AP Mnkeni, P Soundy and MO Brutsch

Contents

Solar drying of fruit and vegetables

This publication is about

Advantages of solar drying

Methods of drying

Advantages of solar dryers

The drying process

Packaging and storing

Specific products

Fruit

Vegetables

Recipes for dried fruit and vegetables

For thousands of years people have sundried fruit and vegetables to preserve for leaner times. New technologies brought changed techniques, but at present the increasing demand for healthy, low-cost natural foods and the need for sustainable income, are bringing solar drying to the fore as a useful alternative for surplus products.

Save for tomorrow what you do not need today!

This publication is about

• Preparation and pretreatment of different fruit and vegetable types for drying

• How to dry these using simple solar dryers

• Proper storage of dried fruit and vegetables

• Recipes for dried fruit and vegetables.

Advantages of solar dryers

• Drying is faster because inside the dryer it is warmer than outside.

• Less risk of spoilage because of the speed of drying. (if the drying process is slow the fruit start to ferment and the product is spoilt).

• The product is protected against flies, pests, rain and dust.

• It is labour saving. The product can be left in the dryer overnight or during rain.

• The quality of the product is better in terms of nutrients, hygiene and colour.

The drying process

Precautions

Predrying treatments

Selection

• Use only ripe, good-quality fruit and vegetables.

• Select fruit and vegetables individually.

• Discard rotted, damaged or diseased fruit and vegetables.

• Remember, processing cannot improve poor-quality fruit or vegetables.

Washing

• Clean all working surfaces before handling fruit or vegetables.

• Water for cleaning must be treated with a household bleach solution.

• Prepare the cleaning solution as follows:

  - Pour 50 parts of clean water in a clean bucket (e.g. 20 litre).

  - Add one part of any household bleach (e.g. 400 ml) containing chlorine

  - For safety reasons plastic gloves should be worn when mixing the solution.

• One bucketful of the treated water (20 litre) is enough for cleaning 20 kg of fruit.

• Use a fresh cleaning solution every day.
  

• Selected fruit and vegetables should be washed and scrubbed individually in the treated water, while plastic gloves should be worn.

• Care must be taken to avoid breaking the skin of the fruit during cleaning and thereby contaminating the flesh.

• Washed fruit and vegetables should be placed into a clean basket or bucket and taken to the peeling or blanching area.

Blanching

Before drying, all vegetables should be blanched in steam to halt the action of enzymes. However, blanching of fruit is optional. Steam blanching is recommended because it prevents the loss of some nutrients and the products being dried from adhering to each other. Do not underblanch, because the enzymes will not be inactivated totally and the dried vegetables will deteriorate during storage.

Procedure

Pour several centimetres of water into a large cooking pot that has a close-fitting lid. Heat the water to boiling and place over it, high enough to keep clear of the water, a wire rack or basket holding a layer of the vegetables (not more than 5 cm deep). Cover and let the vegetables steam for half the required time, then test to make sure all pieces are reached by the steam.

A sample from the centre of the layer should be wilted and feel soft and heated through when it has been properly blanched.

• Remove the vegetables and spread them on paper towelling or clean cloth to remove excess moisture while you steam the next load. Cover with towelling while waiting for further treatment or before taking them to the drying trays.

Peeling

• Hygiene is of utmost importance when peeling.

• Peeling should not take place in the area where the raw materials are washed.

• The area should be swept thoroughly and washed before handling the fruit.

• Peeling knives and working surfaces should be cleaned in fresh bleach solution before use.

• The operator should wash his/her hands and arms thoroughly with clean water and unperfumed soap.

• Clean, sharp stainless steel knives must always be used.

• Careful peeling with minimum removal of the flesh is important.

• Peelings and seeds should be disposed of as soon as possible because they attract flies and other insects.

• Peelings can be used as animal feed or as mulch, or be buried if there is no alternate use.
  

Cutting and slicing

• Thickness of fruit pieces depends upon the kind of fruit being dried.

• Thicker slices will dry at a slower rate than thinner pieces.

• Very thin pieces tend to stick to the drying trays and will be difficult to remove.

• Thicker pieces may not dry fully and may subsequently deteriorate after packing.

• Packages of dried pieces of varying thickness appear relatively unattractive.

• Cutting knives and working surface have to be cleaned with a bleach solution before use.

• Slices should be placed in clean bowls which have been rinsed with clean water ready for loading onto the drying trays.

• Before loading the trays, these have to be brushed clean and washed.

Dryers

A basic box-type low-cost solar dryer can be constructed at home or by village artisans. It is made of wire-mesh trays in a wooden framework surrounded by a clear plastic sheet. The solar cabinet dryer type has a surface of 10 m² and is capable of drying 20 to 35 kg of fresh produce (depending on commodity) over a period of 3 to 4 days. Smaller portable models of the dryer can be constructed depending on available funds for the dryer, construction and the purpose of drying (home consumption or marketing).

For further information
available on solar drying
contact
Mr MD Brutsch at the University of Fort Hare
Tel: (040) 602 2131
Fax: (040) 653 1730
E-mail: mbrutsch@ufh.ac.za





Tray loading

• Trays should be washed and cleaned to remove any fragments of dried fruit or contamination.

• Start to load during slicing rather than waiting until all the fruit has been sliced or cut. (This reduces the problem of sticking together in the bowls and will allow drying to start as soon as possible.)

• Lay the pieces of fruit on trays carefully and close to each other without overlapping to ensure the trays are loaded fully.

• Keep flies away and load trays quickly and continuously.

Dryer loading

• The dryer should be positioned in a level area unobscured by trees or buildings so that it is fully exposed to the sun throughout the day.

• If the wind blows predominantly in one direction for long periods the dryer should be placed end-on to the wind. This will reduce the cooling effect of the wind blowing direct into the drying cabinet, lengthening drying times. It will also reduce the possibility of dust entering the cabinet.

• Before loading, the inside of the drying cabinet should be swept clean and then wiped out with a clean, damp cloth.

• The plastic covers outside should be brushed or washed clean of dust because dirty plastic will reduce dryer performance and increase drying times.

• The doors should be closed immediately after each tray has been loaded and not left open until the next tray is fetched.

Drying

• During the first few hours of drying, particularly during very hot and sunny weather, fruit may dry at such a rate that moisture condenses on the inside of the plastic covers.

• This can be avoided by opening the loading doors slightly (20 mm) to improve air circulation. The gap should, however, be covered with mosquito mesh.

• Doors should be kept open for a minimum period of time and closed again as soon as the weather becomes cloudy.

• In poor weather drying will stop. Rain will rapidly cool the dryer and this will result in a moisture film on the cover because of condensation. It will be some time before the dryer functions again after the sun breaks through. Therefore, protect the dryer from rain.

• Under fine and sunny conditions the fruit slices should be dry after 2 full days in the dryer. However, it is essential to test slices. If the slices are not sufficiently dry, they will become mouldly in a short time. A test for dryness is conducted for specific products.

• If the slices are not sufficiently dry, the process should be allowed to continue for 1 or 2 hours before checking again.

• The final moisture content of dried fruit should be approximately 10 % (on a wet basis).

Unloading the dryer

• When the fruit is considered to be dry, the dryer should be unloaded as soon as possible. This must not be carried out in the early morning because dew and high humidity overnight may cause condensation of moisture onto the fruit. The best time to unload is in the afternoon on a sunny day.

• Trays should be removed from the dryer and taken to a clean and covered area for removal of the dried product.

• The operator must wash his/her hands and ideally wear clean gloves when handling the fruit.

• The dried fruit should be stored temporarily in clean dry baskets before packaging so that the product can cool down.

Packaging and storing

Packaging should be carried out immediately after unloading and cooling because the dried slices will reabsorb moisture and be susceptible to attack by insects and other pests.

Proper storage should take place in the absence of moisture, light and air.

• The use of brown paper bags folded tightly and then placed inside plastic bags is recommended.

• Store in small quantities to avoid large-scale contamination.

• Pack carefully to avoid crushing the vegetables.

• Glass containers are excellent, but these should be kept in a dark area.

• Each bag or glass container should be marked clearly with labels containing the date of packaging.

• The dried products must be stored in a cool, dry and clean area which is secure and protected against rodents and other pests.
  
  

Specific products

Fruit

Vegetables