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Feed the Future Innovation Lab for Collaborative Research on Horticulture

DryCard™ indicates food dryness

The DryCard™ is an inexpensive device developed by UC Davis researchers for determining if dried food is at a low enough moisture content to prevent mold growth during storage. Moldy food has a bad taste and may be contaminated with harmful toxins. Molds will not grow if the air around a product is lower than 65 percent relative humidity.

DryCard shows dryness with color

Photo: Close-up of DryCard with color indicator visible

The indicator strip on the DryCard turns blue or grey when adequately dried, or pink if product is too wet for safe storage.

Making news on campus

The DryCard was recently featured in the UC Davis Egghead blog and in the UC Davis Postharvest Technology Center newsletter. Read the blog post: "10-cent DryCard to Help Farmers Keep Harvest Safe From Mold"

How does DryCard work?

The DryCard incorporates a cobalt chloride humidity indicator strip that changes color with changing relative humidity. The DryCard is based on the concept that relative humidity of air around a product reflects the moisture content of the product. (This concept is called "equilibrium relative humidity.")

How to use a DryCard

Place the DryCard and a sample of the dried product in a moisture tight container, such as a sealed plastic bag or a jar. The card will display a measure of the relative humidity within the sealed container in approximately 20 minutes. If the indicator strip on the card turns pink, then the product is too wet for safe storage. If the strip turns blue or grey, then the product is adequately dried.

Caution with food products that have just been dried: If a food product has just been removed from a dryer, it will take time for the moisture content to equalize within the product, thus allowing an accurate measurement of its equilibrium relative humidity. Immediately after drying, a product will have an overly dried surface, giving an erroneously low estimate of equilibrium relative humidity. For example, rice must be held for about 4 hours after being removed from a heated air dryer before the humidity of the air around the rice reflects its true moisture content. Larger diameter products take longer to reflect their moisture content after drying. For example, large tree nuts require about 24 hours of moisture equalization before an accurate measurement can be taken. Determine moisture equilibration time by placing a DryCard with a sample just out of a dryer and observe how much time is needed for the card to reach a constant color.

If the DryCard indicates the product is too wet to be stored safely, then the product should be used immediately or dried further before storage.

DryCard is reusable: A DryCard can be reused many times. Store the card in a plastic bag to prevent accidental contact with water which will make the card difficult to read. The humidity strip contains cobalt chloride. Do not remove the strip from the card or leach the cobalt chloride by placing it in water.

Related drying technologies

Drying can be an important part of postharvest handling, processing and storage for both food and seed. The Horticulture Innovation Lab has worked with a number of drying-related technologies, including:

More information about the DryCard

For more information on manufacturing and using the DryCard™ or to obtain samples for testing, please e-mail Michael Reid, msreid@ucdavis.edu, or Jim Thompson, jfthompson@ucdavis.edu.

We will continue to update this webpage with more information, so please check back.