VARIATIONS IN ALFALFA JUICE SOLUTE CONCENTRATIONS AS WATER IS EVAPORATED

ABSTRACT

Alfalfa juice was extracted from plants during the graving phases of the first and second crops. The alfalfa juice was dried and solute concentrations were measured periodically during the process using freezing point depression (FPD) as an indicator. Solute/solvent weight ratios were calculated from the freezing point depression of fresh unconcentrated 'uice extracted from both firsc and second crops. At the time of harvest, tie FPD was a function of this ratio. The first and second crops behaved distinctively different during the drying process, with the second having a larger soluce concentration and a greater FPD at a given moisture content. The appearance of preci itate during the drying process and the general1 large FPD makes this materiay a questionable product for drying vitg low temperature processes.     

Relationship of Product Structure, Sorption Characteristics, and Freezing Point of Atmospheric Freeze-Dried Foods

Drying is an important unit operation in processing of foods with a long shelf life. The drying process influences product properties and quality; the products may shrink, break, or undergo rheological, physical, and biochemical changes. Important parameters responsible for product quality changes during drying are temperature, relative humidity, and residence time. Studies of thermal and mass transfer properties during drying are essential for understanding the changes in product quality and for designing and dimensionalizing the drying process.

Drying kinetics, sorption properties, shrinkage, and freezing point depression were determined during atmospheric freeze drying (AFD) of pieces of apple, turnip cabbage, and cod. Adsorption rate and sorption isotherms were determined in the end product. The drying temperature affected the physical properties. Drying at -5°C resulted in a larger shrinkage than drying at -11°C. GAB modeling was used to characterize the sorption properties of the products. No typical sigmoidal shape was found of the moisture sorption isotherms of the products, which is in accordance with the Guggenheim constant found from the same results. Experimental data on freezing point depression were used to find product constants E and b in the Schwartzberg equation for the freezing point depression. Freezing point depression, as a function of the dry matter content, was determined using Schwartzberg's equation and a component composition model (CCM). The result indicates an influence of structural effects on freezing point depression.     

Relationship of Product Structure,Sorption Characteristics,and Freezing Point of Atmospheric Freeze-Dried Foods

Drying is an important unit operation in processing of foods with a long shelf life. The drying process influences product properties and quality; the products may shrink, break, or undergo rheological, physical, and biochemical changes. Important parameters responsible for product quality changes during drying are temperature, relative humidity, and residence time. Studies of thermal and mass transfer properties during drying are essential for understanding the changes in product quality and for designing and dimensionalizing the drying process.

Drying kinetics, sorption properties, shrinkage, and freezing point depression were determined during atmospheric freeze drying (AFD) of pieces of apple, turnip cabbage, and cod. Adsorption rate and sorption isotherms were determined in the end product. The drying temperature affected the physical properties. Drying at ?5°C resulted in a larger shrinkage than drying at ?11°C. GAB modeling was used to characterize the sorption properties of the products. No typical sigmoidal shape was found of the moisture sorption isotherms of the products, which is in accordance with the Guggenheim constant found from the same results. Experimental data on freezing point depression were used to find product constants E and b in the Schwartzberg equation for the freezing point depression. Freezing point depression, as a function of the dry matter content, was determined using Schwartzberg's equation and a component composition model (CCM). The result indicates an influence of structural effects on freezing point depression.     

A generalized correlation of solute inclusion in ice formed from aqueous solutions and food liquids on sub‐cooled surface

Solute inclusion in an ice layer formed on a smooth stainless steel surface under sub‐cooled flow conditions has been studied experimentally. The effects of ice growth rate, solution velocity and bulk concentration of solute on the solute inclusion in the ice layer have been investigated using a controlled flow surface crystallizer (CFSC). For several aqueous solutions (sucrose, NaCI, glycol, fructose), food liquids (skim milk, whole milk, orange juice) and particulate suspension (potato starch particles), the effective molecular weights were calculated and their effects on solid inclusion determined. This effect was then conveniently related to the freezing point depression (FPD) of each liquid tested. A generalized empirical correlation has been established for all the liquids tested.     

冷冻干燥过程相迁移和相分布的孔尺度网络模型与模拟

构建了51×51二维孔-喉网络模型对冷冻干燥过程的升华干燥阶段进行模拟。与传统的连续介质模型相比,孔网络模型的特征是具有跟踪干燥过程中物料内部的干燥前沿和相分布的能力。采用网络模型预测了牛肉和火鸡肉的干燥曲线,并模拟了不同冻结速率的火鸡肉在干燥过程中形成的相分布。讨论了模型的计算特性,并分析了孔径分布对相分布特性的影响。结果表明：网络模型可较好地预测升华干燥阶段,可在孔尺度上揭示干燥过程的动力学机理,将为准确地判断升华干燥与解析干燥的转变点提供理论计算基础。     

Effects of freezing pretreatment and hygroscopicity of fluidizing particles on drying characteristics of carrots in fluidized bed under reduced pressure

Abstract

A method for low-temperature drying with high drying rate was developed for heat-sensitive foods and agricultural products. A freezing pretreatment was combined with fluidized bed drying under reduced pressure. Cylindrical carrot samples were frozen and then placed without thawing in a fluidized bed of hygroscopic porous silica gel particles that adsorbed water from the sample during the drying process. The effects of the freezing pretreatment and hygroscopicity of fluidizing particles on the drying characteristics of carrots were examined. A higher drying rate was achieved when carrots samples were subjected to freezing pretreatment than without it. At 12?kPa, the volume change was smaller in carrots subjected to freezing pretreatment than in untreated samples. A larger amount of water was absorbed during rehydration by carrots dried at 12?kPa than at 101?kPa within 120?min. The properties of dried carrots were affected not only by freezing pretreatment but also by the pressure applied during the drying process.     

用升华与蒸发联合法干燥食品的物理性质

Drying is an important unit operation in processing of biological resources. The drying process may influence the product properties and quality, which may shrink, break or undergo rheological, physical and biochemical changes. The important parameters responsible for such changes are drying conditions, type of drying technology and residence drying time. Thermal conductivity, thermal-mass diffusivity, enthalpy, porosity and density are the main material property and heat-mass transfer parameters, which are essential for understanding the changes in product quality and for designing and dimensioning the drying processes. In this paper physical properties of food products undergoing a combined sublimation and evaporation were studied. Pieces of vegetables and potatoes were dried in a heat pump fluidized bed dryer at combined modes with temperatures below the freezing point in the beginning and a final drying step at temperatures above the freezing point. Samples of products were tested at different moisture contents with respect to physical properties. Physical properties of leek and potato samples were measured and mass diffusivities were determined from drying kinetic data. Based on bulk density and rehydration measurements it was clearly observed that drying temperature and modes influenced the final product physical properties. The potato cuberun dried with initial atmospheric freeze-drying step had rehydration ability 430% above a run dried only above the freezing point. The average effective mass diffusivity for 5 mm slabs of leek was 0.5×10^-11m^2·s^-1 for the sublimation stage and 2.2×10^-11m^2·s^-1 for the evaooration stage.     

冷冻干燥过程强化中冷冻阶段优化的研究进展

冷冻干燥产品质量高,但时间长、能耗高。本文综述了冷冻干燥过程强化中冷冻阶段的优化方法,控制冷冻速率、调节冰晶成核和退火处理可以获得大而均匀的冰晶从而提高升华干燥阶段速率,但物料内部比表面积的减小会降低解吸干燥阶段速率,这类常规的冷冻阶段优化方法对弱吸湿性的物料有一定的强化效果。有机溶剂具有较高的蒸气压,作为共溶剂时可以增加传质推动力,但较低的有机溶剂残留量要求阻碍了其进一步应用。“初始非饱和多孔介质冷冻干燥”的技术思想是将液体物料首先制备成具有一定初始孔隙的冷冻物料,然后再进行冷冻干燥。物料具有的初始孔隙为水蒸气的迁移提供了便捷的通道,而且纤薄的固体基质也有利于结合水的解吸,可以同时强化升华干燥阶段和解吸干燥阶段。该技术思想是过程低消耗和产品高质量的完美结合,为解决冷冻干燥过程速率低的问题提供了新的方案。     

A New Variable Diffusion Drying Model for the Second Falling Rate Period of Paddy Dried in a Rapid Bin Dryer

The objectives of this article is to propose a new drying model for the second falling rate period known as the variable diffusion controlled period that follows after the first falling rate period and to propose a new method to determine the second critical moisture content that separates these two periods. Experimental work on paddy drying at minimum fluidization velocity was carried out in a rapid bin dryer. The effects of operating temperatures (60-120°C) and bed depths (2-6 cm) on the paddy drying characteristics were investigated. It was found that the normalized drying rate of paddy was proportional to the normalized moisture content in the first falling rate period but in the second falling rate period, the normalized drying rate of the material varies exponentially with the normalized moisture content. The different relationship between the normalized drying rate and the normalized moisture content in the first and second falling rate periods indicate that two different mechanism of moisture transport are at work. The new exponential model of the second falling rate period and the linear model of the first falling rate period were found to fit the experimental data very well. Derivation from variable diffusion equation shows that the linear model is the result of constant diffusion coefficient whereas the new exponential model is the result of linear diffusion coefficient. This also implies that the first falling rate period is a constant diffusion controlled period and the second falling rate period is a variable diffusion controlled period. In addition, drying kinetics data of a drying process that fits the exponential model over a very slow drying period will show that the drying process is under the effect of a linear diffusion coefficient. It was also found that the proposed new method to determine the second critical moisture content that distinguishes between the first and second falling rate periods by using a sudden change in the value of the drying rate gradient to a much lower value at that point is more rigorous and yet simpler than the method of determining the specific location of the receding drying boundary since it is based on the behavior of the actual drying kinetic data.     

Kinetics of Dehydration of Green Alfalfa

Artificial drying is an important step in processing of green crops in order to preserve their freshness and nutrients for longer time at relatively lower costs. Forage crops, tea and tobacco are the major green crops where the commercial drying is a major operation in their processing. Fresh green alfalfa at about 75 to 80% moisture is subjected to drying in different types of dryers.ln the case of alfalfa, the raw material consists of leaves, stems, chops and fine stems, each varying distinctly in their physical and structural characteristics. The moisture content is reduced from initial level to about 10%. The drying air temperatures range from 40 b 800°C; the lower temperatures are used inconveyor dryers whereas high temperatures are used in rotary drum drycn.The results on drying behavior, and changes in physico-chemical propedes during drying for components of green alfalfa over the temperature range of 40 m 800°C are presented in this paper. The optimum temperature for drying from the stand point of color and protein solubility was found to be 175°C.     

EXERGY ANALYSIS FOR THE FREEZING STAGE OF THE FREEZE DRYING PROCESS

The mathematical expressions for exergy and the exergy analysis of the freezing stage of the freeze drying process are presented. The exergy analysis indicates that very substantial reductions in the magnitudes of the total exergy loss and of the exergy input due to the heat that must be removed during the freezing stage, can be obtained when the freezing stage is operated through the use of a rational distribution in the magnitude of the temperature of the cooling source. The rational distribution in the magnitude of the temperature of the cooling source should provide significant savings in the utilization of energy during the freezing stage of the freeze drying process as well as satisfactory freezing rates that form ice crystals that are continuous and highly connected and their shape and size are such that the pores of the porous matrix of the dried layer generated by sublimation during the primary drying stage, have a pore size distribution, pore shape, and pore connectivity that are appropriate to allow high rates for mass and heat transfer during the primary and secondary drying stages of the lyophilization process.     

干燥技术在制备纳米粉体中的应用

由于纳米粒子的表面效应,用湿法制备出的纳米粉体的前躯体在干燥过程中极易形成团聚,已经不能采用传统的干燥方法。根据粉体干燥的理论,阐述了纳米粉体干燥过程的团聚机理,综述了微波干燥法、超临界干燥法、冷冻干燥法、共沸蒸馏法、喷雾干燥法等在制备纳米粉体中的应用,并指出了各种方法的优缺点。     

Kinetics of Dehydration of Green Alfalfa

ABSTRACT

Artificial drying is an important step in processing of green crops in order to preserve their freshness and nutrients for longer time at relatively lower costs. Forage crops, tea and tobacco are the major green crops where the commercial drying is a major operation in their processing. Fresh green alfalfa at about 75 to 80% moisture is subjected to drying in different types of dryers.ln the case of alfalfa, the raw material consists of leaves, stems, chops and fine stems, each varying distinctly in their physical and structural characteristics. The moisture content is reduced from initial level to about 10%. The drying air temperatures range from 40 b 800°C; the lower temperatures are used inconveyor dryers whereas high temperatures are used in rotary drum drycn.The results on drying behavior, and changes in physico-chemical propedes during drying for components of green alfalfa over the temperature range of 40 m 800°C are presented in this paper. The optimum temperature for drying from the stand point of color and protein solubility was found to be 175°C.     

Cracking during freeze-drying in dense freeze cast ceramics: Role of drying rate

Cracks can form during the freeze-drying of freeze cast ceramic suspensions while attempting to produce dense ceramics. The suspensions contain alumina particles dispersed in cyclohexane. The rate of drying is controlled by the pressure and temperature during drying (slow drying at atmospheric pressure and −15°C and fast drying under vacuum while the temperature slowly increases from −80°C to room temperature). X-Ray micro-computed tomography was used to characterize internal crack formation. Cracks were found to occur during freeze-drying rather than during freezing. Both slow and fast drying produced cracks, although two different morphologies were observed. Mechanistic models are proposed for the formation of both types of cracks. The rate of freezing was found to influence the formation of cracks. Slow freezing tended to reduce the formation of drying cracks because the slower freezing produced a more heterogeneous distribution of particles and porous regions, which tends to allow stress to be relieved by opening up existing pores rather than forming cracks in the more homogeneous fast frozen bodies.     

A MATHEMATICAL MODEL FOR PARALLEL FLOW DRYER FOR SLABS WITH HIGH THERMAL DIFFUSIVITY

An analytical model for the process is developed. The thermal diffusivity of the drying slabs is assumed infinite and the moisture diffusivity constant during the entire drying process.

With specified initial and boundary conditions, the mathematical model yields a two-part solution for the diffusion equation. The first part is valid for the initial drying during which the surface moisture content exceeds the value of fiber saturation. This part of the solution is used until the surface moisture content drops to the fiber saturation value. The moisture profile at the end of this period is used as the initial condition for the second period of drying which takes place under hygroscopic conditions.

Two simplifying assumptions are adapted for the hygroscopic region: 1. The dependence between the surface temperature and the moisture content is linear. 2. Constant (average) absorption heat is used during this second drying period.

For both parts of the solution, the surface moisture gradient is proportional to the local temperature difference between the drying air and the slab surface. This temperature difference can be expressed by means of a water mass balance equation for the part of the dryer between the slab in-feed and the point considered and by using the thermodynamic properties of the humid air.     

A MATHEMATICAL MODEL FOR PARALLEL FLOW DRYER FOR SLABS WITH HIGH THERMAL DIFFUSIVITY

An analytical model for the process is developed. The thermal diffusivity of the drying slabs is assumed infinite and the moisture diffusivity constant during the entire drying process.

With specified initial and boundary conditions, the mathematical model yields a two-part solution for the diffusion equation. The first part is valid for the initial drying during which the surface moisture content exceeds the value of fiber saturation. This part of the solution is used until the surface moisture content drops to the fiber saturation value. The moisture profile at the end of this period is used as the initial condition for the second period of drying which takes place under hygroscopic conditions.

Two simplifying assumptions are adapted for the hygroscopic region: 1. The dependence between the surface temperature and the moisture content is linear. 2. Constant (average) absorption heat is used during this second drying period.

For both parts of the solution, the surface moisture gradient is proportional to the local temperature difference between the drying air and the slab surface. This temperature difference can be expressed by means of a water mass balance equation for the part of the dryer between the slab in-feed and the point considered and by using the thermodynamic properties of the humid air.     

Measurement of bound water content in sludge: The use of differential scanning calorimetry (DSC)

The bound water content of original and frozen activated sludge and an inorganic sludge were measured via differential scanning calorimetry (DSC) and drying tests. Original sludge contained a significant level of bound water which was reduced by about 50% after freeze/thaw treatment. A simple thermodynamic interpretation showed that liquid–solid binding strength played an important role in depression of the freezing point of bound water in a sludge. The choice of a threshold temperature in a DSC test was equivalent to definition of a threshold binding strength dividing bound/non-bound water content. The possible close relationship between the bound water content measured via DSC and the drying test is discussed. The importance for sample uniformity was also examined.     

天然气长输管道干燥剂干燥投产工艺分析

干燥剂法干燥投产在陆上和海底输气管道投产中应用较为普遍。清管器的运行控制是干燥过程的一个重点和难点,必须准确的估算清管器运行压降,同时干燥剂的窜漏量也是一个十分关注的参数。概述了干燥剂法干燥工艺过程,重点分析清管器运行压降和窜漏量的理论计算方法,为干燥设计和施工操作提供了理论指导与依据。     

Experiments and insights of desalination by a freezing/thawing method at low subcooling

Desalination by freezing/thawing method was a very important method to obtain fresh water from sea water. In this work, desalination by freezing/thawing method was conducted with initial sodium chloride of 3.5 wt% in consideration of stirring speed, freezing time and subcooling. The subcooling ranged from 1.2 K to 4 K. The optimum conditions for desalination in this work were stirring speed of 200 r·min⁻¹, freezing time of 120 min, and subcooling of 3 K. The results also showed that sodium chloride cannot be effectively removed by once freezing/thawing process. The maximum removal efficiency of sodium chloride was 64.3%. Two major reasons resulting in the impurity of obtained melted water by freezing/thawing method were proposed. The first reason was the inevitable adhesion of salt solution to the surface of ice, which could be removed easily by distilled water flushing. The second reason was that salt solution was heterogeneously wrapped in the accumulated ice, which was difficult to be removed by distilled water flushing. The liquid flushing method was proposed to verify the conjecture, and the results were in accordance with the two reasons mentioned above. Additional method, such as multiple flushing liquid method, was suggested to be used during the freezing/thawing process for effectively removing sodium chloride, and obtaining fresh water.