Etiket Arşivleri: Drying

Drying: Principles and Practice ( Professor A. S. Mujumdar )


Introduction to food processing; drying ‐ Fundamentals

Classification‐general ideas

Why so many dryer types?

Key criteria for classification

Criteria for dryer selection

Different dryer types

Energy related issues in drying

Special/Innovative dryers


Food Processing

• Need of food processing ‐ to avoid the spoilage of foods due to various reasons; to increase shelf life; to make food products available through out the year

• The spoilage could be due to physical damage, chemical damage, microbial attack

• Various food processing methods – Freezing, canning, preserving in syrup, food irradiation, salting, vacuum packaging, dehydration

• canning and freezing – best way to retain the taste, appearance, and nutritive value of fresh food (Cost involvement)

• Drying/Dehydration – very much cost‐effective; product takes much less storage space than canned or frozen foods; Some dehydrated products have very good rehydration properties

Drying ­ Fundamentals

Removal of a liquid from a solid/semi­solid/liquid to produce solid product by thermal energy input causing phase change (Sometimes converts solid moisture into vapor by sublimation eg. Freeze drying with application of heat.)

Needed for the purposes of preservation and storage, reduction in cost of transportation, etc.

Most common and diverse operation with over 100 types of dryers in industrial use

Competes with distillation as the most energy­intensive


Drying Foods ( Jananne Finck )

Drying Foods

Jananne Finck, MS, RD.

Nutrition & Wellness Educator

Springfield Center

Drying is New Again

One of the oldest methods of preserving food

Alternative to canning & freezing

Simple, safe and easy to learn

Can dry year round

No refrigeration needed

Takes little space

How Drying Preserves

Removes moisture from food so bacteria, yeasts & molds can’t grow and spoil food

Slows down action of enzymes but doesn’t inactivate them

Temperatures for Drying

Optimum temperature for drying food is 140°F

If higher temps are used, food cooks instead of drying

Avoid “case hardening” – dried outside and moisture is trapped inside – mold results

Helps Drying…

Low humidity aids drying

Increasing air current speeds up drying, removes air around food

Ways To Dry Food

Foods can be dried in the sun, but often doesn’t work well in Illinois…need several days of high temp and low humidity

Dry in an oven

Dry in food dehydrator

Drying Out-Of-Doors

Sun Drying

Dry fruits, high sugar & acid make them safe

Don’t dry vegetables or jerky/meat outdoors

Need hot, dry, breezy days

Temp of 85ºF. or higher, for several days with humidity below 60%

Need to cover, watch materials used and control for insects and pests


Solar Drying

Need to make a dryer

Need to stir and turn food several times a day

Need several days…

Vine Drying

Dry beans, lentils and soybeans

Leave bean pods on vine until beans inside rattle

If not dried can dry further in oven or dehydrator


Freezer Method – seal food in freezer plastic bags. Freeze at 0ºF for at least 48 hours

Oven Method – Place food in single layer on fray or in
shallow pan. Place in oven preheated to 160ºF for 30 minutes

Drying Foods Indoors

Food dehydrators


Convection ovens

Conventional ovens

Food Dehydrators

Small electrical appliance for drying foods indoors

Electric element for heat & fat and vents for air circulation

Dry foods fast at 140ºF

Buy at department stores, mail-order, garden supply, internet


Costs vary depending on features. Some are expandable & extra trays can be purchased. Twelve square feet of drying space dries about a half-bushel of produce.

Major disadvantage of dehydrator is limited capacity.