Freeze Drying of Apple Slices with and without Application of Microwaves

In this study, Fuji apple slices were dehydrated using freeze drying (FD) combined with microwave assisted with vacuum drying (VMD). The optimal parameter for the diversion point of moisture content from FD to VMD process was at the moisture level of 21%, and for VMD the optimal parameter for vacuum pressure was at 9.15 kPa and microwave power density was at a level of 3.18 w/g. The results show that the two-step technique can significantly reduce total FD time required by up to 40%, while the nutritional value of the dried apple chips remained unchanged compared to FD used alone.     

Optimal Control of the Primary Drying Stage of Freeze Drying of Solutions in Vials Using Variational Calculus

The problem of operating freeze drying of pharmaceutical products in vials placed in trays of a freeze dryer to remove free water (in frozen state) at a minimum time was formulated as an optimal control problem. Two different types of freeze dryer designs were considered. In type I freeze dryer design, upper and lower plate temperatures were controlled together, while in type II freeze dryer design, upper and lower plate temperatures were controlled independently. The heat input to the material being dried and the drying chamber pressure were considered as control variables. Constraints were placed on the system state variables by the melting and scorch temperatures during primary drying stage. Necessary conditions of optimality for the primary drying stage of freeze drying process in vials are derived and presented. Furthermore, an approach for constructing the optimal control policies that would minimize the drying time for the primary drying stage was given. In order to analyze optimal control policy for the primary drying stage of the freeze-drying process in vials, a rigorous multi-dimensional unsteady state mathematical model was used. The theoretical approach presented in this work was applied in the freeze drying of skim milk. Significant reductions in the drying times of primary drying stage of freeze drying process in vials were obtained, as compared to the drying times obtained from conventional operational policies.     

Optimal Control of the Primary Drying Stage of Freeze Drying of Solutions in Vials Using Variational Calculus

Abstract

The problem of operating freeze drying of pharmaceutical products in vials placed in trays of a freeze dryer to remove free water (in frozen state) at a minimum time was formulated as an optimal control problem. Two different types of freeze dryer designs were considered. In type I freeze dryer design, upper and lower plate temperatures were controlled together, while in type II freeze dryer design, upper and lower plate temperatures were controlled independently. The heat input to the material being dried and the drying chamber pressure were considered as control variables. Constraints were placed on the system state variables by the melting and scorch temperatures during primary drying stage. Necessary conditions of optimality for the primary drying stage of freeze drying process in vials are derived and presented. Furthermore, an approach for constructing the optimal control policies that would minimize the drying time for the primary drying stage was given. In order to analyze optimal control policy for the primary drying stage of the freeze-drying process in vials, a rigorous multi-dimensional unsteady state mathematical model was used. The theoretical approach presented in this work was applied in the freeze drying of skim milk. Significant reductions in the drying times of primary drying stage of freeze drying process in vials were obtained, as compared to the drying times obtained from conventional operational policies.     

冷冻干燥过程的数学模型

本文以物料衡算和热量衡算为基础，作适当的简化，导出了所需参数较少，但能准确地预测冷冻干燥全过程时间的数学模型，通过对胡萝卜和土豆的实验研究，检验了模型的适用性和准确性。     

Optimal Control of the Secondary Drying Stage of Freeze Drying of Solutions in Vials Using Variational Calculus

Abstract:

The problem of operating freeze drying of pharmaceutical products in vials loaded on trays of freeze dryer to obtain a desired final bound water content in minimum time is formulated as an optimal control problem. Two different types of freeze dryer designs were considered. In the type I freeze dryer design, upper and lower plate temperatures were controlled together, while in the type II freeze dryer design, upper and lower plate temperatures were controlled independently. The heat input to the material being dried and the drying chamber pressure were considered as control variables. Only the scorch temperature was considered as a constraint on the system state variables during the secondary drying stage, because all the free water content (frozen water) is removed from the solid matrix during the primary drying stage of freeze drying. Necessary conditions of optimality for the secondary drying stage of freeze drying process in vials were derived and presented by using rigorous multidimensional unsteady-state mathematical models. The theoretical approach presented in this work was applied in the freeze drying of skim milk. Significant reductions in drying times of the secondary drying stage of the freeze drying process in vials were observed and more uniform bound water and temperature distributions in the material being dried were obtained compared to the conventional operational policies.     

How to Meet the Freeze Drying Standard in Combined Drying Processes: Pre and Finish Drying of Carrot Dice

Freeze-drying (FD) processes are well known to produce high-quality consumer products. Major problems are long drying times, high investment costs, and resulting maintenance and operating costs that make FD a very costly process. In this study, the potential of air drying (AD) and microwave vacuum drying (MVD) is tested to reduce freeze-drying times and make better utilization of the investment by combining FD with alternative lower cost drying processes using a pre or finish drying step while preserving the qualwity of the final product. Using carrot dice as a sample product, predrying did not lead to the desired qualities and reduction in drying time, whereas finish drying produced promising results. FD/MVD was 40% faster than pure FD and achieved freeze-drying standards regarding volume retention, shape, color, and rehydration. The good performance of microwave vacuum finish drying can directly be traced back to a time effect that is possibly linked to a characteristic collapse time of the cellular matrix. In order to use this effect, a stable skeleton must have developed at the point of process changeover, changeover has to be immediate, and finish drying has to be quick.     

Foam-Mat Freeze Drying of Egg White—Mathematical Modeling Part II: Freeze Drying and Modeling

Foam-mat freeze drying is one of the promising methods of drying, which utilizes advantages of both freeze drying and foam-mat drying. Egg white with its excellent foaming properties makes a suitable candidate for foam-mat freeze drying. Experiments were conducted to study foam-mat freeze drying of egg white, in an effort to determine the suitability of this method. Xanthan gum (XG) at 0.125% concentration was used as stabilizer for foaming. The results showed that the addition of xanthan gum during foaming has a positive impact in reducing the total drying time and also produces excellent quality egg white powder. The addition of stabilizer also plays an important role in improving drying. Simple models were applied for determining drying time and diffusion coefficients during freeze drying.     

Studies on the Microwave Freeze Drying Technique and Sterilization Characteristics of Cabbage

The vacuum freeze-drying (FD) technique used in the food industry can yield a high-quality product, but it is very expensive and requires a long processing time. Besides, the quantity of microorganisms in FD products can often exceed the required standard. As a result, it will be important to develop a new freeze-drying technique. In this article, cabbage was used as a model material, and the microwave field was used as a heat source to supply sublimation heat so that the drying time was shortened greatly. The effect of the microwave sterilization during the drying process was evaluated. Effects of the pressure, thickness of material being dried, and the input microwave power on such indices as drying time and the microorganism number were studied. Compared with the method of ordinary freeze drying, microwave freeze drying (MFD) can greatly reduce the drying time and has a notable sterilization effect.     

Studies on the Microwave Freeze Drying Technique and Sterilization Characteristics of Cabbage

The vacuum freeze-drying (FD) technique used in the food industry can yield a high-quality product, but it is very expensive and requires a long processing time. Besides, the quantity of microorganisms in FD products can often exceed the required standard. As a result, it will be important to develop a new freeze-drying technique. In this article, cabbage was used as a model material, and the microwave field was used as a heat source to supply sublimation heat so that the drying time was shortened greatly. The effect of the microwave sterilization during the drying process was evaluated. Effects of the pressure, thickness of material being dried, and the input microwave power on such indices as drying time and the microorganism number were studied. Compared with the method of ordinary freeze drying, microwave freeze drying (MFD) can greatly reduce the drying time and has a notable sterilization effect.     

Effects of Different Drying Methods on the Quality Changes of Granular Edamame

In this article, the quality changes of the granular fruits and vegetables dried by vacuum microwave drying, freeze drying, hot air drying, and combined hot air-vacuum microwave drying are investigated, and the quality parameters compared on the basis of vitamin C and chlorophyll contents, shrinkage and rehydration capacity, color, texture, and microstructure changes. The quality parameters of products dried by vacuum microwave drying are slightly lower than those obtained by freeze drying, but much better than those obtained using conventional hot air drying. The quality characteristics of product dried by combined hot air-vacuum microwave are significantly improved compared to those simply hot air-dried.     

Effects of Different Drying Methods on the Quality Changes of Granular Edamame

In this article, the quality changes of the granular fruits and vegetables dried by vacuum microwave drying, freeze drying, hot air drying, and combined hot air–vacuum microwave drying are investigated, and the quality parameters compared on the basis of vitamin C and chlorophyll contents, shrinkage and rehydration capacity, color, texture, and microstructure changes. The quality parameters of products dried by vacuum microwave drying are slightly lower than those obtained by freeze drying, but much better than those obtained using conventional hot air drying. The quality characteristics of product dried by combined hot air–vacuum microwave are significantly improved compared to those simply hot air–dried.     

Freeze Drying of Saffron (Crocus sativus L.)

Saffron obtained from the dried stigmas of a flower scientifically known as Crocus sativus L. is considered to be the most precious and expensive agricultural product due to its labor-intensive harvest and post-harvest processing. The post-harvest processing such as dehydration and storage conditions determine stability, quality, and economical value of the final product. The contents of crocin (degraded carotenoids) and safranal (carotenoid oxidation products) are the key components that characterize color, taste, and aroma characteristics of saffron. In this work, the quality parameters such as crocin and safranal contents of commercial saffron that were obtained by using the freeze-drying method and natural sun drying were studied. The sarfanal contents of the samples dried in a freeze dryer were found to be five times higher than the safranal contents of the samples dried naturally under the sun, while crocin contents of the samples dried in a freeze dryer were about 40% higher than the crocin contents of the samples dried naturally under the sun. These encouraging results indicate that the freeze-drying process can be used effectively for dehydration of saffron by minimal loss of safranal and crocin contents.     

Drying of Apple Slices in Atmospheric and Vacuum Freeze Dryer

Atmospheric freeze drying (AFD) and vacuum freeze drying (VFD) of Fuji apples was investigated. A factorial design was used in each case, with particle size, freezing rate, infrared (IR) radiation, and air temperature as factors. The effect of these factors on moisture content, duration of drying, rehydration properties, color, and texture were determined. The drying curves were fitted with both the simplified constant diffusivity model (SCDM) and the modified Page model, and rehydration curves were tested with the Peleg model, resulting in R² higher than 0.96. Antioxidant capacity, polyphenols, and ascorbic acid content in the final product were determined and compared with those obtained in a tunnel dryer. A sensory evaluation was performed.

The drying times obtained for VFD were shorter than the drying times obtained for AFD. The particle size and IR application were found to be the significant parameters that affect duration of drying for both AFD and VFD (duration of drying was proportional to particle size and inversely proportional to IR application). Air temperature affected drying time only during the secondary drying stage. Estimated D_eff values were on the order of 10^?10 (m²/s). In AFD, R² higher than 0.81 was obtained for the SCDM, and R² higher than 0.96 was obtained for the Page model. In VFD, a better fit was obtained (R² > 0.97) for both models. AFD produced nutritional alterations similar to convective drying. Sensory quality was not altered by AFD or VFD.     

Freeze Drying of Aluminum Nanoflakes from Cyclohexane Suspensions

Hydrophobic aluminum nanoflake slurries were freeze dried using varying freezing conditions, aluminum concentrations, and shelf temperatures. The effectiveness of using cyclohexane as a carrier to produce dry, de-agglomerated aluminum powders was determined from light scattering measurements. Significantly less aggregation was observed in the dried aluminum powders when cyclohexane was used as opposed to water, with most aggregation being prevented by freeze drying dilute aluminum slurries at low shelf temperatures. A significant increase in the primary rate of freeze drying was also observed when particles were dried from cyclohexane compared to particles dried from water.     

介电材料辅助的微波冷冻干燥的实验研究

设计、加工和装配了一套实验室规模的微波冷冻干燥装置,旨在实验验证介电材料对微波冷冻干燥液体物料的强化作用。介电材料用烧结的碳化硅(SiC),石英玻璃作为介电材料的参照物;甘露醇,一种典型的药物赋形剂被选为待干溶液中的溶质。实验结果表明使用介电材料可以有效地强化微波冷冻干燥过程。与传统冷冻干燥相比干燥速率大大加快,在试验条件下干燥时间可以节省20%。微波加热逐渐生效并且主要体现在干燥过程的后半部分。当溶液中的固含量很低或者固体物质具有很小的介电损耗因子时,如果不用介电材料,微波加热的效果不明显。     

关于真空冷冻千燥机新型环保制冷剂的思考简

本文首先叙述了国际上对制冷剂的分类,然后讲述了国内生产的真空冷冻干燥机采用环保型制冷剂的工作进程,最后介绍了欧盟对环保型制冷剂的最新要求及我们的对策及思考等。     

Combined Microwave-Vacuum and Freeze Drying of Carrot and Apple Chips

A combination of microwave-vacuum (MWV) drying and freeze drying was investigated as potential means for drying carrot and apple chips. The sample was first dried by microwave-vacuum to dehydrate some amount of internal free water and then by freeze drying to a final moisture content of less than 7% (wet basis). Chemical properties (carotene and vitamin C retention) and physical properties (shrinkage, color, texture, and rehydration ratio) of carrot and apple slices dried by this method were evaluated and compared with those dried by freeze drying alone, MWV drying alone, and conventional hot air drying, respectively. The comparison showed that the carotene retention of carrot slices and the vitamin C retention of apple slices dried by the current method were close to those of freeze-dried carrot and apple slices and much better than those of conventional hot air–dried ones. The samples prepared by the current method exhibited very close rehydration capacity, color retention, and texture with those of the freeze-dried ones but with a little higher shrinkage. However, the samples still showed the attractive external appearance without marked warp.     

Trends in Microwave-Assisted Freeze Drying of Foods

Microwave-assisted freeze drying (MFD) can be accomplished in two distinct ways: freeze drying assisted concurrently with microwave application (MFD-1) and freeze drying and assisted microwave/vacuum microwave drying in two consecutive separate drying stages (MFD-2). MFD is a rapid dehydration technique that can be applied to certain foods, particularly to seafoods, solid soup, and fruits and vegetables. MFD involves much less drying time and energy consumption than conventional freeze-drying methods. Currently, this technology has been successfully used to dry many food materials and has potential in the food industry. Increasing concerns over product quality, energy savings, and production costs have motivated researchers and the industry to adopt MFD technologies. The advantages of MFD include shorter drying time, energy savings, improved product quality, and flexibility in producing a wide variety of dried products. However, current applications are limited to small categories of foods due to high startup costs and relatively complex technology compared to conventional freeze drying. This article presents a concise review of recent progress in MFD R&D and makes recommendations for future research to bridge the gap between laboratory research and industrial applications.     

A Combination of Freeze Drying and Microwave Vacuum Drying of Duck Egg White Protein Powders

High energy consumption during freeze drying (FD) is a major concern that limits its application on common food product manufacturing. In this research, fresh duck egg white protein (FDEWP) powder and desalted duck egg white protein (DDEWP) powder were obtained by a combined two-stage tandem drying technologies (FD and microwave–vacuum drying [MVD]) in order to reduce energy consumption while maintaining good product quality. The results showed that the drying time for the FDEWP and DDEWP powders was significantly decreased by FD + MVD compared to those obtained by the FD-only process. The FDEWP powders dried by FD + MVD had a better color (higher L* and lower b*), lower apparent density, and lower foaming stability but higher emulsifying index than those dried by FD only. The DDEWP powder dried by FD + MVD had a product quality similar to that of FDEWP powder, suggesting that the DDEWP powder could be widely used as a food ingredient.     

Comparison of the Retention of 6-Gingerol in Drying of Ginger Under Modified Atmosphere Heat Pump Drying and other Drying Methods

A heat pump dryer using normal air, nitrogen, and carbon dioxide was selected to dry sliced West Indian ginger (Zingiber officinale Roscoe) rhizome (3 mm in thickness). The drying characteristics were compared with each other and inert gas heat pump drying showed an improved effective diffusivity. Quantities of the main pungent principle (6-gingerol) of ginger, extracted from these dried samples, were determined by high-pressure liquid chromatography (HPLC). The evaluation included dried samples obtained by heat pump, modified atmosphere heat pump, freeze drying, and vacuum drying. Retention of 6-gingerol increased in the order of normal air drying, freeze drying, nitrogen drying, carbon dioxide drying, and vacuum drying. From this point of view, inert gas also showed a better retention of flavor compared to most other types of drying.