Once a plant is harvested it starts a process of decay that will result in the
inevitable death of the organism and the deterioration of its organic matter. This
process is even more accentuated if the harvested tissue is a particular organ
(fruit, leaf, root, etc.) instead of the whole plant. In an ideal world this fact
should not be a problem, we would grow our vegetables and fruits in our
backyard and consume them fresh every day. In real life most of the food we
consume has been grown hundreds or even thousands of kilometres away from
the shop where we bought them. Therefore we are daily confronted with the fact
that food crops, after they are grown, need to be transported to intermediate
destinations, stored, transported again and distributed before finally reaching the
consumer. Aside from the natural/physiological reasons stated above,
sociological aspects need to be considered; the backyard self-sufficient growing
strategy will not work nowadays because many of us do not have a backyard.
The world is evolving from a predominantly rural population to a new
demographic distribution based on high human concentration on urban areas.
Areas of food production are therefore far away from areas of food consumption.
We are consequently fighting a continuous battle to bring the right food to the
right place with a minimum of losses. Sadly, although important advances have
been made in post-harvest technology, we are still losing the battle.
Post-harvest problems can ac
count for substantial losses, the magnitude of
which depends on the crop, the country and the year. It is important to stress that
post-harvest losses are one of the most significant factors limiting agricultural
production in third-world and developing countries. Whereas technically
advanced countries such as the USA, Japan, Australia and European countries can apply relatively sophisticated technologies to minimise losses; developing
countries cannot afford them. In addition, many of the developing regions in the
world are situated in the tropics, where high temperature and humidity
exacerbate the problem.
Senescence, the final stage in the life of a plant (or a particular organ) was once
thought to have an essentially chaotic nature in which cell components break
down without a particular order. Nevertheless, recent advances in our understanding
of the process have revealed that senescence is a very well programmed
developmental stage, involving highly coordinated cellular events that require the
sequential action of many genes. Our fundamental knowledge of senescence has
greatly improved in the last few years but we are still far from having a good
understanding of the underlying biochemical and molecular mechanisms. There
are two kinds of senescence processes, a natural one that comes after the end of
the useful life of an organ is reached, and a stress- or environmentally-induced
senescence. Fruits will develop and ripen to entice predators and ensure seed
dispersal. When the seeds contained in the fruit are not viable any more, a natural
senescence process will eventually end up with the spoilage of the fruit. Old
leaves shrivel and fall while new ones develop to keep the photosynthetic capacity
of the plant. These events are just natural processes pre-programmed in the genetic
code. Harvesting plant tissue for commercialisation causes a series of stresses in
the detached tissue that will inexorably trigger senescence. Natural and induced
senescence do not always share the same mechanisms.
The main objective of post-harvest technology is to increase the useful life of a
particular foodstuff but increasing the useful life is a very ambiguous term. For
some produce it means keeping the tissue turgor for longer (we all want our
lettuces, broccoli and apples to have that fresh crispy feel), for other foods such as
mangoes and bananas it means keeping the right degree of softness for longer
without over-ripening (we like our mangoes soft but not mushy). Horticultural
produce is extremely heterogeneous with a large variety of plant tissues being
commercialised such as fruit, leaves, flowers, roots and tubers. Even though
senescence has some common features, each tissue has specific characteristics and
therefore needs to be studied separately. This is the reason why there is not a single
universal post-harvest treatment useful for all horticultural crops. Leafy vegetables
must be treated in a different way from fruits and even fruits have a wide variety of
post-harvest problems depending on the fruit and even the commercial variety.
From a biotechnological point of view it is therefore important to establish the
nature of the crop and the particular problem before establishing the approach to
be attempted. It is also important to emphasise that the same problem can be
tackled with very different approaches. Metabolic engineering can target internal
processes but is not restricted to endogenous genes. Modern genetic engineering
techniques allow us to cross species barriers (and even kingdom barriers) and
therefore genes that would not normally be accessible by conventional breeding
can now be incorporated into the plant species being targeted.
Post-harvest technologies such as atmosphere control and (CO2
humidity), refrigeration, irradiation, etc., have proven useful in controlling post-harvest losses, but the implementation of many of the existing and new technologies is
quite difficult in many developing countries due to the lack of infrastructure and
specialised personnel in rural areas. Many crops are also grown in small farms
meaning that a single grower is not able to afford the economic cost of setting up
treatment plants and has no access to specialised packaging requirements.
Regional centres, either governmental or private, can alleviate the situation but
require a level of organisation commonly non-existent.