We have heard from our grandmother that they used to dry various foods on the roof in the summer. So, the preservation of fruits and vegetables through drying dates back many centuries and is based on sun and solar drying techniques. Over 20% of the world perishable crops are dried to increase shelf-life and promote food security and they play an important role in the food supply chain. The poor quality and product contamination lead to the development of alternate drying technology. Let’s hop on to what exactly is Drying Of Fruits and Vegetables.
Food drying is the process of removing water from food by hot air circulation, which inhibits the growth of bacteria and enzymes and preserves it for a long time.For a layman, we can say that the preservation technique that attempts to remove moisture is Drying/Dehydration.
Generally, terms drying & dehydration are used alternatively but there is a difference between these two. Drying refers to the removal of free water from the compound whereas dehydration is the removal of all the water from the compound.
Types of drying
Dryers are classified according to drying process into:
Adiabatic drying – Solids are dried by direct contact with hot air
Non-adiabatic drying – External medium is used to transfer heat.
Based on the above two process different types of drying are:
Sun drying –
Sun and solar drying have been practiced extensively since ancient times. The grains are spread in thin layers on paved grounds and expose it to sun and wind for drying. In addition to cereals, other products like fruits, spices, oilseeds, vegetables, and fish are naturally dried in the sun by spreading them. It is a slow and inexpensive process but environmental contamination, insect infestation, and microbial deterioration cause low quality food.
Tray drying – Food material is arranged in a thin layer in shallow mesh or perforated trays. Hot air is blown through a system of ducts & baffles to promote uniform air distribution. Generally, it requires low capital and maintenance cost. But the poor quality of food near to heat source dries more rapidly.
Drum drying – In this food is spread over the heated drum surface. When the drum rotates, with food remains on the drum surface for the greater part of the rotation, during this time the drying takes place, after dried material scraped off.
Fluidized bed drying – In this drying heat is transferred to food material by convection. Hot air is blown upwards directly underneath the food, causing it to flow and remains suspended against gravity. There may also be a horizontal air flow helping to convey the food through the dryer.
Freeze drying – also called lyophilization. It is based on sublimation principle. It freezes the material first, then reduces pressure and adds heat for frozen water in the food to sublimate. There are 4 stages in this: Pretreatment, freezing, primary drying, and secondary drying. It is an excellent method for preserving heat-sensitive food material. Freeze-dried product is easy to reconstitute also. Its only drawback is its cost.
Spray drying – Slurry of liquid or fine solid material is injected into a blast of hot air in the chamber, water evaporates and solid part of product remain in powder form. Air and solids may move in parallel or countercurrent flow. It is an excellent method for products which are damaged by exposure to heat for a long time, as the process occurs very rapidly.
Vacuum drying – here moisture is removed by means of creating a vacuum. It involves reducing pressure below the vapor pressure of water surrounding the product and increases drying rate. It is the wide application for hygroscopic, heat sensitive materials.
Osmotic drying – It is a simple and effective method for preservation of fruits & vegetables. Sugar syrup is generally used as a solute for fruits and brine for vegetables in this method. It is the process of removal of water from fruits & vegetables to an osmo-active solution and transfer of solute in the food material. This is preferred method due to their color, aroma, nutritional constituents and flavor compound retention value.
Microwave drying – In this electromagnetic radiation of frequency 915 and 2450MHz directly interact with food and generate heat. Pressure gradient & concentration gradient play an important role in moisture transfer. Mostly it is used in combination with other drying methods for better quality. This is not common in food industries due to technical problems.
Advantages of drying | Drying Of Fruits and Vegetables
- Stop the growth of bacteria, yeast & mold by removing moisture.
- Slowdown enzymatic activity
- Minimize packaging requirement
- Reduce transport weight
What foods can you dehydrate?
Dehydration is the excellent way to food preservation. Fruits like apples, grapes, figs, berries, plums, banana, mango, and vegetables like onions, potatoes, tomatoes, corn, beets, and carrots can be dehydrated as whole or thin slices or its puree can be dried. They require pretreatments before keeping it for drying. Different herbs which add flavor to food are dehydrated for long use.
For every product, there is drying curve which describes drying characteristics at a specific temperature, velocity and pressure condition.
Three phases of drying –
- Initial rate period – initial heating of equipment and the product to be dried occurs.
- Constant rate period – Free moisture is removed easily and unbound moisture will come-off at constant rate
- Falling rate period – Rate of moisture removal decreases with time
Process for Drying Of Fruits and Vegetables :
Factors affecting drying
Temperature – Ideal temperature is 60-700C, higher temperature is not desirable
Humidity – humidity affects the drying time
Type of dryer – adiabatic and non-adiabatic dryers affects the product.
Air velocity & direction of air flow – rate of air velocity and direction of air flow affects the drying time and product quality.
Nature and size of food – physical, chemical composition of the material and its size affect the quality.
When water is added to the dried product it forms similar product as it was fresh. It is used as the quality characteristic of dried products.
Rehydration Ratio = Weight of dried sample ÷ Weight of sample after rehydration
Drying is the most efficient technology for preservation of food and retains all the nutrients except vitamins. Various advanced drying technologies like superheated steam drying, impinging steam drying, contact sorption drying, pulse combustion drying has been developed to overcome the drawbacks of conventional dryers.
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