By Arshi Siddiqui and Khan Chand*
Drying is the oldest method of food preservation. Fruits and vegetables have essential dietary nutrients such as vitamins, minerals and dietary fibers which are considered important for a balanced diet. These nutrients reduce the risk of possible deficiencies and help to maintain a healthy lifestyle. Fruits and vegetables have a high moisture content of more than 80%. These are classified as highly perishable commodities and are more prone to spoilage. Drying is a suitable alternative to preserve food in a stable and safe condition by reducing water activity, extending the shelf-life much longer than that of fresh produce. Due to the perishable nature of fruits and vegetables, it is necessary to preserve them and drying is one such method for post-harvest management especially in developing countries like India where cold storage facilities are poorly established. Dried foods are tasty, nutritious, lightweight, easy to prepare, easy to store and use.
Since prehistoric times the preservation of fruits and vegetables is done by the sun and solar drying techniques. But due to the poor quality and product contamination, the alternate drying technologies are developed. The most applicable method of drying includes freeze, vacuum, osmotic, microwave, infrared and hybrid drying.
Modern Drying Techniques
Osmosis is known as a partial dehydration process. In this technique, foods to be dried are placed in a hypertonic solution which causes a difference in concentration and causes the water content of foods to be driven out from the sample to the solution. The change in physical, chemical, nutritional values, taste and structural properties of the final product may be occurred due to the transfer of mass during osmosis. This technique is more advantageous than other conventional methods of drying as it requires less energy to carry out the procedure. Osmotic drying also retains maximum nutrients and has lower product thermal damage as it is conducted at a lower temperature.
Vacuum drying technique is used for dehydration of highly perishable commodities. Vacuum drying technique has a higher rate of drying, lower temperature, and better retention of rehydrating capacity and less energy usage. This technique has good product quality and nutritional value as it has better retention of product colour, taste and other contents such as vitamins and volatile aroma in comparison with conventional drying methods.
Freeze drying is one of the best methods for drying of fruits and vegetables. It is the process of drying a substance through freezing and removal of solvents associated with direct sublimation. Due to the unavailability of liquid water and very low temperature, the superior quality end product is obtained and also all the deterioration activity and most of the reactions involving the microbes are completely stopped. In the first phase of freeze-drying, about 90% of the water present in the fruits is removed. Freeze-dried fruits have the ability of rapid rehydration. Also, the organoleptic property of the rehydrated product is almost similar to the fresh product. Freeze drying is advantageous as it has minimum volume reduction, minute chemical change, and minimum loss of volatile components. Freeze-dried products can be stored for a long period too. These are stable and have the ability to be used as antioxidants and colourants. High cost and energy consumption during freezing, drying and condensing process are the disadvantages of the freeze-dried products.
Microwave radiation is electromagnetic radiation in the frequency range of 300 MHz to 300 GHz, with 2,450 MHz being the most commonly used frequency. Loss of heat-sensitive components of food can be caused by long drying time and high temperature. Microwave drying is efficient as it requires less time and temperature to remove the moisture content in foods. Microwave drying is more advantageous separately or in combination with other methods of drying such as vacuum drying. Microwave drying is more beneficial than conventional drying due to a short drying period, better quality product and having flexibility in the drying of a wide variety of products.
Infrared drying is another method for moisture removal from foods having a wavelength range of 0.75 and 1000µm. In infrared heating, the charge is built up in the electronic state as well as in the vibrational and rotational state at the atomic and molecular levels. This caused the heating of food without any changes in the temperature of the air surrounding the food. Infrared drying is energy efficient and has minimum drying period, temperature uniformity in the products and high quality of end product.
Hybrid drying/combined drying
Many researchers analyzed that drying techniques which were developed from the combination of two or more drying systems have benefits of both from individual or multistage. Drying systems which are in a combination known as hybrid systems of drying resulted in improved quality of the product. Hybrid drying system enhances the product quality and reduces the chances of product degradation. The hybrid drying system is more energy-efficient, reduces the duration of the processing, cost-effective and easy to operate.
Combination of the microwave with spouted fluidized bed system of drying improved the drying uniformity and thus reduced the burning. Similarly, non-thermal sources such as ultrasound, ultraviolet and pulse electric field-assisted with convective hot air drying system is an efficient tool in processing. The various studies found a hybrid drying system more feasible and economical.
- Ahmed et al. (2013). Different Drying Methods: Their Applications and Recent Advances. International Journal of Food Nutrition and Safety, 4(1):34-42.
- Hasan et al. (2019). Modern drying techniques in fruits and vegetables to overcome postharvest losses: A review. Journal of Food Processing and Preservation, 43(12):1-15.
- Maisnam et al. (2017). Recent advances in conventional drying of foods. Journal of Food Technology and Preservation, 1(1):25-34.
- Sagar V.R. and Suresh Kumar P. (2010). Recent advances in drying and dehydration of fruits and vegetables: a review. Journal of Food Science and Technology, 47(1):15-26.
* Mr. Khan Chand is Assistant Professor and Ms. Arshi Siddiqui is M. Tech student at Department of Post Harvest Process and Food Engineering, College of Technology, G B Pant University of Agriculture and Technology, Pantnagar, Uttarakhand