Tag Archives: sun drying

Should Sun Drying be Rejected as a Food Unit Operation

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 atĀ 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.

Some Good Data on Sun and Solar Drying

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. All of this is to minimise the costs of electricity in domestic houses, which also have a significant impact on reducing theĀ business gas prices of a particular household.

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.

The two images in this post are from the online journal at http://www.ajfand.net/Volume12/No7/Mercer11020.pdf