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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 guide

Two DryCards, one DryCard indicator shows 
'dry' and the other shows too wet for storage

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.

 

Sign-ups

Interested in working with the DryCard?

Please sign up to receive notifications as the DryCard becomes more widely available and to express your interest in working with the DryCard — whether as a potential user, partner, manufacturer, distributor or researcher.

 

News about the DryCard

DryCard news articles are available on the Horticulture Innovation Lab's blog.

 

DryCard wins innovation challenge in Africa: This new invention was recognized as a top emerging technology to reduce food loss in Africa — beating more than 200 entries to win the grand prize at an all-Africa event in Nairobi. UC Davis news release: DryCard Invention Wins Competition to Reduce Food Loss in Africa

 

Story behind the invention: Michael Reid and Jim Thompson have long worked together on postharvest technologies. The pair's latest invention is the DryCard, inspired by a trip to a market in Tanzania focused on postharvest quality. Read: Inventing a low-cost solution to reduce moldy foods.

 

The DryCard was also featured in the UC Davis Egghead blog: 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 30–60 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 the 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 30–60 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:

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Are you interested in...

Visit the DryCard sign-ups to register your interest and to receive notifications as the DryCard becomes more widely available.

The Horticulture Innovation Lab team continues to work with partners to test and improve the DryCard in real-world situations and to explore options for making this tool more widely available. Please sign up to express your interest in working with us.

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.

We will continue to update this webpage with more information, as additional updates are available.