment in cold (20 ± 2 °C) and in hot (90 ± 5 °C) NaCl solution. To further improve the heating efficiency, the conductivity was adjusted during the treatment. With an increasing power input during OH, increasing texture softening rates were observed. Compared to conventional cooking, an up to 8-times faster texture softening rate occurred. Treatments at 300 kHz resulted in a better texture uniformity compared to OH at 12 kHz. Compared to the conventional cooking, OH treatments resulted in significant (p ≤ 0.05) higher weight losses and a higher texture softening at similar cooking degrees. By adjustment of the liquid conductivity and power input during the treatment, textural properties were improved, and weight loss could be reduced.Industrial relevanceOhmic heating (OH) is an alternative heating technology with the potential to reduce processing times and energy consumption. It can overcome limitations in the heat transfer particularly given for large-volume solid foods and provide a uniform heating. However, limited attention has been given to the effects of the product and process parameters on the heating uniformity during OH treatments of solid foods. The present paper highlights the importance of the electrical conductivity and the frequency of the applied electric field on the texture uniformity. The obtained results contribute to the understanding of the interaction of the electric field and the food matrix, which is the basis for the implementation of tailored and targeted processing concepts in the industry.     

Continuous flow radio frequency heating of particulate foods

Small whole carrots, carrot cubes, and potato cubes suspended in 1% (w/w) CMC solutions were heated by a 40.68-MHz, 30-kW continuous flow radio frequency (RF) unit that has an applicator tube with an inner diameter of 14.6 cm (5.75 in.) and a height of 2.1 m (82.5 in.). Solution temperatures and the residence time when the particles entered and left the system were recorded. An infrared camera was used to record the temperature distribution within the particulates. A relatively nonuniform temperature distribution (a local difference up to 8 °C) was seen within carrot cubes (2 cm each side). However, the temperature distributions within small whole carrots and potato cubes (2 cm each side) were relatively uniform (a local difference of 1 to 4 °C). The structure at the center of the big carrot used to make carrot cubes may result in different dielectric properties from the rest of carrot and therefore slower heating at these areas. Although the initial temperature inside the particles (7 °C) was significantly lower than that of the solution, the exit temperatures (averages ranging from 38 to 60 °C) within the particles were close to the solution temperatures. Uniform temperature distribution within the particles makes RF a promising heating source in thermal processing, especially aseptic processing of liquid and particulate foods.     

Partition of proteins in the aqueous guar/dextran two-phase system

The partition of proteins (bovine serum albumin, β-lactoglobulin, micellar casein) has been examined in the guar/dextran aqueous two-phase system. The influence of parameters like pH, ionic strength (NaCl), and polymer concentration has been investigated while the protein concentration was increased up to 2% w/w. For globular proteins (bovine serum albumin, β-lactoglobulin) the general trends of partitioning established in the literature with other systems like polyethyleneoxide/dextran were nearly verified. The dextran-rich bottom phase was found to be slightly more concentrated in protein than the guar-enriched top one. However, micellar casein displayed a different behavior, giving a more uneven partition and phase separation phenomena at concentrations above 0.5% w/w. This difference has been attributed to the larger size of the particle. Meanwhile, the modifications of the guar/dextran phase diagram have been investigated. It was shown that the addition of globular protein or micellar casein up to 2% w/w did not affect the thermodynamic features of this system.     

Influence of Increasing Brine Concentration in the Cod-Salting Process

ABSTRACT: Cod-salting studies were accomplished using 20 and 25% w/w brine concentrations, and a combination of both. Cod weight increased in the case of the 20% concentration and decreased in the case of the saturated. The heavy-salted cod obtained using a combination of both solutions had a higher yield than that obtained when using saturated brine. The weight change standard deviation values were related to the NaCl concentration driving force between the cod liquid phase and the brine, and the structural parameters. The Force/Area values, determined by means of a cut test, showed an exponential increase with respect to the NaCl concentration in the cod liquid phase.