Food Engineering
Saeede Hamidi; Nafiseh Zamindar; Nayyere Gholipour Shahraki
Abstract
IntroductionThermal processing is an important method of canned food production (Farid & Abdul Ghani, 2004). Estimation of the heat transfer rates is essential to obtain optimum processing conditions and to improve product quality. In addition, a better understanding of the mechanism of the heating ...
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IntroductionThermal processing is an important method of canned food production (Farid & Abdul Ghani, 2004). Estimation of the heat transfer rates is essential to obtain optimum processing conditions and to improve product quality. In addition, a better understanding of the mechanism of the heating process will lead to an improved performance in the process and to some energy savings (Abdul Ghani et al., 1999). Computational fluid dynamics (CFD) is an efficient way to study flow behavior and temperature distribution of thermal processing in the food technology (Ghani et al., 2003). As the semi-rigid aluminum packaging market recently has been introduced, there is limited information about the temperature distribution during the heating process of such containers. In this paper the temperature distribution was predicted and location of cold zone was determined. The effect of headspace (air and water vapor) in heat transfer mechanism was investigated. Materials and MethodsPhysical propertiesMalt extract properties such as density, specific heat, thermal conductivity and viscosity values are necessary for the equations solution. Viscosity and density of the sample was measured as a function of temperature (Vatankhah et al., 2015). Specific heat and thermal conductivity of sample were estimated using the mass fraction of its constituents. For simulation, the experimental results were applied by piecewise-linear method in the material part of the software to describe viscosity, thermal conductivity and specific heat. Experimental methodologyFor the experimental, a thermocouple probe was located at point (0, 0, -2.76) in a semi rigid aluminum based packaging to measure the temperature distribution inside the container. Then the package was filled with malt extract (°Brix ~ 60) and then the package was sealed at 280 °C using Alcan machine. Another thermocouple was placed near the containers, in the water cascading Barriquand steriflow retort. The thermocouples were attached to Ellab data logger by PT100 cables. The data logger was connected to a personal computer and E-val 2.1 software was used to export time temperature profile of each thermocouple in 1 min intervals. Geometry and meshingGambit 2.3.30 was used to develop geometry and set of grid (0.2 cm, and 0.1 cm mesh size) was performed. Then software of fluent 6.3.26 with 3-D, double precision, pressure-based solver, implicit formulation, unsteady time, laminar flow was applied to solve the system of the governing equations (Vatankhah et al., 2015). Boundary conditions and initial valuesUnsteady temperature function was imposed to all faces of the geometry in 1 min time intervals. No-slip boundary condition was supposed for velocity components relative to boundaries. The boundary conditions used at top surface, bottom surface and side walls included: T = Tw, Vx = 0, Vy = 0 and Vz = 0. The initial temperature was assumed as the first temperature which was measured by the thermocouple at the starting time of processing. Solution methodologyFluent software was used to solve the Navier-Stokes and energy equations simultaneously. A preset convergence limit of 10−3 for continuity and momentum equations and 10−8 for the energy equation were used, in order to achieve an appropriate convergence. The under-relaxation factors were adjusted smaller than 1 to obtain a good convergence of the numerical solution. SIMPLEC algorithm was used for pressure-velocity coupling. Results and DiscussionThere was no significant difference between predicted and experimental temperatures for point (0, 0, -2.76) in models with and without head space using t-test (p<0.01). Temperature contours of predicted models (with headspace) were similar to model without headspace at the different stages of the process. Simulation result showed slowest heating zone located in (0.02 <X< 0.8, -1 <Y< 0.3 and -3.27<Z< 3.27) for model of malt extract with headspace and in (-3.58 < X< 3.76, -3.44 <Y< 0.48 and -3.46 <Z< -3.05) for model of malt extract without headspace. ConclusionThe heating process of malt extract in semi rigid aluminum container during thermal processing was simulated successfully using CFD. The CFD based model showed that the position of SHZ was located in the third end of the container.
Fatemeh Heidari Dalfard; Masoud Taghizadeh; Seyed Mohammad Ali Razavi
Abstract
Introduction: Malt extract is one of the malt products obtainedfrom concentrations of water soluble extract of grains such as barley and is a proper alternative to white sugar. It has a high diastasis properties andcontainshigh amount of different vitamins specially the group of vitamin B as well as ...
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Introduction: Malt extract is one of the malt products obtainedfrom concentrations of water soluble extract of grains such as barley and is a proper alternative to white sugar. It has a high diastasis properties andcontainshigh amount of different vitamins specially the group of vitamin B as well as high amount of fermentable sugars. Therefore, malt extract has high nutritional value and is recommended by nutritionist for children and people who are having growth problems. In addition, it has a high potential to be used as sweetening agent in different food products such as cookies, biscuits, ice cream, chocolates etc. Knowledge on the thermo-physical properties of malt extract such as density, specific heat, thermal conductivity as well as thermal diffusivity is highly necessary for the designing of processing equipments, formulation of derived products, heating and chilling processes, and other unit operations such as pasteurization, concentration, dehydration as well as final safety and quality of formulated products. Since, no research work has been reported on thermo-physical properties of malt extract, the aim of this study was to determine different thermal properties of malt extract as well as investigation of the effect of temperature and soluble solid contents (SSC) of the studied properties.Materials and methods: Barley malt was purchased from local market. 150 gr of malt was first grounded and added to 600ml water at 46°C and stored for 30 minutes. Then, its temperature was increased using a heater to reach 70°C. 300 ml water was then added to the mixture and stored at 70°C for 60 minutes. Then, the mixture was cooled at room temperature and filtered to gain a sweet solution. The obtained solution was concentrated to 60, 70 and 80 degree of Brix for further experiments.Specific heat and thermal conductivity of samples were determined using a differential scanning calorimeter (DSC). DSC is a powerful tool which is able to spontaneously measure different thermal propertiesof samples such as specific heat, thermal conductivity, glass transition temperature, melting point, crystallization point etc. as a function of time and temperature at the desired temperature levels. Density of samples was also measured using a 50ccvolumetric pycnometer. 25 grams of samples were first solved in hot water and then were placed in an isothermal bath to measure the density. Thermal diffusivity of samples was determined using the following equation:∝=k/〖pC〗_p Results and discussion: The obtained results on specific heat measurement showed that decreasing SSC from 80 to 60% and increasing the temperature from 25 to 90°C would increase cp from 2.074 to 3.063 kJ/kg°C in a linear manner. Following equations were obtained to predict specific heat as a function of temperature:C_p=2.756+0.004T R^2=0.893 ، X_s=60C_p =2.245 +0.005T 〖 R〗^2=0.868، X_s=70C_p=2.066 +0.001T R^2=0.75، X_s =80Thermal conductivity measurements were also showed that decreasing SSC from 80 to 60% and increasing temperature from 25 to 90°C would increase the K values from 0.1196 to 0.347 W/m°C in a linear manner. Increasing temperature would increase molecular movements and therefore it elevates the heat transfer velocity and K increases. Following equations were obtained to predict thermal conductivity as a function of temperature:K=0.152 +0.003T R^2=0.761، X_s=60K =0.097+0.002T R^2=0.851، X_s=70K=0.114+0.001T R^2=0.706،X_s =80In order to develop a model to predict thermal conductivity of malt extract based on its soluble solid content and temperature, multiple regressions was used. The obtained model was a two-parameter linear model with R2 of 0.858. The results showed that 1% increase in soluble solid content percentage would cause an increase of 6% in K, while 1% increase in temperature would cause only 49% increase in thermal conductivity value.Density measurements were also showed that increasing density from 60 to 80% and temperature from 25 to 90°C would increase density of malt extract. Following equations were obtained to predict density as a function of temperature:P=1328.699-0.402T R^2=0.999، X_s=60 P=1375.451-0.290T R^2=0.999، X_s=70 P=1426.201-0.286T R^2=0.998،X_s =80Thermal diffusivity of samples was also determined using indirect method for soluble solid content of 60 to 80 and in the temperature range of 25 to 90°C. It was found that thermal diffusivity would increase linearly by decreasing soluble solid content and increasing temperature. Following equations were obtained to predict thermal diffusivity as a function of temperature:∝=5.176〖×10〗^(-8)+0.049×〖10〗^(-8) T R^2=0.783، X_s=60∝=2.993×〖10〗^(-8)+0.050×〖10〗^(-8) T R^2=0.929، X_s=70∝=3.125×〖10〗^(-8)+0.036×〖10〗^(-8) T R^2=0.94،X_s =80The results of the present work were in agreement with the results reported by other researchers confirming that both SSC and temperature have significant effect on thermo-physical properties of malt extract.
