Tag Archives: solar drying

Amazing Free eBook on Solar Drying Available Online

The book entitled Solar Drying: Fundamentals, Applications and Innovations is some  pages long and covers a range from fundamentals to case studies.

Www.arunmujumdar.com - PublicationsSolar Drying Fundamentals Applications and Innovations pdf

click the image to open the pdf version of the eBook

The following chapters are essentially stand alone books covering a range of topics.

  • Principles, Classification and Selection of Solar Dryers
  • Solar Drying of Fruits and Vegetables
  • Solar Drying of Major Commodity Products
  • Solar Drying of Fishery Product
  • Quality Characteristics of Solar Dried Products
  • Recent Advancements in Solar Drying

Its really worthwhile getting this book to keep as a reference.

A Comprehensive Solar Drying Manual – Free Online Information

I happened onto this manual just now and it got me to thinking about solar drying again.

click the image to download the manual

The document was prepared as a training manual for a Zimbabwian project funded by the Austrian Embassy titled Establishment of a production, sales and consulting infrastructure for solar thermal plants in Zimbabwe.

Its a comprehensive manual covering many designs and processes which you will see in illustrations from other documents you have read over the years. The 110 page book contains descriptions of many different dryers as well as examples of many different crops and their dryers. It also presents a great deal of technical data on drying but even though the title of the project nothing about markets, packaging and selling.

Solar drying should be all the rage currently giving its potential to impact global food shortage and climate change – but we still seem to be where we were decades ago.


Free Online Information – Processing Aspects of Solar Drying in Small Scale Fruit Processing Enterprises.

Some while ago I linked to a FAO based document on evaluating the viability of small scale drying of tropical fruits.

Now I am linking to a very comprehensive document on Processing Aspects of running running such a business.

Producing solar dried fruit and vegetables for micro and smallscale rural enterprise development Processing aspects | TECA

from: TECA at FAO
(click image for full story online)

This is an incredibly detailed, systematic and detailed document that will ensure the processor is able to run a business that produces quality and safe solar dried products.

It covers the range of issues including:

  • Solar drying techniques
  • Meeting custmer requirements
  • Hyginene and food safety
  • Procurement and storage of fresh produce
  • Preparation
  • Operation of the dryer
  • Product quality
  • Management of processing operations
  • in a simple style which is illustrated by the extracts below on preparing fruit and controlling the drying process.

    Cutting and slicing

    It is important to cut and slice the peeled fruit to a particular thickness depending upon the kind of fruit being dried, as thicker pieces will dry at a slower rate than thinner pieces, very thin pieces tend to stick to the drying trays and will be difficult to remove, thicker pieces may not dry fully and subsequently deteriorate after packing and packages of dried pieces of varying thickness appear relatively unattractive and may be difficult to market.

    The optimum thickness of slice for drying varies for different produce. From experience gained so far with processing operations in Uganda, it is suggested that fruit is sliced with thicknesses as follows: pineapple 2 3 mm, mango 2–3 mm, banana 5mm and tomato 3–5 mm.

    Cutting knives and working surfaces need to be cleaned in bleach solution before use. Knives should be kept sharp and fruit handled as gently as possible during cutting to minimise further bruising. Slices should be placed in clean bowls which have been rinsed with clean water, ready for loading onto the drying trays. For tomatoes, it is better to place the cut slices directly onto the drying tray.

    Any bruised or otherwise damaged pieces should be removed at this time since they will result in a discoloured and unsaleable product. Their removal at this stage, rather than after drying, will maximise throughput and hence profitability. Bruised and damaged fruit should be disposed of with the peelings. The stones from mangoes should be similarly discarded.

    Before loading the trays, they need to be brushed clean and washed free of debris before being used.

    Key points on cutting/slicing operations

    >Wash hands in clean water using unperfumed soap
    >Ensure slices are of the correct thickness and even
    >Discard any damaged or discoloured slices
    >Handle slices with care, store slices for drying in clean bowls.


    During the first few hours of drying, particularly during very hot and sunny weather, fruit may dry at such a rate that moisture condenses on the inside of the plastic covers. This can be avoided by opening the loading doors slightly (20 mm) to increase air circulation, but covering the gap with mosquito mesh. This will only be necessary, if at all, during the sunniest part of the day (10:00 to 14:00), and only during the first day of drying. Doors should be kept open only for a minimum period of time and closed again as soon as the weather becomes cloudy. The loading doors should never be left open during the night.

    In poor weather, drying will stop. If it rains, it will rapidly cool the dryer and this will usually result in the cover fogging over due to condensation. It will be some time before the dryer will start to work again once the sun returns. Ideally, putting a portable, locally-constructed thatched cover over the dryer in poor weather will help operations. (This can also be used when the dryer is not in use to help protect it and prolong its life).

    Under fine and sunny conditions, the fruit slices should be dry after two full days in the dryer. However, it is essential to test slices; if they are not sufficiently dry, they will become mouldy and completely unsaleable in a short time. Judging dryness is an Important skill and not an easy one to describe; it is easier to show. As a check, several slices should be removed from the dryer and allowed to cool for several minutes. The fruit may be sufficiently dry if it is not possible to squeeze out moisture from the slices, tearing a piece in half, the middle is no longer moist and slices are kneadable and pliable, but do not stick together. If the slices are not properly dried, drying should be allowed to continue for one to two hours and then checked again.

    The final moisture content of the dried fruit should be around 10% (on a wet basis). Simple, low cost equipment is not available for measuring moisture content, but it is critical to achieve a low final moisture content for safe long-term storage. The buyer of the products will check it as a critical factor when assessing quality. This is particularly the case if the products are to be sold into export markets; poorly dried products will undoubtedly result in future loss of sales to buyers and consumers.

    Key points on drying operations

    > Open doors if moisture condenses on covers for short times only (the gap should be covered with mosquito mesh)
    > Check slices are dry before removing trays
    > Never leave loading doors open at night
    > Prevent insects entering the chamber.

    Solar Drying of Fish – Free On Line Information series

    The use of dried and smoked fish is an important part of the diet of many poorer communities with access to fish either marine of fresh water.

    Emeka Okafor of Timbuktu Chronicles, a really good blog with a focus on business successes in Africa, presents some technical information on fish smoking and drying.

    Timbuktu Chronicles_ Search results for preservation.jpg

    from: Timbuktu Chronicles
    (click image for full story online)

    Drying and Smoking are two critical technologies for those who have access to fresh fish which they wish to process and store for later use. They are critical technologies for resource poor people and cheaper and more environmentally sustainable than freezing, chilling and canning.

    The first video is a bit of an eyeopener and actually focusses on potraying the existing processing of waste fish parts from a Nile Perch processing factory on Lake Victoria in Uganda.

    The second by the Kenya Marine and Fisheries Research Institute is an introduction to fish smoking from fish buying to dried fish storage.

    Free Online Information – Before You Start Solar Drying

    If you search the blog you will see that drying is the frequent subject of posts. This is because it is an effective and simple preservation technology that can (depending on the process used) be implemented with little or no capital investment.

    Given the dual problem of food shortages and soaring energy costs, it seems that now would be a good time for extension and project officers to be focussing on sun and solar drying. I am thinking of preparing a post and webpage that will try and present a practical linked decision tree for sun and solar drying.

    In the mean time this article on the FAO website is extremely useful.



    TECA - Technology for Agriculture.jpg


    from: FAO
    (click image for full story online)



    The article has its origins in work done in Uganda on a simple cabinet dryer. This first article in a series focuses on the approach an entrepreneur would take to evaluate the potential of entering the dried fruit business.

    Detailed costs and calculations are supplied. These need to be adjusted for the entrepreneurs local conditions and costs especially as this new summary document is undated and the original work was done in the 70s. However, the process outlined, checklists and calculation methods are all very useful.

    The article supplies sufficient technical information to be able to do the evaluation but not enough to build a dryer and start drying. This is the subject of additional articles.

    The information provided and the people involved are linked to the export of dried pineapple, mango and banana to Europe which was very successfully introduced on a large scale in Uganda. An interesting warning in the article is that the local selling price is normally 2 to 6 times lower than the export price.

    Free Online Information – Echonet Solar Dryer

    I believe now is an appropriate time to focus on drying and solar energy. Drying is the simplest and one of the most effective methods of preserving food, allowing it to be stored without cost and equipment. The energy of the sun is free to anyone able to use it effectively and can even be tracked by solar monitoring system if one wants to know the exact benefit and power surge its helping to produce. In the current situation where energy and food are both expensive and likely to continue increasing for some years the sun and solar drying of food is a very useful technology.

    Although we will cover some of the basics over the next while, I want, in this post, to illustrate how simple it is to dry. I use a document distributed by Echonet to illustrate what can be done with little effort and little cost.




    from: ECHONET
    (click image for full story online)


    This pdf document is an extremely useful and practical manual for the solar drying of foods. It presents a simple drawing and describes a method of construction based on specified materials. And opportunistic firms such as Renew Energy have been researching on products like the ones above to lay out and demonstrate the many uses of solar energy. As is obvious from the photograph below, this is not a flash and expensive piece of equipment from industry, but rather a device anyone with a little mechanical aptitude could construct and even redesign to make use of other materials which might be locally available.



    from: ECHONET
    (click image for full story online)


    As well as information on how to build the dryer the manual contains detailed information, originally from the University of Georgia, on drying and how to dry. The information covers the basics including hygiene, pretreatment, determining dryness and conditioning but also provides fruit by fruit and vegetable by vegetable information on preparation, drying conditions, storage and even how to reconstitute.

    I am continuing to try and get permission to email the documents I discuss to those requesting copies and having difficulty with connectivity – however, for now it is probably best to make contact through the website and request that the document is emailed.

    There is one other option, which is to use the web by email system that I will describe in a future post.

    Solar Dryer Detailed Design – Link

    Following on the link to a simple overview by the GATE programme of GTZand the article on 3 different solar dryer designs a final article gives detailed design information and drawing for the cabinet dryer.

    01-0701.pdf (page 1 of 10).jpg

    from: GTZ-GATE Programme
    (click image for full story online)


    This document details constuction down to wood sizes, joint details and glass securement, detailed drawings and materials lists.

    Three Solar Dryer Designs – Link

    Following on the link to a simple overview by the GATE programme of GTZ, the article below gives practical details of three solar dryer designs.

    02-0603.pdf (page 1 of 5).jpg

    from: GTZ GATE
    (click image for full story online)


    This paper presents the characteristics of three solar driers. A 15 US$ tent drier, a 400 US$ box drier, and a 5,000 US$ tunnel drier are discussed.and then the drier is placed over them. The first day of drying should be sunny to produce a quality dried product.

    Solar Drying Overview – Link

    The German Appropriate Technology and Ecoefficiency Programme (GATE) Programme of GTZ (the German Development Agency) has a good overview of sun drying as a food technology process.

    02-0602.pdf (page 1 of 8).jpg

    from: GTZ – GATE
    (click image for full story online)


    It presents the technical background of drying, describes and compares different drying operation and equipment, gives an idea of costs, tabulated advantages and disadvantages and gives a good list of references.

    In this time of potential food shortages and rising energy cost, using the sun’s free energy to preserve food which might otherwise be wasted, is at first glance very attractive. The article provides some of the information needed to consider the pros and cons.

    One should remember though that the bulk of the worlds dried fruit is sun dried and we have no concerns about its safely and quality.