SOLAR DRYING OF FRUITS AND FISH



Solar tunnel drier and drying technology for quality dried fruits and fish



Diagram



Picture

Introduction :

Agricultural products in Bangladesh are dried by traditional method of sun drying and the quality of the dried products is low. Considerable losses ranging from 10 to 25 % can often occur. To avoid insect infestation and larva attack and for safe storage, died fish producers usually apply insecticides including DDT. The contaminated dried fish creates a wide spectrum of environmental and health hazards. Drying of agricultural products is still the most widespread preservation technique and it is becoming more and more an alternative to marketing fresh fruits and fish since the demand of high quality dried fruits and fish is permanently increasing allover the world. Fruits such as pineapple and mango are produced in large quantities and incomes derived from these products are normally minimal due to inadequate conservation and storage, and lacking marketing structures. Large quantities of fish and fish products are also being spoiled due to lack of proper preservation and storage facilities. All the areas in Bangladesh receive abundant solar radiation and natural convection solar drier appears to have potential for adoption and application in Bangladesh . The high weather dependent risk and drying limitations due to extremely low buoyancy induced air flow of natural convection solar driers stimulated the development of efficient technologies for production of quality dried fruits and DDT free dried fish using forced convection solar drier.

Solar Tunnel Drier :



The drier consists of a flat plate air heating collector, a tunnel drying unit and a small fan to provide the required air flow over the product to be dried. These are connected in series. Both the collector and the drying unit are covered with plastic sheets. Black paint is used as an absorber in the collector. The products to be dried are placed in a thin layer on a plastic net in the tunnel drier. Glass wool is used as insulation material to reduce the heat loss from the drier. The whole system is placed horizontally on a raised platform. The air at required flow rote is provided by two dc fans operated by one photovoltaic module. As the air is passed over the product rather than through the product in the drier, the power requirement to drive a fan is low. To prevent the entry of water inside drier unit during rain, the cover is fixed like a sloping roof. Solar radiation passes through the transparent cover of the collector and heats the absorber. Ambient air is forced through the collector. Heat is transferred from absorber to air in the collector and heated air from collector while passing over the products absorbs moisture from the products. Solar radiation also passes through the transparent cover of the drier and heats the products in the drier. This enhances the drying rate and the temperature in the drier.

Construction of solar tunnel drier :

The design of solar tunnel drier developed at the University of Hohenheim , Germany has been modified to make it economic and simple in construction using locally available materials. The standard size of the drier is 2m x 20m with 20 m2 drying area. All parts of the drier including back insulation and metal frames are designed using modular concepts which facilitates easy transport and installation.

First step: Determine the length and width of both collector and the drier unit. The floors and the frames over the sides of the collector and the drier can he constructed using MS sheet and timber. Glass wool is to he sandwiched between two MS sheets with groove and tonge system.

Second step: Paint the floor of the collector with black paint.

Third step: Determine the dimensions of rooflike structure or frame of both the collector and drier. This frame may be constructed by using MS sheet. Place uv stabilized plastic foil on the rooflike structure and fix the sides of the plastic foil to frames of both collector and drier. Aluminum U-channels may be fixed to the metal frames of the collector and the drying unit and the sides of the plastic foils are fixed to the aluminum U-channels by using rubber ropes.

Forth step: Add any available insulation material on the back of the collector to reduce the heat losses.

Fifth step: Fit electrical motor operated fan to the entry of the collector and the solar module to the electrical motor.

Drying of pineapple, mango and fish An extensive laboratory and field level studies at three locations in Bangladesh were conducted to develop the technologies for solar drying of pineapple, mango and fish during the period 1998-2000 under the financial support of Bangladesh Agricultural Research Council and packages of technologies have been developed for production of quality solar dried pineapple. mango and fish. The success story of solar drying of pineapple, mango and fish has been published in national and international journals and an extension of these packages of technologies using the approach of Grameen Bank is proposed.

Drying of pineapple and mango :

(1) Ocean the fruits to be dried and cut the products into slices of 10 mm thickness for optimum drying rate. To reduce the non enzymic browning during and storage pretreat the sliced products by burning elemental sulfur in a box by exposing the sliced products to the fumes of SO2 for half an hour (4mg of elemental sulfur per kg of fresh fruit).

(2) Place the sliced products on the wire mesh inside the drier in a single layer.

(3) Dry the products in the solar tunnel drier from 8 am to 4 pm till the desired moisture content is reached.

(4) Maintain the drying temperature within 65°C by changing either the air flow rate or collector length.

(5) Store the dried products in sealed polyethylene bags after the drying process is complete.

Drying of fish :

(1) Ocean the fish to be dried. Silver jew fish may be pretreated with salt in the proportion of 1:3 to 1:10 (salt to fish).

(2) Place the clean or salted fish on the wire mesh inside the drier in a single layer.

(3) Dry the products in the solar tunnel drier from 8 am to 4 pm till the desired moisture content is reached.

(4) Maintain the drying temperature within 50°C by changing either the air flow rate or collector length.

(5) Store the dried products in sealed polyethylene bags after the drying process is complete.