Papers describing food drying techniques normally list the obvious contamination risks of drying in the open air as a major disadvantage and and based on this dismiss its application. However, the large volumes of food that are sun dried every year and the savings in energy and capital bely this quickly reached conclusion.
Searching the literature and regulations doesn’t give any guidance on the advisability of establishing a new sun drying business.
However, the two videos below show the large scale use of sun drying and the associated technologies applied at Cecilia’s Farm, near Prince Alfred in South Africa.
It seems that the critical point about the sun dried business they run, is that all fruit is washed before further processing and or packaging. Arguably this makes it comparable to fruit which is often not peeled, packaged salad and leafy vegetables. These are grown in the same environment and simply washed before eating.
The massive advantage of sun and to a lesser extent solar drying is that they produce shelf stable foods with minimal environmental impact and low processing and packaging costs.
I have always been a promoter of sun and solar drying, because they allow people to convert perishable fruit, often available at low or no cost during the season, into a stable product that can be stored until the next season at almost no or low cost.
This article provides some good information on the drying process, that helps in deciding how to actually dry.
The article is an in depth one and gives some really interesting data on drying rates. It compares theoretical with measured rates and is then able to model the progress of drying with this data. The graph below is a really clear indication of the main difficulty of solar powered systems – they only work for a part of the day.
This is particularly important in drying, were it means that sun drying carries on for 3 days. This is because, as the graph shows, drying actually only carries on for a fraction of the day. This fraction depends on the location. This is obvious, but for me only really became clear when I saw this graph!
This has implications for how you run your drying. First of all, it's no good having a nice social day picking, transporting, washing, selecting and preparing your fruit and getting it out into the sun in the late morning or even worse the afternoon. If you do you are going to need four days to dry. More importantly the fruit will be wetter at the end of drying on the first day and therefore more likely to spoil overnight. So rise early and get the fruit ready for the moment when drying can start. Secondly, because the whole drying period until your fruit is shelf stable is many times longer, the cleanliness and hygiene of the plant become more important to avoid spoilage and loss.
For some time I have been slowly promoting the sun drying process because of its low capital cost and zero energy consumption. So the Drying Process has a very low carbon footprint! But here is someone who’s taking it even further!
They claim a 7 to 10 day drying period is at the heart of their product quality! The long drying period means drying is gentle and they even speak of rehydration during cool moiste evenings.
The logical argument against sun drying is the potential of it being soiled either by microorganisms, insects, rodents, birds or just dust and dirt. The chance of these occurring is proportional to the time the tomato is out in the open – so it makes you think!
I have always said a 3 or 4 day drying period is necessary if a stable clean product is to be produced, but this seems to say different. Possibly there is a cleaning and sorting step involved – I will be following up and giving more information.