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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 displays dryness with color guide

Photo: Close-up of DryCard with color indicator visible

How can you check if something is dry? The indicator strip on the DryCard turns blue or grey when food is adequately dried, or pink if food product is too wet for safe storage.

 

Blog: Story behind inventing the DryCard

Michael Reid and Jim Thompson have worked together before on postharvest research and to design a more efficient solar dryer. The pair's latest invention is the DryCard, inspired by a trip to a market in Tanzania focused on postharvest quality. Read the whole story: "Inventing a low-cost solution to reduce moldy foods."

 

The DryCard was also 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"

 

Fact sheet

A one-page fact sheet is available with basic information about the DryCard, its uses and benefits:

New, low-cost indicator of food dryness: DryCard

 

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 the DryCard to check dryness

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 20–30 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.

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.

How to use DryCard when drying food products

Using the DryCard in conjunction with the drying process requires a different procedure to accurately determine product dryness. Immediately after drying, a product will have an overly dried surface, giving an erroneously low estimate of equilibrium relative humidity. For food products that have just been removed from a dryer, it will take time for the moisture content to equilibrate within the product, thus allowing an accurate measurement of its equilibrium relative humidity.

If a DryCard is placed in a sealed container with product that has just been removed from drying, the color of the humidity strip will continue to change toward a wetter indication for many hours.  The color will not stabilize until the moisture within the product has equilibrated. 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. (This source of error also effects the reading of electronic moisture meters.)

There are two ways to deal with this issue:

  1. Break up or grind a sample of the product, then put all of the sample in a container with a DryCard. Wait 30-60 minutes, until the paper strip no longer changes color to read the indicator. The wait time will depend on the type of product, how finely it is broken up and the magnitude of the moisture gradient within the product. Experience with an individual product will tell a user how much wait time is needed for an accurate reading.
  2. Take a reading with DryCard after 20–30 minutes, but realize that the commodity is actually wetter than indicated. With experience the dryer operator will learn the color associated with a properly dried product. 

Inventing the DryCard

Michael Reid and James Thompson, UC Cooperative Extension specialists with UC Davis, created the DryCard as an easy-to-use tool that farmers could use to measure food dryness. Repackaging cobalt chloride paper with a color guide and directions increases the usability of the paper as a low-cost indicator.

Read more about the story behind this invention on the Horticulture Innovation Lab's blog: "Inventing a low-cost solution to reduce moldy foods."

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

To share basic information about the DryCard with others, there is a one-page fact sheet about DryCards (PDF). There is also a 5-minute informational video about the DryCard, for presentations.

For more information on manufacturing 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, as additional updates are available.