SR article

How to dry fresh products

In this article we will have a closer look on how to dry fresh products in a healthy way. Since this is a preliminary survey of our project in Gambia, our main focus will be on products which are present in this country.

Temperatures:

First of all, different products are best dried on different air temperatures. To minimize the loss of vitamine C, fruit and vegetables are best dried at 55/60 degrees Celsius. Whereby fish and meat are better dried at higher air temperatures so that the forming of microbacteriae during the first stages of drying is being minimized. At last herbs, seeds, nuts, spices and flowers are best dried at temperatures between 30 and 40 degrees. If air temperatures are too high nuts and seeds might become rancid and herbs, spices and flowers tend to loose their aroma and colours.

Packaging:

The packaging of dried products is essential for their preservability. The packages ought to be airtight and moisture proof. Otherwise the dried products might absorb water and the drying proces will be in vain. Especially tomatoes and onions should be packed immediately, because they absorb moisture from the air very quickly. The packages should be stored in the coolest, darkest and driest place possible.

Drying fruits:

Fresh and ripe fruits are most suitable for the drying proces. When the fruits are feeling leathery and though, they have been dried enough. You could choose to continue the drying untill they are crispy, if you would like to make fruit powder.

The preperations needed before the drying proces are as follows:

  • Wash the fruits and remove imperfections.
  • If the fruit has a skin, peel it. Also remove stems et cetera. If the fruit has a natural protective wax coat, like prunes, figs or grapes, put the fruit for 30 seconds in boiling water. The skin then bursts a bit, which increases the speed of the drying proces.
  • Cut the fruit in pieces (for thickness, see tabel 1 below).
  • Remove pits if there are any.
  • Pre-treatment if necessary. Some fruits tend to turn brownish when they are being sliced. You can think of apples, pears or bananas. To maintain its original colour during the drying proces, sprinkling some lemon or lime juice over the pieces helps. This also helps to maintain more vitamine C inside the fruit pieces.
  • Dry the fruits.

Tabel 1 gives an overview of the recommended slice-thickness and drying time of fruits which are present in The Gambia:

Fruit

Slice-thickness

Drying time

Banana

+/- 10 mm

6 – 12 hours

Mango

+/- 10 mm

6 – 16 hours

Orange

+/- 6 mm

2 – 12 hours

Coconut

+/- 10 mm

3 – 8 hours

Tabel 1: Recommended slice-thickness and drying time of fruits

Drying vegetables:

Just like fruits, fresh and ripe vegetables are most suited for drying. So after picking it is best to dry them immediately. When the vegetables are though or crisp (depending on the type of vegetable) they are ready.

They preperation needed for drying vegetables are quite similar to drying fruits. However it has to be kept in mind that the preperation differs with each vegetable. For example corn must be peeled and removed from the cob, while with onions you have to remove the skin first. The two basic steps that must be taken before drying vegetables are:

  • Wash the vegetables and remove rotten parts or imperfections.
  • Peel, slice and cut the vegetabeles.
  • Dry the vegetables

The recommended slice-thickness and drying time for vegetables present in The Gambia are the following:

Vegetable:

Slice-thickness:

Drying time:

Corn

-

6 – 12 hours

Egg plant

+/- 13 mm

5 – 12 hours

Onions

+/- 10 mm

6 – 12 hours

Green pepper

+/- 13 mm

5 – 12 hours

Hot pepper

In half (mild) or in whole

3 – 20 hours

Potatoes

+/- 10 mm

6 – 12 hours

Tomatoes

+/- 10 mm

6 – 12 hours

Tabel 2: Recommended slice-thickness and drying time of Gambian vegetables

Drying fish:

There are several ways of drying fish. A good dried fish in ideal conditions has a storage life of several years. One way of drying it contains putting marinated fish in the fridge for 4-8 hours. Since there is a high probability that there will be no fridge available in The Gambia, we will not have a closer look at this method. Also fish are best dried, when they are just caught fresh.

The preperations of drying fish consist of the following steps:

  • Clean the fish thoroughly. Especially the skin of fish with a lot of slime should be cleaned very will, since bacteriae love to grow on this slime after the fish is dead.
  • Put salt to prevent the growth of bacteriae. This is also the step where you can marinate the fish with spices if you like.
  • Optionally: Put the fish in a bucket which is closed for insects and put a weight on top of it. The fish will soak up the salt and loose some of its water.
  • Dry the fish.

When designing a solar dryer for drying fish, it should also be taken into account that seizes of fish differ from seizes of fruits and vegetables. There should be enough space to dry the fish.

Drying meat:

Drying meat does not differ much from drying fish. Since meat is scarce for a lot of people in The Gambia, most people eat the meet directly after slaughter. For this reason we shall not have a detailed look at the meat drying proces.

Conclusions:

Although there are differences, the general idea of drying fresh products stays the same. The first step is to make sure you have the freshest products available. The next step is to clean them and slice them. If necessary you can pretreat the product with lemon juice or salt. All that is left is to put the products in the dryer! Since vegetables, fruits and fishes have different recommendable drying temperatures, it will be hard to make a solar dryer which dries all products.

Source:

The handbook of ‘American Harvest’ by Scott Reinhard (see below)

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