Measurement of Convective Heat Transfer Coefficients During Food Freezing Processes

Any prediction of freezing time for a food product depends on accurate heat transfer coefficients. The purpose of this investigation was to examine experimental procedures for accurate determination of convective transfer coefficients during freezing of food products. Measurements of heat transfer coefficients during cooling of any acrylic transducer and ground beef with similar size and shape were conducted at air temperatures below initial freezing point of the product. The results indicate that accurate measurements of the coefficients can be achieved through nonlinear regression analysis of temperature histories within an acrylic transducer.     

Optimal Sterilization Temperatures for Conduction Heating Foods Considering Finite Surface Heat Transfer Coefficients

Optimal sterilization temperatures are defined as the processing temperatures which result in a minimum surface cook-value after achieving the desired degree of sterility. They were calculated as a function of product heating rate, surface heat transfer coefficient, initial food temperature, heating medium come-up-time, z-value for the quality factor and target F_o-value. Different one-dimensional heat transfer shapes were considered. Compared to the other variables, initial temperature and heating medium come-up-time had little influence on optimal processing temperature. Regression equations were developed relating optimal temperatures with all relevant variables.     

Simplified Equations for Transient Temperatures in Conductive Foods with Convective Heat Transfer at the Surface

Simple equations were developed to predict the values of some characteristic transcendental and Bessel functions, and hence the temperature history in regular solid foods of finite and infinite dimensions under unsteady state conduction with convective heat transfer at the surface. For various combinations of Biot (0.02- 200) and Fourier (>0.2) numbers, the mean error involved in predicting the unsteady temperature ratio using the developed equations was less than 0.1% as compared to the original models. Equations were presented for temperature at any location as well as the mass average temperature. The characteristic functions were related to the f and j parameters from heating and cooling curves.     

Thermodynamic Analysis of the Freezing and Thawing of Foods: Enthalpy and Apparent Specific Heat

On the basis of the principles of freezing point depression, equations for the temperature rate of ice formation, apparent specific heat, and enthalpy were derived and expressed in terms of solids content and temperature. A unique characteristic constant for each food system can be determined experimentally from the calorimetric measurements. The new equations differ only slightly with similar equationspreviously developed by Bartlett in 1944 and by Schwartzberg in 1976. Reliability and accuracy of the methods are demonstrated with literature data for meat, fish and fruit juices. Enthalpy and apparent specific heat values for a range of temperature between 20° and 40°C and solids content between 10 and 50% are presented.     

Effect of Ultrasonic Vibration on Fluid-to-Particle Convective Heat Transfer Coefficients

Studies were conducted to determine the potential of ultrasonic energy as a heat transfer enhancing agent in sterilization applications. Metal transducer particles were immersed in warm liquids with and without sonic agitation, and temperature histories were recorded. Convective heat transfer coefficients were calculated from the solution to the lumped convective heat transfer equation. Results indicate that fluid-to-particle heat transfer was significantly enhanced by the use of sonic energy, and that within the range of variables investigated, the amount of enhancement was dependent more on natural convection and standing wave patterns than particle size and energy input per unit mass.     

优化拟合法计算黑液蒸发器传热系数

以四效混流蒸发器为背景，从系统的静态模型出发，提出了一种采用优化拟合的原理来计算蒸发器传热系数的算法。该算法可根据过程控制机采集的现场工作数据，实时地算出蒸发器的传热系数。该算法适合在集散型系统中监控机上运行，用于实时监控蒸发器的工作状况。     

Modeling Flow and Heat Transfer During Freezing of Foods in Forced Airstreams

ABSTRACT: Mathematical modeling of food freezing has been limited to the modeling of the internal heat transfer where the external convective heat-transfer coefficients are assumed or empirically estimated. Previous procedures followed to solve the external boundary layer in tandem with the internal heat transfer were constrained by numerical complexities due to the transient nature of the heat transfer, requiring unsteady formulation for the flow. In this article, attempts have been made to decouple the flow and heat transfer equations for the external boundary layer flow over a food product being frozen. The flow equations have been solved as a steady-state problem using Falker-Skan transformations of the boundary layer equation. The heat-transfer equation for fluid flow is solved as an unsteady-state problem in conjunction with the internal heat transfer and phase change inside the product undergoing freezing. The model is validated for a case of air-impingement freezing.     

Prediction of Freezing and Thawing Times of Foods Using a Numerical Method Based on Enthalpy Formulation

An explicit numerical method, involving enthalpy formulation, to predict temperature distribution in foods during freezing and thawing was developed. The accuracy of the proposed method was validated using published experimental data obtained for freezing and thawing of Tylose. The enthalpy formulation avoids the problems of strong discontinuity experienced when the apparent specific heat formulation is used in predicting temperatures for situations involving phase change. The proposed method predicts temperatures in good agreement with experimental data. The computer code can be easily programmed on a desk-top computer for use in teaching and research on predicting freezing and thawing rates in foods.     

Estimation of Heat and Mass Transfer Coefficients During Air-Freezing of Cucumber

Freezing of foods involves coupled heat and mass transfer. It is essential to optimally design the freezing equipment and maximise the efficiency of the freezing process. Therefore, it is necessary to estimate the heat and mass transfer coefficients. In this study, the surface heat and mass transfer coefficients were estimated employing the Hilbert equation as well as the corresponding thermal and diffusive boundary layers during freezing of unpeeled cucumber at low air temperatures (?10 to??18 and??25°C). The estimated mean heat transfer coefficients ranged from 10.99 W m^?2 K^?1 for 0.5 m s^?1 up to 40.07 W m^?2 K^?1 for 5.0 m s^?1. The respective mass transfer coefficients ranged from 8.98 up to 32.40 m s^?1. The estimated heat transfer coefficients were compared with the respective ones calculated from Dincer and Dincer and Cengeli during air-cooling (22 to 2°C) of unpeeled cucumbers of similar size as well as other agricultural cylindrical products.     

响应面法优化食品浸渍速冻冻结液配方

为优化食品浸渍快速冻结的冻结液配方,以甜菜碱、丙二醇、氯化钠为主要组分,添加少量甘氨酸、低聚木糖、甘露醇,组成低温冻结液,在单因素试验基础上,运用Box-Behnken试验设计原理进行试验设计,探讨浸渍快速冻结的冻结液配方的最佳配比。结果表明：以占冻结液总质量分数计,由21%甜菜碱、15%丙二醇、10%氯化钠、1%甘氨酸、1%低聚木糖、1%甘露醇和水组成的多元冻结液,冻结点可达－38.10℃,可用作食品浸渍快速冻结的冻结液,用于水产品及冷冻方便食品的冷冻加工。     

压力辅助解冻过程相变及传热模拟

本文比较了食品中冰在常压和高压下的解冻过程.利用显热容法处理潜热释放,分析了不同因素对处理效果影响,考虑了不同样品尺寸不同压力条件下解冻时间.计算得到了不同操作条件下的解冻温度曲线,结果表明,相变温度降低是影响压力辅助解冻过程解冻时间缩短的主要因素,同一样品的解冻时间与处理压力成二次曲线关系.     

Thermodynamic Analysis of the Freezing and Thawing of Foods: Ice Content and Mollier Diagram

On the basis of the freezing point depression equation, an ice content equation was derived. The required parameters are similar to equations for enthalpy and apparent specific heat previously developed. Validity and accuracy of the equation are demonstrated with experimental data for meat, fish and fruit juices. Cohesive data for enthalpy, apparent specific heat and ice content are calculated for a wide range of temperatures between 20 and -40°C. The calculated results are consistent with values obtainable through enthalpy-moisture content-temperature (Mollier) diagrams developed by Riedel.     

Determination of Retortable Pouch Heat Transfer Coefficients by Optimization Method

A computer-based optimization technique was developed for determining the apparent heat transfer coefficient (h) for retortable pouches heated by circulated hot water under overriding air pressure. The technique incorporated actual process data, a finite difference model and an optimization criteria to converge on h values. A water flow channel was constructed to fit inside a vertical retort which allowed direct water exposure on the pouch surface at a quantifiable velocity. Because the optimization technique was not dependent on analytical equations, it could be applied to such areas as future heat transfer studies of new retortable pouch materials, designing new pouch-holding cassettes and improving water flow patterns in retorts.     

A Method to Determine Initial Freezing Point of Foods

An accurate initial freezing point T_sh is required for reliable analysis of a food freezing or thawing process. Two previous formulae for accurately estimating enthalpies, one for temperatures above T_sh and another below T_sh, were used for this development. The estimation of T_sh was based on the mathematical formulation of continuity in food enthalpies estimated by both formulae at T_sh. The method was validated by estimating T_sh values of aqueous solutions of sucrose and NaCl at different solutions. There was fair agreement in the T_sh values of fresh fruit and vegetables, lean beef meat and lean fish meat estimated by the proposed and a previous method. One potential advantage of our new method was the direct estimation of T_sh through analysis of food enthalpies collected at different temperatures.     

香菇脆片真空油炸过程中传热规律研究

真空油炸过程中同时发生热量和质量传递现象,这些现象对产品的品质和安全控制极为重要。该文研究真空度、油炸温度、预处理(漂烫、漂烫+浸渍+涂膜)对香菇脆片真空油炸过程中对流传热系数(h)的影响,结果表明:h值的变化趋势是先增大后减小;达到峰值的时间都在300 s～600 s;油炸温度越高,峰值越高;真空度越高,峰值越高;在真空油炸后期,油炸温度和真空度低的香菇脆片对流传热系数值高;油炸条件相同,漂烫处理香菇脆片的最大对流传热系数值高于漂烫+浸渍+涂膜处理香菇脆片。     

Surface Heat Transfer Coefficients for Steam/Air Mixtures in Two Pilot Scale Retorts

Surface heat transfer coefficients (h) for steam/air mixtures were studied in a vertical, positive flow retort and a horizontal, forced circulation Lagarde retort, using rectangular metal bricks. Influences of test brick orientation and fractional steam content (S), temperature, flow rate and direction of the heating medium on h values were studied. In both retorts, steam content was found to be the major factor (p < 0.05) influencing h, while temperature had no effect (p > 0.05). Flow direction and flow rate in the positive flow retort, and brick orientation in the Lagarde retort also influenced h. Exponential equations, h = a exp (bS), described relationships between h and S, with a and b dependent upon retort type, flow rate, flow direction and test brick orientation.     

Determination of Heat Transfer Coefficients for Continuous Belt Freezers

Air-product heat-transfer coefficients for belt freezers were determined experimentally. Measurements were performed for foods (beef hamburgers) and a model substance (copper disk) using a prototype tunnel. Air flow parallel, as well as perpendicular, to the belt, was employed; in the latter case moving upwards or downwards through the belt. From the measured temperatures, heat-transfer coefficients were calculated using appropriate mathematical methods. Values obtained for both types of materials were compared, and the usefulness of the test substance was discussed. From these data, simple and precise regressions were obtained, useful to predict the coefficient for the above mentioned types of air flow and valid over a wide range of operating conditions in belt freezers.     

Gas-Particle Heat Transfer Coefficient for the Fluidization of Different Shaped Foods

Gas-particle heat transfer coefficients for the fluidization of diced potatoes and potato strips were measured in a batch fluidized bed in which wet particles were dried under constant rate period conditions. Thus, the surface temperature of particles was equal to the wet-bulb temperature of the air. The minimum fluidization voidage and velocity were also measured as well as the bed expansion characteristics. Results are expressed through Colburn's factor, as a function of a modified Reynolds number and an Archimedes number, allowing for the correlation of data for different shaped foods (potato cubes and strips, peas, etc.) into a single equation.     

Impact of Overall and Particle Surface Heat Transfer Coefficients on Thermal Process Optimization in Rotary Retorts

ABSTRACT: This study attempts to examine the significance of recent research that has focused on efforts to estimate values for global and surface heat transfer coefficients under forced convection heating induced by end‐over‐end rotation in retorting of canned peas in brine. The study confirms the accuracy of regression analysis used to predict values for heat transfer coefficients as a function of rotating speed and headspace, and uses them to predict values over a range of process conditions, which make up the search domain for process optimization. These coefficients were used in a convective heat transfer model to establish a range of lethality‐equivalent retort temperature–time processes for various conditions of retort temperature, rotating speed, and headspace. Then, they were coupled with quality factor kinetics to predict the final volume average and surface quality retention resulting from each process and to find the optimal thermal process conditions for canned fresh green peas. Results showed that maximum quality retention (surface and volume average retention) was achieved with the shortest possible process time (made possible with highest retort temperature), and reached the similar level in all cases with small difference between surface and volume average quality retention. The highest heat transfer coefficients (associated with maximum rotating speed and headspace) showed a 10% reduction in process time over that required with minimum rotating speed and headspace. The study concludes with a discussion of the significance of these findings and degree to which they were expected.     

Microwave Assisted Thawing of Model Frozen Foods Using Feed-back Temperature Control and Surface Cooling

An apparatus was developed that combined microwave energy and cold air with different ambient temperatures to reduce thawing time and avoid run-away heating during microwave assisted thawing. Effects of microwave power level, sample thickness and surface air temperature on thawing time were investigated. The microwave power was cycled on and off using two temperature control schemes to maintain a predetermined temperature gradient based on hot and cold points. Thawing time was reduced by as much as a factor of seven compared to convective thawing at ambient temperature when appropriate conditions were used.