ITDG 
Interdev Agroalimentaire  

Small-scale processing of dried fruits

	Type de fiche	Techniques/Technologies
	Mise en œuvre	ITDG(Royaume Uni)
	
	Zone géographique	Global; 
	Mots clés	PAPAYE; GOYAVE; MANGUE; SECHAGE; ANANAS; BANANE; ABRICOT

	Echelle d’action	Régional (transnational)
	Public(s) cible(s)	Small- and medium-scale food processors

Contexte
Drying fruits is a popular and relatively simple way of preserving and adding value to fruits.  Most fruits can be dried, but some are more popular in the dried form than others.  The dried fruits have a soft, rubbery texture and a sweet taste with the characteristic flavour and colour of the fruit that has been used.  The fruit pieces can either be eaten as snack foods or can be used for confectionery and baked goods.  Dried fruits are wholesome and nutritious and are popular in the health foods and breakfast cereal markets.

Preservation relies on the removal of water from the fruit by soaking in a sugar syrup and by drying.  An acid dip or sulphur dioxide may also be used to reduce the number of micro-organisms.

Problématique / Objectifs
When producing dried fruits, it is essential that the raw material is of top quality and at the correct stage of maturity.  If the fruits are overripe they ar ewasily damaged and may be difficult to dry.  If they are underripe they have a poor colour, flavour and appearance.  Harvesting and transport of the fruits should be done as carefully as possible to prevent spoilage, bruising and loss of quality.  Specially designed low cost boxes can be used to protect the fruit.

The size of fruit pieces affects the time needed for pre-treatments such as sulphuring or dipping and for drying.  The pieces should be of a uniform size so that they all dry at the same rate.  The size of the fruit pieces is of particular importance during both the initial and latter stages of drying.  In the early stages, smaller pieces have a greater surface area from which the water can evaporate, thus speeding up the drying time.  In the second stage, the smaller the pieces of fruit, the shorter is the distance that the moisture has to move from the inside to the surface of the fruit.

Some fruits can dry too quickly during the initial stage which leads to a condition known as case hardening.  This occurs when the outer layer of the fruit dries quickly, producing a hard layer that impedes the movement of water from the inner layers to the outside of the fruit.  Thus, the fruit is hard on the outside with a soft, moist centre, which is not good as the fruit will have very poor keeping qualities.  This condition is most common with fruits with a high sugar content. Case hardening can be controlled by reducing the rate of drying during the initial phase _ this requires a dryer with thermostatic control. It can also be controlled by altering the size of the fruit pieces.

Red fruits or fruits that are expected to turn brown after drying (for example dates and grapes) should not be treated with sulphur dioxide or sulphite dips as these bleach the colour.  In other fruits, the use of sulphur dioxide and sulphite helps to protect the strong bright colour.

Choice of dryer
Sun drying is a traditional method of preservation carried out in hot dry regions.  At the most basic level, the fruit pieces are placed in the sun on a clean flat surface. If climatic conditions are favourable _ ie it is hot and humidity is low - drying is very rapid. However, the prodcut will become contaminated b
Description/Mise en oeuvre
A process flow sheet for dried fruits is available (ref link)

The process includes the following basic steps:
  selection and cleaning of fruit
  peeling and cutting
  acid dip or sulphur dioxide treatment
  mix with sugar syrup
  dry
  pack and store

Selection and cleaning of fruit
The fruit should be harvested at the correct stage of maturity.  Under-ripe, overripe and mouldy or damaged fruits are discarded.  The fruit is washed in clean water to remove dust, dirt and debris. The fruit is sorted according to the stage of maturity, colour, shape and size.  This is done to ensure that the fruit in one batch is uniform with respect to the stage of maturity as this has an impact on the rate of drying.

Peeling and cutting
The washed fruits are peeled to increase the rate of drying.  The layer of peel on the surface acts as a barrier to the movement of water out of the fruit.  Peeling is usually carried out by hand using a stainless steel knife (other metals will stain and discolour the flesh of the fruit).  For larger scale operations, a variety of fruit peeling machines are available.
The fruit is then cut into smaller pieces (about 0.5-1.0cm cubes) to speed up the rate of drying. 

Acid dip or sulphur dioxide treatment
Acid dipping or sulphuring is an optional step, but it helps to prevent browning of the fruit during drying.  To acid dip, the fruit pieces are dipped in lemon juice or citric acid for 5-10 minutes.  For sulphuring, there are two methods that can be used _ burning sulphur or using a sulphite dip. Neither should be used with red fruits as they bleach the colour.
a) Burning sulphur. Sulphur is burnt in a sulphuring cabinet or tent to give off sulphur dioxide gas.  The rate to use is 350-400g sulphur per 100kg fruit (2-3 tablespoons per kg fruit).
b) Sulphite dip. Either sodium or potassium metabisulphite (0.3-0.45g per litre of water) is dissolved in water to produce a 0.2-0.8% sulphite dip. The length of time needed for sulphuring depends on the shape and size of the fruit pieces.

Mix with sugar syrup
The fruit pieces are boiled and soaked in a sugar syrup as this helps to remove some of the water and therefore reduces the length of time required in the dryer.  The fruit pieces are boiled in a 60-70% sugar syrup solution for 10-15 minutes.  They are then left to soak for up to 18 hours. During this soaking time, up to half the water is removed from the fruit. Sugaring also gives good colour retention and a sweeter, blander product.  However, the lower acid levels may lead to spoilage if the food is not properly dried and packaged.  To make this process economical, the sugar solution must be re-used three or four times.  The concentration must be brought back to 60-70% by the addition of more sugar.  Care must be taken not to burn the syrup as this will spoil the flavour of the fruit.

Dry
The choice of dryer depends on the scale of operation and the types available locally. The drying rate depends on the temperature, humidity and speed of air, the type of fruit and the size of the fruit pieces.  Generally, temperatures of 50-60 C are used.

Pack and store
Dried fruit is very hygroscopic, that is it readily absorbs water from the surrounding air, which leads to spoilage.  The fruit must be packaged in moisture-proof packets and heat sealed.  It is stored in a cool dry place away from direct sunlight.

Résultats/Eléments chiffrés
The following equipment is required:
Wash tank
Boiling pans (optional)
Sulphur cabinet or tent (optional)
Dryer
Heat sealer
Polythene or polypropylene packaging

Enseignements
The use of sulphur dioxide is controlled by law in some countries.  Processors need to check the local regulations (and those of the countries they are exporting to if appropriate) before using this chemical.

Although the drying of fruits is a fairly simple way of preserving fruit, it is not sufficient to preserve it just to use up a seasonal glut of fruit.  It is essential to carry out a detailed market study to determine demand for the product before starting production.

Auteur : Sue Azam Ali
Saisie le : 19/03/2002 

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