Drying of Foamed Biological Materials: Opportunities and Challenges

An overview is provided on hot air drying of foamed materials in a thin layer (foam-mat drying), foam-spray drying, microwave-assisted drying of liquid foams, as well as microwave drying of frozen foams with and without dielectric solid inserts, used as complementary heat sources. In particular, the mechanisms of heat and moisture transport during the drying of foams are identified. The effects of foam characteristics (e.g., foam density and stability) and drying conditions (temperature, air velocity) on drying kinetics and product quality are examined, and the differences between the drying of non-foamed and foamed materials are discussed.     

US-Assisted Convective Drying of Biological Materials

The aim of this work is to promote ultrasound as an energy source that is suitable for the enhancement of drying processes, and in particular of biological materials. The study aims at a more profound recognition of the interaction between ultrasonic (US) waves and the tissues of fruits and vegetables, which may contribute to an intensification of moisture removal during their drying. Absorption of acoustic energy causes heating and structural changes of the drying material due to a series of rapid material compressions and decompressions. The research hypothesis is based on the expectation that the ultrasound waves may enhance moisture removal from the fruits and vegetables during drying due to a special “heating effect,” “vibration effect,” and “synergistic effect.” Convective ultrasonic-assisted drying tests were conducted experimentally on a new hybrid dryer with ultrasonic equipment, and the effects of ultrasonic enhancement by drying are presented based on a drying model and assessed numerically.     

Microwave-Assisted Thin Layer Drying of Wheat

Drying characteristics of Canada Western Red Spring (CWRS) wheat were studied using a domestic microwave convective oven. The effects of microwave power level, grain bed thickness, and initial grain moisture on the drying kinetics were investigated. Wheat samples with initial moisture levels of 0.18 to 0.29 kg water/kg of dry matter were dried for different drying periods of 180 to 360 s. The moisture loss data were recorded at regular short intervals. Then moisture loss data were fitted to various models (Page equation, modified drying equation, and Midilli equation) to study the drying kinetics of wheat. The results showed that wheat moisture loss increased with increasing microwave power level. A mathematical model was developed by coupling mass and energy balances, resulting in a system of non-linear equations. The predicted moisture loss data from the developed model were compared by fitting to experimental microwave data that were in good agreement.     

Surface Stickiness of Drops of Carbohydrate and Organic Acid Solutions During Convective Drying: Experiments and Modeling

Abstract

Drying kinetics of low molecular weight sugars such as fructose, glucose, sucrose and organic acid such as citric acid and high molecular weight carbohydrate such as maltodextrin (DE 6) were determined experimentally using single drop drying experiments as well as predicted numerically by solving the mass and heat transfer equations. The predicted moisture and temperature histories agreed with the experimental ones within 6% average relative (absolute) error and average difference of ± 1°C, respectively. The stickiness histories of these drops were determined experimentally and predicted numerically based on the glass transition temperature (T _g ) of surface layer. The model predicted the experimental observations with good accuracy. A nonsticky regime for these materials during spray drying is proposed by simulating a drop, initially 120 µm in diameter, in a spray drying environment.     

Surface Stickiness of Drops of Carbohydrate and Organic Acid Solutions During Convective Drying: Experiments and Modeling

Drying kinetics of low molecular weight sugars such as fructose, glucose, sucrose and organic acid such as citric acid and high molecular weight carbohydrate such as maltodextrin (DE 6) were determined experimentally using single drop drying experiments as well as predicted numerically by solving the mass and heat transfer equations. The predicted moisture and temperature histories agreed with the experimental ones within 6% average relative (absolute) error and average difference of ± 1°C, respectively. The stickiness histories of these drops were determined experimentally and predicted numerically based on the glass transition temperature (T_g) of surface layer. The model predicted the experimental observations with good accuracy. A nonsticky regime for these materials during spray drying is proposed by simulating a drop, initially 120 µm in diameter, in a spray drying environment.     

奶粉的喷雾冷冻干燥研究

研究开发了实现喷雾冷冻干燥工艺的装置,并以奶粉为物料,进行了试验研究。结果表明,料液可以在-40～-30℃实现快速冷冻,且采用该喷雾冷冻干燥装置可将干燥时间从原来真空冷冻干燥的48h缩短为10h。最后,对获得的干燥产品进行了粉体形态的测试和分析。     

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.     

Experimental Study on Microwave Vacuum Drying of Two-Porosity Level Media

Abstract

Microwave vacuum drying experiments were performed on a laboratory scale dryer with a two-level porosity material: a packed bed of porous alumina beads. The incident microwave power and the vacuum pressure level were fixed, the main varying parameters being the beads diameter and porosity, and the mean pore diameter. The drying kinetics and the evolution of the product temperature are presented. The drying kinetics can be divided into two main periods. The first one corresponds to the drying from the bed voids according to evaporation mechanism that we describe with a stagnant film law. The second one corresponds to the drying from the particle pores and we divide it into two parts: we suggest that the former is dominated by capillarity driven moisture transport, and the latter is limited by the desorption kinetics of the few water layers left.     

Experimental Study on Microwave Vacuum Drying of Two-Porosity Level Media

Microwave vacuum drying experiments were performed on a laboratory scale dryer with a two-level porosity material: a packed bed of porous alumina beads. The incident microwave power and the vacuum pressure level were fixed, the main varying parameters being the beads diameter and porosity, and the mean pore diameter. The drying kinetics and the evolution of the product temperature are presented. The drying kinetics can be divided into two main periods. The first one corresponds to the drying from the bed voids according to evaporation mechanism that we describe with a stagnant film law. The second one corresponds to the drying from the particle pores and we divide it into two parts: we suggest that the former is dominated by capillarity driven moisture transport, and the latter is limited by the desorption kinetics of the few water layers left.     

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.     

Experimental Validation of Thin‐layer Drying Models

The present work was aimed at investigating the applicability of the proposed correlation equations that link the coefficients of the thin‐layer equations. Materials of different properties were chosen and dried in four types of laboratory dryers. The experimental data were approximated applying the commonest thin‐layer equations: the Lewis equation, the modified Page's model, and Fick's second law. The best match was achieved with the modified Page's model. The applicability of the stated literature correlations was confirmed. Irrespective of the match between measuring data (X = f(t)) and Lewis equation, the evaluated drying constant K allowed evaluation of the parameters of the employed two‐parameter models.     

Manufacturing Better Quality Food Powders from Spray Drying and Subsequent Treatments

In this article three major aspects of spray-dried food powders are discussed. We first address several practical problems involved during spray drying that may greatly influence product quality. The second issue identifies how an accurate drying kinetics model can form a useful tool to predict changes in the physical and biological quality aspects and the microstructure of the particle during processing. Dryer-wide simulations using the accurate drying kinetics model can significantly reduce the number of experimental trials for optimizing the process. To date, such success has been restricted to production runs for pilot-scale or small-scale industrial operations. The final issue addresses some of the challenges encountered when evaluating the functionality of the spray-dried powders during their reconstitution. The superior functionality of the spray-dried food product needs to be established more scientifically, which can help commercial operations to achieve high-quality reconstitution.     

Drying of Diced Carrot in a Combined Microwave-Fluidized Bed Dryer

Drying of diced carrot (∼90% wet basis) was carried out in a laboratory microwave fluidized-bed dryer (MFB) and in a standard fluidized-bed dryer (FB). It was found that the drying time in the MFB dryer is 2-5 times shorter than in the FB dryer. Drying efficiency (DE) is a function of moisture content, microwave power and temperature of drying agent. Higher values of DE are obtained for MFB dryer. For both drying systems the water removal was proceeding in two-stage falling rate period (except short initial term).     

Research and Development Needs and Opportunities in Freeze Drying

ABSTRACT

Experimental and theoretical research and development studies are preposed and presented in important areas of the freeze drying process. The practical importance of the synergistic integration of advanced theoretical models and experiments is indicated, and the potential contributions of the proposed     

Research and Development Needs and Opportunities in Freeze Drying

Experimental and theoretical research and development studies are preposed and presented in important areas of the freeze drying process. The practical importance of the synergistic integration of advanced theoretical models and experiments is indicated, and the potential contributions of the proposed     

Effect of Convective Air Attributes with Microwave Drying of Soybean: Model Prediction and Experimental Validation

The heat and mass transfer that occurred during drying of soybeans by a combined process using microwave (MW) and convective hot air was studied. A coupled mathematical model was developed to simulate this phenomenon. The soybean samples were re-wetted to 20% wet basis, the selected level of initial moisture content (IMC), and then dried in a domestic microwave oven under various MW power levels from 300 to 390 W, using inlet air with relative humidity of 35, 55, 75, and 95%. The simulated moisture loss profiles obtained from the coupled model compared well with those obtained in the experiments. Results showed that the drying rate decreased from 6.235 × 10^?5 to 6.192 × 10^?5 kg water/(kg wb s) as the inlet air temperature increased from 30 to 60°C. Furthermore, the drying rate was observed to increase from 6.192 × 10^?5 to 6.211 × 10^?5 kg water/(kg wb s) as the relative humidity (RH) increased from 35 to 95%.     

Optimizing Drying Conditions for the Microwave Vacuum Drying of Enzymes

The aim of this study was the methodological use of experimental planning for the optimization of microwave vacuum drying of enzymes using α-amylase as a model. A factorial in star designwas used to optimize the microwave vacuum–drying process, and the variables were power output and vacuum pressure. The material dehydrated by this technique was analyzed with regard to its enzymatic activity, water activity, and moisture content. Response surface methodology was used to estimate the main effects of vacuum pressure and power on the enzymatic and water activities. The experimental in star design revealed that microwave vacuum drying is influenced mainly by power. The dehydrated product showed high enzymatic activity and low water activity.     

An Experimental Study on Freeze-Drying Behavior of Strawberries

In this article, the freeze-drying behavior and the drying kinetics of strawberries were investigated. Drying experiments were performed in a freeze-drying experimental setup constructed in the Department of Mechanical Education, Faculty of Technical Education, Gazi University, Ankara, Turkey. In experiments, 5- and 7-mm-thick sliced strawberry samples have been used. Some models, which were given in the literature, have been used to predict the drying behavior of strawberries. Experimental data findings were fitted for determining the best model to represent the drying behavior of strawberries and the drying kinetics used in these models. It was seen from the correlation coefficient, reduced chi-square, and root mean square error (0.9984, 2.688 × 10^?4, 0.015 and 0.9976, 4.669 × 10^?6, 0.002, respectively) that the Page model could sufficiently describe the drying behavior of 5- and 7-mm strawberry samples.     

Partial Crystallization Behavior during Spray Drying: Simulations and Experiments

Simulations of crystallization behavior in a spray dryer have been performed using modifications to a model developed by previous workers and applied to a Buchi-290 laboratory-scale dryer. The potential crystallization behavior has been modeled using Williams-Landel-Ferry kinetics. Explorations using the model have suggested that the air inlet temperature is an important variable affecting crystallization in the dryer. The explorations suggested drying conditions that permitted reasonable drying while controlling the degree of crystallization. These conditions were examined and tested experimentally, also showing that the apparent degree of crystallinity was affected by the inlet air temperature over the range of inlet temperatures from 134°C (∼55% crystalline) to 210°C (∼76% crystalline). The simulation also predicted the trends in the experimental results from previous workers where their experimental results are considered in terms of the measurement techniques used in each case, suggesting that the simulation is a reasonable tool for developing operating conditions for drying equipment to give low or high degrees of crystallinity in the products.     

Theoretical Drying Model of Single Droplets Containing Insoluble or Dissolved Solids

A literature survey of published single-droplet drying models has exposed their various shortcomings. An advanced theoretical model of drying of single droplets, containing either insoluble or dissolved solids, has been developed and numerically solved. The validation of the developed model has been accomplished by comparison of the predicted temperature and mass histories with published experimental data for slurries of silica and skim milk solution under different conditions. The results of simulations showed a good agreement between the calculated curves and experimental points in all case studies.