Mathematical Evaluation of Process Schedules for Aseptic Processing of Low-Acid Foods Containing Discrete Particulates

A quantitative methodology was developed for evaluating thermal process schedules for low-acid foods containing particulates of any shape. The methodology uses finite element analysis to determine temperature distributions within particulate foods subjected to timevarying boundary conditions. Simulations were conducted to study the effects of particle size, residence time distributions within heat exchanger and holding tube, and convective heat transfer coefficients. Results indicate that all these factors have significant effects on the thermal process schedule required to achieve commercial sterility. For large particles, simulations indicate that little credit can be given for lethality within the heating section.     

Electrical and Thermo-Physical Properties of Meat Ball

Ohmic heating is an interesting method for heating meat ball; however, its effectiveness relies on the electrical and thermo-physical properties of the product. Therefore, this research aimed to determine the electrical conductivities and thermo-physical properties of pork-meat balls. The results showed that electrical conductivities of samples could be well-fitted into the mathematical models as a function of temperature and the ratio of each ingredient due to their high R² and low RMSE. The addition of salt and sodium tripolyphosphate led to a significant increase in electrical conductivity. The specific heats and thermal conductivities of meat balls were measured, whereas the convective heat transfer coefficients were determined by a lump-heat capacity technique. An equation was developed to estimate the convective heat transfer coefficients as a function of temperature difference between particle and surrounding fluid.     

Ohmic heating behaviour and electrical conductivity of two-phase food systems

Ohmic heating behaviour and electrical conductivity (EC) of two-phase food systems were studied. Food systems were comprised of a liquid phase using 4% w/w starch solution with 0.5% w/w salt, and a solid phase containing carrot puree and cubes of different sizes (6 and 13 mm) in different concentrations (30 and 50% w/w). A set of experiments was carried out for a wide range of particle concentrations from 0 to 60% w/w. With respect to the particle location, experiments were conducted using 30% w/w cubes (6 mm) placed in parallel, in series and in well-mixed conditions with the liquid phase. Ohmic heating was applied to the food systems using a static cell (20-mm-long Teflon cylinder with 35 mm diameter and 25 mm wall thickness) at a constant voltage gradient of 12.5 V/cm. Electrical conductivity values were calculated as a function of particle size, concentration, location and temperature. It was observed that the heating time increased along with particle size and concentration. Overall values of EC ranged from 0.2 to 1.8 S/m, increasing with the process temperature as it ranged from 20–80 °C, and decreasing as particle size or concentration increased. With respect to the particle location within the ohmic cell, the thermal behaviour was different when the two phases were in parallel, in series or in mixed condition. However, there was no significant difference between overall values of EC when liquid and solid phases were separated as compared with the mixed condition.     

含绝缘体物料的欧姆加热过程模拟

欧姆加热利用食品物料本身的电阻抗来产生热量,是一种内加热方式,与传统加热方式相比,具有快速均匀的加热特点。但食品往往不是一个均匀体系,不同部位的电导率的差异也会造成欧姆加热升温的不均匀性,研究不均匀体系欧姆加热时的温度分布对于欧姆加热技术的开发利用有重要意义。本文利用有限元方法,模拟了含有绝缘体的物料的欧姆加热过程,并用实验进行了验证。结果表明,有限元模型能够很好地模拟欧姆加热过程,整个加热过程中,模拟预测的温度与实验测得温度接近,最大差值为8K。同时运用有限元模型模拟了绝缘体的位置和方位对温度场的影响,绝缘体在物料中的位置和方位影响物料中电流密度的分布,温度场最高点位置始终出现在电流密度最高的区域,而最低点与电流的加热作用和物料间的热传导作用有关。     

INFLUENCE of PARTICLE ORIENTATION ON the EFFECTIVE ELECTRICAL RESISTANCE and OHMIC HEATING RATE of A LIQUID-PARTICLE MIXTURE1

The ohmic heating rate of a liquid-particle mixture is expected to depend on particle orientation. A mathematical model was developed to determine the extent of orientation dependence for the case of a cubic particle. the particle and liquid resistances were determined by taking incremental sections through the particle and using the electrical conductivity to calculate the effective resistance. Modeling and experimental results indicate that for a cubic particle, orientation effects are slight; both with regard to effective resistance and heating time. However, this result is not expected to apply to particles of large aspect ratios.     

Quantifying Enhancement in Heat Transfer Due to Natural Convection During Canned Food Thermal Sterilization in a Still Retort

Thermal sterilization of canned viscous liquid foods using saturated steam is enabled by natural convective heat transfer. However, the governing equations for two-dimensional convective heat transfer may be only rigorously solved by numerical calculations. On the other hand, if conduction is assumed to be the only mode of heat transfer, the thermal sterilization problem has analytical solutions for simple boundary conditions. However, the conduction model may not be appropriate in describing thermal sterilization of even viscous liquid foods and may cause considerable error in the prediction of the important parameters such as slowest heating zone (SHZ) temperature and lethality. The longer time for sterilization recommended by the conduction model may lead to overprocessing and an unacceptable food product. The objective of this work is to quantify the faster temperature rise in the food can due to natural convection when compared to the temperature rise obtained by only conductive heating. The consequent enhancement in lethalities is also reported. In addition, this work’s objective is to investigate how quickly the natural convective heat transfer effects begin to dominate over the solely conduction heating mode. The volume-averaged temperature as well as the SHZ temperature variations with time was calculated for the convection-augmented mode using computational fluid dynamics (CFD) simulations. Lethality values were then calculated based on volume-averaged temperature as well as the SHZ temperature. Food cans of different aspect ratios and food medium thermal conductivities are considered in this analysis. For the food system investigated, the critical Fourier number at which the transition to convection-augmented mode of heat transfer occurred is identified and explained from scaling considerations. In the conduction-dominated mode, it was possible to use analytical solutions to predict the volume-averaged and SHZ temperatures of the liquid food undergoing thermal sterilization. The Nusselt number correlation developed by Kannan and Gourisankar (2008) was used in the lumped parameter transient heat transfer model to predict the volume-averaged temperatures in the convection-dominated region. The volume-averaged temperatures from this approach were found to be in good agreement with the CFD simulation results. The time predicted for the SHZ to reach the minimum sterilization temperature was significantly lower when convective heating was also considered. The volume-averaged temperature and SHZ temperature enabled an estimation of overall sterility levels attained and minimum sterility levels prevalent inside the can, respectively. Even though the volume-averaged temperature increase due to convection was only about 10 K, the resulting accumulated lethality values were higher by an order of magnitude. The increase in SHZ temperatures was much higher in the convection-augmented mode, and consequently greater integrated lethalities were attained. The simple conduction model that is amenable to analytical solution cannot be used to approximate the heat-transfer-related phenomena even for “quick estimation” purposes when convection effects are significant. This precaution is found necessary even for the reasonably high viscous carboxy methyl cellulose system, whose average viscosity values ranged between 13 and 3 Pa s during the course of the sterilization process.     

A theoretical analysis on microwave heating of food slabs attached with ceramic plates: Role of distributed microwave incidence

A detailed analysis has been carried out to study efficient heating processes due to microwaves for one dimensional food samples placed on ceramic plates with various distributions of microwave incidence. The enhanced heating effects due to microwave propagation within samples have been illustrated via average power within a sample vs sample thickness diagram. The maxima in power, also termed as ‘resonances’ is observed for specific sample thicknesses. The spatial temperature distributions illustrate the presence of local thermal heating effects for various cases. Either one side incidence or both sides incidence with suitable distribution of microwave incidence may be preferred for various food materials attached with ceramic plates. One side incidence may cause overall larger heating rates for beef samples and the heating rates for oil samples would remain invariant irrespective of various distributions of microwave incidence. It is observed that beef samples would exhibit greater thermal runaway specially with SiC plate in presence of both sides distributed microwave incidence whereas oil sample would exhibit smaller thermal runaway effects with both sides equi-distributed microwave incidence irrespective of any ceramic plates. Current analysis has been illustrated for low and high dielectric food materials (beef and oil) and based on our analysis, a generalized heating strategy for any materials due to uniform plane waves may be derived.     

Mathematical modeling of ohmic heating of two-component foods with non-uniform electric properties at high frequencies

The ohmic heating (OH) of two-component foods (consisting of mashed potato and mashed potato with 1% NaCl) configured using four different filling patterns (parallel, series, and two concentric patterns) was investigated experimentally and by modeling. The electrical conductivities of the samples at 50 Hz and 20 kHz were obtained in the range of 20–85 °C. The differences between the thermal behaviors of the two-component foods were analyzed using internal thermal images and temperature profiles from four positions. A three-dimensional OH model for high frequencies was developed based on electric field analysis using Maxwell's equations. The model agreed sufficiently with the measured results and was able to confirm that the current applied to the sample follows the path that presents minimum electrical resistance, which is independent of the filling pattern configuration. Analysis of the heat flux density provided a better understanding of the heating behavior of two-component foods during OH.Industrial relevanceAlthough several mathematical approaches used to simulate the OH process of solid foods have been published in recent years, the simulation of temperature and electric field distributions using simplified methods, such as by the solution of Laplace's equation, is still a problem for two-component solid foods with special configurations of the internal components treated using an OH process at high frequencies. This problem can be solved using a more fundamental solution based on Maxwell's equations and electric field analysis via computer simulation. A comprehensive mathematical simulation of the thermal behavior and the effect of the orientation of the current within the two-component foods can be used to accurately predict the heat during OH. Therefore, the modeling of heating patterns of complex foods can assist in the design of food sterilization and pasteurization processes. Moreover, it will contribute in industrial applications for a better design of OH systems and electrode configurations for rapid and uniform heating.     

A MODEL FOR HEATING of LIQUID-PARTICLE MIXTURES IN A CONTINUOUS FLOW OHMIC HEATER1

Design of safe thermal processes in a continuous flow ohmic heater requires mathematical model development, since temperature monitoring of particles in continuous flow is not presently possible. A mathematical model previously developed and verified for a static heater was modified to predict temperatures of fluids and particles within a continuous heater with high particle concentration suspensions. Results for constant voltage simulations indicate that the presence of large populations of low electrical conductivity particles results in slow heating of the entire mixture rather than a single phase alone. In all these cases, particles tend to heat faster than the liquid. However, if isolated low-conductivity particles enter the system, the danger of underprocessing exists; hence particle conductivity is a critical control point. Simulations also indicate the role played by residence time distribution and liquid-particle heat transfer coefficients in the above cases.     

具有“核壳”结构的乳清分离蛋白-黄原胶复合颗粒的构建

采用乳清分离蛋白（whey protein isolate,WPI）和阴离子多糖黄原胶（xanthan gum,XG）作为基础原料制备复合颗粒,通过改变离子强度、温度、混合方式等条件,对WPI-XG复合体系浊度、粒径、Zeta电位进行分析,探究影响复合颗粒形成的主要因素,并通过激光共聚焦显微镜观察颗粒的结构。结果表明：低离子强度（≤20 mmol/L）可促进WPI-XG复合物的形成;温度越高,复合颗粒的粒径越大;先单独热处理WPI溶液后再与XG溶液混匀制得的颗粒具有更大的蛋白质多糖比、更小的粒径和更大的电位绝对值;剪切会使颗粒的粒径和电位绝对值下降。最终根据浊度、粒径、电位和共聚焦显微镜的结果,得出剪切后的75 ℃热处理WPI颗粒与XG复合形成更为完整的“核壳”结构。     

Evaluation of key parameters during construction and operation of an ohmic heating apparatus

This study aimed to design, build and validate an ohmic heating apparatus in bench scale and evaluate its performance on liquid food processing. The equipment developed showed adequate performance, monitoring data and heating products satisfactorily. Three ohmic cells were developed and tested in the device. When acerola and blueberry pulps were heated in the ohmic heater, a nonlinear behavior of electrical conductivity with temperature was observed. This behavior is associated with bubble formation caused by water boiling due to temperature gradients inside the cells. This phenomenon is influenced by a number of factors, such as solids content, electric field strength, ohmic cell size and agitation. Two of the ohmic cells developed, when placed on agitating devices, are suitable for a uniform heat treatment of liquid foods. The ohmic heating apparatus developed is adequate to conduct studies to better understand this technology and its applications on food processing.Industrial relevanceThe apparatus of ohmic heating for liquid foods designed and tested in this work can be used to evaluate fundamental parameters, such as electrical conductivity of the product, heating time and process homogeneity. It also provides a tool to monitor processing effects on the quality of end products in order to find the best conditions for a continuous ohmic heating process in an industrial scale.     

A Comparison of the Effectiveness of Sonication, High Shear Mixing and Homogenisation on Improving the Heat Stability of Whey Protein Solutions

Upon ultrasonic treatment at 20 kHz, protein aggregates in a dairy whey solution were broken down. In addition, when sonication was applied to a heated solution of denatured and aggregated proteins, there was a dramatic reduction in viscosity and aggregate size, which was maintained after re-heating. This observed heat stability may be due to shear forces that are induced by acoustic cavitation. To determine whether high shear mixing or homogenisation is able to cause similar effects to that of acoustic cavitation, sonication, high shear mixing and homogenisation were performed on 5 wt% whey protein concentrate solutions at identical energy density levels, which was based on the power drawn in each system. Homogenisation provided similar particle size and viscosity reductions as sonication while high shear mixing was less efficient in decreasing particle size. Cavitation was shown to be absent in both the mixing and homogenisation configurations, indicating that the shear forces generated are responsible for the observed particle size and viscosity reduction. In addition, heat stability was achieved in all systems indicating that a combination of heat treatment and any method that generates high shear forces can be used to improve the heat stability of whey proteins.     

通电加热技术在食品中的应用研究动态

通电加热是利用食品物料本身的电阻抗来产生热量的加热技术,是一种内加热方法。与其他加热方式相比,具有加热快速、均匀的特点。文章介绍了通电加热装置的主要组成,总结了电导率的影响因素,概述了通电加热过程中食品的温度分布模拟,归纳整理出通电加热在热烫、脱水、蒸发、发酵、提取、解冻、灭菌等方面的应用,并对食品通电加热技术的应用前景进行了展望。     

Influence of the mixing process on surimi seafood paste properties and structure

The aim of this study was to understand the effect of the mixing operation in the manufacturing process of surimi seafood. The approach was to work on an actual product and equipment representative of that used in the industry. Surimi seafood paste was characterized in different mixing durations by measurements of particle size, protein solubility and water absorption, temperature and viscoelastic properties, in order to follow the structural modifications and the interactions generated by the mixing process.Two major periods are highlighted during the process: in the first step, the frozen surimi melts; therefore, the shredding of fibers causes a huge reduction in particle size and protein solubilization. Then, temperature increases due to viscous heating. Temperature augmentation leads to denaturation and aggregation of proteins. This then render to the formation of a gel network that will be broken by the ongoing mixing operation. This clearly shows that, during this second step, overmixing affects the functionality of proteins. This functionality is countered by the gelling due to the increase in temperature and subsequent degradation in the mixing process during the gel formation.     

高压均质对EGCG-OSA玉米淀粉复合体的结构及其抗氧化特性的影响

为探究高压均质作用对EGCG-OSA玉米淀粉复合体的影响，分别采用加热、高压均质以及先高压均质后加热的方法制备了EGCG-OSA玉米淀粉复合体，利用傅里叶红外（FT-IR）、X射线衍射（XRD）与电镜扫描（SEM）对其结构、形貌进行表征；利用高级流变仪以及粒度仪对其流变性进行研究；通过测定复合物的DPPH和ABTS自由基清除率比较其抗氧化活性。结果表明:三种复合体均有很高的抗氧化性，并且三种复合体的抗氧化性随储藏时间下降的幅度较小，其中用先均质后加热方式制备的复合体具有最高的抗氧化性，在30 d后DPPH清除率仍能达到89.63%，ABTS自由基清除率达到98.66%；FT-IR与XRD结果表明复合物并没有产生新的化学键和官能团；流变性测试结果则表明:经过高压均质作用后的复合体，黏度下降幅度更大，粒径也有所减小。综上表明，通过先高压均质后加热方式制备的复合体具有更高的抗氧化性和更为优良的储藏稳定性，研究结果能为EGCG在淀粉类食品的加工及生产中的应用提供理论及实验基础。     

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.     

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.     

小麦面条和面过程面絮特征与检测研究进展

和面作为面条制作的关键性工艺，形成的面絮直接决定面条最终品质，但面絮品质缺少客观检测方法，制约了面条工业化发展。面絮形成是谷物粉体颗粒与水分混合形成团聚体的湿法造粒过程，受到面粉自身组分、水分加入量、和面方式和辅料的影响。面絮的颗粒度较为均匀、水分在面絮内部和面絮之间分布均匀、面筋网络结构充分有效包裹淀粉是良好面絮的共同特征。面絮状态图像深度学习分析把面絮形成分为润湿黏连、聚集成形、破裂分散、稳定平衡4个阶段，稳定平衡阶段在精准判别和面终点中起到至关重要的作用。目前面絮颗粒大小评价以目测、时间控制、感官评价为主，还无法实现仪器精确测定或在线质量检测。通过图像分析，用面絮颗粒图像的阴影面积可以间接反映面絮状态，指导面条工业化生产。本文综述了小麦面条和面过程中面絮的基本特征，分析了面絮形成过程中的水分动态变化、颗粒度变化、面筋网络结构变化及其影响因素，讨论了面絮与面条品质之间的关系，介绍了图像检测方法、其他检测方法在面絮检测中的研究进展。     

细菌耐热性测定中热滞后修正的新方法

本文为细菌耐热性值测定中热滞后修正发展了一个新的方法。方法的发展是基于迭代计算,研究结果表明新方法可以在被测定细菌固有的Z值下进行热滞后修正,从而获得精确的细菌耐热性值。 为实施热滞后修正的迭代计算,发展了计算机程序。此计算机程序可用于任何目前使用的细菌耐热性测定装置,从而为细菌耐热性值的测定计算提供了简便,正确而又快速的方法。 文章表明细菌耐热性测定装置存在的热滞后对耐热性测定值,D和Z值都有较大影响。因此,运用本文发展的方法在细菌耐热测定中进行热滞后修正对正确制订热杀菌条件有重要作用。     

烟草颗粒热解与释烟特性影响因素研究

[目的]为探索加热状态下烟草颗粒热解和烟气释放规律.[方法]采用导热系数测试仪、热分析仪(TGA)和锥形量热仪(CONE)表征分析了甘油添加量、水分含量以及粒径对烟草颗粒导热特性、热解过程和烟气释放特性的影响.[结果](1)样品导热系数随水分和甘油含量的增加以及粒径减小整体上呈现不同程度的上升趋势,不同水分与甘油含量样...