The usual purpose for drying food it to be able to preserve perishable food for some time with refrigeration, pasteurizing, or special packaging. This depends on having removed enough moisture. It is also important in some cases not to dry the food too much reducing the flavor or nutritional quality. So the question of how dry is dry enough, and how to measure "dryness" is an important one.
Several of the sources I found (see Reference information) give some guidelines. The UC Davis dryer plans and manual say that high sugar content fruits (like mangoes and papayas) should be dried to 20% moisture content and dried leafy or thin sliced vegetable should be dried to 10% moisture. That source also notes that the fruit and thicker vegetables should be pliable but dry ("leathery" is one way to describe this), and thin vegetables will be brittle.
While the food can be weighed with a simple balance beam or dietary scale before and after drying to determine the amount of moisture lost, this measurement does not tell us the moisture content of the food. As noted in the Terms and definitions, to determine moisture content, we need to know the percent dry matter of the food. While we may be able to estimate the percent dry matter from published data, it is a quantity that varies even with a particular kind of food. Determining the percent dry matter from a sample of food we are drying would require a scale and drying the food for 24 hours at high heat.
The amount or percentage of moisture lost is something we can determine if we have a scale, and with experience we can use it to guide us. Based on experience (and separate tests--see below) if we remove 80-85% of the moisture from mangoes and papayas, they will store well. For tomatoes (whole fruit cut into wedges) the moisture removed should be higher--90-95%. Based on only one test, sweet bananas were well preserved with 73% moisture removal.
Another test is to measure what level of relative humidity develops when the fruit is stored in a sealed container. Besides a sealed container, this test requires a way to measure the relative humidity without opening the container. We have done a few tests using the small relative humidity meters shown in the photo above and they confirm that products deemed dry enough based on appearance and percent moisture loss meet the standard noted in the UC Davice reference--65% or less after the relative humidity has reached equilibrium in the sealed container. This level is sufficient to prevent the growth of mold if the dried product is stored in a sealed container. Note that 65% relative humidity is below the levels listed for mold formation for several common food spoilage modes listed in the VITA dryer manual.
The UC Davis manual also mentions color indicator cards, called DryCards, which could be placed inside containers of dried food to test the equilibrium relative humidity. If these become available and are not too expensive, they might be a good way to check food dryness.
Note that this discussion has focused on food moisture levels at the end of the drying process when the food will then be stored in sealed containers. Also, it is focused on food spoilage by mold. The following notes address situations that occur during the drying process, and for food spoilage organisms other than mold.
Wetter materials like fruit are likely to take more than one day to dry. Ideally, we would take the food out of the dryer and refrigerate it overnight, but that takes a lot of time and assumes we have a refrigerator (and that there is enough space in the refrigerator). The experience with the direct dryer (and to some extent with the indirect dryer, although there hasn't been enough testing yet) is that if the first day has had good drying conditions and the food has formed a dry skin on the surface, it can be left in the dryer overnight to continue drying until the next day. But we have had a couple of instances of mold formation when the food went in later in the first day, or cloudy/rainy conditions shortened the drying day and continued on the second day. Getting the food into the dryer early in the day and observing weather conditions carefully will be important for managing the drying process. We also hope to research supplemental heating methods for drying during rainy periods or at night.
The measures of what is "dry enough" have focused on spoilage and hazards caused by mold. Bacteria can also be an issue. Besides basic hygiene in preparing and handling the food, there is a concern about bacteria producing toxins in food while it is drying. Some of our partners have asked questions about Bacillus cereus, which can cause food poisoning (gastroenteritis). Based on what we know so far this is of particular concern with starchy foods like grains, cassava or potatoes, or a cooked stew or gravy, and would arise of the bacteria was present and the food did not get hot enough in the drying process. We are still investigating to make sure we understand the issues. Concerns with Bacillus cereus have not appeared in any of the dryer references we have been using.