Drying fruits tubers spices

1. DEHYDRATION
OF
FRUITS / VEGETABLES / SPICES / GRAINS
Dr Devinder Dhingra
Indian Council of Agricultural Research, New Delhi, India
Mobile: 9249049
Email: dhingrafiji@gmail.com

2. DRYING / DEHYDRATION
Purpose
 Preservation – enhances shelf life / reduces PHL
 Preparation of instant, convenience and new food
products – flour / soup / buns / cake / snacks / pasta etc.
 Reduction in weight & volume
 Lowers the cost of packaging, storing and transportation

3. SUITABILITY FOR DRYING
Commodities Suitability for drying
Onions, peppers, chilli, cassava, Uto,
Kumala, Ginger, Turmeric, Coconut kernel,
All kind of grains (&seeds)
Excellent
Carrots, Mushrooms, peppers (green or
red), potatoes, garlic, bitter gourd (karela)
Good
Beans (green), okra, beets, turnips,
pumpkins, peas, tomatoes
Fair to good
Yams, sweet potatoes, cabbage, beans
(lima)
Fair

4. Commodities Suitability for drying
Egg plant, squash(summer),
asparagus
Poor to fair
Spinach, cucumber, Kale, celery,
cauliflower, mustard (green)
Poor
Broccoli, lettuce, radishes, squash
winter
Not recommended

5. •Successful drying depends on - heat , air dryness and good air circulation;
slice thickness, layer thickness and turning upside down
•Select fresh fully ripe fruits / tender vegetables
•Wash, peel and cut fruits/vegetables/tubers etc.
•Pre-treat fruit pieces by dipping in ascorbic acid, citric acid, lemon juice or
sodium metabisulfite solution (why we do it?)
•Pre-treat vegetables by dipping in hot boiling water
•Some products do not need any pre-treatment
•When dry, allow to condition for some time before packaging and storage
•Package dried produce in tightly sealed containers and store in a cool dry
place

6. VEGETABLE DEHYDRATION PROCESS
SELECTION & SORTING
WASHING
PEELING *
CUTTING *
BLANCHING*
COLOUR PRESERVATION *
DRYING OR DEHYDRATION
SWEATING*
SCREENING *
INSPECTION *
PACKAGING
PM
PM
PM
VOC
SO2
VOC
PM
VOC
* MAY NOT OCCUR FOR ALL
FRUITS AND VEGETABLES

7. Pre treatment: BLANCHING (HOT WATER DIP)
Vegetable except onions, garlic, peppers etc. are blanched
Checks action of undesirable enzymes.
(Peroxidase and catalase enzymes are inactivated)
Improves rehydration of dried vegetables.
Cleans product / Reduces micro-organisms
Addition of Na2HCo3 to blanching water::::: when okra, green peas, and
some other green vegetables are blanched.
PROCESS: Immersion in hot water (95-100) for few minutes or exposure to
steam.
Disadvantages : Loss of ascorbic acid, vitamins, water soluble components.

8. Pre-treatments
Ascorbic Acid pre-treatment: Ascorbic acid (vitamin C) is an antioxidant
that keeps fruit from darkening and enhances destruction of bacteria during
drying.
Dissolve approx 35 g ascorbic acid (2½ tablespoons) in 1 litre of cold water
(room temperature) and dip the cut pieces for 10 minutes. 1 litre solution
will be good enough for 5 kg cut fruit pieces.
Drain the fruit pieces and dry.

9. Pre-treatments
Citric Acid or Lemon Juice Pre-treatment: Citric acid or lemon
juice may also be used as anti darkening and antimicrobial pre-
treatments. Prepare the citric acid solution by stirring 1 teaspoon
(5 grams) of citric acid into 1 litre of water.
For the lemon juice solution, mix equal parts of lemon juice and
cold water (i.e., 1 cup lemon juice and 1 cup water).
Cut the peeled fruit directly into the citric acid or lemon juice
solution.
Allow to soak for 10 minutes, then remove with a slotted spoon,
drain well and dehydrate.

10. Pre-treatments
Sodium Metabisulfite Pretreatment: Sulfur and sulfite compounds
have been used for centuries to prevent discoloration and reduce
spoilage during the preparation, dehydration, storage, and
distribution of many foods.
Stir 1 tablespoon (21 grams) sodium metabisulfite (food grade)
into one litre of cold water.
Cut the peeled fruit directly into the sodium metabisulfite
solution.
Allow to soak 10 minutes, then remove with a slotted spoon, drain
well and dehydrate.

11. Pre-treatments
Cracking Skins
Fruits such as grapes, prunes, small dark plums, cherries, figs,
and firm berries have tough skins with a wax-like coating. To
allow inside moisture to evaporate, crack or “check” skins before
drying whole fruits.
To crack skins, dip fruit in briskly boiling water for 30 to 60
seconds, then dip in very cold water.
Drain on absorbent towels before placing on drying trays.

12. Drying ratio:- Prepared : Finished dry
Onions : 8-10:1
Corn : 3:1
Peas : 4:1
Spinach : 15:1
Beets : 10:1
Sweet potatoes : 4:1
Potatoes 8-10 : 1
Carrots 15 : 1
Cabbage 18-20 : 1
Cassava 4:1
YIELD OF DRIED PRODUCT

13. DRYING METHOD
Drying Trays: Drying trays may be simple or complex, purchased or built.
Good air circulation without reaction between food and tray is most
important.
•Arrange pre treated fruits on drying trays in single layers, pit cavity up.
• Place the trays under open sun. Dry at 140 degrees F (60°C) in an oven or dehydrator
•Thinner slices and smaller pieces will dry more quickly than larger, thicker
•pieces or whole fruits.
• If possible, stir food and turn large pieces over every 3 to 4 hours during the
drying period.
•Fruits scorch easily toward the end of drying. Therefore, it’s best to turn the
power off when drying is almost complete and open the door wide for an
additional hour before removing pieces.

17. Testing for Dryness
Foods should be dry enough to prevent microbial growth and
subsequent spoilage. Dried fruits should be leathery and pliable.
To test foods for dryness, remove a few pieces and let cool to room
temperature. When warm or hot, fruits seem more soft, moist and
pliable than they actually are. Squeeze a handful of the fruit. If no
moisture is left on the hand and pieces spring apart when
released, they are dry.

18. Post-Drying Treatment: Conditioning
When drying is complete, some pieces will be more moist than others due to
their size and placement during drying.
Conditioning is a process used to evenly distribute the minimal residual
moisture throughout all pieces. This reduces the chance of spoilage, especially
from mold.
To condition, place cooled, dried fruit loosely in large plastic or glass
containers, about two-thirds full. Cover and store in a warm, dry, well-
ventilated place for two to four days. Stir or shake containers daily to separate
pieces. If beads of moisture form inside, return food to drying trays for further
drying, then repeat conditioning process.

19. DRYING PROCESS OF SELECTED PRODUCTS
Cassava (dry matter around 30%)
Selection …Washing….Peeling…washing …Slicing
….washing….spreading in trays under sun ….turning….
Conditioning .. Packaging….grinding…flour
Selection …Washing….Peeling…washing …Grating/Shredding
….squeezing….spreading in trays under sun ….turning….
Conditioning .. Packaging….grinding…flour

20. Breadfruit

21. DEHYDRATION PROCESS OF SELECTED PRODUCTS
POTATOES
Temp. storage (darkness & low temp 50C)
Peeling – mechanical or chemical (10-15% lye solution)
Slice thickness: 5-6 mm
Blanching : time 2-5 min
Sulfiting: Dipping in KMS solution (8000 ppm SO2) for 1 min. 2% NaCl
water solution
Dry at 650 C

22. CARROTS
Roots with red cores (not woody)
Remove stalk & tips, wash, scrape cut into slices 5-6 mm
thick
Blanching – Dip slices in boiling water containing 50g salt
(NaCl) / litre for 3 minutes
Cooling in running water
Sulfiting:Dipping in KMS solution (2000 ppm SO2) time 3
min (optional)
Dried to a moisture content of about 6%. (12:1)

23. ONION
Varieties : Colored / Red or White
Remove tops / roots
Washing
Cut into 3 mm thick slices
Cut at right angle to the core of onions
Washing
Dry up-to 5% moisture content (9:1)
Cooling
Packing
Labeling
Storage

24. TOMATOES
Ripe red firm tomatoes
Dip slices in a solution containing 0.7% KMS + 10% salt for 3 min
Empty the tomatoes and cut in 6-8 mm slices and place in drier
Drying ratio (raw material : finished dry) 25:1
(40g of dried product is obtained from 1 kg of fresh tomatoes)
Cooling
Packaging (in 100g cellophane bags)
Labeling
Storage in dark place
Flaking (gives better color & look), by rubbing through sieve.

25. Ginger rhizomes

Washing

Surface drying

Slicing

Drying Solar drying
 Tray drying/Mechanical
Milling

Ginger Powder

Packaging
Dehydration of ginger

26. Turmeric
Washing --
Boiling the rhizomes in clean water for 30 minutes--
Drain-
Sun dry (metallic sound)
Conditioning
Polishing (rubbing to remove some peel)
Storage
Grinding

31. THANK YOU

32. QUALITY EVALUATION OF DEHYDRATED VEGETABLES
Moisture Content : below 5 percent for most products : 5-8 %
Grind sample to pass 20 mesh sieve, sample 2g. Dry in vacuum oven at
700C for 2 h.
Enzyme tests:
Catalase: 2g of well mixed, coarsely broken sample placed in a large test
tube with 20 cc of distilled water. After about 15 min soaking, add ½ cc of
0.5% Hydrogen Peroxide.
If catalase is still active, vigorous evolution of O2 will ensue within 2-3 min.

33. Peroxidase: ½ cc of 0.5 % Hydrogen peroxide and ½ cc of 0.5% guaiacol
solution in 50 per cent alcohol.
If pieces remain unchanged in colour for 15 min, the test is considered
negative, or blanching is considered adequate. If pieces show a reddish or
reddish brown colour within 3 min, the test is positive.
For carrots use a 0.5 % solution of benzidine in 50% alocohol instead of
guaiacol solution