Two Dimensional Heat Conduction in Food Undergoing Freezing: Predicting Freezing Time of Rectangular or Finitely Cylindrical Food

Since there was no procedure available for estimating freezing time of rectangular or finitely cylindrical food through manual calculations, the present study was initiated to develop such a procedure. For this development, our computerized model was used together with statistical techniques. A design for screening tests was used to identify eight dimensionless groups, which influences significantly the freezing rate, out of 22 independent groups. Among the eight groups, only two were related to the thermophysical properties of food. Through the joint application of a central composite design of experiments and of the computerized model, algebraic formulae containing the eight significant groups were derived for the freezing time estimation. Their validity was verified through experimental freezing processes.     

Parametric Analysis for Predicting Freezing Time of Infinitely Slab-Shaped Food

A computer program was developed in order to predict freezing times of homogeneous food which may be approximated by infinite slabs. For this solution we assume temperature dependent thermophysical properties, together with symmetric or nonsymmetric convective and radiative heat exchange imposed at the boundaries. Through application of a statistical analysis, all dimensionless groups that have a significant influence in the rate of heat transfer were selected and used to develop a regression equation for the prediction of freezing times, by applying a central composite experimental design. This equation is applicable to the wide range of parametric values encountered during commercial operations. The reliability of the computer program and regression equation developed was verified experimentally.     

Effect of Biot Number and Freezing Rate on Accuracy of Some Food Freezing Time Prediction Methods

Existing food freezing time prediction methods yield widely differing results at high Biot numbers (internal resistance controlling), a situation that could not be previously resolved due to lack of data. Freezing tests were carried out on slabs of Tylose gel in a plate freezer to obtain more data, which alone and in combination with previous data, were used to test the methods of Cleland and Earle, Hung and Thompson, Pham, and finite differences. Good agreement was obtained with Pham's methods and with finite differences. Biot number, freezing rate and supercooling had negligible effect on the accuracy of these methods. The accuracy of the finite difference method has been significantly improved by our new thermal property data for Tylose gel.     

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.     

平板状食品冻结时间的数值预测

采用有限差分法获得平板状食品冻结时间的数值解。对凝固区（两相区）的参数进行处理,使食品在整个冻结过程可以选用统一的差分格式;通过凝固区释放热量的累积效果,判断凝固区界面的移动;对常见的平板状食品的冻结过程进行计算机模拟来预测冻结时间,并通过实验来验证可靠度。结果表明,这种计算方法可以应用在实际生产中。     

食品微生物检验中两种均质方法的比较

目的比较食品微生物检验中两种均质方法对检验结果的影响。方法采用旋刀式均质方法和拍击式均质方法均质,测定细菌、霉菌回收率,并对经两种均质方法检验的食品微生物项目结果进行分析。结果细菌回收率旋刀式均质方法略高于拍击式均质方法,霉菌回收率拍击式均质方法略高于旋刀式均质方法。结论食品微生物检验过程要根据样品的不同形态特点和检验项目选择合适的均质方法。     

两种前处理方法在钠含量检测中的对比研究

针对食品中钠含量检测的国标前处理方法(湿法消解)操作繁琐、效率低下的问题,开展干灰化法和微波消解两种前处理方法的对比研究,以加标回收率评价方法的可行性,还考察了原子发射法检测钠含量过程中调整单色器的标液浓度、标准曲线浓度范围、样品稀释倍数等方面的影响。结果表明,用于调整单色器的钠标准溶液选择1 mg/L优于2 mg/L;选择浓度范围(0~2 mg/L)进行标准曲线的二次曲线拟合结果较好;合适的稀释倍数就是使得稀释后样液浓度接近用于调整单色器的中间浓度的稀释倍数;在酱油、火腿肠、乳饮等三种不同钠含量水平的食品中,微波消解前处理的测定结果均比干灰化法高,就乳饮而言,微波消解的回收率96.88%,而干灰化法的回收率87.18%,两种前处理方法的估算结果无显著差异。因此,干灰化法、微波消解可以作为食品中钠含量检测的前处理方法,是对国标方法的补充。     

两种分析塑料食品包装材料中抗氧化剂的方法

采用气相色谱/氢火焰离子化法(GC/FID)和高效液相色谱/二极管阵列法(HPLC/DAD)分别对塑料食品包装材料中BHA、BHT、TBHQ三种酚类抗氧化剂进行定性定量分析。分别用正己烷及乙腈超声提取塑料食品包装材料中抗氧化剂,并对超声提取时间、次数及温度进行优化。结果表明:在本试验条件下,抗氧化剂添加量为6~30mg/kg范围内,气相色谱法测定BHA、BHT、TBHQ三种成分的平均添加回收率分别为95.8%~97.2%、93.1%~96.3%、91.5%~95.8%;相对标准偏差分别为1.55%~2.38%、1.95%~2.60%、2.35%~3.14%;检出限分别为0.40、0.32、0.72mg/kg。高效液相色谱法测定BHA、BHT、TBHQ三种成分的平均添加回收率为94.5%~96.2%、90.5%~93.7%、88.2%~91.6%;相对标准偏差分别为1.87%~2.58%、2.05%~2.72%、2.62%~3.38%;检出限分别为0.30、1.25、0.90mg/kg。两种方法皆准确、灵敏、重现性好,适用于塑料食品包装材料中抗氧化剂的检测。     

Water-Oil Absorption Index (WOAI): A Simple Method for Predicting the Emulsifying Capacity of Food Proteins

An index for describing the relative hydrophilic-lipophilic character of proteins was proposed based on the measurement of the “spontanoeous water and oil uptake”. The water-oil absorption index (WOAI) showed a defined correlation with emulsifying capacity (EC) for most proteins making it possible to predict EC in a simple way. Maximum emulsion capacity was achieved when the WOAI was nearly two, that is when the protein absorbed twofold more water than oil. Proteins with WAOI greater than two had increasingly hydrophilic characteristics and concomitantly increasingly low EC. Proteins with WOAI lower than two showed increasingly lipophilic characteristics and concomitantly increasing low emulsifying capacity. The finding of this study supports the concept that emulsifying capacity of proteins depends on the suitable balance between the hydrophilic and lipophilic characteristics, rather than merely high values for each one.     

实验室用简易流化床

冲剂的制备离不开沸腾干燥设备,但一台沸腾制粒机多则几十万元,少则几万元,作为一般的教学和科研单位,难以支付如此高的费用。再者,在教学和科研活动过程中,实验操作特点是制备的样品品种多,而每种样品所用的物料量却很少,因此,大型的工厂用设备也不适合教学特别是科研工作的需要。本研究采用实验室所拥有的常规器材,制造出小型的沸腾干燥设备,适合配方研究、工艺参数探讨等研究使用。1　材料与方法11　基本器材层析柱、电阻丝、空气压缩机、压力调节器、塑料切割器、旋风分离器、硬橡胶管。12　工艺流程图工艺流程见图1。操作过程如下:打开…     

A Converging-Front Model for the Asymmetric Freezing of Slab-Shaped Food

In food freezing situations where heat flows at different rates from the two surfaces of a slab, a Plank-type converging-front model enabled the position of the thermal center and hence the freezing time, to be calculated. The thermal center was defined as the point where the freezing fronts meet, i.e. the last point to cross the “mean freezing temperature”. Finite-difference calculations showed that this model successfully accounted for the effect of asymmetry in heat transfer coefficient or coolant temperature and enabled previously developed simplified prediction methods to be applied to asymmetric situations.     

A zTurbulent Conjugate Heat-transfer Model for Freezing of Food Products

ABSTRACT: A 3-dimensional conjugate heat-transfer model for the analysis of freezing food products has been developed. The food-freezing process is a multi-medium, multi-phase, and transient heat-exchange phenomenon in connection with the cooling flow around the food items. The developed numerical model couples the energy equation with the Navier-Stokes equations outside the food items to simulate the velocity distribution around the food items and the heat flux across the food surfaces. The conjugate heat-transfer methodology and enthalpy method was used to solve the energy equation across the fluid-solid interface into the food item. The heat convection in the fluid and conduction in the foods are implicitly coupled to predict the heat-transfer rate and the enthalpy change during its freezing process. The conjugate heat-transfer model presented here is applicable to perform various heat-transfer calculations involved in the design of storage and refrigeration equipment and to estimate the process time required for freezing of foods. The article presents the mathematical model used, the outline of the numerical scheme, and the results of computations. The model-predicted results are compared with the experimental data available in the literature. Overall good agreement was obtained.     

食品微生物快速检测技术

食品安全是人类健康的重要保障与前提。因此,建立食品病原微生物的快速检测方法对于食品安全及人类健康有着重要的意义。简要的综述食品微生物快速检测技术的研究进展。     

用两种方法评价四种食品抗氧化剂的抗氧化活性

分别用DPPH·分光光度法及TBARS法研究了BHT、TBHQ、TP与VE的抗氧化活性。结果显示它们对DPPH·皆有清除作用,清除活性为TBHQ>TP>VE>BHT;对亚油酸的脂质过氧化也都有抑制作用,抑制活性为BHT>VE>TBHQ>TP。不同测试体系中它们表现出的抗氧化活性不同,值得注意。     

简单形状食品冻结时间计算方法的分析研究

冻结时间对食品的冻结质量和能耗有重要意义.通过对几种简单形状食品冻结时间计算模型和相应公式的分析和计算,对它们在特定工况下应用的准确性和可靠性进行了比较,并对相关计算参数的选取给出了说明和建议.     

辐照食品的热释光分析鉴定方法研究

本文研究了用热释光分析仪鉴定食品辐照与否的具体方法和鉴定标准。在分离样品中的矿物质后,测量其热释光发光曲线的发光峰面积积分值G1,矿物质经1kGy的剂量照射后,再测量其热释光发光曲线的发光峰面积积分值G2;以G1/G2之值作为样品是否经过辐照的判断依据。通过对国家允许辐照的6大类食品中85个样品的检测分析得出:当G1/G2≥0.10时,可判定样品是辐照过的;当G1/G2<0.10时,判定样品是未辐照过的。     

Characterization of Ice Crystals in Pork Muscle Formed by Pressure-shift Freezing as Compared with Classical Freezing Methods

ABSTRACT: Cylindrical specimens of fresh pork muscle packed in plastic bags were frozen by air blaster freezing (ABF), liquid immersion freezing (LIF), and pressure-shift freezing (PSF) (100 to 200 MPa). Sample internal temperature and phase transformations were monitored at center, midway, and surface locations. ABF and LIF resulted in large irregular ice crystals, causing serious muscle structure deformation. PSF ice crystals were generally small and regular, but differed along the radial direction. Near the surface, there were many fine and regular intracellular ice crystals with well-preserved muscle tissue. From midway to the center, ice crystals were larger in size and located extracellularly. Ice crystal formation was affected by super-cooling during/after depressurization and subsequent freezing.