Solar Dehydrator

A solar dehydrator is an enclosure of screened trays, built so sunlight heats the air passing through it. Drying preserves food by a simple rule: take enough water out of a piece of fruit and the moulds and bacteria that spoil it can no longer grow. The enclosure is what separates it from fruit on an open screen. A reflective inner surface lifts the air roughly 11 to 17 C (20 to 30 F) above the outside temperature, and vents at each end draw fresh air through to carry the moisture away.

The weather decides whether it works, not the build. It asks for constant direct sunlight, an outdoor temperature of at least 30 C (86 F), better still 38 C (100 F), and relative humidity - the amount of water the air already carries - below 60 percent. A run takes days, not hours, and where those three do not coincide the load does not dry slowly, it spoils: there is no way to force the heat or the airflow, and the food moulds first. Temperature traps at both ends. Too cool at the start and micro-organisms survive in the still-damp food, which is why a powered dryer begins near 63 C (145 F). Too hot and the outside hardens and seals the moisture inside, so a piece tests dry and spoils from the middle.

A dryer is only the middle of the process. Many fruits are treated first, dipped in ascorbic or citric acid or blanched in water or syrup. Food dried by sun or solar heat is not safe as it comes off the tray: it is pasteurised afterwards, in an oven at 71 C (160 F) for half an hour or by freezing the packed product at -18 C (0 F) for two days, so a second heat or cold source must be free at harvest time. Fruit is dry when it is pliable and no beads of moisture appear as a piece is pressed; vegetables when they are tough or brittle. Upkeep is hygiene: trays washed between loads so sugar residue cannot feed mould and insects, and glazing kept clear so the chamber holds its heat.