Donald G. Mercer, Ph.D., P.Eng.Department of Food Science

University of Guelph© Donald G. Mercer 2012

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• Objectives• Introductory Observations• Why do we dry food products?• How do we dry food products?• Factors influencing drying• Effects of drying on the product

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• Home Dehydrators• Solar Dryers• Final Comments• Open Discussion

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The purpose of this presentation is to examine the “art” and the science involved in the dehydration of fruits and vegetables.

We will examine the use of home dehydrators and home-made solar dryers along with the important aspects of their operation.

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Food drying is something that everybody seems to try and do.

However, very few people ever seem to take the time to learn enough about it to understand it, and to do it well.

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Many of the things you need in order to dry foods are things that you already know from your everyday life.

You just have to know how to translate these skills to a food process.

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Any of you who have ever dried a load of laundry probably already know the basics of drying technology.

e.g., surface area, thickness, etc.

(more about this later)

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Many of the things that affect cooking foods and getting heat into them can also be applied to getting moisture out of them.

e.g., particle size, thickness, etc.

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• to achieve a desired moisture level• to reduce spoilage• to increase shelf life• to reduce weight (shipping etc.)• to enhance convenience• to alter inherent attributes

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Direct Application of Heat:- over an open fire- one of the oldest methods of drying

Indirect Application of Heat:- hot air heated prior to contacting product- perhaps the most popular drying method

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Solar:- may be classed as indirect

application of heat- significant enough to be

mentioned on its own- may pre-date drying by fire

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There are many factors that can influence the drying of a material or food product.

Such factors would include the following:

(by no means a complete list) list)

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Product attributes:

- Product composition / structure

-e.g., compare structure of a turnip to that of an apple.

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Product attributes: Particle size

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Product attributes: Particle shape (characteristic dimensions)

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Product attributes: Particle shape (characteristic dimensions)

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Product Configuration

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Product attributes: Porosity

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Product attributes:- Surface characteristics

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Moisture content:

Tomatoes contain 95% moisture by weight.

Need to remove 472 g water from 500 g fresh tomatoes to get 28 g dried tomatoes at 10% moisture

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Moisture content:

Apples contain 84% moisture by weight.

Need to remove 411 g water from 500 g fresh apples to get 89 g dried apples at 10% moisture

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Product attributes:

- Seasonal variation

- etc.

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Dryer attributes:

- type of dryer

- temperature of air

- relative humidity of air

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Consider drying of mangoes:

A 350 gram mango contains 298 grams of water.

Proper drying conditions must be used.

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Dryer attributes:- volumetric rate of air flow

- linear velocity of air flow

- direction of air flow

- retention times

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Stagnant Boundary Layer:

Without sufficient air movement, a stagnant layer of moisture-saturated air clings to the surface of the product.

This layer must be swept away and replaced by fresh unsaturated air to promote drying.

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Stagnant Boundary Layer:

Acknowledging the presence of the stagnant boundary layer is the key to successful drying.

Action must be taken to address this in all drying applications.

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In addition to individual factors and attributes, there are interactions between and among various factors

e.g., particle size (or volume) to surface area ratios

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This is where you can use your everyday knowledge of drying.Here’s the proof:

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Ideally, drying should have absolutely no deleterious effects on the product whatsoever.

If the product is to be consumed in its dried state, there are certain product standards and expectations that must be met.

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If water is to be added back into the product, then the rehydrated or reconstituted product must meet the original product quality and performance attributes as closely as possible.

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In some cases, the dried product may be expected to exceed the initial food material in certain key attributes such as functionality or taste.

e.g., Instantized potatoes

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Drying presents the opportunity for a host of negative factors to be introduced into a food product.

These may include:

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Nutritional degradation:Many vitamins are unstable and highly susceptible to heat damage.

Flavour changes:Caramelization due to browning or Maillard Reaction. This may or may not be desirable.

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Colour change:

Natural sugars in apples can lead to browning.

Vitamin C can help reduce the impact.

Use a lemon juice dip or commercial products.

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Case hardening:

Excess surface drying creates a hard outer "shell” that prevents or restricts moisture loss.

The primary cause is excessively high temperatures.

Try to stay below 55°C (approx. 135°F)

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Case hardening:

For fruits and vegetables, try to stay below 55°C (approx. 130°F to 135°F).

For herbs, which tend to be quite sensitive to temperature, try to keep the drying at about 45°C (approx. 110°F).

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Case Hardening:

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Case Hardening:

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Enhanced Sweetness:

Apples contain 10.5% sugar and about 84% moisture.

500 g fresh apples would contain 420 g water, 52.5 g sugar, and 27.5 g other material.

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After drying: at 10% moisture

89 g of dried apple

Including:52.5 g sugar9 g of water27.5 g other material

Sugar is now about 60% of weight.

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There are two very popular brands on the market today.

American Harvest / Nesco

Excalibur

Both offer excellent drying features and are well-suited to drying fruits, vegetables, and herbs.

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Before making a purchase, visit each company’s website to see the specific features and different models available.

Do not under-estimate your drying needs. Purchasing a unit without sufficient capacity can ruin your drying experience.

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Some manufacturers produce models without fans. This may create problems with uneven drying and excessively long drying periods.

Commercial forced-air units have a pre-set fan speed which is appropriate for most dehydration needs.

Be sure to select a unit with an adjustable thermostat.

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A built-in timer is nice, but not a necessity.

Some models are expandable by purchasing additional trays which can be stacked on top of each other.

Units that blow the heated air across the upper and lower surface offer the best drying properties.

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Be sure to consider the ease of cleaning the dryer before you buy it.

If you want to make fruit leathers and perform other specialized frying, be sure the model or unit you select has the capability to do that.

Special trays are required fro fruit leathers.

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Fine mesh inserts may be required for small particles.

Small carrot slices may fall through coarse mesh due to contraction during drying.

A plastic mesh insert will avoid this problem.

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These dryers are very popular and offer most of the features you will need.

A typical unit has a cabinet with nine slide-in trays (the trays are large and square).

There is an adjustable thermostat on the cabinet.

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Air is forced across the top and bottom surfaces of the food by a built- in fan.

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The trays should be rotated and moved from one location to another (i.e., top to bottom) at regular intervals during the drying process to enhance uniformity.

You can easily do this when checking the progress of your drying.

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These dryers are also very popular and offer most of the features you will need.

They have circular trays which stack on top of each other.

The basic unit comes with six trays plus a bottom and top.

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The top of the unit contains the fan, thermostat, and timer on some models.

The built-in fan forces heated air through a circulation system so that it travels across the top and bottom surfaces of the food.

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The order of the trays can be changed during the drying process if there is a concern about uneven drying.

You can easily do this when checking the progress of your drying.

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Be sure to dry things like onions or garlic in the garage.

Drying onions is an experience that will bring tears to your eyes.

Drying garlic is memorable, to say the least.

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Wash, peel (optional), and core the highest quality apples available.

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Cut into ¼ inch (6 mm) thick slices.

Dip slices into lemon juice, lemon juice concentrate, or a commercial preparation to reduce browning.

Place rings on drying racks with even spacing and no overlap.

The rings may just touch each other.

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Rings on drying rack.

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Dryer indicator at time “zero”.

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Dryer indicator showing 130° F (~55°C)

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Dried apple rings

without peels with peels

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Dried apple slices should be allowed to cool to prevent “sweating”.

They should then be placed in an air- tight container and stored in a cool, dry, location out of the light,

Storage of apple rings is generally not a problem. They are usually eaten before there is any risk of spoilage.

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Here, the sun provides the heat to the incoming air by means of a heat collector.

There are many different designs limited only by the builder’s imagination.

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Solar drying is not as easily controlled as forced-air drying using a home food dehydrator.

Extra care must be taken to avoid problems (more later).

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Homemade solar dryer showing heat collector, glass- enclosed drying chamber, and solar- powered fans.

Weighing assembly is on top of dryer.

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Solar dryer with horizontal air flow pattern.

Solar fans draw air through the heat collector and drying chamber before exhausting it.

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“Solar Flex Dryer” developed by Malnutrition Matters for use in developing countries

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A commercial solar dryer used in Tanzania. Note the black metal walls and “poly” roof.

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Fresh air enters through opening near base.Exhaust air escapes under eaves.

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Plastic mesh racks support product for drying. Product is “turned” regularly.

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Another Tanzanian dryer.Basically a wooden frame covered in “poly” sheeting with mesh-covered openings to allow air to enter and escape.

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Plastic mesh racks support product being dried.

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Major design flaw: No openings in the bottom of dryer to allow entry of air.

Natural convection currents cannot assist in removal of moist air from drying chamber.

Product is said to frequently “burn” (i.e., “case harden”)

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Drying is dependent on the temperature and velocity of the air.

Air movement is a major problem with many solar dryers (as is the case with forced-air dryers as well).

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Dirt and other contaminants

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Mold growth due to insufficient drying

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Home dehydration of food products (fruits, vegetables, and herbs) is an effective way to enhance their shelf-life.

With new developments in home food dehydrator technology, this processing method is now available to everyone at a relatively low cost.

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Sources of information on home food dehydration are readily available on-line and in book form.

Recipes show how to compensate for the difference in water content between dried materials and their fresh counterparts.

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As a general rule, any meat products should be properly cooked before being dehydrated to avoid unnecessary health risks.

Be sure to keep accurate, organized records in order to repeat successes and avoid failures.

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Always label your products with information such as the date and product description, along with processing conditions and a description of your raw materials.

This way, you can link production batches to your records.

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Have fun, and enjoy your home food drying experiences.

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THANK YOU

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