Drying Methods

Quality requirements, raw material characteristics, and economic factors determine selection of a drying method as well as the operating conditions for a given fruit. Several drying methods are commercially used, each better suited for a particular situation.

The selection of drying methods depends on the following factors:

• Physical form of raw material: whole fruit (eg, cranberry, grape, plum, etc), sliced form (eg, banana, mango, kiwi, etc), liquid, paste, slurry, pulp, thick liquid, and large versus small aggregates

• Properties of raw material: sensitive to oxidation, sensitive to temperature damage, thermoplastic residues, and so on

• Susceptibility to microbial attack

• Sugar content

• Presence of a "skin" that acts as a barrier for water removal (eg, grapes, blueberries)

• Desired product characteristics: powder, instant solubility, retention of shape, and so on

• Value of the finished product: low, medium, or high price

The following three basic types of drying process are used for fruits:

1. Sun drying and solar drying are practiced for certain fruits, such as prunes, grapes, and dates.

2. Atmospheric dehydration processes, including:

a. stationary, or batch, processes (using kiln, tower, and cabinet dryers)

b. continuous processes (eg, tunnel, belt trough, conveyor, and fiuidized bed)

c. spray dryers, suitable for making powders from fruit juice concentrates, and drum, or roller, dryers, useful for drying fruit juice concentrates, slurries, or pastes

3. Subatmospheric dehydration techniques, including: a. vacuum dehydration processes, which are useful for the processing of low-moisture fruits with high sugar content, such as peaches, pears, and apricots b. freeze drying, which ensures high flavor retention and minimal damage to product structure and nutritional value; the finished product with open structure permits fast and nearly complete rehydration

More than 200 types of dryers are available commercially to process the diverse physical forms of foods and required product characteristics. For drying fruits, conventional types of dryers are commonly used because of their simplicity of construction and operation, as well as low cost. Most products are seasonal in nature, which means that dryers are operated only over a fraction of the year. Thus, the units must have low capital costs for economic reasons.

Sun Drying

Sun drying of fruit crops is still practiced unchanged from ancient times. This method is limited to climates with hot sun and dry atmosphere, and to certain fruits such as prunes, grapes, dates, figs, apricots, and pears. These crops are processed without much technical aid by simply spreading fruits on the ground, on racks, or roofs and exposing them to the sun until dry. Because sun-dried products generally have moisture levels no lower than 15 to 20%, they have a limited shelf life. It is a slow process, unsuitable for producing high-quality products, and dust, dirt, and insects often contaminate the finished products.

Solar Drying

In recent years considerable interest has been focused on the use of solar energy because of the rapid increase of fuel costs. In this method, solar energy is used alone or may be supplemented by an auxiliary energy source. A simple method of accelerating the sun-drying rate of fruit is to paint the trays black. A solar trough application of mirrors to increase solar energy and indirect solar dryers applying solar collectors are also recommended to improve the efficiency of this process.

Kiln Drying

The natural draft from rising heated air used for drying fruit is the oldest type of dehydration equipment still in commercial use. Kiln dryers generally have two levels: gas burners on the lower floor provide heat, and the warm air rises through a slotted floor to the upper level. Food materials such as apple slices are spread out on the slotted floor in a layer about 25 cm deep and turned over periodically by fork until they reach the desired moisture, usually between 16 and 24%.

Cabinet Drying

Cabinet dryers are arranged for batch operation and are usually held at constant temperature, though humidity may decrease during the drying process. They consist of an enclosed chamber fitted with a variable heater and a fan for air stirring, together with deflectors for airflow adjust ment, outlet air louvers, and adjustable inlet air louvers. Cabinet drying is a particularly useful research tool for establishing the drying characteristics of a new product, prior to a large-scale commercial run.

Tunnel Drying

The equipment is similar to a cabinet dryer except that it allows a continuous operation along a rectangular tunnel through which move tray-loaded trucks. The tunnel dryer provides rapid drying without injury to fruits and permits a uniform drying process; therefore, it is widely used in drying fruit.

Tunnel dryers are classified by the direction in which the air traverses the product. In a parallel flow unit, the fresh material encounters the driest and warmest air initially, and leaves the drier at the coldest end; in a counter-current flow unit, the air direction is opposite to the movement of the product, so the dry product leaving the drier encounters hot dry air. Multistage dryers consisting of three, four, or five dryer stages are also used. Such systems are flexible and can achieve close to optimum drying conditions for a wide variety of products.

Conveyor Drying

Conveyor dryers are continuous processing equipment and consist of an endless belt that carries the material to be dried through a tunnel of warm circulating air. The speed of the conveyor is variable to suit both the product and the heat conditions. Furthermore, process conditions are usually controlled by designing the system in sections, thus allowing different flow rates, humidities, and temperatures to be set in each section, and by rotating the product when it moves from one section of the belt to the next.

This drying method has the advantage of essentially automatic operation, which minimizes labor requirements. The conveyor dryer is best adapted to the large-scale drying of a single commodity for the whole operating season. It is not well suited to operations in which the raw material or the drying conditions are changed frequently, because extensive hours of start-up and shutdowns make it difficult to produce satisfactory products.

Spray Drying

Spray drying involves the dispersion of liquid or slurry in a stream of heated air, followed by the collection of the dried particles after their separation from the air. The process is widely used to dehydrate fruit juices.

The general construction of a spray dryer incorporates four main features:

1. a heater and at least one fan to produce air at the required temperature and velocity;

2. an atomizer or jet to produce liquid particles of the required size;

3. a chamber in which the liquid droplets are brought into intimate contact with the hot air; and

4. a means of removing the product from the air stream.

The final product is delivered as a free-flowing powder. Drum Drying

In drum drying, which is suitable for a wide range of liquid, slurried, and pureed products, a thin layer of product is applied to the surface of a slowly revolving heated drum. In the course of about 300 degrees of one revolution, the moisture in the product is flashed off, and the dried material is scraped off the drum by a stationary or reciprocating blade at some point on the periphery. The residence time of the product in the drier is on the order of two to few minutes.

Drum drying is an inexpensive method; however, its commercial application is limited to less heat sensitive products. Its usefulness for fruit dehydration is quite limited because the high temperature required, usually above 120°C, imparts a cooked flavor and off-color to most fruit products. Also, the high sugar content of most fruit juices makes them difficult to remove from the drum dryers because of their high thermoplasticity.

Vacuum Drying

Dehydration under vacuum has special merits for certain fruits in terms of final quality. Drying can be carried out at lower temperatures than with air drying, and heat damage is minimized. Furthermore, as drying is carried out with virtual absence of air, oxidation of the fruit is virtually eliminated.

Vacuum-drying systems have the following components: (1) a vacuum chamber, (2) a heat supply, (3) a vacuum-producing unit, and (4) a device to collect water vapor as it evaporates from the food. The vacuum shelf dryer, the simplest type of vacuum drier, is used to process a wide range of fruit products, including liquid, pastes, discrete particles, chunks, slices, and wedges. The equipment consists of a vacuum chamber containing a number of shelves arranged to supply heat to the product and to support the trays on which the product is loaded into the chamber. The shelves may be heated electrically or, more often, by circulating heated fluid through them. The vacuum chambers connect to suitable vacuum-producing equipment, located outside the vacuum chamber, which may be a vacuum pump or a steam injector.

Another essential part of a vacuum dryer that has a vacuum pump is a condenser, which collects water vapor to prevent it from entering the pump. A steam injector, which is often used to create vacuum, is an aspirator in which high-velocity steam jetting past an opening draws air and water vapor from the vacuum chamber.

Because of the high installation cost and operating cost of vacuum driers, they are used only for high-value raw materials or products requiring reduction to extremely low levels of moisture without damage.

Freeze Drying

In freeze drying (also called lyophilization) the moisture is removed from the fruit by sublimation, that is, converting ice directly into water vapor. Therefore, no transfer of liquid occurs from the center of the mass to the surface. As drying proceeds, the ice layer gradually recedes toward the center, leaving vacant spaces formerly occupied by ice crystals.

The processing involves two basic steps: (1) the raw fruit is first frozen in the conventional manner followed by (2) drying to around 2% moisture in a vacuum chamber while still frozen. The most common type of freeze-drying equipment is a batch chamber system similar to a vacuum shelf dryer but with special features to meet the needs of the freeze-drying process.

Freeze dehydration produces the highest-quality fruit products obtainable by any drying method. The porous nonshrunken structure of the freeze-dried product facilitates rapid and nearly complete rehydration when water is added. The low processing temperatures and the rapid transition minimize the extent of various degradative reactions such as nonenzymatic browning and also help to reduce the loss of flavor substances.

The industrial application of freeze drying to a wide range of fruits has been limited because of the requirements of high capital investment, high processing costs, and the need for special packaging. This process is suitable only for high-value products.

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