Prediction of Thermohydric History of Whey Protein Concentrate Droplets during Spray Drying

In this study, we analyzed heat and mass transfer phenomena occurring during spray drying of a whey protein concentrate at pilot scale. Conservation equations were written for both liquid droplets and humid air. Predicted results were then compared with experimental data: particle final moisture content, outlet air temperature, and humidity. The good adequacy found between experimental and predicted data allowed us to use the predicted values as a good indicator of thermohydric history followed by a droplet during drying. These results can also be used to help to find optimal process settings during production of spray-dried powders with specific properties.     

单液滴干燥过程中载体组分对植物乳杆菌的保护作用

喷雾干燥是目前制备益生菌微胶囊最常用的方法,而研究喷雾干燥塔内液滴的干燥历程有一定的难度。本试验中采用单液滴干燥(SDD)技术模拟喷雾干燥条件,以不同DE值糖浆和不同熔点棕榈油包埋植物乳杆菌,研究90℃和110℃两种干燥条件下对植物乳杆菌的保护作用。研究结果表明,两种干燥条件下含高熔点棕榈油的液滴中植物乳杆菌的存活率均高于低熔点棕榈油的液滴。扫描电镜观察到干燥后的单液滴颗粒表面均无游离的植物乳杆菌。结合液滴干燥过程中水分含量变化推断糖浆的DE值越高越利于表面结构的稳定。对液滴干燥参数以及乳状液的热稳定分析表明,植物乳杆菌的存活率主要与液滴温度有关,棕榈油的熔点越高在干燥过程中吸收的热就越多,从使植物乳杆菌的存活率越高。     

单液滴干燥过程中阿拉伯胶乳液的干燥特性

喷雾干燥塔是一个完全封闭的设施,难以对干燥过程中的液滴取样分析。本试验以阿拉伯胶包埋油脂的水包油(O/W)型乳液为样液,采用单液滴干燥技术(SDD)模拟喷雾干燥条件,通过分析乳液液滴在干燥过程中的形态收缩和干燥动力学参数的变化,探究干燥气流速度和乳液中固形物含量对颗粒的表面形成机制的影响。结果表明,在0.70,0.90,1.10 m/s 3种干燥气流速度下乳液液滴的干燥行为差异很大。在1.10 m/s的干燥气流速度下,液滴水分蒸发最快,表壳形成最早,最终所得颗粒收缩率最小。乳液总固形物含量对液滴干燥行为的影响也较为明显。随着总固形物含量的增高,乳液液滴水分蒸发越慢,表壳形成越早,最终所得干燥颗粒形态越大。本试验对于干燥动力学模型的研究和喷雾干燥产品品质的改善具有指导性意义。     

Enzyme inactivation kinetics: Coupled effects of temperature and moisture content

Enzymes are often dried for stability reasons and to facilitate handling. However, they are often susceptible to inactivation during drying. It is generally known that temperature and moisture content influence the enzyme inactivation kinetics. However, the coupled effect of both variables on enzyme inactivation over a broad temperature–moisture content range is still not well understood. Therefore, the inactivation of β-galactosidase in maltodextrin matrix is investigated using a newly developed method. An improved kinetic modelling approach is introduced, to predict the inactivation over a large range of temperature–moisture values. The model assumes a two-step inactivation mechanism involving reversible unfolding and irreversible inactivation. The model is able to describe the inactivation kinetics of β-galactosidase accurately, showing the temperature-dependent kinetic transition from reversible unfolding to irreversible inactivation limited. Application of this approach can provide immediate understanding of the effect of processing on enzyme inactivation and indicates the processes’ critical points, which offers the possibility for optimisation.     

Hole and vacuole formation during drying of sessile whey protein droplets

Morphological development from droplet to particle during drying has strong influence on powder structure and functionality. We study the evolving morphological properties of whey protein droplets during single sessile droplet drying experiments as a well-defined model for spray drying. Sessile drying droplets were visualised with a camera and subjected to varying drying conditions such as temperature, initial protein concentration, presence of airflow and droplet rotation. The final particles were imaged by SEM and X-ray tomography. Under all conditions used, the droplets initially shrink steadily while at a specific point a hole nucleates. Subsequently, a vacuole develops until a rigid hollow particle is obtained. The location of the hole was found strongly dependent on the presence and the direction of the applied air flow. We hypothesise that in the early drying stage a skin forms, which becomes more rigid when the hole nucleates. The hole forms at the position where the local modulus of the skin layer is minimal and/or at the point below the skin where the local pressure is minimal, and that after the hole has nucleated, the vacuole develops mainly by evaporation of water through the hole.     

Drying kinetics and survival studies of dairy fermentation bacteria in convective air drying environment using single droplet drying

The drying and survival kinetics of Lactococcus lactis ssp. cremoris in a convective air drying environment were measured using single droplet drying experiments. Tests were carried out at five different drying temperatures (45–95 °C) at a constant air velocity (0.5 m/s) and within 2.4–11% relative humidity. The effect of protective agents (10% w/w) of lactose, sodium caseinate and lactose:sodium caseinate (3:1) was also evaluated. The thermal inactivation kinetics parameters in convective air drying and isothermal water bath heating were determined and compared. The results showed that the final temperature attained by the droplet affected the survival of the bacteria significantly, however, most of the bacterial death occurred in early stage of drying while evaporative cooling kept the drop temperature relatively low. At higher droplet temperatures (?65 °C) the bacterial cultures were inactivated by both dehydration and thermal stresses. At lower droplet temperatures (?55 °C) the rate of change in droplet moisture content had much stronger effect on the bacterial survival. Lactose and sodium caseinate, as protective agents, enhanced the survival of bacterial cells significantly at all the test conditions. The lactose:sodium caseinate (3:1) mixture synergistically enhanced the survival of the bacterial cultures. The death of these bacteria followed first-order kinetics during convective single droplet drying as well as during isothermal water-bath heating. However, the inactivation energy in convective single droplet drying (181.3 kJ/mol) was much higher than the inactivation energy in isothermal water bath heating (16.8 kJ/mol) within the medium temperature of 45–95 °C.     

阿拉伯胶/乳清蛋白乳液的干燥行为特性的研究

为了研究液滴/颗粒在喷雾干燥过程中的干燥机理,本实验制备以阿拉伯胶和乳清蛋白为壁材的乳液,采用单液滴干燥技术(single droplet drying,SDD)模拟喷雾干燥环境,对单个液滴/颗粒的干燥过程进行分析,探究气流温度和壁材种类对液滴/颗粒干燥行为和所得颗粒性质的影响。结果表明:气流温度为130 ℃时所得颗粒形态最大,110 ℃次之,90 ℃下所得颗粒最小,气流温度高会促进表壳快速形成,进而抑制颗粒收缩,故所得颗粒较大。此外,增大气流温度有利于提高液滴/颗粒的干燥速率。与阿拉伯胶乳液相比,乳清蛋白乳液液滴降速干燥阶段出现早,所得颗粒收缩程度较小。研究结果表明干燥气流温度和壁材种类对表壳形成过程和颗粒形态有显著的影响。     

雨生红球藻喷雾干燥工艺的建立与放大

为了建立具有应用价值的雨生红球藻喷雾干燥工艺,本文使用采收后的雨生红球藻藻泥为原料,进行中试(蒸发能力25 kg·h?1)和生产规模(蒸发能力250 kg·h?1)实验,建立并放大了雨生红球藻的喷雾干燥工艺.结果表明,在中试规模下,进风温度在130～180℃范围内对藻粉理化指标没有显著的影响;出风温度在50～75℃范围...     

PREDICTION OF SPRAY-DRIED PRODUCT QUALITY USING TWO SIMPLE DRYING KINETICS MODELS

Increasing attention has been given to spray drying because of its popularity in the manufacturing of various powders with prerequisite quality. A large number of theoretical and experimental studies have been published during the last five decades to study spray‐drying operations. Despite this progress, there is still lack of information on the selection of an accurate and simple drying kinetics model to characterize the droplet drying process. This article has particularly focused on the comparison of two simple and effective drying kinetics models: the characteristic drying‐rate curve and reaction‐engineering approach (REA) models. This article reported mathematical models that can be used to design the spray‐drying operation and to predict the particle's characteristics for realistic dryer‐wide situations. Both the physical and biochemical quality attributes were investigated regarding their sensitivity to the model parameters used in the two drying kinetics models. The REA model was found to deliver more realistic predictions during the modeling of the droplet drying process for all the sets of conditions tested.     

A model for heat transfer in cryogenic food freezing

An experimental study of the heat transfer between liquid nitrogen sprays and a model food (a gelatine slab) was carried out under conditions similar to those in a cryogenic freezer. Measurements of heat flux and local mass flow rates of the spray were made at various liquid N₂ pressures and various temperature differences between the spray and the food surface.
At higher spray pressures, the heat transfer coefficient increases with the mass flux density of the liquid available at the food surface. The quantity of liquid nitrogen sprayed onto the solid surface, the mean droplet size, spray velocity, surface coverage and the mean temperature difference between the boiling nitrogen and the food surface are major factors influencing the rate of heat transfer during the freezing process. Although the heat transfer coefficients at the food surface are much less than those obtained for individual droplets, the model provides useful data.
The results are critically discussed in relation to cryogenic freezing of foods.     

Spray-dried whey protein/lactose/soybean oil emulsions. 2. Redispersability, wettability and particle structure

In the present paper redispersion and wettability experiments of spray-dried whey protein-stabilized emulsions are presented. Emulsion droplet size after redispersion gives information about eventual coalescence between emulsion droplets in the powder matrix during drying or storage, resulting in an increase in emulsion droplet size after redispersion. Results from redispersion experiments are combined with previously presented knowledge about powder surface composition and particle structure to elucidate internal processes in the powder matrix and external processes on the powder surface during drying and storage of whey protein powder. The results show that with addition of lactose to whey protein-stabilized emulsions, emulsion droplet structure remains intact in the powder matrix during drying since the emulsion droplet size in the redispersed spray dried emulsion is unchanged. In the absence of lactose there is a growth in emulsion droplet size after redispersion of the spray-dried whey protein-stabilized emulsion, showing that a coalescense of emulsion droplets occurs during the drying or redispersion process. Storage of the whey protein-stabilized powders in a humid atmosphere (relative humidity 75%, 4 days) induces changes in some powders. When the powder contains a critical amount of lactose there is a remarkable increase in emulsion droplet size after redispersion of humid stored powders compared with the emulsion before drying and with the redispersed dry stored powder. In addition, there is a release of encapsulated fat after humid storage of lactose-containing powders detected by electron spectroscopy for chemical analysis. For powders which do not contain any lactose there is no increase in emulsion droplet size after storage in a humid atmosphere compared with the redispersed dry stored emulsion. Addition of only a small amount of lactose prevents coalescence of emulsion droplets and the subsequent increase in droplet size during drying. If the lactose content is kept rather low neither an effect on the droplet size after storage under humid conditions nor a release of fat onto powder surfaces is detected. Furthermore, wettability of the spray-dried whey protein-stabilized emulsions by water is presented. It is concluded that it is beneficial to wettability in water to have as high a coverage of lactose on the powder surface as possible. In addition, a review of particle structure for powders of various composition is presented.     

The Relative Activation Energy of Food Materials: Important Parameters to Describe Drying Kinetics

An effective drying model should be accurate and require a small number of experiments to generate the parameters. The relative activation energy of various food materials, important drying kinetic properties used in the reaction engineering approach, is evaluated and summarized. The reaction engineering approach is then implemented to model the global and local drying rates of food materials. By using the relative activation energy, the reaction engineering approach describes the (R² higher than 0.99) global drying rate of food materials well. The reaction engineering approach can be coupled with a set of equations of conservation of heat and mass transfer to model the local drying rate of food materials. The relative activation energy is indeed proven to be accurate to model the local drying rate. While the predictions are accurate, the reaction engineering approach is very effective in generating the drying parameters since the relative activation energy can be generated from one accurate drying run. Different drying conditions of the same material with similar initial moisture content would result in the similar relative activation energies. The drying kinetics parameters generated here are readily used for design of new equipment, evaluating the performance of existing dryers, and monitoring the product quality.     

Experimental studies and kinetics of single drop drying and their relevance in drying of sugar‐rich foods: A review

Levitation and free‐flight techniques applied to investigate the drying kinetics and morphology of single drops containing dissolved solids and suspensions are reviewed. A review of works related to receding interface model proposed to quantify the drying kinetics of single drops along with techniques to measure the kinetic parameters such as moisture diffusivity, thermal conductivity and specific heat capacity is presented. Problems associated with spray drying of sugar‐rich compounds are briefly discussed and possible links of stickiness and flavor retention with glass transition temperature (T_g), temperature history, drying rate and morphological changes including skin formation, as monitored through single drop experiments, are explored.     

干燥方式对蛋清蛋白理化性质和功能特性的影响

为探究干燥方式对蛋清蛋白（egg white protein,EWP）功能特性的影响及其内在机理,分别通过喷雾干燥与真空冷冻干燥制备蛋清蛋白粉,并对其蛋白结构、理化性质与功能特性进行研究。结果表明,与蛋清液（EWP-C）相比,喷雾干燥使蛋清蛋白（EWP-P）的内源性荧光强度降低,表面疏水性和表面游离巯基含量增大。傅里叶变换红外光谱分析显示,EWP-P的α-螺旋、β-折叠和β-转角分别为16.30%、25.72%和40.23%,冷冻干燥蛋清蛋白（EWP-D）分别为20.43%、24.32%和35.69%。不同pH下,EWP-D的溶解度均高于EWP-P,表面张力小于EWP-P。此外,EWP-P的接触角为99.62°,高于EWP-D（接触角为65.97°）,表明喷雾干燥能显著提高蛋白的疏水性（P<0.05）。EWP-D在不同pH下的乳化性、乳化稳定性以及起泡性均大于EWP-P,但起泡稳定性更小,这与EWP-D较高的溶解性与较低的表明疏水性有关。荧光倒置显微镜及激光共聚焦扫描显微镜分析表明EWP-D乳液的微粒更小,分布更均匀,其稳定性高于EWP-P。综上,喷雾干燥蛋清蛋白的β-折叠结构较多,表面游离巯基含量和表面疏水性较高,具有较好的凝胶性;冷冻干燥蛋清蛋白的表面疏水性较小,且表面张力小、溶解度大,具有更好的乳化能力与起泡性。     

Multiscale Modeling for Food Drying: State of the Art

Plant‐based food materials are mostly porous in nature and heterogeneous in structure with huge diversity in cellular orientation. Different cellular environments of plant‐based food materials, such as intercellular, intracellular, and cell wall environments, hold different proportions of water with different characteristics. Due to this structural heterogeneity, it is very difficult to understand the drying process and associated morphological changes during drying. Transport processes and morphological changes that take place during drying are mainly governed by the characteristics of and the changes in the cells. Therefore, to predict the actual heat and mass transfer process that occurs in the drying process and associated morphological changes, development of multiscale modeling is crucial. Multiscale modeling is a powerful approach with the ability to incorporate this cellular structural heterogeneity with microscale heat and mass transfer during drying. However, due to the huge complexity involved in developing such a model for plant‐based food materials, the studies regarding this issue are very limited. Therefore, we aim in this article to develop a critical conceptual understanding of multiscale modeling frameworks for heterogeneous food materials through an extensive literature review. We present a critical review on the multiscale model formulation and solution techniques with their spatial and temporal coupling options. Food structure, scale definition, and the current status of multiscale modeling are also presented, along with other key factors that are critical to understanding and developing an accurate multiscale framework. We conclude by presenting the main challenges for developing an accurate multiscale modeling framework for food drying.     

Towards a maximal cell survival in convective thermal drying processes

Freeze-drying is a traditional approach of producing desiccated microorganisms. Industrial mass-production of desiccated microorganisms, however, pursues a more economic drying process, which leads to an increasing research interest in the thermal drying processes of microorganisms. These thermal drying processes include spray drying, fluidized bed drying, vacuum drying, air-convective drying, etc. Cells being thermally dried suffer from simultaneous heat and dehydration stresses. To maximize the cell survival and to study the mechanisms of cell inactivation during such processes, it is necessary to have understandings of both the chemical engineering principles involved and the biological properties of the microorganisms to be dried. This review attempts to give a balanced discussion on both aspects. Factors influencing cell viability during thermal drying are discussed in two groups, viz., intrinsic factors related to the microorganisms being dried and extrinsic factors related to process conditions. Finally, modeling of the inactivation kinetics is briefly reviewed. This article aims to bring together some common observations and findings for different organisms subjected to thermal drying, and discuss the mechanism underlying these observations. Efficient drying which is associated with fast removal of water content has economical benefits, but hurts cells. The desiccation of microorganism thus is a balance between these two considerations and needs to be strain-specifically optimized.     

Microencapsulation by Spray Drying: Influence of Emulsion Size on the Retention of Volatile Compounds

ABSTRACT: The influence of the mean emulsion droplet size on flavor retention during spray drying was studied. d- Limonene, ethyl butyrate, and ethyl propionate were used as the model flavors. Gum arabic, soybean water-soluble polysaccharides, or modified starch blended with maltodextrin, were used as the wall materials. The increasing emulsion droplet decreased the retention of flavors. The distribution curve containing larger emulsion droplets shifted toward a smaller size after atomization, indicating that the larger emulsion droplets would be changed in size during atomization and result in decreasing flavor retention. For ethyl butyrate and ethyl propionate, the retention had a maximum at the mean emulsion diameter of 1.5 to 2 and 2.5 to 3.5 μm, respectively.     

喷雾干燥及热风干燥对南瓜粉营养特性和感官品质的影响

以新鲜南瓜为原料,分别研究了喷雾干燥及热风干燥两种不同干燥工艺对南瓜粉营养特性和感官性能的影响,重点研究了喷雾干燥的干燥温度、压缩空气流量与改良剂添加量(南瓜固形物与麦芽糊精比例)及热风干燥的干燥温度,物料厚度与改良剂添加量(南瓜固形物与麦芽糊精比例)三个因素的影响,以南瓜粉成品的Vc和β-胡萝卜素含量及感官评分作为评...     

Modelling the Dynamic Inactivation of the Probiotic Bacterium <Emphasis Type="Italic">L. Paracasei</Emphasis> ssp. <Emphasis Type="Italic">Paracasei</Emphasis> During a Low-Temperature Drying Process Based on Stationary Data in Concentrated Systems

In order to better understand inactivation of cells during a drying process, the inactivation kinetics of concentrated Lactobacillus paracasei ssp. paracasei (F19) was measured under stationary conditions for different combinations of water activities and temperatures in a water activity range of a _w = 0.23–a _w = 0.75 and temperatures between 4°C and 50°C. It was shown that the inactivation kinetics of the probiotic bacterium L. paracasei at moderate temperatures could, for all conditions, be formally described by a first-order reaction with activation energies that are much lower than for thermal inactivation (E _a = 61 kJ/mol). With regard to the water activity, the reaction rate constants exhibit a maximum inactivation rate at intermediate water activity a _w = 0.52. As this behavior has direct implications for the stability of cells in a drying process, the stationary data were used to model the inactivation during test vacuum drying processes, where both temperature and water activity dynamically change. It is shown that—depending on the drying rate—dynamic effects have to be taken into account when modeling the survival during drying. Nevertheless, the model based on stationary inactivation data is capable to predict the characteristics of inactivation during a drying process. Therefore, it can serve as basis to optimize the drying process with regard to maximum survival of cells. However, a further refinement of the model with regard to the drying rate is necessary.