Herbs in Africa
Part 2.
Dehydration
Ivor Hughes continues a series of articles which outline the basic
requirements, for small scale, sustainable cultivation, and processing
techniques, for rural communities. For Part One enter here.
Dehydration
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.
Photosynthesis
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.
Watch out for the third part in this series next month at Science in Africa.
For Part One of this Series: 
For Part Three of this Series:
More information:
www.herbdatanz.com Herbdata New
Zealand
|
