Herbs In Africa

Part 2
Part~1     Part~3      Part~4      Part~5     Part~6
Ivor Hughes

A series of articles which outline the basic requirements, for small scale, sustainable cultivation, and processing  techniques, for rural communities.


Dehydration as an art is very old, the origins of which are lost in time.  As a science it is relatively young being less than 100 years old. As a process  it is fundamental to most herb growing operations.

The drying phase is the point at which an otherwise satisfactory crop may be ruined or its economic value considerably reduced and yet it is the one  process which is most often botched with some quite appalling materials  appearing in the market place.

The Benefits of Dehydration

The ownership of a dehydrator confers upon the herb grower a degree of  market flexibility which is unmatched by any other branch of horticulture. Some  of the benefits are as follows:

The crop is stabilised and may be stored for up to nine months.
No necessity to sell the crop onto a glutted market.
The crop bulk is reduced with good savings on transport.
The crop is greatly increased in value.
The marketing options are considerably expanded.

The Aim Of Dehydration

Good dehydration practice seeks to preserve the herb metabolites in as near to their natural state as possible. Therefore the water content of the material must be quickly and efficiently reduced to a level where biochemical reactions cease and micro-organisms are unable to function. The temperatures  employed must be so regulated that the metabolite and cosmetic integrity of the material is not damaged, therefore the grower must not only have knowledge of dehydration theory and the apparatus employed but must also understand the characteristics of the material upon which they work.

The Living Herb

As living entities herbs are incredibly complex. A single cell with the addition of a few basic elements can manufacture in seconds a dazzling array of intricate compounds, even one of which could take a modern research laboratory many years of painstaking work to reproduce, if indeed they could be reproduced at all. It is well that we remember that the chemical expertise demonstrated by  a single blade of grass is as yet beyond our knowledge.


The word 'photosynthesis', means literally, 'made from light' and by that ultimate transmutation the green plant may be seen as the supreme planetary  alchemist. The green plant alone has mastered the secret of the transmutation of sunlight, water and carbon dioxide into food. All life forms are dependant on  the power of the leaf.

There are certain kinds of bacteria that are classed as autotrophs ie, able to synthesize food from inorganic molecules such as hydrogen sulphide; however the hydrogen sulphide which is used instead of water, is produced from  the breakdown of green plant protein by sulphide bacteria, so they too are  dependent on the green plant for life. In that respect they may be seen as the  bacterial equivalent of the fungus.

Primary and Secondary Compounds

Primary compounds such as carbohydrates, proteins, lipids and nucleic acids are to be found in all living organisms, whereas the natural distribution of the secondary compounds such as alkaloids and glycosides etc, is somewhat  more sporadic, however the secondary compounds are produced in great variety by the green plants. Several thousand of them have been identified, what is surprising is that they have been synthesized from just 6 major chemical groups, yet they are able to elicit all known pharmacological responses.

General Plant Metabolism

Metabolism is the term applied to the sum total of chemical activity that occurs within the plant. Herbs like humans must eat, drink, respire, reproduce and die. Water as the arbiter of life performs the same function within the herb as does the blood stream within the human system. The plant is able to exercise a high degree of control of the water throughout its system. When the weather is hot and water plentiful, its rate of transpiration is rapid. Experiments have shown the metabolic rate of an organism is considerably increased by a rise in the temperature, the rate of many reactions being doubled by a 10�C  rise.

The evaporation effect of transpiration is cooling therefore the plant is able to modify its internal temperature and reduce its metabolic rate. When the  weather is hot and water is short, the plant is able to reduce its rate of  transpiration and within limits control its metabolic rate.

The herbs, stem and upper surface of its leaves are covered in much the same way as human skin with thousands of tiny pores called stomata (pl.). the stoma may be opened or closed by 2 sickle shaped guard cells, which line the edges of the stoma.

The guard cells are activated by internal or environmental cues or a combination of both. The stomata allow the exchange of gas and vapours between the plant and the atmosphere.

The skin or epidermis that surround the stomata secretes a waxy cuticle  that inhibits the evaporation of water from the epidermal area of the leaf. For that reason around 95% of the plants respiration and transpiration is via the stomata, however in young or partially developed leaves, or for shade loving  plants, then the cuticular exchange of gas or vapours could be as high as  50%.

The stomata are normally open during daylight hours and closed at night,  however they will also close if the plant is in anyway damaged or subjected to  environmental stress.

Water vapour on being discharged from the stomata will linger around the  plant and form what is known as the boundary layer. The depth of the boundary layer will vary from specie to specie and will only form in still air, a light air movement being sufficient to disperse it.

Enzymes, the plant chemists

Enzymes are classed as complex proteins and nearly all chemical reactions  that occur within a living organism are ordered by enzymes.
Enzymes are sensitive to temperature (thermolabile), with many being destroyed at  temperatures in excess of 70�C. The minimum temperature at which enzyme activity will cease is 0�C or freezing point. The optimum temperature at which enzyme activity is at its greatest is 30�C.
The temperatures given are not absolute for it is not only the degree of temperature that is important but also the duration.
As previously stated, metabolism is the sum total of chemical reactions within the plant. Metabolism is of two orders;
Anabolic ~ which is a constructive process involving the building up of complex molecules from simpler structures.
Catabolic ~ which is akin to a destructive process in which complex molecules are broken down into their component parts.

Harvesting shock initiates intense biochemical activity, the dynamic equilibrium of the herb is disrupted as the enzymes commence to fire in random order. Anabolic reactions cease and the catabolic reactions predominate as the herb starts to die. During that process, the important secondary compounds are  systematically reduced to primary compounds and from there they decompose to the original elemental state.
Therefore from harvesting onwards the herbs potency, or lack of it, is a function of time, with up to 35% of the herbs pharmacological activity reduced in 12 hours, however this biochemical activity must be mediated by water. Remove the water and the biochemical activity will cease.

Dehydration Times and Temperatures.

The water content of the freshly harvested herb must be swiftly and efficiently reduced to 8 or 9% of its total, at which point enzyme activity will cease, the herbal material may then be considered stable.
Heat is necessary for the evaporation of water, however the method, the degree and the duration of the heat applied is of prime importance in the production of a quality crop.  Many of the herbs secondary compounds are thermolabile (Decompose) when exposed to excessively high temperatures, conversely low temperatures are equally destructive because the extended drying time promotes excessive enzyme activity.  As a general rule drying times in excess of 10 hours are detrimental.

Dehydration, Basic Information

Differing species of herb exhibit differing characteristics not only in shape and form but also in the structural composition of its parts, eg soft, hard, fleshy, dense, fibrous, waxy, thick, thin etc; all of which may be considerably modified by geo-climatic factors which will vary from site to site,  therefore it is not possible to raise the treatment of any specie to the level of dogma. Good dehydration practice is as much an art as it is a science and because of the considerable number of variables involved it has not been possible to reduce the practice to a series of tidy mathematical equations. The human mind can weigh and judge imponderables, then arrive at a working solution,  therein lies the art. Therefore the operator of dehydration equipment must  temper the science with observation and experience.

Part~1      Part~3       Part~4       Part~5

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These articles have been published in Science in Africa
Next in this series:
Extraction of Herbal Material