https://horticulture.ucdavis.edu/drycard
New, low-cost indicator of food dryness: DryCard
In developing countries, mold growth on dried foods can result in postharvest losses for farmers and unsafe foods for consumers. Food will not mold if it is properly dried, but smallholder farmers do not have access to a cost-effective way of assessing food dryness. Farmers who are able to assess dryness before storing can reduce postharvest losses and better provide consumers and their families with safe, higher quality foods.
How the DryCard™ works
The DryCard incorporates a cobalt chloride humidity indicator strip that changes color depending on the level of relative humidity. In an airtight container, the relative humidity of air around a product reflects the dryness of the product (this concept is called equilibrium relative humidity).
To use, place the DryCard and a sample of the dried product in an airtight container, such as a sealed plastic bag or a jar. After a brief wait, the card indicator will display a measure of the equilibrium relative humidity. Match the color of the strip with the scale on the card. If the indicator strip turns pink, then the product is too wet for safe storage. If the strip turns blue or grey, then the product is adequately dried. Store the card in a plastic bag to prevent contact with water. If cared for properly, the DryCard can be reused many times