JULY 16, 2009 ISSUE
Solar Food Dryers Designed at ASU Preserve Food Worldwide
About 60 solar food dryers based on a design by an ASU technology professor are being used worldwide. Two women in Afghanistan are using the food dryer to dry fruit. Photo courtesy of Robert Foster
A solar food dryer designed in the early 1990s during an ASU study abroad program in Honduras has become an economical and efficient food preserver used worldwide. Currently, the design is being used to build 60 solar food dryers in Afghanistan.
According to Dennis Scanlin, creator of the solar dryer design and a professor of appropriate technology at ASU, drying is the oldest method of food preservation.
“For several thousand years people have been preserving dates, figs, apricots, grapes, herbs, potatoes, corn, milk, meat and fish by drying,” said Scanlin. “Until canning was developed at the end of the 18th century, drying was virtually the only method of food preservation. It is still the most widely used method.”
Solar dryers, according to Scanlin, are easy to build with locally available tools and materials and operate simply by natural convection. The solar food dryers designed at ASU are basically wooden boxes with vents at the top and bottom. Food is placed on screened frames that slide into the boxes. A properly sized solar air heater with south-facing plastic glazing and a black metal absorber is connected to the bottom of the boxes. Air enters the bottom of the solar air heater and is heated by the black metal absorber. The warm air rises up past the food and out through the vents at the top.
The dryers produce temperatures of 130 to 180 degrees Fahrenheit, which is a desirable range for most food drying and for pasteurization. With these dryers, it’s possible to dry food in one day, even when it is partly cloudy, hazy and very humid. Inside, there are 13 shelves that will hold 35 to 40 medium-sized apples or peaches cut into thin slices.
“Food scientists have found that by reducing the moisture content of food to between 10 and 20 percent, bacteria, yeast, mold and enzymes are all prevented from spoiling it,” said Scanlin. “The flavor and most of the nutritional value is preserved and concentrated, and these foods can be preserved for several years in many cases.”
In addition, Scanlin explained that dried foods take up less room than fresh, don’t require any special storage equipment and are easy to transport.
Scanlin started working with solar food dryers in the early 1980s while a graduate student at West Virginia University. When he came to ASU in 1984, he continued to design and build them with his students. In the early 1990s, Scanlin designed a dryer for a solar energy education project he and ASU professor of anthropology Jeff Boyer were working on in Honduras.
With the assistance of Chuck Smith, director of the Sustainable Development Program at ASU and a graduate student in the ASU Department of Technology at the time, Scanlin built the first prototype of a solar food dryer and subsequently several more during the 1992 study abroad trip in Honduras.
During the 1993-94 academic year, Scanlin and Smith redesigned the dryer for the United States’ latitudes and built another version. Between 1993 and 1999, ASU students, technology Professor David Domermuth and Scanlin built several other dryers and conducted a series of experiments to improve the dryer’s performance and reduce the cost and complexity of the construction.
The team’s designs and experiments were described in several articles that were published during this time. Since then these articles have become quite influential and the “Appalachian Dryers” have been built in many regions of the world. Scanlin frequently receives inquiries about this work and responds to questions about the dryer’s design, construction and performance from individuals all around the world.
For more information about ASU’s Department of Technology, click to www.tec.appstate.edu.
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