Processing of Horticultural Crops (1+2)

Exercise

Practical 30 - Dehydration of Vegetables

Aim: To perform practical on drying/dehydration of different vegetables.

Theory: Drying refers to the method of removal of moisture content from the food to a level at which the activities of food spoilage and food poisoning micro-organism are inhibited. Therefore, reduction in water activity of the food is the main principal of preservation by drying. In the dehydrator initial temperature is generally kept at 43^oC which is then gradually increased to 60-66^oC for vegetables. The product shall be prepared from wholesome vegetables free from blight, insect infestation and fungal discoloration. Only edible portion of the vegetables shall be used and it shall be free from stalks, peels, stems, and extraneous matter. The dried/dehydrated vegetable may contain permitted preservative. The finished product shall be of good edible quality and shall reasonably reconstitute to its original shape and quality on boiling for 15 minutes to an hour. The finished product shall be free from visible mould, insect or larvae. Kind of dry vegetable shall be declared on the label. Acid insoluble ash shall not be more than 0.5%.

Raw material, ingredients and utensils required

1. Vegetables like cauliflower, cabbage, spinach, coriander, potato, carrot, radish red chillies, onion, garlic, methi, etc. are most commonly dried.
2. Stainless steel knives, utensils for blanching and dipping in KMS, dehydrator, sulphur fumigation chamber, drying trays, solar drier etc.

Procedure for drying:
Follow the procedure for preparation of vegetables for drying as given in Table 30.1 which include the steps like

1. Selection of vegetables

2. Sorting, washing, peeling (hand peeling, steam, hot water, lye peeling or abrasive peeling and slicing).


Table 30.1: Schedule for preparation of vegetables for drying and dehydration

Vegetables	Preparation/ pretreatments	Treatment before drying	Drying temperature (0C)	Drying yield
Cauliflower	Wash, remove stalk, stems, break flowers florets into pieces of uniform size	Blanch in boiling water for 4-5 minutes, immerse in 1% KMS solution for one hour and drain	55-600C or sun dry or use solar drier	3-4
Cabbage	Wash, remove stalk, outer leaves and cut into fine shreds	Blanch for 5-6 min., immerse in 0.5% KMS solution for 10 minutes & drain	55-60 or sun dry or use solar drier	5-7
Beans	Wash, cut into small pieces	Blanch for 3-4 minutes and immerse in 0.5% KMS solution for 10 minutes & drain	55-60 or Sun dry or use solar drier	14-16
Onion	Remove the outer peel and cut into round thin pieces	Dip for 10 minutes in 5% salt solution	60-65 or sun dry or use solar drier	10.0
Green leafy vegetables	Wash, sort, trim off rough stems and stalk, shreds	Blanch for 2-3 minutes in boiling water	60-65 or sun dry or use solar drier	-
Potato	Wash, peel, cut into 10 mm thick slices.	Blanch for 4-5 minutes and immerse in 0.5% KMS	60-65 or sun dry or use solar drier	14.0
Tomato	Wash, cut in to pieces	Blanch for 30-50 seconds, peel and cut into round slices 10 mm thick	60-65 or sun dry or use solar drier	3-4
Carrots	Scrap peel, cut stalk and tip cut 0.5cm thick slices	Blanch in boiling common salt solution (2-4%) for 2-4 minutes	68-74 or sun dry	5-6
Bitter gourd	Scrap peel of bitter gourd, cut 0.6 cm thick slices	Blanch in boiling water for 7-8 minutes	66-71 or sun or solar drier	6-8
Ladies finger	Use whole, halves or discs	Blanch in boiling water for 4-8 minutes	63-68	8-10

3. Blanching: Blanching of vegetables is carried out to inactivate enzymes. The vegetables are kept in boiling water or under steam for a pre-determined period followed by immediate cooling.

4. Use either sulphuring or sulphiting for treating vegetables with sulpher dioxide before drying.
a) 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 prepared vegetable for sulphuring. Generally 3g sulphur for each kg of prepared vegetables 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.

b) Sulphiting: Place the prepared vegetables in a solution of potassium meta-bisulphite (0.5-2% KMS) and keep for 30-45 minutes.

5. Drying: Place the treated vegetables in thin layers on the trays and keep either in sun light, solar drier or in mechanical dehydrator. Allow the drying process to continue till a constant weight loss. Turn the material frequently on the trays to achieve uniform drying. Drying time depends upon the tray load and drying temperature.

6. Sweating:-Keep the dried product in cloth bags for 10-15 days for moisture equalization. Pack the sorted and graded product in air tight containers.

7. Yield of dried products: The yield of dried product generally depends upon the total solid content of the fresh product. Percent drying yield for some vegetables is given in Table 30.1.

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

FPO specifications

Preservative – Sulphur dioxide - Not more than 2000 ppm
Ash – Acid insoluble ash - Not more than 0.5%
Rehydration ratio - Reconstitute to original shape and quality by boiling for 15- 60 minutes
The dried product shall be free from visible mould, insect or larvae.