Dried Vegetables

There are many vegetables that are suitable for drying, like leafy vegetables such as cabbage, but also tuber and root crops. For drying, the preference is given to vegetables with the following properties:

• Good and strong flavor, because flavoring agents are lost during drying;
• High solids content, so that a high efficiency (yield) is obtained;
• Strong colour, making it possible for the consumer to recognize the vegetables;
• Good shelf life, making it possible to process the product in phases.

During the drying process, it is important that the temperature of the product that is being dried, does not become too high, in order to preserve the taste, the nutritional values and its appearance. However, the product must be sufficiently dried, so that there is no possibility of bacterial growth and enzymatic reactions can no longer occur. For this reason, the water activity (a_w) in the vegetables to be dried, is brought back to 0.15 to 0.20 a_w.

The drying speed depends on:

• Air speed;
• Humidity;
• Air distribution;
• Distribution of the product;
• The temperature of the product to be dried.

The drying rate decreases when the top layer of the product is keratinised. This means that it has a dry outer layer.

Dried vegetables are used, among others things, in instant soups and sauces. It is important that the vegetables have good water absorption capabilities.

Dried vegetables production

Before the vegetables are dried, they undergo a number of processes. Depending on the type of vegetables, one or more of the following processes are applied.

Washing

In several washers, depending on the type of vegetables, unwanted parts such as sand, small stones and residues of the pesticides are removed. Moreover, with this step a considerable reduction in the total bacterial count can be achieved.

Peeling

The outer peel of tuber and root crops is generally removed and discarded, since its not or less suitable for consumption.

Cutting

In general, the vegetables are reduced in size before they are dried. In this way the surface is increased, so the drying rate is increased. However, this will make the aromas escape faster.

When cutting machines or tools are used, it is important that the blades are sharp. Blunt knifes can damage the product too much, which may cause unwanted enzymatic discolouration, which in turn makes for an unattractive product. 

Sorting

Most types of vegetables can be sorted according to size, so that the final product obtained is as homogeneous as possible. This is beneficial during the drying process.

Sulfiting

To prevent browning and oxidation, white vegetables can be sulfited. The vegetables are then dipped in or sprayed with a sulfite solution. Citric acid or ascorbic acid is often used for non-white vegetables, because sulfur dioxide (SO₂) will cause decolourization of these vegetables.

Blanching

By blanching the vegetables to be dried in advance, flavour and colour changes due to enzymatic and non-enzymatic reactions are prevented. Moreover, blanching makes the vegetables soft and pliable. Preference is given to steam blanching, so the loss of dry substance by leaching is reduced. The chance of leaching can be further minimised by using superheated steam or saturated hot air.

Drying

Vegetables can be dried in various ways. Which drying method is used, depends on the type of vegetables. The most important drying methods will now be discussed further.

Drum Dryer
Mainly starch-containing products, which can easily form a film, such as beans and potatoes, are dried using a drum dryer. A drum dryer is a contact dryer, meaning that the material that has to be dried is separated with a metal wall from the heating source. The heat required to evaporate the water from the product is supplied through conduction. The drum dryer consists of a hollow metal cylinder, which rotates around its longitudinal axle. The steam is conducted via a hollow shaft to the inside of the cylinder. The steam releases its heat and condenses to the inner wall. The product that has to be dried is attached to the roller in a thin film layer. After about three-quarters roller rotation, the dried layer is scraped off with a knife. The drying time takes a few seconds to 1 minute.

Drum drying can also be done under vacuum conditions. Because the cooking temperature will be lowered considerably, heat damage to the product is reduced.

Tunnel Dryer
The vegetables that are going to be dried are placed on carts in a drying tunnel. When the wet material of a new cart comes into contact with dry fresh air, it is called a direct current dryer. This drying method is used for temperature sensitive foods. In addition, there are also counter-current tunnel dryers, which allow for a shorter drying time to be obtained.

In general, a tunnel dryer is divided into sections, making it possible to reheat the drying air. It is possible to have a combination of, for example, direct current and counter-current tunnel dryers. The drying time ranges from 2 hours to 2 days.

Kiln dryer
A kiln dryer is especially suitable for drying root vegetables, and consists of a number of superposed mesh screens, through which the dry air is guided in an upward direction. This is counter-current drying air, since the wet products are placed on top of the mesh screens.

The disadvantage of this type of dryer is that the filling, removing and tumbling of the product, very labor-intensive is, which causes an inconsistent product quality.

Belt dryer
With a belt dryer the wet products are dried on a mesh belt in a layer thickness of 5 to 15 cm. The dryer consists of several sections. In the first section very hot air is blown from below through the wet products. In the last two sections, the band runs slower, which creates a thicker product layer. The dry air is blown from above on to the product. The outgoing dry air cannot be much hotter than the maximum product temperature, in order to prevent heat damage.

There are singe-belt and multi-belt dryers. The single-belt dryer is suitable for the drying of thin, mainly foliar, vegetables. Multi-belt dryers consist of multiple bands that run one above each other at different speeds.

Turbine-band dryers are used to save floor space. This band dryer consists of a vertically placed spiral shaped mesh belt, on which the products slowly move up through the drying room.

Freeze Dryer
In a freeze dryer, sublimation is applied, meaning, the direct tranformation of ice (the solid phase) into water vapor (the vapor phase). In this way, water is extracted from the product to be dried, while its aroma, taste, color, structure and nutritional values ​​remain well preserved. Moreover, the dried product has a long shelf life and absorbs water more quickly. The product must first be frozen, making this way of drying expensive in comparison to other methods. However, this method is, because of the above-mentioned advantages, typically used for leeks, onions, mushrooms and cauliflower.

In a freeze dryer, the vegetables are placed on a cart, in many thin layers, after which the whole is frozen. Next, the cart is placed in a freeze-drying room, in which a pressure of 0.001 bar is applied. After starting with a relatively high heating temperature, this temperature will gradually be reduced, to prevent the heated product surface from becoming too hot.

Expansion Dryer
Before the vegetables are placed in an expansion dryer, they are dried with hot air until they have a moisture content of about 40%. In the expansion dryer the product is heated up while rotating until it reaches an elevated pressure level. Opening the dryer at this time, causes the water and steam, present in the product, to expand and presses the vegetable tissue apart. The moisture content of the dried vegetables can be optionally lowered even further.

This method is mainly applied to vegetables which get a too compact structure when dried with hot air, which could potentially hinder its water absorption capabilities.

Sorting

The dried vegetables are sorted to color, shape and defects using one or more of the following sorting methods:

• Manual sorting;
• Vibrating mesh screen;
• Electronic sorter;
• Wind sifter.

Packing

In general, the dried vegetables are packaged in tin-plated cans, barrels of hardboard or steel or multiple paper bags with a polyethylene layer in-between. To prevent oxidation of the product and corrosion of the container, the air in the package is often replaced with nitrogen. The product can also let to be vented first and then gassed.

Gassing is a method to slow down any fungal growth. In the absence of oxygen or in poisoned air by, for example, 20% carbon dioxide (CO₂), fungi cannot develop further.

Storing

Dried vegetables must be kept in a cool and dry place, because these products are very hygroscopic.