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close this book Food chain - Number 19 - November 1996
View the document Contents
View the document Greetings
View the document Food processing training - some problems and possible solutions
View the document Booklines
View the document Principles of food dehydration
View the document Simple methods of quality assurance
View the document Research notes
View the document Food preservation by the Turkana people.
View the document Ayib (traditional cheese)

Principles of food dehydration

It is important to appreciate that drying foods by traditional methods is very area specific, and these methods have evolved over many years to take account of the local climate.

We have heard from a number of readers that they find Food Chain articles very useful for teaching and training purposes. Therefore we have decided to introduce a training element to Food Chain, which, while aimed particularly at trainers, should also be of interest to those involved in food drying.

The preservation of foods by drying is the oldest and most common method used by mankind. Usually foods are dried by exposing them to the heat of the sun either on the ground on mats or trays raised off the ground, or by hanging them under the eaves of huts and houses. In very damp humid areas such as the Amazon, it is necessary to use also the heat of cooking fires in order to fully dry the food. In these cases preservation is assisted by the preservative action of chemicals in the smoke.

It is important to appreciate that drying foods by traditional methods is very area specific, and these methods have evolved over many years to take account of the local climate, commodities available and social habits of the inhabitants. A wide range of foods are traditionally preserved including fruits, vegetables, nuts, herbs, spices, medicinal plants, milk products, meat and fish. While the prime objective is to preserve excess produce for times of need, the reduction in weight is also of prime importance for nomadic peoples.

WHY PRESERVE FOODS BY DRYING?

Nowadays many foods are dried not solely for food security, but for commercial reasons, and this change has far reaching implications. The most important is that in many cases the foods to be processed are not selected for being in balance with the local ecology and climate, but for their market potential. This means, for example, that fruits which grow well in a humid tropical area have to be dried under conditions in which traditional sun drying simply will not work. Therefore new technologies have to be developed to make the drying process more independent of local climates, in other words -artificial dryers are needed.

Other considerations of commercial dehydration are profitability, buyers' demands for quality, and packaging for a long shelf life.

When drying foods commercially the most important consideration is the value added (per unit weight) by processing which must be sufficient to cover all the costs involved. These include:

• losses of rejected material

• labour

• fuel

• depreciation of equipment

• packaging

• rent, taxes and other statutory obligations.

Finally there must be a profit for the enterprise.

Foods can be divided into three broad groups based on the value added through processing by drying. In the case of cereals, legumes and root crops, very little value is added per ton processed. For example, a ton of rice that is well dried in an artificial dryer will be worth very little more than a ton which has been sun dried on mats at the farm. With such crops drying can only be economic either at a very large scale in silos, or in climates in which sun drying is totally impossible due to rain or high humidity.

More value per unit weight is added to foods such as vegetables, fruits and fish; and considerably more to high value crops such as spices, herbs, medicinal plants and nuts. These are the products that are most suitable for small and medium scale enterprises, as sufficient value can be added from processing at small scale to cover costs and make a profit.

HOW ARE FOODS PRESERVED BY DRYING?

The majority of fresh foods contain between 70 and 95 per cent moisture and it is this water that allows micro-organisms such as moulds, yeasts and bacteria to multiply, causing rot and spoilage. If the moisture content is reduced to between 5 and 25 per cent (depending on the type of food), there is insufficient moisture to support micro-biological growth and the food is preserved. The micro-organisms however, are not dead, they simply remain dormant and should the moisture content rise by absorbing water from the atmosphere after drying they will start to multiply again. Hence the need for good protection by packaging.

As mentioned above, the necessary reduction of the moisture content to preserve a given food is wide, ranging up to 25 per cent. The reason for this is that it is not the actual amount of moisture in a food that is critical hut the amount of the total moisture that is available for microbiological growth. Some food components, in particular sugars and salt, are able to chemically hind water making it unavailable to micro-organisms and this is the reason that fish and meat is usually salted, and some fruits are sugared prior to drying.

Some foods, in particular those containing fats and oils, deteriorate through the action of enzymes (naturally occurring components of living matter) that give rise to rancidity, colour loss and flavour changes. A reduction in moisture content slows down the rate of such enzymatic actions and so prolongs the shelf life of the commodity.

The micro-organisms however, are not dead, they simply remain dormant and should the moisture content rise by absorbing water from the atmosphere after drying, they will start to multiply again.

DRYING THEORY

The process of dehydration involves the removal of water by evaporation from the surface of the food to the surrounding air and the rate of drying depends on:

• the size of the food particle and its surface area. Moisture is lost from the surface of the food particle, so the smaller its size the less is the distance that the water has to move to the surface.

• the nature of the food. Products that contain sugars for example, tend to bind moisture which results in slow drying rates. Fibrous leafy materials dry very quickly, whilst starchy materials dry more slowly and fruits containing sugars even more slowly.

• the condition of the air used. Dry air is able to absorb more moisture than humid, damp air.

• the quantity of air passing over the food. A given volume of air can absorb a given quantity of moisture, therefore the greater the volume of air passing over the food, the greater the removal of moisture

• case hardening is a phenomena which occurs mainly in starchy foods when the surface dries relatively rapidly and forms a barrier to moisture moving from the interior of the food particle to (and through) the food surface. Case hardening can dramatically reduce drying rates.

• the over-all geometry of the drying system. Efficient removal of moisture depends upon efficient contact between the air and the material being dried.

Table 1. Kg of water that can theoretically be removed per kg of air.

Temperature

RH%

Water removed (Kg)

20

90

0.6

30

50

7.0

40

28

14.5

50

1 5

24.0

PROPERTIES OF AIR

Air contains water vapour, the amount present being expressed as its humidity which can he expressed in two ways Relative humidity and Absolute humidity.

Relative humidity (RH%) is the most commonly used description. Absolutely dry air has an RH of 0% while air that is saturated with moisture has an RH of 100%. The amount of moisture that air can absorb is very temperature dependent and Table 1 shows the effect of heating air with an RH of 90% at 29°C on the RH % and the amount of water that the air can absorb to be saturated (that is reach 100% RH). Table 1 clearly demonstrates that a small amount of heating of the air has a dramatic influence on the quantity of moisture it is capable of absorbing from food.

Absolute humidity is the actual weight of water vapour present in a given air. It is usually expressed in kgs of water per kg of dry air. While the term is less commonly used, an understanding of absolute humidity is essential when undertaking theoretical drying calculations such as designing a drier. Absolute humidity does not change with temperature because no water is added or lost by simply heating air.

This article was written by Barrie Axtell, Technical Editor of Food Chain and a director of Midway Technology.