Dehydration of Food Preservation

The process of dehydration

heat transferred to the food removing water from ood

Preservation principle of Dehydrated foods

chemical, enzymatic and microbial reactions require freewater, which is taken away by dehydration

Differences between Concentration and Dehydration

Both use removal of free water from foods, but dehydration is the removal of as much water as possible for a long storage life

Changes during dehydration

1. Shrinkage

2. Case hardening

3. Chemical changes

Shrinkage

water moves to interior where it evaporates

Case hardening

rapid drying causes compounds (such as sugars) to form a hard, impermeable case around food piece

Chemical changes

• a_w drops so sugars and amino acids become highly concentrated and react w each other, turning food brown and generating new flavours

• nutrient and flavour loss

5 factors affecting dehydration

1. Surface area (surface to volume ratio)

2. Temperature of drying air

3. Air velocity

4. Humidity of the drying-air

5. Atmospheric pressure and vacuum

Surface area (surface to volume ratio)

Smaller food piece, more rapid the rate of moisture loss-more surface to volume ratio- faster the moisture loss

Temperature of the drying air

higher temp. will increase Dehydration (DH) rate

Air velocity

Maximize velocity of heated air moving around the food particles

Humidity of the drying air

The drier the air, the more moisture it can absorb % RH of drying air determines the final moisture content of food

Atmospheric pressure and vacuum

the physical weight of the air pressing down on the outside of the food

Sun drying

• food under mats under sun

• slow

• high variability between products

Tray (Air) Drying

• food inside insulated chamber w hot air

• fast

• high consistency w minimized nutritional loss due to shorter exposure

Spray Drying

• Tiny droplets of food are sprayed into a stream of heated air (fine powder)

• ultra-fast

• powdery

Drum Drying

• semi-solid food is applied on a rotating heated drum then scraped off

• fast process

• can result in burning, vitamin loss and protein denaturation

Freeze Drying

• food frozen solid then put in vacuum where ice subliminates to water vapour

• very slow

• excellent rehydration, food shape retained, flavour retained, expensive

Vacuum Microwave Drying

• food placed under vacuum to lower boiling point of water while microwave penetrates food to heat internal water molecules

• fast

• minimal nutrient loss, rehydration, expensive and flavour & colour retention

Deep-fat frying

• water inside food flash boils into steam and escapes while oil moves inward to fill the empty cellular voices behind, evaporating water

• fast

• high fat content, crispy

Extrusion drying

A slurry of food – steam heating, under pressure → release pressure → steam ↑ (puffing)

Packaging requirements for dehydrated foods

1. Protect against moisture absorption

2. Physical protection

3. Protection from O₂ and light