Drying of Horticultural Produce - Operation of Dryer
Drying foods has received the most widespread acceptability of all food preservation methods of food preservation and techniques have been passed from one generation to another based on experience. Today, a variety of dried foods are available in the market and are quite popular among the consumer, especially spices and herbs. Dried tea leaves, fenugreek (methi), Curry tree leaves (sweet neem), mango pulp (amchur), dried pomegranate kernels (anar dana), amla powder, Triphala, grapes, etc. are among the most commonly used products in our kitchen. Dried fruits and fruit leathers may be used as snack foods while dried vegetables may be added to soups, prepared vegetables, and pulses to improve taste and flavor. Most food-spoiling agents (micro-organisms) including biochemical reactions need water to act. When foods are sufficiently dehydrated (dried), microorganisms cannot grow and foods will not spoil. Moreover, dried foods are good sources of quick energy and wholesome nutrition, since the only thing lost during preservation is moisture. Exposure to high-temperature heated air during drying operation and blanching vegetables before drying (to destroy enzymes) can result in some loss of water-soluble vitamin C, and B-complex. Methods used for drying food have become sophisticated over time. It wasn't until 1795 that the first dehydrator was introduced, in France, for drying fruits and vegetables.
Drying methods
Drying, like other preservation methods, requires energy. Foods can be dehydrated by various means: the sun, a solar dryer (figure 1), an electric dehydrator, or a microwave oven (for herbs only). Unless sun drying is possible, the energy cost of dehydrating foods will add to the cost of processing. Solar drying is a process of drying foods by harnessing the heat energy of the sun in a special dehydrator that not only increases the temperature but also, improves the airflow. So, it is a modification of sun drying in which the sun's rays are collected inside a specially designed unit with adequate ventilation to remove moist air. The temperature in the unit is usually 15 to 20oC higher than in open sunlight, which results in a shorter drying time. While solar drying has many advantages over sun drying, lack of control over the weather is the main problem with both methods.
Hot air drying is the most practical way to dry the vegetables at commercial level. It requires initial investment and incurs operating costs but protects the foods and works continually with better control of air temperature and airflow. Drying times in dehydrators vary considerably depending on the amount of food dried, moisture content, room temperature, and humidity. Some foods require several hours and others may take more than a day. Prolonging drying time or interrupting drying time may result in spoilage. It is important to control air temperature and circulation during the drying process. If the temperature is too low or the humidity too high the food will dry more slowly than it should and microbial growth can occur. Watch temperatures closely at the beginning and end of the drying period. If the temperature is too high at first a hard shell may develop on the outside, trapping moisture on the inside. Temperatures that are too high at the end of the drying period may cause food to scorch. Temperatures between 50oC to 70oC are recommended for drying fruits and vegetables. Temperatures up to 70oC may be used initially but should be lowered as food begins to dry. At least for the last hour of the drying period, the temperature should not exceed 50oC.
Types of Dehydrators
Usually, dehydrators come in two designs where one has a horizontal airflow since the heating element and fan are located on the side while the other model has a vertical airflow with the element and fan located at the base. Dehydrators with a horizontal flow of air have several advantages – mixing of flavors is reduced which allows several foods to be dried simultaneously, there is no dripping of liquid and all trays are heated equally. Vertical flow dehydrators cannot prevent mixing of flavors and this can increase the time for drying different types of foods.
Important Parameters for Successful Drying
Speed
For a good-quality product, vegetables and fruits must be prepared for drying as soon as possible after harvesting. They should be blanched, cooled, and laid out to dry without delay. Foods should be dried rapidly, but not so fast that the outside becomes hard before the moisture inside has a chance to evaporate. Drying must not be interrupted. Once you start drying the food, don't let it cool down in order to start drying again later. Mold and other spoilage organisms can grow on partly dried food.
Temperature
During the first part of the drying process, the air temperature can be relatively high, that is, 65oC to 70oC so that moisture can evaporate quickly from the food. Because food loses heat during rapid evaporation, the air temperature can be high without increasing the temperature of the food. But as soon as surface moisture is lost (the outside begins to feel dry) and the rate of evaporation slows down, the food warms up. The air temperature must then be reduced to about 60oC. The temperature should be high enough to evaporate moisture from the food, but not high enough to cook the food.
Humidity and Ventilation
The higher the temperature and the lower the humidity, the more rapid the rate of dehydration will be. Humid air slows down evaporation. If drying takes place too fast, however, "case hardening" will occur. This means that the cells on the outside of the pieces of food give up moisture faster than the cells on the inside. The surface becomes hard, preventing the escape of moisture from the inside. Moisture in the food escapes by evaporating into the air. Trapped air soon takes on as much moisture as it can hold, and then drying can no longer take place. For this reason, be sure the ventilation around your oven or in your food dryer is adequate.
Uniform Drying
Drying the food evenly takes a little extra effort and attention. Stirring the pieces of food frequently and shifting the racks in the oven or dryer is essential because heat is not the same in all parts of the dryer. For the best results, spread thin layers of uniformly-sized pieces of food on the drying racks.
Space Requirements
Dehydrators vary in size and can be square or rectangular. You will need to find a convenient space to operate it. Place the dehydrator near an electrical outlet so that extension cords are not necessary. There should be space to open the door and easily load and rotate the trays. When deciding where to put the dehydrator, keep these points in mind
- The food will give off humidity
- The fan will create noise
- The appliance will give off heat
Steps
- Fenugreek (Kasuri methi)
Step 2
- Sorting, trimming, and cleaning
Step 3
- Hot water blanching
(0.1% MgO, NaHCO3, 0.5% KMS solution for 2 min)
Step 4
- Hot air drying
(Temperature 55- 60oC, 6-7 hrs)
Step 5
- Dried product
(MC 5%)p
Step 6
- Packaging and storage
Vital Specifications of Evacuated tube solar dryer:
The evacuated tube-assisted solar dryer consists of a solar collector made of the evacuated tube for heating the ambient air up to 60 C required for turmeric drying. The hot air produced is sent into the poly house attached to the collector. In the poly house, the raw material is spread in a single layer on the perforated trays put on a rack. The capacity of the dryer may vary from 3-7 quintals. In this dryer, turmeric can be dried almost half the time required for drying in the open sun. The quality of the produce is much better. The cost of the dryer can be 2-3 lakhs. This technology has been approved by PAU.
Packaging and storing dried foods
Package dried foods in glass jars, food-grade plastic storage containers, or plastic food storage bags. Make sure the package has an airtight seal. It is a good idea to package dried foods in small amounts because after the package is opened, the food can absorb moisture from the air and quality deteriorates. Store containers of dried foods in a cool, dark, dry area. Exposure to humidity, light, or air decreases the shelf life of foods. The lower the temperature, the better: Foods stored at temperatures under 35oC will keep approximately one year for every 10oC drop in temperature, and the shelf life of dried product increases three to four times.
Mahesh Kumar, Gurveer Kaur, and Tarsem Chand
Department of Processing and Food Engineering
Punjab Agricultural University, Ludhiana
Source:- AD Communication
