UNIVERSITY OF SOUTHERN MINDANAO
4. Drying of Foods
(ND 223 – Fundamentals of Food Technology)
IAN JADE A. FLORES
Faculty, Department of Food Technology
College of Human Ecology and Food Sciences, USM
Topic Outline
• Drying/Dehydration
• Advantages of dried and dehydrated foods
• Mechanisms of drying/dehydration
• Methods of drying and dehydration
• Stages of drying
• Types of food to dry
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Intended Learning Outcomes
• Discuss and apply the theories, principles and methods of
drying and dehydration
• Enumerate different equipment used in the process
• Produce dried/dehydrated food products
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ND 223 - Fundamentals of Food Technology
Drying/Dehydration
Food preservation process that consists of removing the
food’s water, which effectively inhibits the growth of
microorganisms
Drying: general term applied to all methods of
removing moisture in the food
Dehydration: drying by artificially produced heat under
controlled condition
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Advantages of Dried/Dehydrated Foods
• Easier transport and handle
• Convenience
• Shelf-stable
• Can be eaten as is or be rehydrated
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Advantages of Dried/Dehydrated Foods
• Easier transport and handle
• Convenience
• Shelf-stable
• Can be eaten as is or be rehydrated
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Mechanism of Drying/Dehydration
• Involves simultaneous processes:
• Getting heat into the food
• Getting water out of the food
• In drying it is important to maximize drying rates while
minimizing the losses in product desirable
characteristics
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Factors Affecting Drying Rate
• Humidity
• Surface area
• Temperature
• Air velocity
• Ventilation/moist air outlet
• Atmospheric pressure and vacuum
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Methods of Drying/Dehydration
• Sun drying – traditionally carried out by the sun
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Methods of Drying/Dehydration
• Solar drying – uses equipment to collect sun’s rays in a
unit designed to ventilate moisture
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Methods of Drying/Dehydration
• Commercial drying–
• Conventional drying
• Tray/cabinet drying
• Tunnel drying
• Spray drying
• Roller/drum drying
• Vacuum drying
• Osmotic drying
• Freeze-drying
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Methods of Drying/Dehydration
• Conventional drying – uses heat to evaporate water
within kilns or drying rooms
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Methods of Drying/Dehydration
• Tray/Cabinet drying –
food is spread out thinly
on trays and heating may
be by an air or by radiation
from heated surfaces
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Methods of Drying/Dehydration
• Tunnel drying – food is placed on trays or “cars”
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Methods of Drying/Dehydration
• Spray drying – used to dry
liquid foods
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Methods of Drying/Dehydration
• Roller/Drum drying – the liquid food is poured over
the very hot surface of a drum dryer, resembling a
large barrel
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Methods of Drying/Dehydration
• Vacuum drying – dehydrates
foods to very low moisture level
through the use of a vacuum
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Methods of Drying/Dehydration
• Osmotic drying – the food is soak in high concentration
of syrup or sugar that osmotically draws water from
being dried
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Methods of Drying/Dehydration
• Freeze drying – the food is freeze
first then placed it in a vacuum,
where the ice sublimates to a
vapor; yield the highest quality
dried products process is more
costly
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Stages of Drying
• Constant Rate Period
surface water quickly
evaporates
• Falling Rate Period
as the moisture content
decreases, the rates of
drying become very slow
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Types of Food to Dry
• Fruits, Vegetables &Herbs
• Fish & Meat
• Juices & Liquid extracts
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Case Hardening
• The cells on the outside of the pieces of food give up
moisture faster than the cells and inside.
• This will prevent the water in the inner part of the
piece from vaporizing, resulting in high moisture,
making it susceptible to spoilage
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Sulfur dioxide
• Either in sulfite solution or sulfur dioxide gas
• used in dried fruit to preserve their natural color,
prevent spoilage and protect against loss of Vitamin A
& C.
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Testing for Dryness
• Required aw or moisture content
• Weighing
• Squeezing
• Time:
• Fruits: 4 – 10 hrs
• Vegetables: 4 – 12 hrs
• Herbs: 2 – 3 days (air dried); 2 – 3 hrs (cabinet drying)
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Conditioning/ Sweating
• process where the dried products are placed in a
closed container lined with muslin or cheesecloth for
atleast 24 hrs to allow the moisture from the under
dried pieces to be absorbed by the over dried pieces.
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Packaging of Dried/Dehydrated Foods
• The most practical packaging for dried food is Low
Density Polyethylene (LDPE) with thickness of at least
0.003 inch (3 mm).
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UNIVERSITY OF SOUTHERN MINDANAO
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