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.     

THERMAL TRANSPORT IN A MULTIPLE JET IMPINGEMENT OVEN

Three‐dimensional numerical simulation of flow field and heat transfer in a jet impingement oven with multiple jets was carried out. Distribution of local heat transfer coefficient on a model cookie in the oven cavity was obtained from the predicted flow and temperature fields, and compared with the experimentally measured values. Effects of air temperature (400, 450K) and air velocity (2.5, 5, 10 m/s), on the surface heat transfer coefficient were studied. Numerical predictions showed good agreement with the experimental results. It was found that heat transfer coefficient was a strong function of air velocity while air temperature had no effect on the heat transfer coefficient. Local heat transfer coefficient values ranged between 11 and 66 W/m²K whereas average heat transfer coefficient values ranged between 24 and 42 W/m²K. Results also showed that because of the presence of the suction fan in the oven, flow field near the model cookie surface was quite different from that for a jet impinging on a flat surface.     

Baking of Sponge Cake: Experimental Characterization and Mathematical Modelling

Sponge cake is a sweet bakery product that begins as a fluid batter and, during baking, transforms into a porous solid, presenting an important volume expansion. The aim of this work was, first of all, to study experimentally the influence of operative conditions (natural and forced convection; oven temperature, from 140 to 180 °C; steam addition) on volume expansion and the heat transfer dynamics during baking of sponge cake. It was observed that an increase in oven temperature, airflow and steam injection produces an increase in volume expansion. Secondly, a mathematical model was developed to simulate heat transfer coupled with volume expansion. Both experimental and simulated temperature profiles verified that the last region to achieve a correct degree of baking is the one near the crust around the axial axis. In consequence, the minimal baking time was defined as the average time at which this region reaches 95–98 °C. The baking time was strongly affected by the effective oven temperature, with a slight influence of the convection mode.     

INDIVIDUAL HEAT TRANSFER MODES IN BAND OVEN BISCUIT BAKING

The individual modes of heat transfer, e.g., conduction, radiation and convection, are considered for the processing of products in conventional band ovens. A specific theoretical model is considered for the baking of biscuits in an indirect fired oven. Values of individual heat transfer constants in the theoretical model and major effects of the individual modes of heat transfer were determined using lab scale heating devices. Extrapolating these results to a band oven baking process, the model indicated a heat transfer profile of about 20% heat transferred by conduction, about 45% by radiation and about 35% by forced convection in the band oven, with about half the heat being absorbed as sensible heat, and about half as latent heat.     

Development of an improved heating system for industrial tunnel baking ovens

This objective of this work was to develop, test and optimise the design of a novel gas-fired radiant burner suitable for incorporation into industrial tunnel ovens. Computational fluid dynamics (CFD) simulations have been used to model the burner and baking chamber environment, and in particular to predict radiation heat fluxes incident on the top surface of the food, both across the width of the baking chamber and along its length. Data from thermocouple sensors attached to a full-scale 40 kW prototype burner have been used to validate the CFD model. Initial results presented here show that CFD model predictions agree with experimental data to within 10%. CFD simulations have indicated that the new burner is capable of delivering irradiation to a travelling conveyor more uniformly than existing radiant burner designs. The effects of oven chamber humidity and surface emissivity on radiation heat transfer have been quantified.     

Modeling of simultaneous heat and mass transfer during convective oven ring cake baking

The convective oven ring cake baking process was investigated experimentally and numerically as a simultaneous heat and mass transfer process. The mathematical model described previously by the authors for cup cake baking was modified to simulate ring cake baking. The heat and mass transfer mechanisms were defined by Fourier’s and Fick’s second laws, respectively. The implicit alternating direction finite difference technique was used for the numerical solution of the representative model. Prior to the utilization of the developed model in predicting the temperature and moisture profiles for ring cake baking, the results of the numerical model were compared with analytical results involving only heat or mass transfer with constant thermo-physical properties. Excellent agreement was observed. The numerical temperature and moisture contents predicted by the model were compared with the experimental profiles. They agreed generally reasonably well with the experimental temperature and moisture profiles.     

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.     

Estimation of confidence interval of pasteurizing values of conduction-heated Sous Vide food in a combination oven

The thermal diffusivity of potato and the apparent heat transfer coefficients (h) in a conduction chiller and in a combination oven at 85, 90 and 95°C and vario-steaming mode were determined experimentally. Based on K-S tests and assumption, the variabilities of all six parameters (thermal diffusivity, h values for heating and cooling, initial food temperature, process medium temperatures and z value) were represented by normal distribution only (model A) or by both normal and gamma distributions (model B). The mean P₇₀ values and their 95% confidence intervals for three oven temperatures, estimated from experimental temperature histories, were 1510 ± 101, 1590 ± 131 and 1590 ± 170 min (for each oven temperature, n = 100) respectively. Good agreement was obtained between these estimations and those predicted using the Monte Carlo procedure and models A and B.     

Modelling of coupled heat and mass transfer during a contact baking process

A mathematical model of coupled heat and mass transfer of a contact baking process is developed. In the current model formulation, a local evaporation of water is described with a reaction-diffusion approach, where a simultaneous diffusion and evaporation of water takes place. The resulting coupled model equations (unsteady state heat transfer, liquid water and water vapour) were solved using the Finite Element Method (COMSOL Multi-physics® version 3.5). During the baking process, local temperatures and overall moisture loss were measured continuously. The model - predicting temperature, liquid water content in the product and water in the vapour phase - was calibrated and partially validated using data obtained during baking of a representative food model (a pancake batter) under controlled conditions on a specially designed experimental rig. The unknown parameters in the model equations were estimated using the standard least squares method by comparing the measured with the predicted temperature profile. Good agreement was achieved between model predictions and the experimental values.     

Computational fluid dynamics (CFD) modeling of an electrical heating oven for bread-baking process

Radiation is the most dominant heat transfer mode in an electrical heating oven. A 3D CFD model for an electric heating baking oven was developed. Three different radiation models namely, discrete transfer radiation model (DTRM), surface to surface (S2S) and discrete ordinates (DO) were employed for the simulation of the electrical baking oven. All models predicted almost similar results, which tallied well with the experimental measurements. During the full heating cycle, the oven set-point temperature was reached after 360 s. Lower temperature zones occurred near oven wall due to lower air flow. Based on preliminary evaluation of applicability, the DO radiation model was selected for bread baking simulation and validated with the experimental measurement of bread temperature. Bread simulation was carried out to study the profiles of temperature and starch gelatinization of crust and crumb of the product. This study indicated the baking process to be complete at 1500 s when the temperature of bread-center reached 100 °C.     

Modeling heat transfer for disinfestation and control of insects (larvae and eggs) in date fruits

A space-and-time dependent model describing the impact of heat transfer on insect survival rate was developed for analyzing the heat disinfestation process of dates. A 2D axial-symmetric domain was defined with five sub-domains: the bulk date pulp, a thin air layer and the date pit, a larva (located in the air layer) and eggs (on the date surface). The model included convective heating and microwave heating (915 MHz). Dielectric properties of the date pulp, egg and larvae were measured. The model predictions showed good agreement with experimental results in the case of the “biological model” (Ephestia kuehniella Zeller) in dates subjected to hot air treatment (70 °C). Sensitive analysis showed that the date size and water content greatly affected the heating time process, during hot air and microwave treatments. Moreover, regarding the eggs’ specific surface, it is advisable to combine convective hot air with microwave heating to induce egg mortality on the date surface.     

Decomposition kinetics of umami component during meat cooking

We developed a kinetic model for the decomposition reaction of inosine monophosphate (IMP), which is a umami component, and obtained kinetic parameters based on the amount of IMP in an isothermal experiment. The amount of remaining IMP decreased with heating time, and its reduction rate was the highest at 40 °C. We assumed that the activity of IMP decomposition enzyme is temperature-dependent above 40 °C, and constant below 40 °C. The predicted results using this kinetic model are in good agreement with the experimental ones. Unsteady-state three-dimensional heat transfer analysis of meat during sous-vide cooking was conducted, and the distribution of remaining IMP was predicted. By the end of sous-vide cooking, the ratio of the amount of IMP in the interior of the meat decreased, whereas at the surface region, it was almost the same as the initial value, because the surface temperature reached the inactivation temperature immediately.     

Baking of muffins: Kinetics of crust color development and optimal baking time

Baking is a decisive stage in the production of bakery products, in general—muffins, in particular—for most of the quality attributes of the final products depend on it. The aim of this work is to model the kinetics of muffin crust color development during baking and to evaluate the feasibility of this kinetic model to predict the baking times. Surface color is represented by the Browning Index, and the effect of baking temperature (from 140 to 220 °C) and process convective characteristics (natural convection, forced convection, and steam-assisted forced convection) are analyzed. Minimal baking times are calculated from experimental core temperature measurements. The modeling of browning kinetics, which incorporates the optimal crust color determined by sensory analysis, allows the estimation of optimal baking times. For all the tested conditions t _op?>?t _min, assuring a product whose inner structure was already totally baked. Finally, minimal, half, and optimal baking times present an exponential dependence with the oven temperature. Besides, there are no significant differences between both forced convection modes.     

Heat transfer modelling in a refrigerated display cabinet: The influence of operating conditions

The product temperature in refrigerated display cabinets is very variable and directly influences the safety and quality of food products. This variability is due to the type of display cabinet (vertical, horizontal), the position of the product inside the cabinet, fluctuations in the ambient temperature in supermarkets (day/night, opening hours, seasons, air conditioning, etc.) and the surrounding conditions of the display cabinet (in front of another refrigerated display cabinet or in front of grocery products, etc.).This study proposes a heat transfer model to predict the load temperature in an open vertical display cabinet. Heat transfer by conduction, convection and radiation was taken into account in the model. Four load positions were considered: top/front, top/rear, bottom/front and bottom/rear where the product temperatures are significantly different. The model was validated by comparing predicted values with experimental ones. Then it was used to predict the influence of various parameters such as the ambient temperature in the supermarket, the radiation temperature of the surrounding walls and the internal/external infiltration rates. Through integration of quality and microbiological predictive models, this approach can be used as a numerical tool for quality and sanitary evaluation of products displayed in display cabinets. The developed model will be further used in the modelling of the cold chain, which comprises several types of refrigeration equipment.     

Prediction of cooking times and weight losses during meat roasting

In this work, a meat oven cooking model is developed, and its ability to predict three main process variables – evaporative loss, dripping loss and cooking time – is evaluated. Heat transfer is modelled by Fourier’s law, while the internal moisture content variation is modelled as a function of water demand, which depends on the water holding capacity of beef. Experimental cooking of semitendinosus muscle samples was carried out in a convective oven to obtain general information about the process and to assess the model accuracy. Simulations were done by means of the finite element method, using three-dimensional irregular geometries as simulation domains. The model predictions were in good agreement with the experimental ones; the average absolute relative error was 3.91% for cooking time prediction, and 7.96% for total weight loss prediction.     

Heat damage of water biscuits from einkorn,durum and bread wheat flours

To limit heat damage and improve the nutritional properties of bakery products, furosine, glucosylisomaltol, hydroxymethylfurfural, furfural, sugars, α-amylase, β-amylase and colour were assessed during the production of water biscuits from three einkorn, three bread and one durum wheat flours. Heat damage indices, colour and a_w development during baking (from 25 to 75 min duration) of water biscuits from one bread wheat were also studied.     

Modeling of baking of thin layer of cake using the lumped reaction engineering approach (L-REA)

Baking is a simultaneous heat and mass transfer process commonly applied in the food industry. It is desirable to have a simple, accurate and robust model in order to assist efficient process design and evaluation of product quality. The reaction engineering approach which was proven to be accurate to model several challenging drying cases, was implemented in this study to model the baking of thin layer of cake. The equilibrium activation energy (ΔE_v,b) was evaluated according to the baking oven temperature and corresponding humidity. It was then combined with the relative activation energy (ΔE_v/ΔE_v,b) to produce a unique relationship. Results of the modeling indicate that the L-REA can describe the profiles of moisture content and temperature very well. While the results are accurate, the model itself remains simple. Another significant application of the REA for modeling the processing of food products has been made by this research.     

Bakery product characteristics as influenced by convection heat flux

Energy consumption and product quality changes are often observed as the ratio of the convection to the conduction modes of heat transfer varies in industrial baking ovens. Air and oven-wall temperature profiles as well as air velocity can affect the convection/radiation heat transfer and hence the quality of the baked products. A programmable pilot-plant oven was used to establish five baking profiles by measuring the total heat flux and the convective component using a special heat flux meter called an h-monitor. The purpose was to keep the total heat flux delivered to the h-monitor constant while varying the convective component from 27% (for the standard profile) to 11%, 22%, 33% and 37% by modifying air characteristics and wall temperatures. Industrial cupcakes were baked using the five established baking profiles and then evaluated in terms of quality parameters. Compared to the standard profile, a 10% reduction in volume expansion and a 30% increase in texture properties were observed for extreme oven conditions; top colour was always darker but more uniform for the conditions with less convection. The moisture content of the middle part of the cake was always higher than that of the top, bottom and sides. Baking industries are interested in using the pilot-scale oven to modify baking profiles for the purposes of quality improvement, product development and energy savings, rather than having to engage in high-cost trial and error practices on the production site.     

Natural convective cooling of cheese: Predictive model and validation of heat exchange simulation

The proposed FEM model describes natural convective air cooling of cheese curd and cheeses with different sizes, chemical composition and initial temperature, including temperature-dependent functions accounting for the variation of specific heat capacity and thermal conductivity of cheeses. Both the calculated convective heat transfer coefficients (from 3.58 to 15.15 W/m² K) and the ratio between Grashof and square Reynolds numbers confirmed that heat exchange was natural convective. The model permitted to accurately predict the transient temperature change into the cheese, as shown by the mean RMSEs values (from 0.34 to 2.29 °C). Higher RMSEs values (up to 3.29 °C) were obtained for cheese curds, because some deviations from the assumptions of the model occurred. These higher RMSEs values for cheese curds quantify the importance of compliance with the model’s assumptions to ensure a best fit between the simulated and experimental data.     

Consolidation of heat transfer coefficients of viscoelastic simulated food solutions in helical exchangers

Heat transfer of viscoelastic liquids in helical exchangers attracted limited work in the past. Most heat transfer equations proposed do not reduce to the Graetz-Leveque equation for the straight tubes. Heat transfer coefficients were obtained for seven copper helical coil heat exchangers with different diameters and diameter ratios. Hot water was used as the heating medium; and dilute polyacrylamide solutions were used to simulate the food solutions. Results showed increased heat transfer coefficients but the magnitudes were lower than those obtained by previous workers. A unified form of the Graetz-Leveque equation obtained was:Nu = 1.75 Gz^1/3[1 + 0.5421Dn^0.45(d/D)^0.54] for water. The heat transfer equation for the 250 ppm solution is represented by, Nu = 1.75 Gz^1/3[1 + 0.3515Dn^0.45(d/D)^0.54]; and for the 500 ppm solution the results can be represented by Nu = 1.75 Gz^1/3[1 + 0.3615Dn^0.45(d/D)^0.54]. Viscoelasticity reduces heat transfer performance.