food preservation teaching outline - Oregon State University ...

FOOD PRESERVATION TEACHING OUTLINE
SP 50-543
Revised April 2013
Dehydrated Foods
How it Works
A. One of oldest methods of preservation
B. Removes moisture from food so microorganisms cannot grow and spoil food.
C. Dehydration slows enzyme action but they are not destroyed.
Methods of Dehydration
A. Dehydrators
Have an electric element for heat.
Fan for air circulation.
Thermostat for temperature control
Durable trays.
Discuss advantages & disadvantages of different models.
B. Sun or solar drying
Good option for high acid foods.
Not recommended for blanched low-acid foods (vegetables) and meat/fish.
Disadvantages: hard to control temperature and humidity.
Need to be pasteurized to kill insects and eggs.
- Freezer method: place dried food in freezer bags and freeze for 48 hours.
- Oven method: place dried food, single layer on cookie sheet in preheated oven
160°F for 30 minutes.
C. Oven method:
Most costly
Prop open door on electric ovens
Convection ovens work best
D. Microwave not recommended except for drying small batches of herbs. (See SP 50-921
Drying Herbs.
Steps in Drying Fruits and Vegetables
A. Select sound, ripe fruits and young tender vegetables.
B. Wash thoroughly.
C. Sort, pit, trim, peel, etc.
D. Slice into uniform pieces ⅛ – ¼ inch thick.
E. Reasons for pre-treating fruits and vegetables.
Preserve color and flavor
Minimize nutrient loss
Stop decomposition (enzyme action)
Ensure more even drying
Extend storage life
Page 1

Enhance destruction of harmful bacteria.
F. Methods of pre-treating fruit – See PNW 397 pg. 7 for amounts
Choice of method is a matter of preference.
Salt/vinegar soak (2-4 Tbsp. salt and vinegar/gallon of water). Soak for 3-5 minutes.
Drain well
Ascorbic acid / citric acid dip
Syrup/water blanching
Salt solution dip
Honey dip
Sulfiting
Sulfite caution: label foods using sulfites because some individuals are very allergic.
G. Pre-treating vegetables
Why?
- Inactivate the enzymes causing vegetables to mature.
- Prevent vegetables from becoming bitter.
- Prevent off flavors.
- Cleans and softens them – easier to rehydrate.
Methods of blanching.
- Steam blanch
- Water blanch
- Follow time outlined (PNW 397 pgs. 20-22)
- Drain well and pat dry
- Some vegetables need no pre-treatment (onions, mushrooms, zucchini squash,
green peppers, hot peppers and herbs).
- Canned fruits and frozen vegetables can be dehydrated without pretreatment.
H. Drying Methods
Dehydrator
Oven
Sun
I. Test for Doneness
Fruits—leathery and pliable because of high sugar content
Vegetables—most are hard and brittle. Green peppers, sweet onions, tomatoes and
mushrooms dry more like fruit.
J. Storage
Conditioning (PNW 397 Pg. 13)
Packing
- Thoroughly cool foods.
- Pack into clean, dry, insect proof containers (e.g. metal cans with tight fitting
lids, glass jars, moisture-vapor proof freezer containers and bags.
- Need to tape edge of lid to prevent insect infestation.
- Store in cool, dark, dry location.
K. Problems
Case hardening caused by drying at too high a temperature. A crust forms on outside
of food and moisture cannot escape.
Browning: can be prevented by using a pretreatment, storing in dark location. Food
safe to eat.
Page 2

Mold—caused by food not dried enough or not stored properly so that moisture was
re-absorbed by food.
Sugar crystals—often happens with overripe fruit. Safe to eat. Will dissolve if
cooked.
Insect infestation.
L. Rehydration and using dried foods
Fruits
- Can be eaten as is—no rehydration needed.
- Can be rehydrated by:
1. Pouring boiling water or juice on fruit. Let stand 5-10 minutes.
2. Steam fruit over boiling water 3-5 minutes.
3. Soak in cold water for several hours.
4. Cover with water and bring to boil in microwave. Let stand 5-10 minutes.
5. Soaking fruit first in cold water, then cooking gives a more tender product.
- Chopping fruits
1. Freeze fruit first then crush with hammer or meat cleaver.
2. Put frozen fruit in blender or food processor.
3. Cut pieces of fruit with scissors or sharp knife. If fruit tends to stick coat
blade with a little cooking oil.
Vegetables
- Dried vegetables will be tenderer if soaked before cooking. Can take 15 minutes
to 2 hours to plump. Then cook or add to soup or stew.
- Take longer to rehydrate than fruit because more water is lost.
- Do not let soaking vegetables stand longer than 2 hours without refrigeration.
- Vegetable flakes and powders.
- Can use blender or food processor to flake or make vegetable powders – great
seasonings.
Food spoilage
- Rehydrated dried foods should be refrigerated. Very susceptible to spoilage.
Fruit and Vegetable Leather
A. What is it?
B. Leathers are a chewy product usually made from pureed fruits but some vegetables like
tomatoes also make good leather.
C. What fruits to use
Fresh fruits
- Any fresh fruit will work—good place to use culls, overripe fruit or those not
suitable for canning or freezing.
- Cut away all bruises and spoiled portions.
- Citrus fruits should be used in combination with other fruits because they are so
juicy.
- Fresh fruit purees are usually too thin to use as is-need to be cooked to thicken
or blend with drained canned fruit such as pears or apple sauce.
Page 3

Canned fruits
- Good way to use old canned fruits.
- Easiest source of fruit for leather.
- Drain off liquid and puree the canned fruits.
- May add 1 Tbsp. lemon juice to puree for a fresher flavor or combined with
other fruits.
Frozen fruit
- Thaw fruit and drain off all juice.
- Puree the thawed fruit.
- Combine the frozen fruit puree with drained canned fruits or bananas to thicken.
D. Making the puree, tips:
Puree the fruit in a blender or food processor until smooth.
The puree for leather should be the thickness of apple sauce or baby food.
If too thin, cook on low heat until desired thickness is achieved.
For seedy fruit like berries, you might want to sieve the puree to remove some of the
seeds. They become very hard.
Some fruits like raspberries make very sticky leather so best to combine with another
fruit like canned pears, apple sauce or bananas.
E. Drying leather
Line drying trays or cookie sheet with plastic wrap, parchment paper or use trays
designed for fruit leathers.
Spread on drying trays ¼ to ¾ inch thick.
May use toppings like coconut, nut, sesame seeds or seasonings. Sprinkle on puree
before drying.
Dry in a dehydrator, oven or sun. Rotate tray if necessary.
F. Storage
After dry (should be a little tacky but no longer moist). Leather can be rolled and
wrapped in plastic wrap, or cut into 4-6 inch pieces or rolls.
Place rolls or pieces in heavy food-grade plastic bags or containers and store in a
cool, dry, dark place.
Demonstration
Show how to blanch, cool and dry sliced carrots.
Slice and pretreat apples or fruit of your choice.
Page 4

Lab
Dry fruits
- Apples (using a variety of pretreatments):
1. Ascorbic acid dip
2. Commercial pretreatment
3. Salt/vinegar dip
4. Sodium bisulfate (discuss safety concerns for people with respiratory disorders)
- Bananas plain and dipped in lemon juice
- Canned fruit such as pineapple and apricots
- Frozen or fresh blueberries (show how to crack skins). Seedless grapes can also be canned.
- Parsley—using the dehydrator and the microwave.
Make fruit and vegetable leathers
- Canned, fresh, and frozen fruit. Have a variety of fruits available as well as toppings such as
sliced almonds, coconut, and cinnamon.
Make pizza leather
- 15 oz can drained stewed tomatoes
- 8 oz. can tomato sauce
- Dried basil, oregano, and garlic powder.
- Spray the drying trays or plastic wrap with a cooking spray or lightly grease with oil to
prevent the leather from sticking.
Reconstitute dried foods and prepare a dish such as vegetable soup, apple pie or apple-pear
cookies.
See SP 50-587 Enjoying Home Dried Fruits and Vegetables.
Prepared by Nellie Oehler, Extension Family and Community Health, Lane County
Reviewed by Carolyn Raab, Extension Foods and Nutrition Specialist
© 2012 Oregon State University. Oregon State University Extension Service cooperating. Oregon State University Extension Service
offers educational programs, activities, and materials without discrimination based on race, color, religion, sex, sexual orientation,
national origin, age, marital status, disability, or disabled veteran or Vietnam-era veteran status. Oregon State University Extension Service
is an Equal Opportunity Employer.
Page 5