Exercise

Practical 29 - Dehydration of fruits

Aim: To conduct practical on drying of fruits

Theory: Drying generally refers to the method of removal of moisture from the food under natural condition such as sunlight and wind for example open sun drying, shade drying etc. Whereas, dehydration refers to a process of removal of moisture by application of artificial heat under controlled conditions of temperatures humidity and air flow. It also represents drying of agricultural commodity to dry state. For drying, single thin layer of fruit and vegetables, either whole or sliced after primary pretreatments is spread on the trays which are then placed inside the dehydrator or in the open sun for drying. In the dehydrator initial temperature is generally kept at 43oC which is then gradually increased to 66-71oC for fruits.

Raw material, ingredients and utensils required
  1. Fruits like mango, grapes, papaya, apple, apricot, date, aonla and fig etc.
  2. Stainless steel knives, utensils for blanching and dipping in KMS, dehydrator, drying trays , solar drier, sulpher fumigation chamber.
Procedure for drying: Drying generally involves three stages: pre-drying treatments or pre-treatments (Table 29.1), drying and post drying handling, packaging and storage.

Pre- drying treatments

1. Select mature and firm fruits for drying.

2. Sort, wash and peel (where required),

3. Follow schedule for preparation as given in Table 29.1

4. Slice apple and Papaya into thin slices.

5. Cut small bunches of grapes along with rachis.

6. Blanching: Fruits like grapes, plum and apricot are dipped in boiling 0.5% NaOH solution for 7-10 seconds followed by cooling to remove the bloom (waxy layer from grapes and plum) or to remove pubescence (hairy growth from apricots) which otherwise interfere in moisture removal.

7. Sulphuring: The sulphuring is done in sulphur fumigation box which is airtight wooden box of 90×60×90 cm size in which the trays are arranged to place the fruit for sulphuring. Generally 3g sulphur for each kg of prepared fruits and is burnt inside the chamber. Sulphur fumigation is carried out for 45-60 minutes to allow the fumes of sulphur dioxide to be absorbed by the commodity. OR

8. Sulphiting: Place the prepared fruits/ in a solution of potassium meta-bisulphite (1-2% KMS) and keep for 30-45 minutes. After the treatment, the fruits are drained and are placed on the trays for drying.

Table 29.1: Schedule for drying and dehydration of fruits (preparation and pretreatments)

Fruits

Preparation/ pretreatments

Sulphuring/

sulphiting time

Drying temperature

(0C)

Apple

Wash, peel, core, trim and cut into 3-5mm thick slices

30 minutes (1-2% KMS)

60-71 for 6-7 hours

or sun dry

Apricot

Dip in 0.5% boiling caustic soda solution for 7-10 seconds and rinse

1 hour (3g sulphur/ kg fruits)

57-68 for 10-12 hours or sun dry

Aonla

Wash, grate or cut into halves, destone

Salt treatment @ 40g/kg fruits

Sun dry

Banana

Wash, peel, cut lengthwise/round shape 12 mm thick

30 minutes

(1-2% KMS)

55-71 for 10-12 hours or sun dry

Date

Wash, dip in boiling 0.5 % caustic soda solution then rinse

-

45-50 or sun dry

Grapes

Dip in boiling 0.5% caustic soda for 7-10 sec and rinse or dip in 1-2%. ethyl oleate solution

1 hour (3g Sulphur/kg fruits)

55-60 or sun dry

Mango

Wash, peel, cut into 12 mm thick slices

2 hours

(1-2%KMS)

50-60 or sun dry

Papaya

Wash, peel, cut into 6 mm pieces/slices, remove seed

2 hours

1-2 %KMS)

60-65 or sun dry

Prunes

Wash, use whole fruit or cut into pieces, remove seed, pretreatment for 3-5 seconds with boiling 0.5% caustic soda following by cooling.

1 hour (sulphur 3g/ kg fruit)

55-60 or sun dry

29.1
9. Drying: Place the prepared fruits/after sulphuring or sulphiting in thin layers on the trays and keep either in sun light, solar drier or mechanical dehydrator. Allow the drying process to continue till a constant weight loss. Frequently turn the fruits upside down for uniform drying. Mechanical dehydrator takes few hours to dry while sun or solar drier takes longer time for drying. The drying time depends upon temperature used for drying and quantity of material loaded in the drier.

10. Sweating: Keep the dried product in boxes/cloth bag or bins to equalize the moisture contents within the product.

11. Sorting, grading and packing: After moisture equalization for 10-15 days, sort the dried product (remove rachis from grape bunches) and grade on the basis of colour, size, and pack in polythene bags or aluminium laminated bags.

12. Yield of dried products: The yield of dried product generally depends upon the total solid content including TSS of the fresh product. Drying yield of different products vary between 20-25%, grapes, 18-20% apricot, 10-12% apple, 14-20% banana and 3-5% onion etc.

13. Storage: Store the dried products in a cool and dry place.

Osmotic dehydration
  1. Place the prepared fruits (apple slices, apricot and plum) in 70oBrix syrup at room temperature for overnight.
  2. Drain the fruits and rinse in water to remove the excess syrup from the fruit surface.
  3. Place on the drying trays and dry in mechanical drier to a constant weight.
  4. After drying, keep in the cloth bags for moisture equalization.
  5. Pack in airtight bags and store in cool and dry place.
  6. The yield of osmotically dried fruits is more than that of fruits dried without osmosis.

FPO specifications for dried and dehydrated fruits and vegetables

Moisture

Dehydrated fruits- Not more than 20% w/w

Sun dried fruits- Not more than 24% w/w

Preservatives

Sulphur dioxide: Raisins- Not more than 750 ppm

Apricots, peaches, pears, apple and other fruits - Not more than 2000 ppm


Last modified: Friday, 9 March 2012, 6:11 AM