Advanced Drying Technologies for Foods

Arun S Mujumdar, Hong-Wei Xiao, Arun S Mujumdar, Hong-Wei Xiao

• Publisher: CRC Press
• Year: 2019
• ISBN:
  9781000012316
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Food Science
• Index: Technology & Engineering

The goal of all drying research and development is to develop cost-effective innovative processes that yield high-quality dried products with less energy consumption and reduced environmental impact. With the literature on drying widely scattered, Advanced Drying Technologies for Foods compiles under one cover concise, authoritative, up-to-date assessments of modern drying technologies applied to foods. This book assembles a number of internationally recognized experts to provide critical reviews of advanced drying technologies, their merits and limitations, application areas and research opportunities for further development.

Features:

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• Provides critical reviews of advanced drying technologies

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• Discusses the merits and limitations of a variety of food drying technologies

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• Explains drying kinetics, energy consumption and quality of food products

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• Reviews the principles and recent applications of superheated steam drying

The first four chapters deal with recent developments in field-assisted drying technologies. These include drying techniques with the utilization of electromagnetic fields to deliver energy required for drying, for example, microwave drying, radio frequency drying, electrohydrodynamic drying, and infrared radiation drying. The remainder of this book covers a wide assortment of recently developed technologies, which include pulse drying, swell drying, impinging stream drying, and selected advances in spray drying. The final chapter includes some innovative technologies which are gaining ground and are covered in depth in a number of review articles and handbooks, and hence covered briefly in the interest completeness.

This book is a valuable reference work for researchers in academia as well as industry and will encourage further research and development and innovations in food drying technologies.

1 Electrohydrodynamic Drying

Alex Martynenko and Tadeusz Kudra

Dalhousie University

CONTENTS

1.1 Introduction

1.2 Fundamentals of EHD Technology

1.3 Key Factors in EHD Drying

1.3.1 Voltage and Electrode Geometry

1.3.2 Effect of Airflow

1.3.2.1 Electrically Induced Airflow

1.3.2.2 Mechanically Induced (External) Airflow

1.3.3 Effect of Relative Humidity

1.4 Large-Scale EHD Dryers

1.4.1 Prototype EHD Dryers

1.4.2 Pilot-Scale EHD Dryers

1.5 Closing Remarks and R&D Needs

References

1.1 INTRODUCTION

Electrohydrodynamic (EHD) drying refers to removal of water from the wet material exposed to strong electric field due to aerodynamic action of the so-called corona wind, ionic wind, or electric wind. This wind originates from a sharp end of the electrically conducting pin (needle) or fine horizontal wire as a result of ions leaving this pin/wire and impinging the surface of the material being dried.

To avoid ambiguities and misinterpretation encountered in published papers on the phenomena arising from the application of high-voltage electric field (HVEF), a distinction should be made for EHD at the outset from the following:

1. Electroporation (electropermeabilization) where the material is exposed to a series of short-time (microseconds) strong electric field (over 22 kV/cm) pulses which result in discharge of electric sparks perforating the skin and disrupting the cellular structure of fruits, berries, or vegetables, thus facilitating the mass transfer in subsequent processes such as osmotic dehydration (Amami et al., 2005) or extraction of valuable compounds (Sack et al., 2010; Loginova et al., 2011; Boussetta and Vorobiev, 2014).
2. Application of high-voltage pulsed electric fields (PEFs) for inactivation of microorganisms, spores, and enzymes in liquid foods. Comprehensive yet concise information on PEF in various applications to foods is given by Barbosa-Canovas and Zhang (2001).
3. Alteration of the properties of fruits, vegetables, and other biomaterials in order to extend their shelf life, increase germination, and preserve antioxidant capacity and enzyme activity. Such perishable materials are placed between two parallel plate electrodes connected to a high-voltage power supply (Atungulu et al., 2005; Bajgai et al., 2006; Wang et al., 2008).
4. Any technologies based on the movement of ions in HVEF, thus giving rise to EHD micro/meso-pumps, thrusters, coolers, etc. (Seyed-Yagoobi, 2005; Aryana et al., 2016).
5. Dielectric barrier discharge (DBD) where high alternating current (AC) voltage is applied to electrodes with a dielectric material in between electrodes. It generates cold plasma, which is used for sterilization of food products (Mir et al., 2016) or polymeric packaging surfaces (Pankaj et al., 2014).

In contrast, EHD drying/dewatering is further referred only to those HVEF applications, where ionic wind is generated and a specific aerodynamic effect is created on the surface of drying material.

The EHD drying has recently gained an extensive interest due to its nonthermal nature, particularly suitable for heat-sensitive biomaterials such as foods (e.g., vegetables, mushrooms, berries, fruits, tofu, fish, scallop, and beef), medicinal plants (e.g., herbs, plants used in Chinese medicine), and biomaterials (e.g., probiotics, nutraceuticals, enzymes, functional foods, and starter cultures).

With respect to product quality, the following can be quoted as examples: lower shrinkage (Alem-Rajabi and Lai, 2005; Bajgai and Hashinaga, 2001a), higher rehydration ratio (Bajgai and Hashinaga, 2001a), and preserved vitamin content (Bajgai and Hashinaga, 2001b). Moreover, no discernible degradation is generally noted in terms of color (Alem-Rajabi and Lai, 2005; Bajgai and Hashinaga, 2001a, b; Esehaghbeygi and Basiry, 2011). However, Li et al. (2006) reported the distinctive browning of okara cake just under the needle electrode.

Energy consumption in both EHD and combined EHD–hot air drying is claimed to be much lower than in hot air drying (Kudra and Martynenko, 2015). However, it should be noted that the majority of authors report the net EHD energy consumption based on discharge energy calculated from the measured current and applied voltage, which ranges from 90 to 720 kJ per kg evaporated water. These numbers do not reflect the total energy consumed by high-voltage generator and peripheral equipment. Experimentally determined total energy consumption in EHD dryer indicated that 85%–99% energy was lost in the low-to-high direct current (DC) voltage convertor (Martynenko and Zheng, 2016).

Aside from purely experimental research on EHD drying of apples, carrot, potato, tomato, mushrooms, spinach, rapeseed, grapes, blueberry, cranberry, etc., as well as model materials such as water, paper tissue, agar gel, wet sand, and glass beads, theoretical studies on EHD drying are focused on the determination of the ionic wind characteristics, such as space charge and corona current distributions (Zhao and Adamiak, 2005; Ahmedou et al., 2009) or numerical solution of the mathematical model with experimental validation through drying experiments (Huang and Lai, 2010; Heidarinejad and Babaei, 2015; Zhong et al., 2018).

Because of the nature of ionic wind, EHD drying is considered as convective drying with all implications regarding drying kinetics and material temperature. Figure 1.1 presents typical kinetics of material moisture content (X) with time when exposed to air at constant velocity, temperature, and humidity (drying curve). Another curve supplementing the drying curve is the temperature curve illustrating the variation of material temperature with time as shown in Figure 1.1. Comprehensive information on specific features offered by these two types of curves can be found in topical books (Strumillo and Kudra, 1986; Kudra and Strumillo, 1998).

Referring to the drying curve shown in Figure 1.1, the segment A–B represents the so-called initial drying period when the wet material has initial moisture content (X_i) and initial temperature (T_mi). In this initial drying period, the material surface is covered with a water film considered as unbound moisture, so evaporation is controlled by the air boundary layer until the moisture content reaches its value at point B. Characteristically, the material temperature decreases due to evaporative cooling from its initial value to the wet bulb temperature (T_m,wb). This initial drying period in EHD drying is very short and often neglected. One exception is the EHD drying of okara cake, where both the initial drying period and the falling rate period are clearly visible (Li et al., 2006).

At point B, the nonlinear relation X = f(t) turns into the linear drop of moisture content with time that continues until the critical moisture content (X_cr) is reached at point C. The rate of water evaporation between points B and C is constant (constant drying rate period) due to unrestricted transport of liquid water from the material core to its surface. When X < X_cr, the amount of water reaching the material surface begins to decrease gradually so the water evaporation from the material is controlled by the liquid moisture transport from the wet core to the material surface (internal conditions). Since the gradient of moisture concentration reduces with time, the drying rate also decreases and the straight line turns into a curve, which asymptotically reaches thermodynamic equilibrium at X_eq (point E). The falling drying rate period between points C and D ends at the final moisture content X_f . Over the falling drying rate period, the material temperature increases exponentially from the web bulb temperature to the final material temperature T_m,f . Because EHD drying is regarded as a surface phenomenon, it is less efficient in the final stages of drying to remove the moisture trapped deep inside the material.

FIGURE 1.1 Typical drying curve (open circles) and material temperature curve (filled circles) for isothermal convective drying of material.

Depending on the type of a drying material and electric field strength, one could observe drying kinetics curves, different from those shown in Figure 1.1. For example, high moisture biomaterials, such as chlortetracycline known as Aureomycin™ (Na et al., 1999), agar gel (Isobe et al., 1999), or wet paper tissue (Martynenko et al., 2017), show relatively long constant rate drying. In contrast, biomaterials with internally controlled mass transfer, such as champignons (Martynenko and Kudra, 2017), wheat (Cao et al., 2004), rapeseed (Basiry and Esehaghbeygi, 2010), or apples (Martynenko and Zheng, 2016) dry entirely in the falling rate period. However, the majority of food and agricultural products, such as tomato slices (Esehaghbeygi and Basiry, 2011), ginseng (Na et al., 1999), or blanched mushrooms (Dinani et al., 2014), demonstrate both constant and falling rate periods of various lengths, depending on product physicochemical properties.

1.2 FUNDAMENTALS OF EHD TECHNOLOGY

Electrical current in gases attracted the attention of famous scientists, such as Clerk C Maxwell, Joseph J Thomson, and Ernest Rutherford, who studied it theoretically and experimentally (Townsend...

Table of contents

• Cover
• Half Title
• Series Page
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Editors
• Contributors
• Chapter 1 Electrohydrodynamic Drying
• Chapter 2 Drying and Instant Controlled Pressure Drop Swell Drying: Towards High-Quality Dried Foods and Starch-Free Snacks
• Chapter 3 Advances in Impinging Stream Processing of Agricultural and Biological Products
• Chapter 4 Pulse Combustion Drying
• Chapter 5 Advances in Production of Food Powders by Spray Drying
• Chapter 6 Advances in Microwave Drying
• Chapter 7 Infrared Radiation-Assisted Drying of Agricultural Products
• Chapter 8 Radio Frequency Drying of Agricultural Products and Foods
• Chapter 9 Miscellaneous Drying Technologies
• Chapter 10 Advances in Intermittent Batch Drying of Foods
• Index