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During dehydration:

1. Water - is transferred from food to dehydrating environment
2. Heat - is transferred to the food (promoting water removal)

Preservation principle

- dehydrate foods have lower water activity (aw = 0.2-0.6)
- not enough free water for:
- microbial growth (need aw >=0.93)
- staphylococcus aureus (aw 0.85; pathogen)
- mould (need aw >= 0.6)
- enzymatic reactions
- chemical reactions (e.g. Maillard browning) at aw > 0.3

MOs during dehydration

MOs are not killed by dehydration
MOs resume growth after food is rehydrated


- removal of as much water from the food as possible
- imparts a long storage life


- some of the water is removed from the food
- concentrate the food constituents
- concentrated foods are not fully shelf-stable
- requires other forms of preservation: refrigeration, dehydration, thermal processing
- e.g. fruit juices, canned soup, jams, evaporated milk, condensed milk, maple syrup

Changes during dehydration:

- shrinkage (water migrates out and evaporates; decrease aw)
- case hardening
- chemical changes


- water migrates out and evaporates
- water-soluble substances (sugars, acids, and salts) are carried out to surface of dehydrated foods

Case hardening

- rapid drying --> compounds (e.g. sugars) form hard, impermeable case around food
- hard case --> cause rate of dehydration to decrease
- common in high-sugar products (e.g. fruits)

Chemical changes

- browning and flavour changes
- Maillard browning increase as conc. of solutes increase
- denaturation of proteins, aggregation of polysaccharides
- loss of water binding capacity
- loss of water-soluble components
- conc. of surface material
- loss of volatiles (flavour compounds)


Not all dehydrated foods show these changes

Grapes vs. Raisins

- 4.5 lb grapes = 1 lb raisins
- sugar coated, less free water

Factors affecting dehydration:

- surface area: smaller --> more rapid
- temperature of drying air: inc. temp --> inc. rate
- air velocity
- humidity of drying air: drier air absorbs more
- atmospheric pressure and vacuum: decrease boiling point by inc. temp

Methods for Dehydration

- sun drying
- spray drying
- tray and tunnel air drying
- drum drying
- freeze drying
- vacuum microwave drying
- deep fat frying
- vacuum extrusion drying

sun drying

e.g. fruits, vegetables, fish
- dry, warm climates
- inexpensive
- slow method
- invasion by insects, birds, rodents, MOs

spray drying

e.g. powder skim milk, instant coffee, tea, eggs
- liquid foods and purees
- tiny droplets of food sprayed into steam of heat air
- rapid method

tray (air) drying

- trays or racks
- heat air at set velocity
- poor rehydration; case hardening
- food shrinkage, dense product
- quick and inexpensive
- e.g. pasta, vegetables, fruit, spices

drum drying

- semi-solid foods: food paste, purees
- rotated heated drum
- rapid drying
- dried food scraped off

freeze-drying (lyophilization)

- food in frozen state
- vacuum chamber
- radiant heaters
- water sublime: solid --> vapour
- prevent heat damage
- voids from ice crystals
- excellent rehydration
- no translocation of water-soluble consitituents to surface
- shape of food retained (no shrinkage)
- food closely resemble start
- very expensive
- used for high value food
- e.g. instant soups, high quality coffee, space food

vacuum microwave drying

- vacuum (keep temp low = lower bp)
- microwave (rapid energy transfer)
- high quality products
- less nutrient loss
- flavour retention, less colour change
- complete rehydration
- quick drying emthod (min vs. hours)
- very expensive

deep-fat frying

- hot oil --> water evaporation
- food pick up oil
- changes sensory properties
- e.g. snacks, bakery products
- e.g. instant noodles

extrusion drying

- slurry of food - steam heating under pressure --> release pressure --> steam increase (puffing)
- moist heat cause starch gelatinization and cooking
- e.g. breakfast cereals, snacks

packaging requirements for dehydrated foods

- protect against moisture (DH foods hygroscopic; prevent moisture transmission)
- physical protection (prevent crushing, freeze dried foods - porous structure, easily crushed)
- protection from O2 and light (photooxidation)

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