A NUMERICAL APPROACH WITH VARIABLE TEMPERATURE BOUNDARY CONDITIONS TO DETERMINE THE EFFECTIVE HEAT TRANSFER COEFFICIENT VALUES DURING BAKING OF COOKIES

The increasing trade of ready‐to‐eat foods such as cookies highlights an interest in quality defects during baking. Heat (h and thermal diffusivity) and mass (mass transfer and diffusion coefficients) transfer parameters are significant parameters affecting the quality changes. Therefore, it is important to determine these parameters for modeling and process optimization studies. Among these, the h is important, revealing the relationship between the heating medium and product surface. As baking involves a simultaneous heat and mass transfer involving moisture diffusion and heat conduction inside and convective heat and mass transfer outside, a lumped system method may not be an accurate choice to determine the h value. Changes in the product volume and contact heating from bottom of the product also bring extra challenges to the determination of h. Therefore, the objective of this study was to use realistic approaches including simultaneous heat and mass transfer to determine the changes in h. The h_effvalues for the bottom and top surface of the cookies were then determined, applying a numerical procedure where the surface temperature changes were the boundary conditions with evaporation on the surface. The h_band h_t values increased with baking temperature and varied with baking time. The results of this study showed that evaporative mass flux for the top surface, heat flux for the bottom surface and the product's volume changes were significant in the variation of h values.     

Heat Transfer Coefficients on Cakes Baked in a Tunnel Type Industrial Oven

Using an h-monitor, surface heat flux and effective surface heat transfer coefficients were evaluated during baking of two cakes in a tunnel-type multi-zone industrial oven. An average 75–80% of total heat flux was counted as radiation heat. Air-mass temperature outside the boundary layer was determined from the experimental temperature profiles over the h-monitor top plate. In the range of baking temperatures (186–22 5°C), relative air velocities (0.02-0.437 m/s) and absolute humidities (0.0267–0.0428 kg H₂O/kg dry air) heat transfer coefficients were 20 to 48.0 W/m²K. A simple regression model was developed based on experimental data.     

Microwave Frying Compared with Conventional Frying via Numerical Simulation

Microwave heating can be combined with other means of heating to yield a unique heating profile. In the study, microwave frying, a combination of convective and microwave heating, was compared with conventional frying. Frying experiments were performed by inserting a single food sample (chicken breast meat) in the hot oil at 180?±?1°C for both frying methods. Center temperature of the sample and the oil temperature were recorded during both frying methods. Simulations were performed to predict heat transfer coefficients. Processing time was shorter with microwave frying. Simulations revealed a varying convective heat transfer coefficient, which was in the range of 160–490 W/m² K, during conventional frying. Higher convective heat transfer coefficient, 500 W/m² K, compared to conventional frying was observed during microwave frying with the simulations. This is suggested to be due to higher turbulence in microwave frying.     

电加热卷烟传热传质和关键物质释放规律研究进展

为了掌握电加热卷烟的研究现状,为产品开发和改良的相关研究工作提供思路,对电加热卷烟的工作过程和关键物质（水分、甘油和烟碱）释放规律研究及现有的数值模拟研究进行了综述,结果表明:①电加热卷烟主要包含热传导和对流换热两种传热形式,其关键物质释放过程的本质是发生在体系内部复合物理、化学反应的传热、传质和相变问题;②烟草基质在加热过程中会发生许多物理化学过程,如水分蒸发及碳水化合物和大分子物质的热解等;③电加热卷烟工作过程中,水分在大温度范围内均有释放,甘油和烟碱的释放分别主要在150~250 ℃和110~220 ℃区间,关键物质的释放与加热温度、雾化剂质量分数、烟支含水率等多重因素有关;④现有数值模拟工作中对电加热卷烟内部的传热和关键物质释放、烟气在滤嘴段的降温和流动特性以及电加热器表面温度分布进行了研究。      

Simultaneous Determination of Thermal Diffusivity and Heat Transfer Coefficient during Sterilization of Carrot Dices in a Packed Bed

Carrot dices were heated under ultra-high temperature sterilization conditions in a packed bed to quantify the effect of superficial fluid velocities on heat transfer coefficient (h) and obtain a value for thermal diffusivity (α). h and α were determined simultaneously under non-constant heating fluid temperatures using a finite difference technique. Temperature profiles calculated using assumed h and α were matched with measured temperature at two points in a cube. Using best fit value for α of 1.94 × 10^?7 m²/sec, mean h ranged from 600 to 1533 and 359 to 735 W/m²K for 0 and 1.58 cm/sec fluid velocity and 1 and 2 cm cubes, respectively. At 15.2 cm/sec, when a controlled heat transfer, calculated α was 1.86 ± 0.044 × 10^?7 m²/sec.     

Thermal Properties of Beef Loaf Produced in Foodservice Systems

There is a lack of information on thermal and other properties of foods, especially food mixtures. Physical properties of beef loaf prepared following formula and procedures used in alternate foodservice systems were determined: heat capacity (0.88 cal/g°C uncooked, 0.91 heated to 60°C) moisture content (72.1% uncooked, 66.2% heated), thermal conductivity (0.40 w/cm°C uncooked, 0.47 heated), fat (17.6% uncooked, 13.0% heated) and density (1.00 g/cm³ uncooked and 0.70 heated). The surface heat transfer coefficient for a forced convection oven was also determined (62 w/ m²°C.) Times to heat beef loaves to specified ending temperatures in a forced convection oven at 163°C and 176°C were calculated. There was less than 1 min difference in calculated heating time required to reach the desired end temperature. The actual times to reach ending temperature show very good agreement with the calculated times.     

Comparative study of the nutritive value of casein heated by microwave and conventionally

The objective of this study was to compare the net protein utilisation, digestibility and biological value of casein solution heated by microwave or conventionally, ie on an electrically heated plate. Portions (500 ml) of an aqueous solution of casein (100 g litre^?1) were heated to a temperature of 80°C and subsequently kept at 80± 5°C for 2 min. The heating times of microwave and conventional heating were the same. Additional portions were subjected to a so-called abused treatment consisting of repeating the normal microwave or conventional heat treatment three times. Next, the nutritive value of heated and unheated casein was determined in a 10-day feeding study with Wistar rats. The net protein utilisation, digestibility and biological value of unheated casein were 61.1, 100 and 61%, respectively. Comparable values were obtained for casein heated by microwave or conventionally. These results did not therefore indicate any decrease in nutritive quality of the protein by microwave heating or conventional heating under simulated household conditions.     

Heating Surface Temperature and Contact-Heat Transfer Coefficient of A Clam-Shell Grill

The objective of this study was to determine the heating rate, characteristics of temperature distribution and change on heating surfaces, and contact-heat transfer coefficient of a clam-shell-grill under conditions of commercial operational cooking. The information obtained will contribute to improved cooking process specifications and equipment design that ensure the quality and safety of cooked patties. We found that grill surface temperatures dropped to 113 °C (the lowest) from a preset temperature of 177 °C after the frozen patties were placed on the grill surface. The temperature of the patties recovered to only between 129.4 and 144.5 °C at the six locations at the end of cooking process. The maximum temperature difference among the six locations was about 15 °C during most of the cooking period. It is important that grill operators or manufacturers map the temperature distribution on grill heating surface before cooking to determine the sequence of hamburger patty placement and removal and minimize temperature variation in cooked patties. The contact-heat transfer coefficient between the grill and hamburger patty surfaces varied with heating temperature and time during the cooking process. The highest contact heat transfer coefficient value obtained was 832 W/m² °C.     

TEMPERATURE DISTRUBITIONS AND LIQUID-SIDE HEAT TRANSFER COEFFICIENTS IN MODEL LIQUID FOODS IN CANS UNDERGOING FLAME STERILIZATION HEATING

Heat transfer studies were performed with clay suspensions and sugar solutions in 303 × 406 cans heated singly in a flame process simulator. Experimental results show that the can wall temperature does not reach excessive temperatures and that the temperature distribution throughout the can is uniform. The slowest heating point was on the axis near the end of the can but this temperature lagged the center temperature by less than 3°C even with adverse heating conditions (i.e., no head space). The liquid-side heat transfer coefficient in 60% sucrose solution at 30 rpm was measured to be 466 ± 34 W/m²K based on the cylindrical area of the can. Rotational speed had only a small effect on heating rate in the range of 10 to 120 rpm.     

Convective heat transfer in herb and spices during open sundrying

Tinospora cordifolia (herb), Curcuma longa L. and Zingiber officinale (spices) were dried in the open sun and their thermal behaviours investigated. Important thermal parameters, such as product temperature, temperature surrounding the product surface and the moisture removal, were studied during the open sundrying. The effect of the temperature difference between the product temperature and the temperature surrounding the product surface on the convective heat transfer coefficient was studied. The maximum values of convective heat transfer coefficient were 3.9, 3.4 and 3.3 W m^?2 K^?1 with experimental errors of 23%, 19% and 17% for T. cordifolia, C. longa L. and Z. officinale under open sundrying, respectively. The predicted values of the temperature and the moisture removal for T. cordifolia, C. longa L. and Z. officinale were close to the experimental values, the coefficients of correlation (R²) and root mean square percent errors (e) varying from 0.986 to 0.999 and 0.42 to 6.55, respectively.     

ELECTRIC FIELD EFFECTS ON THE THERMAL CONDUCTIVITY OF MILK CHOCOLATE DETERMINED USING THE "MIRROR IMAGE METHOD"

Electrorheological fluids exhibit changes in flow properties when exposed to an electric field. Generally, these fluids display an increase in apparent viscosity and a greater yield stress in the presence of a voltage. Another attribute of some electrorheological fluids is the ability to enhance heat transfer, allowing for greater control over heating and cooling unit operations. This paper examined the effects of an applied voltage on the thermal conductivity of molten milk chocolate, a material known to be an electrorheological fluid. The method used to measure the thermal property was based on the “mirror image’ concept. In this technique, identical disks sandwich a heat source providing a constant heat flux directed evenly between the two sides. Knowing the heat flux and the boundary temperatures of the sample gap, an analytical solution was used to calculate the thermal conductivity and specific heat. An applied electric field of up to 450 V mm^?1 at 60 Hz was found to have no significant effect on the heat transfer properties of milk chocolate. The thermal conductivity was found to have an average value of 0.163 W m^?1 K^?1 over a temperature range varying from 25 to 60C. The measurement technique, however, proved to be a rapid method for determining thermal properties that may be useful in investigating other liquid foods.     

The effects of continuous flow microwave treatment and conventional heating on the nutritional value of milk as shown by influence on vitamin B1 retention

 The effects of continuous flow microwave treatment and conventional heating (using a plate heat exchanger) on the nutritional value of raw milk was estimated by measurement of the vitamin B₁ retention. Microwave heating, without a holding phase, did not modify the vitamin B₁ content of milk, whereas analogous treatment carried out with the plate heat exchanger led to a loss of this vitamin. A holding phase caused a similar loss of vitamin B₁ in milk heated using either system. These results indicate that continuous flow microwave treatment of milk compares favourably with conventional heating because it produce less destruction of vitamin B₁ in milk. This could be attributed to the shorter time taken to reach the final temperature and the lack of hot surfaces in contact with the milk in the case of the microwave system. Received: 16 October 1998     

CONVECTION HEAT TRANSFER COEFFICIENT FOR A RESTRUCTURED PORK/SOY HULL PRODUCT1

Heat transfer coefficients, h, (J/cm²°C min) were determined for a pork/soy hull mixture during heating in a convection oven (142°C) with and without control of the dew point. Data were acquired by heat processing 5 stacked layers of product (10 × 10 × 1 cm) so that heat was only conducted into the top surface. One experiment was designed to determine h when the second layer from the top reached one of 6 temperatures: 20, 32, 44, 56, 68 and 77°C, with no control of the dew point. the second experiment determined h for 60 min of heating for 5 dew point levels: 6, 21, 31, 40 and 50°C. Without the control of humidity, values for h ranged from 0.0170–0.0366 J/cm²°C min. the heat transfer coefficient increased with temperature and heat processing time. Product yield and surface area ratios decreased as h increased. When relative humidity was controlled, values for h ranged from 0.0167–0.0270 J/cm²°C min, decreasing for dew points above 31°C.     

An inverse lumped capacitance method for determination of heat transfer coefficients for industrial air blast chillers

An inverse model using lump body was developed to calculate heat transfer coefficients as a dynamic function of time. The model used sequential function specification algorithm to calculate surface heat flux from transient temperature measurements inside a lump body system. Transient temperature measurements were collected during cooling inside an industrial chiller at two different positions with different air velocities using four replicates for each position. The calculated surface heat flux was found to be very accurate as the maximum value of the root mean squares error (RMSE) for temperature is 0.045 °C, lower than the expected error form thermocouple measurements. The calculated heat flux was then used to calculate heat transfer coefficients as a dynamic function of cooling time followed by calculation of time average heat transfer coefficient using numerical integration. The approach developed here could be a pragmatic powerful dynamic method to model spatial variation of heat transfer coefficients for industrial chillers and freezers.     

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.     

Dry Peeling of Tomato by Infrared Radiative Heating: Part II. Model Validation and Sensitivity Analysis

In an accompanying study, a predictive mathematical model was developed to simulate heat transfer in a tomato undergoing double sided infrared (IR) heating in a dry-peeling process. The aims of the present study were to validate the developed model using experimental data and to investigate different engineering parameters that most strongly influence the rate and uniformity of IR heating. The mode was verified by comparison of the predicted temperature profiles with experimental data for tomatoes with three dimensions. Uniformity of temperature distribution at tomato surface was quantified by surface-averaged temperatures and a derived temperature uniformity index. The predicted temperatures agreed well with experimental data (r ²?>?0.9). Simulation results illustrated that IR heating induced a dramatic temperature increase on the tomato surface, which extended to 0.6 mm beneath (>90 °C) during a 60-s heating period, whereas interior temperature at the tomato center remained low (<30 °C). Sensitivity analysis suggested that strategies to enhance IR heating rate and uniformity can be implemented through varying emissive power, adjusting the distance between emitters, and presorting tomatoes according to size. The validated model provides an effective design tool for better understanding the complex IR radiation heating in developing the innovative IR dry-peeling process.     

A novel method for analyzing grain facility heat treatment data

Use of elevated temperatures (?50°C) in food processing facilities for management of stored-product insects is a viable alternative to fumigation with methyl bromide. Effectiveness of heat treatment in controlling insects is determined by attainment of uniform temperatures between 50°C and 60°C. A unique surface area method was proposed and developed to assess the effectiveness of heat distribution. The pilot flour mill at Kansas State University, Manhattan, KS, was heated with natural gas (positive pressure) and electric (neutral pressure) heaters in June and August 1999, respectively. The proposed surface area method compared the two different heating systems and successfully quantified the under- and over-heated sections of the treated rooms at any given time during the treatments. A two-parameter nonlinear log-logistic equation was used to describe and predict the general trend in the floor surface area that is under 50°C as a function of treatment time, and percentage of floor surface area as a function of maximum floor temperature. With electric heating, time delays for temperature increase were considerably shorter than with gas heating. However, electric heating resulted in substantial amounts of under-heated floor areas (T < 50°C) throughout the facility at the end of the heat treatment. The methods provided here, especially when coupled with the contour maps of temperature, can be used to design and evaluate heat treatment strategies in grain and food processing facilities.     

Convection and radiation combined surface heat transfer coefficient in baking ovens

The combined surface heat transfer coefficient is a determining parameter of convective baking process time and efficiency, as well as the resulting food product quality. By this study, the combined surface heat transfer coefficient term was determined at the convective oven temperature range of 70–220 °C, with fan (turbo) and without fan (static oven) applications. The methods of “Lumped Capacity” and “Time–Temperature Matching” were used. Both methods utilize the time–temperature data at a fixed position of a definite material, during unsteady state heating up period inside the convective oven. The increase in oven temperature and the fan application in the oven derived higher calculated values of surface heat transfer coefficients. Good agreement was observed between both methods and the literature values. The given methods are applicable to other oven types and heating modes.     

Thermal inactivation of Listeria monocytogenes on ready-to-eat turkey breast meat products during postcook in-package pasteurization with hot water

The inactivation of Listeria monocytogenes during postcook in-package pasteurization was evaluated for fully cooked turkey breast meat products (4-kg packages). The products were surface-inoculated to contain 10(7) CFU of L. monocytogenes per cm2 of product surface. The inoculated products were vacuum-packaged in different thicknesses (0.08 to 0.33 mm) of packaging films and treated with hot water at 96 degrees C. After heat treatment, the products were immediately cooled in an ice water bath at 0 degrees C. The relationship between heating time and product surface temperature was determined for different thicknesses of packaging films. The effectiveness of heat treatment for inactivating the pathogen was affected by product surface roughness. About 50 min of heating time was needed to achieve a thermal kill of 7 log10 CFU/cm2 on products with surface roughness up to 15 mm in depth. The cooling time needed after a heat treatment increased with an increasing endpoint temperature of the heated product and the heat penetration depth reached in the product. The cooling time needed to cool the product from 71 degrees C to 4 degrees C was about 2.5-fold the heating time.     

Effects of levels of fat,surimi from sardine (Sardina pilchardus) and heat processing on thermal gelation of meat batters

The object of the study was to analyse the Theological behaviour occurring during heat-induced gelation (measured by thermal scanning rigidity monitor) of meat batters containing various proportions of fat (48, 106, 147 and 208 g kg^?1) and sardine surimi (0, 100, 200 and 300 g kg^?1 fish protein added with respect to total protein present) and subjected to different heat treatments (A, heating from 14 to 76?C at a rate of 1?C min^?1; B, gradual temperature increase at a rate of 1?C min^?1 up to 40?C, maintenance at this temperature for 30 min, then heating from 40 to 76?C at 1?C min^?1; C, samples kept at 4?C for 20 h and then heated in the same way as process A). The results indicate that differences in levels of fat content significantly affect the gelation patterns of pork batters, gels being stiffer the higher the fat content and the lower the water content. A highly significant correlation (r = 0.78, P < 0.01) has been established between fat content and maximum values of G, which occur at the final experimental temperature of 76?C. In contrast, in general, neither addition of surimi nor variation in the heat treatment applied has any major effect on the gelation process in the system as formulated.