Specific Heat, Thermal Conductivity, Density, and Thermal Diffusivity of Mexican Tortillas Dough

A split-split unit experiment was conducted and analyzed to determine the specific heat, thermal conductivity, density, and thermal diffusivity of Mexican corn-based tortillas dough (MCTD) as a function of processing time, temperature, and moisture content (wb). The data indicated that thermal conductivity and thermal diffusivity were significantly influenced by processing time, and respective components of temperature and moisture content. Specific heat and density varied with levels of moisture content and temperature. Thermal conductivity, specific heat, density, and thermal diffusivity could each be predicted separately by a polynomial equation.     

Thermal Properties of Wheat Flour

Data from several sources were pooled to develop a regression equation for predicting the heat capacity of either flour or unmilled wheat at specified temperatures and moisture contents. Similarly, relationships were developed for predicting the heat of hydration of flour at specified moisture contents. Finally, this information was used to prepare a table of enthalpy values for flour.     

Estimation of Accumulated Lethality Under Pressure-Assisted Thermal Processing

A study was conducted to develop an integrated process lethality model for pressure-assisted thermal processing (PATP) taking into consideration the lethal contribution of both pressure and heat on spore inactivation. Assuming that the momentary inactivation rate was dependent on the survival ratio and momentary pressure–thermal history, a differential equation was formulated and numerically solved using the Runge–Kutta method. Published data on combined pressure–heat inactivation of Bacillus amyloliquefaciens spores were used to obtain model kinetic parameters that considered both pressure and thermal effects. The model was experimentally validated under several process scenarios using a pilot-scale high-pressure food processor. Using first-order kinetics in the model resulted in the overestimation of log reduction compared to the experimental values. When the n ^th-order kinetics was used, the computed accumulated lethality and the log reduction values were found to be in reasonable agreement with the experimental data. Within the experimental conditions studied, spatial variation in process temperature resulted up to 3.5 log variation in survivors between the top and bottom of the carrier basket. The predicted log reduction of B. amyloliquefaciens spores in deionized water and carrot purée had satisfactory accuracy (1.07–1.12) and regression coefficients (0.83–0.92). The model was also able to predict log reductions obtained during a double-pulse treatment conducted using a pilot-scale high-pressure processor. The developed model can be a useful tool to examine the effect of combined pressure–thermal treatment on bacterial spore lethality and assess PATP microbial safety.     

Evaluation of Thermal Processing of Retortable Pouches Filled with Conduction Heated Foods Considering their Actual Shapes

A computer model was developed to evaluate thermal processing of a retortable pouch containing a conduction heated food. In this model, a transient 2-dimensional heat conduction equation was solved using a modified finite difference technique for a food-filled retortable pouch considering its actual shape. The model was used to determine process time, mass average sterilizing value and nutrient retention to achieve a prefixed level of lethality in the pouched product for a given temperature profile of the heating medium. Temperature distributions and sterilizing values predicted by the developed model were compared with similar values obtained by applying a finite element technique with the aid of a package program (ANSYS). Close agreements were found between the developed model and the finite element technique.     

丝织品的热老化及其寿命预测

为探究湿、热环境对丝织品文物保存状况的影响,在110、130 ℃条件下分别对桑蚕丝试样进行干热老化和湿热老化模拟实验。借助万能材料试验机、色度计、扫描电子显微镜和傅里叶红外光谱仪对老化前后试样的抗拉强度、颜色、微观形貌和结构进行了分析。同时,提出了丝织品的寿命预测方程。结果表明:试样强度随老化时间呈线性下降,颜色变黄;干热老化和湿热老化后,纤维表面出现不同程度的破坏,且湿热老化后,纤维束交织形成的孔洞周围较其他部位溶蚀显著;纤维二级结构中β-折叠构象含量显著降低,与强度变化趋势一致;低温、干燥条件适于丝织品文物的保存。     

Temperature Prediction during Thermal Processing of Surimi Seafood

ABSTRACT: An interactive temperature prediction for surimi seafood was constructed and validated. The prediction model showed good agreement with experimental temperature profiles. The heat transfer coefficient (h) decreased with an increase of temperature difference (δT) between the sample surface and the water bath. The h had an effect on the convective heat transfer, resulting in a slower overall heat transfer at the beginning of heating. Thermal diffusivity (α) of surimi seafood was determined from experimental temperature profiles. The α increased slightly as the temperature difference (δT) between sample and water bath increased. The low a observed at small δT reduced conduction, resulting in a slower overall heat transfer.     

Kinetic Models for Thermal Inactivation of Multiple Pectinesterases in Citrus Juices

First-order kinetic model of a two-component system was applied to investigate heat inactivation of multiple pectinesterases in citrus juices. The conventional log-linear model was applied to determine D- and z-values of thermolabile and thermostable pectinesterases in citrus juices, respectively, for use in the two-component model. A parameter for concentration of enzyme populations in the juice was included in the model. The two-component model predicted results were in better agreement with the observed heat pasteurization curves than those predicted by the log-linear kinetic model. The two-component model is especially useful for optimizing temperature-time conditions for pasteurizing citrus juices.     

A SIMPLIFIED ANALYTICAL MODEL FOR FREEZING TIME CALCULATION IN BRICK-SHAPED FOODS

A simplified analytical model for the freezing time prediction of brick-shaped foodstuffs was developed. It was assumed in the model that the solution to the unsteady, one-dimensional heat conduction equation with constant thermophysical properties was valid during cooling and freezing for each of the three directions of the brick-shaped food. Cooling and freezing times were calculated by superposition of the solutions of the unsteady, one-dimensional heat conduction equation with constant thermophysical properties for each direction. the latent heat effects were incorporated into an effective thermal diffusivity term. the predictions of the model were compared to the available experimental data on freezing of two- and three-dimensional bricks and to the experimental data obtained in this research for the freezing of ground beef and mashed potato bricks. Mean errors varying between -6.3% and 2.3%, and standard deviations from the mean being between 6.6% and 14.0% were obtained for the data sets considered.     

Effects of body surface area estimates on predicted energy requirements and heat stress

The 0.09 x W(0.67) equation (where W = weight in kilograms) used to estimate body surface area (SA) in cattle energy requirements models was developed using measurements of sheep that weighed between 24 and 38 kg. The SA estimates it produced were compared with those of the equation 0.14 x W(0.57), based on Holstein cattle weighing 41 to 617 kg. The estimate of SA produced by the first equation was 23% greater for a 650-kg cow than that obtained by the second equation. The impact of SA estimates on thermal comfort range and development of cold and heat stress effects was calculated, using a thermal balance model, for a 600-kg cow producing 35 kg/d. Predicted metabolic heat production increment at -10 degrees C ambient temperature with 0.5 m/s wind velocity was 0.44 Mcal/d and 1.21 Mcal/d by the first and second equation, respectively. Predicted lower critical temperature (LCT) was -8.7 degrees C by the first equation and -0.6 degrees C by the second equation, an 8 degrees C difference. The LCT difference between SA estimates increased from 4.6 to 9.4 degrees C, with milk production rising from 10 to 45 kg/d. By the first equation, skin nonevaporative heat loss started to decrease at 15 degrees C and became close to nil at and ambient temperature of 35 degrees C, whereas by the second equation, respective values were 10 degrees C and 39 degrees C, a 5 degrees C shift in the estimated temperature at which thermal stress would start developing. The larger SA and skin water loss predicted by the first equation reduced by 50% the respiratory heat loss involved in maintenance of thermal balance at higher temperatures for a 600-kg cow. The second equation seems preferable for Holstein SA estimation since it is based on Holstein cattle data. It provides more adequate estimates of energy requirement in the cold and of heat stress relief needs for Holstein cattle.     

Thermal Conductivity of Gelatinized Starches

The thermal conductivity (k) of gelatinized high-amylose and high-amylopectin starches was determined, using the heated probe method. The granular starches, mixed with distilled water at 1 to 4 kg water/ kg dry solids, were gelatinized at 120°C. Similar k values which varied from 0.434 to 0.548 W/mK in the temperature range 30 to 70°C were obtained for both starch gels. Experimental data were fitted to six structural models, representing various geometries for mixtures of water and dry starch. The perpendicular model, based on heat conduction in series through layers of dry starch and water, yielded best correlations. The predicted thermal conductivity of dry gelatinized starch (k_s) increased linearly with temperature T (K) according to the equation:      

Predicting Internal Temperature of Canned Foods During Thermal Processing using a Linear Recursive Model

A linear recursive model was developed to represent the heat transfer inside a can during sterilization in a retort. The model was analogous to those used in system identification theory, and could also be viewed as a generalization of Ball's classical approach, the basic hypothesis was the linearity of the heat transfer process. A major interest of the model is the possibility to identify heat transfer characteristics of a product on one test experiment, and subsequently use them for prediction; the model also overcomes most difficulties encountered by Ball's method.     

Correction Factor of Deviant Thermal Processes Applied to Packaged Heat Conduction Food

A procedure was developed to estimate the values of correction factor, Cf, for deviant thermal processes applied to canned or other packaged heat conduction food. Sterilizing values at the thermal center of the food were used as a criterion for this estimation. In this procedure empirical heat transfer parameters, f and j values, were used to estimate Cf values. The devleoped procedure is based on the use of a regression equation, which was obtained through the dimensional and statistical analyses of theoretically determined Cf values. According to a series of experiments which were performed by using 307 × 409 and 211 × 300 cans of 8% bentonite suspension, there was reasonably good agreement in Cf values estimated theoretically and determined experimentally.     

单向导汗织物的热舒适性

为探索单向导汗织物在潜汗条件下的热舒适性,建立织物在潜汗条件下传热传湿规律的数学模型。选用2种典型的单向导汗织物与1种普通织物,利用纺织品热湿物性参数测试装置作对比实验,获得了样品的总传热量、微气候和环境干湿球温度以及模拟的人体皮肤温度等数据。采用二元回归拟合出织物热舒适性能参数：干球系数和湿球系数。发现在潜汗条件下,不仅仅是普通织物正反面的热舒适性能相近,就是具有单向导汗功能的织物,其正反面的热舒适性能差别也不大。     

熏煮火腿中志贺氏菌的热失活特性

为了研究志贺氏菌不同菌株在熏煮火腿中热失活规律,以55、63℃对接种108 CFU/g志贺氏菌的熏煮火腿进行热处理,测定处理后样品中志贺氏菌的残存菌体浓度。应用Logistic模型和3rd degree polynomial模型拟合在两种温度下志贺氏菌的动力学模型,用D值(decimal reduction time)表示志贺氏菌在熏煮火腿中的耐热情况。结果表明:11株志贺氏菌的热失活曲线运用3rd degree polynomial拟合较好,相关系数均在0.98以上。志贺氏菌不同菌株的D值不同,55℃条件下志贺氏菌在熏煮火腿中的D值为4.46~10.66 min,63℃条件下D值为0.48~3.22 min。不同菌株之间抗热性存在差异,其中菌株S-5的耐热性最强,S-6最弱。     

Thermal Conductivity of Potato between 50 and 100°C

Thermal conductivity (TC) of potato was calculated from heat penetration into cylindrical samples between 50 and 100°C. TC ranged from 0.545 to 0.957 W/m°C for potatoes (cv Kennebec) specific gravity 1070 kg/m³ and moisture 80%. Experimental values agreed with reported values. TC, k, varied with temperature in a quadratic polynomial form, and an empirical relationship was estimated. A model based on the concept of thermal resistances in parallel was compared with experimental results. These were used in a heat transfer model that simulated potato frying. Measured and simulated time-temperature curves showed good agreement.     

Effect of pH on the heat resistance of spores comparison of two models

All published models describing the effect of pH on the heat resistance of spores can be regarded either as a linear first degree equation or a linear second degree equation. This work aimed to compare both models from three sets of published data for, Clostridium sporogenes and Bacillus stearothermophilus, respectively. The relative quality of fit of each model with respect to the other depends on the species, the strain and the heating temperature. Parameter estimation was more reliable for the second degree model than for of the simple first degree equation. However, in the case of acidic foodstuffs, predictions obtained from the second degree model are more sensitive toward errors of parameter values. The second degree model is better from the point of view of safety at most frequent ranges of pH of foods. Moreover, for Clostridium botulinum the goodness of fit of this model is clearly higher than that of the first degree equation. If this observation is confirmed by further work, the second degree model in application of standard calculations of heat processes of foods would be preferred.     

Effective Thermal Conductivity of Rice Flour and Whole and Skim Milk Powder

ABSTRACT: The effective thermal conductivity for 3 kinds of powdered food (rice flour, whole milk powder, and skim milk powder) was measured at selected moisture contents, temperatures, and bulk densities. The transient heat probe method using twin probes was used for the determination of the effective thermal conductivity. The effects of moisture content, temperature, and bulk density on the effective thermal conductivity of samples were investigated. The observed values were compared with the values calculated from the thermal conductivity of air and powder, using the 3 kinds of structural model for thermal conductivity, that is, the series, parallel, and random heat transfer models. An improved random heat transfer model, in which a correction term was incorporated in the random heat transfer model, with the term written as a function of moisture content and temperature, was developed.     

大米干热糊化过程的数学模型研究

大米在干热糊化过程中发生剧烈的传热,大米内部存在温度分布并且发生迅速变化,这些变化影响大米的糊化效果。数学模型法可以解决无法直接测量大米内部的温度梯度困难。本研究根据糊化过程传热特征,构建了大米温度梯度变化的偏微分方程,解析并验证了方程的准确性。利用数学模型分析了大米在干热糊化过程温度梯度的变化规律,得出了具有普遍意义的干热糊化过程大米平均温度的标准温度曲线,指导干热糊化过程的控制及设备的设计和放大。     

Two Dimensional Heat Conduction in Food Undergoing Freezing: Development of Computerized Model

A model was developed to simulate two dimensional heat conduction in anisotropic food which was undergoing a freezing process. This model utilized a well verified transient state heat conduction equation. We assumed convective and radiative heat exchanges as well as moisture loss on the boundary surface of food to develop our model. Empirical formulae, whose applicability was verified, were used to estimate the temperature dependent thermophysical property values of food in the model. Since the model is nonlinear, a computer program package was prepared to solve it numerically by applying a finite element method. Sample application of computerized procedure was presented for simulating rectangular or finitely cylindrical food.     

Thermal and chemical inactivation of lactococcal bacteriophages

Ten lactococcal bacteriophages in M17 broth were treated by heat (72 °C, 15 min and 90 °C, 5 min), ethanol and isopropanol at concentrations of 10%, 50%, 75% and 100% (v/v). The heat treatments were not sufficient to inactivate all the bacteriophages studied whereas ethanol, which was also more efficient biocide than isopropanol, at a concentration of 75% could inactivate all the phages in 1 min. The 50% and 75% of ethanol were more effective than 100%, which was also observed for isopropanol. Survival curves’ deviations from linearity as sigmoidal shapes could be described via an empirical model with four parameters. The parameters of the model could be reduced from four to two with a slight loss of goodness-of-fit. The reduced model also provided good fit to describe the isopropanol inactivation kinetics of the phages at concentrations of 50% and 75%. This demonstration may also be a useful tool for further inactivation studies in which ethanol or other biocides are used with sufficient survival data points at selected time intervals. Although more studies should be carried out with different phages, this study confirmed the lethal effect of ethanol, commonly used to disinfect utensils and laboratory equipment, on lactoccocal bacteriophages.