Introductory Chapter: A Comprehensive Review of the Versatile Dehydration Processes

1. Introduction

Water is the most abundant substance on the Earth and the main component of plant and animal tissues, in which it plays a role as a solvent and a reagent. The unique role of water in natural processes is related to its physical and chemical properties and its widespread. As a result, most materials in the natural conditions contain water either as chemically bound or retained in pores due to intermolecular interactions [1]. The presence of water in food and foodstuffs plays a significant role in the physicochemical and biological processes that take place during their storage [2].

Dehydration is a complex reversible and endothermic physicochemical process of removing water from the material, which takes place under conditions of simulated energy exchange (especially heat) and mass transfer between the material and the external environment [3]. The removal of water is a kinetically complex process characterized by either rapid nucleation of water molecules at the reaction boundary phase (RBP) or nucleation at certain locations of the boundary phase (RBP), after which there is an increase in the size of the nucleus, which leads to the removal of water from the material. The most important feature of the dehydration process is the dominant influence of the dehydration product on the mechanism and kinetics of water removal from the material [4].

2. Food dehydration

The water content in food, fruits, vegetables, foodstuffs, and in agricultural products varies in a wide range from 60 to 98% by mass. The dominant content of water indicates the key role of water on the physicochemical, biological, and nutritional and sensing properties of the food. Water is, first of all, the most important medium in which chemicals (ions, salts, vitamins, etc.,) and biological reagents (sugars, proteins, lipids, DNA, enzymes, etc.,) move, collide, and react. In addition, water participates such as (a) reagent and coreagent in a series of degradation reactions (hydrolysis of lipids, Maillard reactions, enzymatic browning, vitamin degradation, etc.,); (b) stabilizes the most important biological structures, (enzymes, proteins, DNA, and cellular membrane); (c) control the growth of pathogens and other microorganisms; (d) significantly changes the physical and chemical properties of the material (thermal conductivity, thermal capacity, electrical and dielectric properties), etc. [2]

In regard to that, it is unambiguous that state of water and its physical–chemical (structural–kinetic) during the dehydration of the material has the key influence on dehydration process, chemical and biological reactions, and nutritional and sensory properties of food. Water in food and foodstuffs exists in three different structural-kinetic states, namely bond water, intermediate, and freeze water. Bound water is formed as a consequence of the formation of hydrogen bonds between water molecules and polar groups on RBP. Free water is formed by a mutual interaction between water molecules and is similar in structure to bulk water. Intermediate water is formed from water molecules that interact weekly with RBP. Different structural-kinetic states of water lead to different interactions between water molecules and chemical components of food [5]. In order to understand and govern food dehydration process, it is necessary to focus further research on expanding and deepening the knowledge to (a) structural-kinetic state of water in food, (b) change in the structural-kinetic state of water due to dehydration; (c) interaction of different states of water with chemical and biological components of food.

The term “drying” is a synonym for food dehydration by application of heat and is the oldest method for preserving food. The main reason for drying food is to extend the shelf life of fresh materials without the use of cooling and storage, because the reduction of water content inhibits growth and the development of spoilage and pathogens microorganism reduces the activity of enzymes and reduces unwanted degradation reactions. The process of drying food also leads to a reduction in the weight and volume of food, which significantly affects the costs of packaging, storage, and transportation of food. Drying food leads to a change in color, texture, and smell compared to fresh material and a reduction in the nutritional value of food [6].

Drying is an energy-consuming process and the cost of used energy compared to other storage methods is relatively high with predictable growth for the near future. Accordingly, with aim to reducing specific energy consumption of drying and obtaining a product with preserved nutritional and sensory properties, a number of conventional (sun, hot-air, spray-draying, freeze-drying, fluidized-bed drying, and osmotic dehydration) and innovative (microwave drying, infrared drying, solar drying, electric and magnetic field dewatering, and ultrasonic dehydration) food-drying methods have been developed. In order to improve the existing technological processes of food drying and developing new ones, further research should be focused on the (a) development and advancement of new processes of uniform volume of heating of food; (b) determination and control of the physical-chemical state of water molecules inside the tissue during dehydration and the ones that move inside the material when leaving; (c) understanding and managing of the process of changing the state of the matrix during dehydration.

3. Kinetics models of hydrogel dehydration

Hydrogels are mainly defined as three-dimensional, cross-linked hydrophilic polymeric networks which have the ability to absorb a significant amount of water or other aqueous fluids (swelling) without dissolving or losing structural integrity [7]. Hydrogels are extremely prominent against other polymeric materials because of their characteristic properties such as smart response to external stimuli, swelling ability, high water content, biocompatibility, adjustable porosity, and mechanical properties. The most outstanding are their high swelling capacity and resemblance to living tissues more than any other type of artificial biomaterials. Because of these distinguishing properties, hydrogels have been widely used in versatile applications from biomedical to green energy. Their use are mostly recognizable in pharmacy, medicine, and biomedical applications [8, 9], especially in controlled and targeted drug release [10], regenerative medicine and tissue engineering, contact lenses, biosensors, etc. [11]. Hydrogels are excellent for applications in biotechnology, environmental protection, agrochemistry, horticulture, cosmetics, as superabsorbents in hygiene products, packaging materials for food storage, in textile materials, in sensor materials, etc. In recent times, the applications of hydrogels and hydrogel-derived materials present novel materials for electrochemical energy conversion systems due to their specific and tailorable physicochemical properties [12].

Due to high water content, hydrogels should be assumed as model systems suitable for modeling the description of the kinetics of food dehydration. Similarly, as in food, water in hydrogels can be classified generally into three types: (a) bound water, which involves strongly bound and weakly bound; (b) associated water, involving strongly associated and weakly associated water; and (c) free water. According to their phase transition behavior, three types of water in hydrogels have been identified: nonfreezing, freezing bound, and free non-bound water [13]. Many physical properties of hydrogels depend on the organization of water within and at the surface of hydrogels [14]. The structures of the polymer network and the embedded water are important factors governing the physicochemical properties of hydrogel materials [15].

Knowledge and governing of the hydrogel dehydration process is of extraordinary practical and theoretical importance. In the literature, the kinetics of dehydration of hydrogels is most often described by the diffusion kinetic model [16]. However, new kinetic models have been developed that can describe hydrogel dehydration more precisely and with a higher degree of reliability. The complex kinetics of dehydration of hydrogels was described by a series of novel kinetic models: distribution apparent energy activation model, Webull’s distribution of reaction times, the dependence of the degree of conversion (α) on the temperature which is defined by the logistic function, coupled single step-approximation with iso-conversional curve. These models were applied to evaluate the dehydration kinetics of different hydrogels: poly(acrylic acid) hydrogel, poly(acrylic-co-methacrylic acid), and poly(acrylic acid)-g-gelatin. It was determined that these new kinetic models can very appropriately describe the kinetics of dehydration of the investigated hydrogels covering the whole range of the dehydration process. The correlations among the values of the rate constants (k), activation energy (Ea), preexponential factor (lnA), with the primary structural properties of the investigated xerogels (hydrogels in dry state) were determined [17, 18, 19, 20, 21, 22].

4. Conclusions

Understanding and governing the possibility of controlling the structural–kinetic states of water in food has a key role (key-role) in the mechanism and kinetics of food dehydration and preservation.

Dehydration is one of the most important operations in food science. Dehydration enables extension of shelf life and preservation of their physicochemical, biological, nutritional, and sensing properties. Understanding and managing the possibility of controlling the structural-kinetic states of water in food has a key role (key-role) in the mechanism and kinetics of food dehydration and the preservation of physicochemical, biological, and nutritional and sensing properties.

Due to their unique properties, hydrogels found versatile application. Knowledge and governing of the hydrogel dehydration process is of extraordinary practical and theoretical importance. Hydrogels are suitable for modeling the kinetics of food dehydration.