8.4 Fruit drying and dehydration technology

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The sequence of operations employed in the production of dried / dehydrated fruit is presented in section 8.4.1.

General technical data for fruit dehydration in tunnels are presented in Table 8.4.1.

TABLE 8.4.1 Technical data for fruit dehydration in tunnels

FRUITS Drying Conditions Finished Product
Load kg/m Temperature C Time Moisture % Yield %
Plums 15 I. 40-50 6 H 18-20 25-35
    II. 75-80 14 H    
Apples (Rings) 10 75-55 5-6 H 20 10-12
Apricots (Halves) 10 70-60 10- 15 15-20 10-15
Cherries (w. stones) 10 55-70 6-8 12-15 25
Pears (Halves and quarters) 15 70-65 15-22 18-20 10-15
15 70-60 10-15 15-20 10-15

For fruit with a high sugar content drying temperatures have to be lower at initial stage and then increase to the maximum acceptable; for fruit with lower sugar level the temperatures are applied in a reverse order.

Some pre-treatments of fruit and vegetables for sun /solar drying are described in table 8.4.2.

Technical data on some osmotically dehydrated products are presented in Table 8.4.3. Moisture and shipping factors for some dried / dehydrated fruit are seen in Table 8.4.4.

TABLE 8.4.3 Technical data on some osmotically dehydrated products

Fruit or vegetable Type of cut Treatment
Banana 5 mm slices 2 hours, 80% sugar
2000 ppm SO2
at 70C
Carrots 10 x 10 x 2 mm dices or

5 mm slices

4 hours, 60% sugar + 10%
4000 ppm SO2
Mango, green 8 mm slices 2 hours, 25% salt
8000 ppm SO2
Mango, ripe 8 mm slices 2 hours, 60% sugar
8000 ppm SO2
Onions 2 mm slices 2 hours, 60% sugar + 10% salt
4000 ppm SO2
Papaya 8 x 8 mm slices 4 hours, 80% sugar
2000 ppm SO2 at 70C
Strawberries Whole 4 hours, 80% sugar
4000 ppm SO2
Sweet peppers, red 6 mm dices 2 hours, 60 % sugar + 10 % salt
4000 ppm SO2

Source: FAO, 969a

TABLE 8.4.4 Moisture and shipping factors for some dried/dehydrated fruits

Products Form moisture, %
Apples 6 nun rings 20
Apricots Caps 17-20
Banana Cut pieces 15
Cherries Whole 12-15
Figs Whole 23
Guava Quarters 6
Mango 15 mm pulp sheets 15
Peaches Caps 15-20
Pears Halves 23
Prunes Whole 18-20
Raisins Whole 17

The moisture contents listed are considered as the best from a technical, practical and commercial point of view for delivery to the market or for shipping and safe for the shelf life needed before buying / consumption by customers / consumers.

All instructions about packing, storage and transport must be followed in order to assure delivery of a safe and high quality product to the market.


8.4.2 Processing of fruit bars (Source: FAO 1992c, FAO 1990a)

The fruit bar processing method developed for FAO only involves a single major operation, which it drying the fruit pulp after mixing it with suitable ingredients. It can be used to produce mango, banana, guava or mixed fruit bars.

A dual-powered dryer, working by solar energy during the day and by electric or steam power at night and on rainy days, with cross-flow movement of air and controlled temperature (from 55 C at the beginning of processing to a high of 70 C), is well suited for dehydration of the pulp to the desired moisture level of 15 to 20%.

Main raw material quantities to prepare approximately 100 kg of fruit bars are as follows:

Type of fruit Fruit required in kg Pulp obtained in kg Sugar required in kg Yield (% of fresh fruit) approx.
Mango 720 360 33 14
Banana 600 360 30 17
Guava 406 325 60 25
Mango + banana 540 + 150 360 35 15
Papaya + banana 500 + 140 336 54 23

Source: Amoriggi (1992), FAO (1990)

Mango fruit bar. - Fully ripe mangoes are selected and washed in water at room temperature. The peeled fruit is cut into slices and passed through a helicoidal pulper to extract the pulp. The required amount of sugar to adjust the Brix (the unit measure for total solids in fruits) of the mixed pulp to 25 degrees Brix is then added.

Two grams of citric acid per kilogram of pulp (or 20 ml of lime or lemon juice) are added to inhibit possible growth of micro-organisms during drying. The mixture is then heated for two minutes at 80 C and partially cooled; the heat treatment serves to inactivate the enzymes and destroy the micro-organisms.

Potassium or sodium metabisulphite is added (two grams per kg of prepared mixture), so that the concentration of SO2 is 1000 ppm. The mixture is then transferred to stainless steel trays which have been previously smeared with glycerine (40 ml/m). Each tray must be loaded with 12.5 kg of mixture per square metre.

Drying could be carried out by a dual-powered dryer for a total of 26 hours:

a) 10 hours by solar energy at about 55 C and

b) 16 hours by electric or steam power at 70 C.

At the end of the drying operation, when moisture content is between 15 and 20%, the pieces of suitable shape and size are wrapped in cellophane paper, packed in cartons and stored at ambient air temperature. Pieces of unsuitable shape and size are further cut into small pieces and used to prepare, along with peanuts and cashews, a variety of "cocktail mixtures".

Banana fruit bar. - Banana varieties which give smooth pulp without serum separation must be used for this purpose. Ripe, suitable fruit is selected. The hand-peeled fruits are soaked in 0.3 per cent citric acid solution for about 10 minutes (lime or lemon juice can replace citric acid). The drained fruit are pulped to obtain smooth pulp.

The rest of the procedure is the same as in the case of the mango bar.

Guava fruit bar. - A mixture of pink and yellow varieties is best suited for preparing the bar. The washed fruit is hand peeled and stem and blossom ends trimmed. The peeled fruit is cut into quarters which are passed through a helicoidal extractor to separate seeds and fibrous pieces (the holes in the stainless steel screen should be between 0.8 and 1.10 mm).

To get the maximum yield of pulp, the material is passed through the extractor twice. After adjusting the refractometric solids to 25 degrees Brix, the fruit bar can be prepared by following the same procedure as for mango pulp.

Mixed fruit bar. - Mango and banana pulp, as well as papaya and banana pulp, can be mixed in a calculated ratio for preparing mixed fruit bars. The rest of the procedure is the same as in the case of pure mango pulp.

Packing and storage. - The dried pulp is removed from the dryer and cut into square pieces of 5 x 5 cm at a thickness of about 0.3 cm. These pieces, arranged in three layers make up blocks of about 0.9 cm thickness weighing between 25 and 28 grams. An unit pack consist of two such blocks and weights between 50 and 56 grams.

Each block is separately wrapped in cellophane and the unit pack is filled in a printed cellophane bag of size 15 x 8 cm. Two hundred unit packs are packed in a master carton of size 34 x 22 x 14 cm, with a net weight of about 10 kg. Shelf-life is about one year at room temperature.

Fruit leathers. - Fruit leathers are manufactured by drying/dehydration of fruit pures into leathery sheets. The leathers are eaten as confections or cooked as a sauce. They are made from a wide variety of fruits, the more common being apple, apricot, banana, cherry, blackcurrant, grape, peach, pear, pineapple, plum, raspberry, strawberry, kiwi fruit, mango and papaya.

A description of procedures for mango, banana, guava and mixed fruit bars is given in this document.

Another product with good potential is ciku leather; ciku fruit is grown in Malaysia.

A standard process is carried out using ripe fruits which are washed, peeled, diced and the seeds removed. The fruits are blanched for 1 minute at 80 C and blended into puree in a food processor.

Ciku leather is prepared by mixing ciku puree with 10% sugar, 10% pre-gelatinous rice flour, 150 ppm sorbic acid an 500 ppm sodium metabisulphite (Na2H2SO4).

The mixture is cooked on a water bath at 60 C and then made into sheets 1.8 mm thick on trays spread with glycerol to reduce stickiness. This is then further dried in a forced-air dehydrator at 45 C for 3.5 hr or until the surface no longer feels sticky when touched with the fingers.

The dried and cooled leathers are cut into 12 x 12 cm squares and wrapped in polypropylene (PP) of 0.1 mm thickness.

8.4.3 Osmotic dehydration in fruit and vegetable processing introduction

Osmotic dehydration is a useful technique for the concentration of fruit and vegetables, realised by placing the solid food, whole or in pieces, in sugars or salts aqueous solutions of high osmotic pressure. It gives rise to at least two major simultaneous counter-current flows: a significant water flow out of the food into the solution and a transfer of solute from the solution into the food. Process variables

Main process variables are

  1. pre-treatments;
  2. temperature;
  3. nature and concentration of the dehydration solutions;
  4. agitation;
  5. additives.

In the light of the published literature, some general rules can be noted:

Synergistic effects between sugar and salt have also been observed.

A pilot plant equipment used for detailed study of process parameters in osmotic concentration of fruits is seen in Figure 8.4.2 (Source: Garrote, R.L. et al., 1992). Applications

The effects of osmotic dehydration as a pre-treatment are mainly related to the improvement of some nutritional, organoleptic and functional properties of the product.

As osmotic dehydration is effective at ambient temperature, heat damage to colour and flavour is minimised and the high concentration of the sugar surrounding fruit and vegetable pieces prevents discoloration.

Furthermore, through the selective enrichment in soluble solids high quality fruit and vegetables are obtained with functional properties "compatible" with different food systems. These effects are obtained with a reduced energy input over traditional drying process. The main energy-consuming step is the reconstitution of the diluted osmotic solution that could be obtained by concentration or by addition of sugar.

Various applications of the technique as a unit operation in the food area are summarised in Pig. 8.4.1 together with the process parameters regarded as optimal in the light of the published literature.

Figure 8.4.1 Applications of osmotic dehydration

Figure 8.4.2 Sketch of a pilot plant used for detailed stud of process parameters in osmotic concentration of fruit

Figure 8.4.3 Flow diagram for osmotic dehydration and vacuum drying of bananas (Bongirwar and Sreenivasan, 1977)



Air drying following osmotic dipping is commonly used in tropical countries for the production of so-called "semi-candied" dried fruits. The sugar uptake, owing to the protective action of the saccharides, limits or avoids the use of SO2 and increases the stability of pigments during processing and subsequent storage period.

The organoleptic qualities of the end product could also be improved because some of the acids are removed from the fruit during the osmotic bath, so a blander and sweeter product than ordinary dried fruits is obtained. Owing to weight and volume reduction, loading of the dryer can be increased 2-3 times.

The combination of osmosis with solar drying has been put forward, mainly for tropical fruit. A 24 hour cycle has been suggested combining osmodehydration, performed during the night, with solar drying during the day. Two-three-fold increase in the throughput of typical solar dryers is feasible, while enhancing the nutritional and organoleptic quality of the fruits.

A two-step drying process, OSMOVAC, for producing low moisture fruit products was described. The osmotic step is performed with sucrose syrup 65-75 Brix until the weight reduction reaches 30-50%.

By osmotic dehydration followed by vacuum drying puffy products with a crisp, honeycomb-like texture can be obtained at a cost comparatively lower than freeze-drying.

Commercial feasibility of the process on bananas has been studied, based on the results of a semi-pilot scale operation; the process scheme is reported in Figure 8.4.3. Osmotically dried bananas retained more puffiness and a crisper texture than simple vacuum dried ones, and the flavour lasted longer at ambient temperature.

The combination of osmotic dehydration with freeze-drying has been proposed only at laboratory scale.


A combination of osmotic dehydration with appertisation has been proposed to improve canned fruit preserves. The feasibility of a process, called osmo-appertisation, to obtain high quality fruit in syrup, has been assessed on a pilot scale.

The key point of this technique is the pre-concentration of the fruit to about 20-40 Brix, that causes, together with the enhancement of the natural flavour, an increase of the resistance of the fruit to the following heat treatment, especially for colour and texture stability.

The products obtained are stable up to 12 months at ambient temperature and show a higher organoleptic quality than canned preserved alternatives.

Furthermore, because of their higher specific weight and diminished volume, the filling capacity of jars or pouches is increased.


The frozen fruit and vegetable industry uses much energy in order to freeze the large quantity of water present in fresh products. A reduction in moisture content of the material reduces refrigeration load during freezing.

Other advantages of partially concentrating fruits and vegetables prior to freezing include savings in packaging and distribution costs and achieving higher product quality because of the marked reduction of structural collapse and dripping during thawing.

The products obtained are termed "dehydro-frozen" and the concentration step is generally carried out through conventional air drying, the additional cost of which has to be taken into account.

Osmotic dehydration could be used instead of air drying to obtain an energy saving or a quality improvement especially for fruit and vegetable sensitive to air drying.

Extraction of juices

An osmotic pre-step before juice extraction was reported to give highly aromatic fruit or vegetable juice concentrates.

Further developments

So far only applications on a pilot plant scale are reported in the literature. For further developments on a larger scale, theoretical and practical problems should be solved.

The industrial application of the process faces engineering problems related to the movement of great volumes of concentrated sugar solutions and to equipment for continuous operations. The use of highly concentrated sugar solutions creates two major problems.

The syrup's viscosity is so great that agitation is necessary in order to decrease the resistance to the mass transfer on the solution side.

The difference in density between the solution (about 1.3 kg/litre) and fruit and vegetables (about 0.8 kg/litre), makes the product float.

Another important aspect, so far not investigated, is the microbiological safety of the process, which should be studied thoroughly before further industrial development.

Osmoappertisation in the processing of apricots

In order to obtain an alternative to the canned fruit preserves and to maintain a high quality of the fruits, a research has been carried out on the osmoappertisation of apricots, a "combined" technique that consists in the appertisation of the osmodehydrated apricots.

This technique could contributes also to the reduction of energy consumption, limits the cost of production and combines "convenience" (ready-to-eat, medium shelf-life) with many market outlets (retail, catering, bakery, confectionery, semi-finished products).

Osmoappertisation combines two unit operations: dehydration by osmosis and appertisation (packaging + pasteurization).

Figure 8.4.4 Osmoappertisation in the processing of apricots

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