Whether by sun drying or dehydration, process of drying consists of following steps:
Selection: The fruits should be in mature condition, sound and firm. Different lots at various stages of maturity must not be mixed together; as this results in a poor dried product.
Sorting: It is done to remove any decayed parts. Damaged parts which have been attacked by insects, rodents, diseases etc. and parts which have bad appearance must be removed.
Washing: Thorough washing of vegetables is required to reduce microbial load.
Peeling: It can be done by peelers, knives or by machines – for thick skinned vegetables – dipping in a solution of lye (dilute NaOH solution) which helps in easy removal of peel.
Subdivision of raw material:It involvescutting or sectioning of fruit or vegetables. It is very important to have all slices in one drying lot of the same size. Uneven slices result in poor quality end product.
Blanching: The product is partially cooked when it is dipped in hot water or by exposing it to steam. Blanching helps in inactivating enzymes and prevents discolouration. It destroys oxidizing enzymes, as a result there is better retention of vitamins A and C, removes pungent flavour and shortens drying time.
Sulphiting/ sulphuring: It involves dipping the material in 1-2 per cent potassium metabisulphate (KMS) or exposing the product to sulphur fumes (done in a closed room), respectively. Sulphur prevents mould growth and preserves colour of light coloured vegetables, repels insects and conserves vitamins A and C. Carotenoids are preserved by coating with starch. Food is exposed to sulphur fumes in such a way that 1000-3000 ppm of it is absorbed. Sulphuring of the fruit is done at the rate of 1.8 – 3.6 kg of sulphur per tonne of fruit. After sulphuring, the trays are kept in the sun with occasional turning of the fruit till it is dried.
Drying: Product is kept for drying.
Sweating:Dried product is kept inheapsin a room for few weeks for moisture equilibrium.
Packing: Plastic or polythene covers can be used. Aluminum foils, cardboard (wax lining is necessary for card board and wooden containers) etc. can also be used. Tin as such is not suitable as it becomes rusty but can be used after coating with inert gas like N2.
Some fruits and vegetables, in particular, banana, apples and potatoes go brown very quickly when left in air after peeling and slicing; this discolouration is due to phenol oxidase. To prevent slices from going brown, they must be kept under water until drying can be started. For certain fruits like grapes, figs etc. alkali (lye) dipping is done to help quicker drying due to appearance of cracks using 0.1-1.5 per cent Na2CO3 solution. Acid dipping for apples and pears is done to prevent browning using 1 per cent ascorbic acid and 1.25 per cent malic acid.