EngineeringScience

Radiofrequency drying- A promising technique which still needs to be explored

Radiofrequency drying is a novel technique used for drying food products. Due to its rapid and uniform drying, less energy consumption, and better penetration, it can be considered a very promising technique for drying. It uses the dielectric property of the material to create a heating effect by means of creating friction due to electromagnetic waves.

Radiofrequency by Definition

This type of drying uses 3KHz-3000MHz. This creates the dipole moment of the polar molecules of water in the product. These molecules, as a move to align with the electromagnetic field, create friction and heat energy within the food product. That heat energy increases the food temperature and hence making the water molecules evaporate from the product. 

Figure 1: Dipole moment of the molecules due to radiofrequency waves

Factors/properties of food that affect the Radiofrequency drying

Food dimensions: – Smaller and thinner products will take lesser time as compared to a product that is thicker in dimensions as more water is to be dried from the thicker product.

Penetration depth: – It ensures uniform heating and also tells about the appropriate thickness to be taken for the product.

Applied voltage: – More voltage leads to higher dipole moment which in turn creates more friction. Higher friction means more heat is generated in the food product.

Dielectric loss factor: – measures the easiness with which the radiofrequency dryer can heat the food product. Water is the most controllable factor in terms of drying. Hence, a reduction in the loss factor of water in the product provides protection against overheating

Permittivity: – Permittivity describes how much energy can be stored in the material.

The temperature of the product: – With an increase in temperature, dielectric loss increases. Hence more radiofrequency waves are absorbed by the product. Hence, the temperature difference would be more accelerating the drying of the product.

Density: – Density of the food product defines the amount of heat required and the time required for drying.

The moisture content of the food: – Higher the moisture content, the more radiofrequency waves are absorbed.

 Radio Frequency Dryer and its mechanism

As shown in figure 2, basic radio frequency applicator/dryer consists of parallel plates on which bulk can be loaded. Stray field applicator consists of rods for thin dielectrics and staggers through the field are applied by rods.

The materials used for applicator and enclosure construction are good conductors which are used in order to obtain optimal energy during processing at the selected operating frequency.

Figure 2:- Electrode configuration in RF applicator

Metal is used to make the enclosure to provide the electrical safety and isolation of the radiofrequency heating by outer environment. 

Figure 3 represents the simple standard parallel plate applicator in which the electrodes are square in shape to ensure adequate temperature distribution cylindrical-shaped Teflon or borosilicate glass.

Figure 3:- Parallel plate applicator

Applications of Radio Frequency Drying

Radio frequency drying, a novel method with a lot of advantages of less drying time and better drying rate, however, lacks in uniform drying of the product due to which the application of radio frequency is still not much in food sector. Many improvements and developments are to be done in the technique and hence this novel technique is rarely used in food processing industry. Post-baking drying of biscuits, crackers, and breakfast cereals are common and most used application of radio frequency drying. Apart from this it is also used to dry out the agricultural products such as millets, rice kernels etc.

Agricultural Drying

The most widely used application of agricultural drying is Alfalfa drying. The radiofrequency mechanism is a sustainable way of drying lacks the uniformity in drying methods and hence at times reduces the nutritional content and textural properties of a material if it is subjected to a higher frequency for a longer time.

However, the less cost of drying and bulk drying mechanism makes the technique suitable for drying agricultural products. An advantage of radiofrequency drying of grain is the absence of a drying medium with a high temperature and the good uniformity of drying throughout the entire grain mass. Fruits, herbs, spices, etc. are dried. Moreover, it is also used to remove the excessive moisture content of the commodity such as rice, millets, etc.

Baking

The most promising and widely used application of radiofrequency drying is observed in the bakery industry.  It is mainly used for post-bake drying of cookies, crackers, and pasta which helps in even out the moisture distribution after baking by targeting the remaining moisture pockets.

Moreover, the problem of surface cracking in post bake drying is also resolved in the case of radiofrequency drying which is often seen if conventional drying is used.

Radiofrequency drying also avoids discoloration and flavor damage as already increased temperature leads to uniform moisture distribution in the product. It ensures faster processing, even and consistent moisture level, less temperature used for drying, and smaller equipment. Moreover, Radio Frequency is highly energy efficient for removing final moisture from dry baked products.

The radiofrequency drying is also used to bake the pieces of bread. For this purpose, the heat resistant pans are used to place the dough and it is then kept in the baking oven operated by a radiofrequency method with controlled airflow and temperature which takes out the excessive moisture leading to the bread baking process.

Drying of spices and herbs

Spices and herbs are low moisture food commodity which is dried in order to enhance the shelf life and also to enhance its aroma and flavor. The radiofrequency method is considered to be an energy-efficient way of drying spices and herbs as it locks the aroma and flavour of the spices and herbs which is often degraded in case of conventional drying. Moreover, it is also cost-efficient when bulk drying of the product is to be done as it uses a volumetric heating mechanism in the drying process.

Recent Developments in Radio Frequency Drying

The amalgamation of many drying methods in order to overcome the disadvantages of the radio frequency heating mechanism is done. Ways to combine the conventional drying systems with the radio frequency drying and to ensure uniform heating, making it less energy driven and more cost-effective.

Parallel combined RF drying

Applying the combination of conventional drying with the radio frequency drying avoids the problem of low heat transfer which is seen in the conventional method and ensures uniform drying which is a clear disadvantage of using the radio frequency drying alone. The most effective combination which is used in food industries for drying is radiofrequency and vacuum drying.

It consists of two parallel plate electrodes in the radio frequency heating system with a vacuum pump attached to it.  Hence vacuum is created inside a radio frequency drying system which in turn resolves the problem of non-uniform drying. The combination of RF-vacuum drying improves the drying rate by controlling the drying temperatures of the commodity also at the last stage of drying which makes it more suitable for heat-sensitive commodities.

Tandem combined RF drying

Tandem combined RF drying also known as hybrid drying is a process in which different drying technologies are deployed at different stages. It ensures better textural properties, better thermal performance, and making it more energy-efficient with uniform drying of the product.

Hence, radiofrequency is widely used for post-baking drying of crackers, biscuits, and breakfast cereal which proves to be a bane as in uniforms the moisture content of the commodity.  It avoids surface cracking, hence is used as a post-baking technique for increasing the product quality.

In Conclusion

Radiofrequency drying is a promising technique to ensure a safe and better drying process for food products. Due to its larger penetration depth, more energy efficiency, and uniform moisture level of the product. Radiofrequency drying due to its high advantage can be considered as a better technique over conventional methods.

 However, radio frequency is a method which is not yet studied to its full potential. The lack of knowledge about the RF dielectric properties of various food materials makes it difficult to ensure proper processing of food product in radio frequency waves.

Parallel combined RF technique, radio frequency vacuum technique and hybrid radio frequency drying are done in order to take the most advantages out of both conventional and radio frequency drying mechanism. However, still there is a need to study more about the technique in order to get the best of applications out of this promising technique.

Written By,

Kadambini Sharma

Student, NIFTEM

Well, this article covers almost everything about radiofrequency. If you want to learn more technological advancements in food technology, like 3D Printing, check out our article!

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