Prediction of Thermal Conductivity of Vegetable Foods by the Effective Medium Theory

The Effective Medium Theory (EMT), known in the field of physics to model conductive properties of heterogeneous chaotic media, has been applied to predict the thermal conductivity of sugar-based fruits and of potatoes as a function of the water content. With the thermal conductivity of the constituents, the composition and porosity as data, the equation derived from the EMT for continuous media was applied to apples, pears, and potatoes. The results when compared with experimental values obtained by the probe method and from the literature showed good agreement, with differences not larger than 8%, though the model appeared to be over predictive, particularly for higher water contents.     

Thermal Conductivity Measurement and Prediction of Particulate Foods

There is reason to question some of the published thermal conductivity data for particulate foods. It is argued in this article, based on thermal conductivity bounds analysis, that the thermal conductivity of particulate foods should range between 0.03 W m^–1 K^–1 and 0.30 W m^–1 K^–1 (provided the porosity is greater than 25%), and that any data outside this range should be rejected. This argument was supported by data for five foods measured as part of this study (cocoa powder, corn starch, whole milk powder, sucrose, and wheat flour), and the majority of published data for particulate foods do indeed fall within this range. The geometric model was found to provide the most accurate thermal conductivity predictions of the models considered, and is recommended for use in first approximation predictions.     

Measurement of the Thermal Conductivity of Foods at High Pressure

The line heat source probe method, widely used for determining thermal conductivity of food materials, was applied in a pilot scale high-pressure unit and the possibility of extending this technique to pressures up to 400 MPa was investigated. Commercially canned tomato paste and apple pulp were used as test products. Probes were calibrated to 1.5% agar gel. A probe specific pressure- and temperature-dependant calibration factor was applied. The accuracy at high-pressure was similar to published values. Given the simplicity of the method, this approach seems very promising for determining thermal conductivity of foods over a range of pressures and temperatures.     

非稳态热线法测定大麦的导热系数

将一根细长的线性热源埋于自然堆积的大麦中 ,通过一套由稳压电源、电位差计、电流表等二次仪表组成的测试装置 ,测定热线的升温速率 ,然后根据非稳态法热线测定的理论模型 ,计算出不同含水率大麦的导热系数。结果显示 ,在常温下 ,含水率在 10 %～ 2 6 .5 3%间的大麦的导热系数在 0 .1344～ 0 .192 1W/m·K之间 ,与其含水率呈现一定的相关性     

Reference Materials for Calibrating Probes Used for Measuring Thermal Conductivity of Frozen Foods

Reference materials which contained either 8% or 10% bentonite pastes and very fine pure copper powder (300 mesh, 30–.55%), provided thermal conductivities ranging from 0.9 to 1.9 W/mK. These materials were used to calibrate probes for measuring thermal conductivity of frozen food materials. Thermal conductivity values measured by the steady-state (parallel plate) method, the unsteady-state heat transfer (heating curve) method, and the probe method agreed within 5%. Although experimental density and heat capacity data of copper-filled bentonite pastes agreed with a mass average value of component properties, measured thermal conductivity values were much lower than the volume fraction average values.     

层析-高效液相色谱法测定保健食品中葛根素的含量

目的 建立测定保健食品中葛根素含量的层析反相高效液相色谱法.方法 D101大孔树脂层析柱(10cm×15cm×1.0 cm)层析,采用ODS C18(250 mm×4.6 mm,5 μm)色谱柱,甲醇 36%乙酸 水=25 3 72作为流动相,检测波长247 nm.结果 葛根素在5.0～50.0μg/ml内呈线性关系,r=O.999 8,方法的回收率为81.6%-100.2%,试样测定RSD为0.84%～1.08%.结论 该方法简便、准确、灵敏,回收率高,分离效果好.     

离子液体作为新型光度增敏剂测定食品中微量铝(Ⅲ)

建立以离子液体溴化十四烷基咪唑盐(C14 minBr)作为新型光度增敏剂测定食品中铝(Ⅲ)含量的方法。在pH值为4.8的Britton-Robinson缓冲液中,离子液体C14 minBr对Al3+-铬天青体系具有增敏作用,形成Al(Ⅲ)-CAS-C14minBr三元配合物,并在621nm波长处有最大吸收,铝的质量浓度在0.001～0.150μg/mL范围内符合朗伯比尔定律,表观摩尔吸光系数ε621=1.942×105L/(mol·cm)。将该法用于粉丝与粉皮中微量铝的测定,结果与火焰原子吸收光度法测定结果一致,回收率在97.8%～101.8%之间。     

The Finite Element Method in Thermal Processing of Foods

A variational finite element approach using triangular simplex and 2nd order quadrilateral elements was employed to analyze several problem areas that are of practical importance in the thermal processing of conduction heating products: (1) A method was developed to calculate conversion factors for thermal process design applicable to glass jars filled with conduction heating products. (2) The overshooting of temperatures after steam-off was studied, indicating that not including the contribution of overshooting to sterilization values in process design, can lead to gross overprocessing. (3) A method of correcting sterilization values to account for harmonious fluctuations in retort temperature was developed. (4) The air cooling of cans by natural convection was analyzed.     

Thermal Conductivity of Fresh and Dried Squid Meat by Line Source Thermal Conductivity Probe

Thermal conductivities (TC) of fresh and dried squid mantle meat determined using a line source probe method varied from 0.52 to 0.04 w/m°C at 30°C. TC decreased with decreasing moisture. TC data correlated with moisture content by an exponential form equation.     

应用流式细胞技术快速检测液态商品中的细菌总数

建立应用流式细胞技术(flowcytometry,FCM)快速检测液态商品牛奶、果汁中的细菌总数的方法。采用膜过滤、离心技术对果汁样品进行前处理,去除影响FCM检测的基质颗粒,使样品达到FCM可检测状态,检测限达到101CFU/mL数量级。对液态商品牛奶进行重复性和验证实验,检测结果经Q检验法和方差分析,在一定范围浓度下,FCM检测液态商品中的细菌总数与平板法成线性相关。FCM是一种比平板法检测细菌总数更加快速、准确的方法。     

Effective Thermal Conductivity of Beans via a Steady-State Method

Abstract

Steady-state concentric cylinder equipment was used to determine the effective thermal conductivity of beans (Phaseolus vulgaris). The measuring cell had no heated end guards and its length to diameter ratio was 10.5. Glass beads were employed to assess the accuracy and repeatability of the experimental system under heat transfer conditions. The results agree well with those reported in the literature so that the system can be considered reliable. Corn was used to verify the system's accuracy under heat and mass transfer conditions. Again the results were satisfactory. Moisture migration was observed and measured during the tests with beans, but this behavior does not compromise thermal conductivity values if both thermal and mass transfer steady-states are correctly interpreted. The effective thermal conductivity increases linearly with increasing grain moisture content. Statistical regression leads to good estimates of the fitted parameters.     

反演法求小麦面团在冻结温度范围内的热导率

为确定小麦面团在冻结温度范围内的热导率,采用差式量热扫描法测量了3个不同水分含量面团的比热,然后利用COMSOL软件反演求出面团在-30~30℃范围内的热导率。结果表明,面团热导率随水分含量增大(38%~44%)而逐渐升高;且随着温度的降低,面团的热导率逐渐降低,但是当面团的温度降至冰点后,面团的热导率急剧上升,然后又呈缓慢上升趋势。通过对面团热导率与水分含量关系的进一步分析,建立了速冻温度范围内面团热导率随温度和水分含量变化的数学模型。     

Maximum Slope Method for Evaluating Thermal Conductivity Probe Data

A published method determines the thermal conductivity, k, of material without probe calibration using the maximum local slope value of a temperature response curve monitored by a line heat source probe. We examined the applicability and validity of this method A mathematical heat conduction model was used for theoretical examination and a thermistor based probe for experimental examination with a computer based data acquisition and reduction system. Glycerol (99.5% pure) and agar gel (0.5%) were test materials. The k values estimated by the maximum slope method were comparably accurate and less statistically variable compared to those determined by two widely used data reduction methods, the time correction and maximum r-square methods.     

Modelling Thermal Conductivity in Heterogeneous Media with the Finite Element Method

Three-dimensional finite element simulations were developed to predict the effective thermal conductivity of theoretical composite materials having complex structures. The models simulated a steady-state thermal conductivity measurement device performing measurements on theoretical materials with varying structures. The structure of a composite was considered to be composed of some simplified basic models. When the geometry, orientation type and number of dispersion are specified, the computer randomly generated the position and orientation for each dispersion and created the geometrical model and finite element mesh. The effective thermal conductivity of the theoretical composite was calculated using this method and compared to the values obtained by simple effective thermal conductivity models methods. The influence of some factors such as the volume fraction and the ratio of the thermal conductivities of the heterogeneities and the surrounding material on the effective thermal conductivity is discussed.     

银杏淀粉的分离和纯化

用重结晶法可以得到纯度较高的银杏直链淀粉和支链淀粉。凝胶过滤色谱表明：银杏直链淀粉的分子量比玉米直链淀粉的小,而支链淀粉的分子量则具有较宽的分布。银杏直、支链淀粉的碘亲和力分别为19．19％和0．13％,蓝值分别是0．85和0．12,γmax为626nm和564nm;银杏淀粉中直链淀粉含量为33％。     

热线法测量食品热导率的实验研究

在食品材料的热导率的测量中,热线法是一种比较理想的方法。本文在介绍热线法的同时,运用自行研制的热线法导热系数测量仪对一些食品材料的热导率进行了测量。并将实验得到的数据同一些经验公式进行了对比,重点研究了食品中的水分对食品材料的热导率的影响,并得到了食品材料的热导率和其含水率关系的经验公式。     

基于COMSOL Multiphysics对含顶隙液态罐头食品热杀菌的模拟研究

基于COMSOL Multiphysics软件,对纯水及1.0%羧甲基纤维素钠溶液的热杀菌过程分别建立了包含及不包含顶隙的2种二维轴对称模型,采用无线温度传感器对比2种模型在罐内相同位置的温度变化曲线以验证模型的准确性。2种模型对温度的模拟值与实测值之间的误差在1.92%~4.89%范围内,且包含顶隙模型的误差更小,表明含顶隙的模型能更好地表征液态罐头食品在热杀菌过程中的物理场变化。以1.0%羧甲基纤维素钠溶液为研究对象,分析2种模型模拟生成的速度场、温度场和致死率场,结果表明,为准确地预测杀菌过程中最小杀菌值出现的位置并进行杀菌工艺的优化,顶隙的影响不可忽略。在含顶隙模型的基础上,对不同顶隙高度及保温温度下1.0%羧甲基纤维素钠罐头内的相对压力进行分析发现,温度越高,顶隙高度越小,罐内相对压力越大。生产厂家可通过模拟仿真的方法得到不同罐头在杀菌工艺中各阶段的压力,以便对杀菌釜压及罐头最大耐压值进行控制,避免出现胀罐或瘪罐等质量问题。     

Prediction of Thermal Conductivity of Fruit Juices by a Thermal Resistance Model

A model for prediction of thermal conductivity of fruit juices was developed. The model was based on the concept of thermal resistances in parallel and compared with other models and experimental values. Effect of temperature and solid concentration on thermal conductivity was discussed. Extension of the model to predict thermal conductivity of fruit juices below the freezing temperature was briefly discussed. The model gave error of less than 10% from the experimental values for juices for which such values were available.     

Detection of 5-hydroxymethyl-2-furfural Levels in Selected Chinese Foods by Ultra-High-Performance Liquid Chromatograph Analytical Method

In this study, an ultra-high-performance liquid chromatography method was developed to detection the levels of 5-hydroxymethyl-2-furfural (HMF) in 227 selected food products obtained from the Chinese markets. The performance of the analysis method was evaluated by some quality parameters such as limit of detection (LOD), limit of quantification (LOQ), linearity, recovery, and run-to-run (n?=?6) and day-to-day (n?=?18) precisions. The LOD and the LOQ of the method in different food matrices ranged from 0.15 to 0.50 and from 0.35 to 1.20 mg kg^?1, respectively. The results from this study showed that HMF was mostly detected in all samples selected. HMF contents in different samples varied greatly according to the raw materials and processing conditions. The highest level of HMF was found in preserved fruits and ground coffee, with average levels of 409.6 and 409.9 mg kg^?1, respectively. Preliminary estimates of HMF exposure from foods in the Chinese population was estimated to be 0.12 mg kg^?1 body weight day^?1, which is relatively low compared with the result reported by JECFA and European Food Safety Authority.     

Measurement of Thermal Conductivity of Edible Oils Using Transient Hot Wire Method

Thermal conductivities of three different edible oils, namely sunflower oil, corn oil and olive oil, were measured at temperatures 25, 40, 60, and 80°C. The measurements were carried out using a hot wire probe method. The calibration of the probe was performed using 0.3% agar gel with water and glycerin. In general, thermal conductivities of oils used in this study are found to be decreasing with temperature. The values of thermal conductivity measured are quite near to each other, the highest and the lowest being respectively 0.168 W/m K for sunflower oil at 25°C and 0.152 W/m K for corn oil at 80°C.