Lesson 6. Principle of drying

6.1 Introduction

Drying is one of the oldest methods of fruits and vegetables preservation. It is currently a versatile and widespread technique in the food industry as well as a subject of continuous interest in food research. Drying is a critical step in the processing of dehydrated products because of the high energy requirement of the process (due to low thermal efficiency of dryers). The main aim of drying fruits and vegetables is the removal of moisture up to certain level at which microbial spoilage and deterioration chemical reactions are greatly minimized. In addition to preservation, the reduced weight and bulk of dehydrated products decreases packaging, handling, and transportation costs. Furthermore, most food products are dried for improved milling or mixing characteristics in further processing. In contrast, with literally hundreds of variants actually used in drying of particulates, solids, pastes, slurries, or solutions, it provides the most diversity among food engineering unit operations.

Currently, dehydrated fruits, vegetables, grains and spices command considerable importance in the Indian and international market. These dehydrated products are the single largest import item in Europe and United States both in quantity and value items, as these products are used by every home, canteen, cafeteria, restaurant and other institutional food establishment. At present, instant beverage powders, dry soup mixes, spices, coffee, and ingredients used in food transformation are the major food products that are dehydrated. Also in India, dehydrated products are required for armed forces. In India, generally the agricultural production exceeds the requirement. Due to lack of proper post harvest management and storage facility for agricultural products, a considerable percentage of it produced goes as waste. Therefore, India needs processing of agro produce to convert them into stable products so as to minimize losses due to waste during the post harvest phase.

Fresh agricultural product is a perishable commodity in tropical countries as higher temperature causes it to wilt and gives a poor appearance. Therefore, the refrigeration and controlled atmosphere storage have been used to increase their storability. The shelf life of agricultural products can also be enhanced by drying. These products are generally dried by hot air. Sun drying is the most common method to preserve the agricultural products in most of tropical countries. However, this technique is extremely weather dependent and has the problems of contamination with dust, soil, sand particles and insects. Also, the required drying time can be quite long. Therefore, mechanical dryers, which are rapid, providing uniformity in drying and hygiene, are inevitable for industrial food drying processes. Agricultural material are dried by several methods like sun drying, hot air drying, fluidized bed drying, heat pump drying, freeze-drying, microwave hot air/vacuum drying, vacuum drying and hybrid drying. For improving quality of dehydrated products the pretreatments like osmotic dehydration, blanching, dipping in chemical solutions and microwave heating are common.

In short the main objectives of drying are:

– Extended Storage Life

– Quality Enhancement

– Ease of Handling

– Further Processing

The drying requires different pr processing operations of the product. These are operations are based on the product requirement. The Figure 1 shows steps to be carried in drying operation.

Figure 6.1. Steps to be carried in drying operation

 6.2 Theory of Drying

As most of drying operations require air to remove moisture from the product, it is necessary to know the some important properties of air related to drying. These properties are used to estimate the drying rate of the product. The properties of air water vapor mixture are also known as psychrometric properties of air.

There are three inter-related factors that control the capacity of air to remove moisture from a food:

1. The amount of water vapour already carried by the air

2. the air temperature

3. the amount of air that passes over the food.

6.3 Fundamental properties of water vapor and air mixtures related to drying

The most important psychrometric properties of air are as follows:

Humidity (H) and relative humidity (RH) are calculated according to the following equation:

where Mw is the molecular weight of the moisture vapor, Mg is the molecular weight of dry air (gas), P is the total pressure, and Pw is the partial pressure of water vapor.

When the partial pressure of the vapor in the gas phase equals the vapor pressure of the liquid at the temperature of the system (T), the gas is saturated. The relative humidity is a measure of moisture saturation. It is defined as the ratio of the partial pressure of water vapor in a gaseous mixture with air to the saturated vapor pressure of water at a given temperature. The relative humidity is expressed as a percentage and is calculated in the following manner:

where Pw0 is the saturated vapor pressure.

The dry bulb temperature (Tdb ) is the temperature of the air as measured by a thermometer freely exposed to the air but shielded from radiation and moisture.

The wet bulb temperature (Twb ) is measured by a gas passing rapidly over a wet thermometer bulb. It is used along with dry bulb temperature to measure the relative humidity of a gas.

The dewpoint is the temperature at which air becomes saturated with moisture (100% RH) and

any further cooling from this point results in condensation of the water from the air.

The relationships between air and water vapor and the psychrometric properties of moist air are commonly found in the form of psychrometric tables and chart which is shown in Figure 5.2.

Figure 6.3: Graphical representation of psychrometric properties of air water vapour mixture

6.4 Fundamental Properties of food product related to drying

In addition to air properties there are some properties of food which play important role in drying. These are:

The moisture content of a material is the weight of water per unit weight of wet solid (wet basis, Xw) or the weight of water per unit weight of dry solid (dry basis, X). They are related in the following manner:

The moisture ratio (MR) is the moisture content of a material during drying. It is usually expressed in a dimensionless form as:

where X is the moisture content at any time t, Xe is the equilibrium moisture content, and X0 is the initial moisture content of the product.

Water activity (aw) is an index of the availability of water for chemical reactions and microbial growth. It can be defined by the following equation:

Moisture content can be classified according to its availability in the food matrix in following types:

1. Bound moisture: Bound moisture is the amount of water tightly bound to the food matrix, mainly by physical adsorption on active sites of hydrophilic macromolecular materials such as proteins and polysaccharides, with properties significantly different from those of bulk water.

2. Free moisture content: Free moisture content is the amount of water mechanically entrapped in the void spaces of the system. Free water is not in the same thermodynamic state as liquid water because energy is required to overcome the capillary forces. Furthermore, free water may contain chemicals, especially dissolved sugars, acids, and salts, altering the drying characteristics.

An important term in drying is the equilibrium moisture content, which is the moisture content of a product in equilibrium with the surrounding air at given temperature and humidity conditions. Theoretically, it is the minimum moisture content to which a material can be dried under these conditions. A plot of the equilibrium moisture content versus the relative humidity or water activity at constant temperature, which is called sorption isotherm, is used to illustrate the degree of water interactions with foods. The value of the equilibrium moisture content for some solids depends on the direction from which equilibrium is approached, and the desorption equilibrium is of particular interest for drying calculations. The equilibrium moisture content for biological materials generally increases rapidly with a relative humidity above 60 to 80% because of capillary and dissolution effects.

6.5 Moisture migration during drying

Water migration in foods is an important phenomenon in drying. During drying heat flows over the product and goes in to the product. This heat increases the temperature of product and moisture which converts the moisture in to water vapor which results in to increase in the vapour pressure that moves moisture towards the surface (Figure 6.5).

Figure 6.5. Heat and moisture flow during hot air drying of food slab.

From the surface the moisture moves in to the environment. To make easy movement of moisture from product surface to environment there should not be any resistance. The resistance is generally arrived if there is more moisture in the air (which is related to relative humidity of air). Therefore, it is always necessary to use the drying air having low relative humidity. For this purpose, most commonly used convection drying method uses the hot air having temperature in the range 50° to 90°C. Due to increase in the air temperature relative humidity decreases which helps to remove the moisture from the product rapidly and high temperature of air transfers heat to product to evaporate the moisture within the product Also, it can be understood as a driving force for drying. It is defined as the difference in partial pressure of water vapor in the air and the pressure of the moisture in the product. This is a simple theory of moisture movement during drying.  Actually the drying is very complicated phenomenon. Moisture in foods is subdivided in to ionic groups, such as carboxyl and amino acids; hydrogen groups, such as hydroxyl and amides and unbound free moisture in interstitial pores and intercellular spaces (Figure 4). Therefore, the moisture movement within product (internal mass transfer) takes place by combination of several phenomenon like vapor diffusion, liquid diffusion, pressure diffusion, capillary movement, flow by evaporation – condensation sequence and gravity flow. Internal mass transfer is generally recognized to be the principal rate-limiting step during drying. After the moisture reaches the surface of the product it can be removed by convection and diffusion in to the atmosphere (external mass transfer). Therefore diffusion is one of the most important transport phenomenons in the drying and dehydration. While drying takes, the heat transfer also takes place in the product either by conduction, convection or radiation as shown in the figure 5.4.

Figure 6.5.1. Internal and external heat and mass transfer during drying

Last modified: Monday, 30 September 2013, 4:22 AM