Testing for Dryness

It’s been difficult for me to explain how to know when foods are properly dried.  I guess I didn’t really know myself, but instinctively understood what works and what doesn’t.  As I have stated before, do not simply go by a time frame, because there are too many variables that affect drying time. However, I feel pretty confident that the following information from the Cooperative Extension Service, Utah State University is accurate and can be trusted.

You can determine when the product is dry

by feel or by calculation

of the amount of water remaining in the product.

By Feel:

Fruits should be dried until leathery, but not hard.

• The time required for drying will range considerably.
• Fruit always feels softer and less dry when warm in the dryer, therefore remove a piece from the dryer and let cool before testing.
• The sample will show no moisture when cut and pressed.
• When a few pieces are squeezed together they fall apart when the pressure is released.
• They have a leathery or suede-like feel.
• High sugar fruits, like figs and cherries, will feel slightly sticky. Fruit leather can be peeled from the plastic wrap.

Vegetables are generally brittle or tough when they are dry enough.

• If there is a question as to whether vegetables are dry enough, reduce the temperature and dry the product a little longer, using a low temperature toward the end of the drying period.
• There is little danger of damage being done by this extra drying time.

By Calculation:

For optimum plumpness of produce while maintaining safety, calculate the percent solids

in the dried product to determine if the product is adequately dry. 

1. Weigh the container that will be used on the scale. (Tray Wt.)
2. If your scales allows you to do so, adjust so that the container weight = 0.
3. Weigh the raw produce in the container (Product and Tray Wt.)
4. Calculate Raw Product Wt. (Raw Wt.):
5. Calculate desired final Wt. of dry product using the following formula
1. (Raw Wt. ) x (Solids %) = Desired

• 90% solids is a good value to use for vegetables.
• Fruits are moister if 80% is used for calculation purposes. Do not use a lower percent value for solids.

Below is a chart showing the Percent Solids in Raw Fruit and Vegetables

Source: USDA Handbook 8-8, 8-11. Composition of Foods. 1982, 1984.

Start by looking up the percentage of moisture in a food item based on the chart below. You will weigh the food item before and after dehydrating to determine the percentage of moisture loss.  Then you can know whether or not it is sufficiently dried. 

Linda's Note: this is an approximation.  If you are unsure, you may use this as a guide until you feel confident in determining by texture whether or not a food item is sufficiently dry.

Fruit	% Solids	Vegetables	% Solids
Apples	16	Beans	10
Apricots	14	Beets	13
Bananas	26	Broccoli	11
Blue berries	16	Cabbage	8
Coconut	49	Carrots	12
Cherries, sour	14	Cauliflower	8
Cherries, sweet	20	Celery	5
Figs	21	Corn	24
Grapes	19	Eggplant	8
Nectarines	14	Mushrooms	9
Peaches	12	Onion	9
Pears	16	Parsley	12
Pineapple	14	Peas in pod	12
Plums	14	Peppers, bell	7
Raspberries	14	Potatoes	21
Rhubarb	5	Spinach	9
Strawberries	9	Squash	6
		Tomatoes	6
		Turnip	7

• For example: Want to dry cherries to 80 % solids (20% water).
• Solids in raw cherries (from Table above) = 14%
• Container = 5 oz.
• Container + cherries = 45 oz.
• Wt. of raw cherries = 40 oz.
• 40 oz. X 14% = 7 oz. final dry weight = 80%

The final weight of the cherries should be 7 oz.

• Since it will be weighted in a 5 oz. container, the weight will be 7+5=12 oz. If you adjusted scales so that container weight = 0, the final weight is 7 oz.
• If fruit is dried to an 80% solids level, it will be safe from microbial spoilage with the exception of mold growth.
• To control mold growth, vacuum pack the dried fruit or freeze the product.
• Linda’s note: based on this information, I personally prefer to dry my foods more (less moisture than recommended). That saves me the risk of  mold growth and the necessity of freezing my dehydrated food.