Agriculture Reference
In-Depth Information
control of temperature through the whole handling chain is
often referred to as the
cold chain
.
sensitive to concentrations well below 100 parts per billion
(ppb). In the United Kingdom, ethylene is known to build
up in packhouses to concentrations near 1000 ppb (= 1 part
per million [ppm]) (Rees, n.d.), which is above the
threshold of sensitivity of most produce. A study conducted
on a range of perishable produce showed a 60% extension
of post-harvest life when stored in <5 ppb compared with
100 ppb ethylene (Wills
et al
. 1999).
The biochemistry of how ethylene controls plant
processes is complex (for an introduction, see Rees &
Hammond 2002). Processes controlled by ethylene can be
classified into two types, System 1 and System 2.
In System 1 ethylene stimulates the process. If ethylene
concentrations are increased the process goes faster, and if
ethylene concentrations are reduced or ethylene is removed
completely then the process slows or stops. This is the case
for ethylene stimulation of the deterioration or senescence
of vegetables, the ripening of nonclimacteric fruit, the
over-ripening or senescence of fruit (both nonclimacteric
and climacteric), and the discolouration of cucumber and
browning of broccoli.
In System 2 ethylene acts as a switch that cannot be
stopped. Thus, ethylene triggers the process. If ethylene
levels are reduced or ethylene is removed completely, then
the process continues. This is the case for the initiation of
ripening of climacteric fruit.
Controlled atmosphere storage and modified
atmosphere packaging
As well as by lowering the temperature, metabolic
processes and development of perishable plant produce can
also be slowed down through modification of the storage
environment, usually by decreasing oxygen concentration,
sometimes with an associated increase in carbon dioxide
concentration. In some cases the atmospheric concentrations
are closely controlled throughout the storage period
(
controlled atmosphere storage
), and in other cases, after
an initial modification period, the atmosphere may be
allowed to alter through respiration of the commodity itself
(
modified atmosphere storage
). Modified atmospheres
may be used on a scale as large as a container or pallet
(100s-1000s kg), or down to the scale of individual
consumer packs (< 500 g). The technologies and how they
are applied to a wide range of perishable plant products are
thoroughly reviewed by Thompson (2010).
Ethylene control technologies
The importance of ethylene control for maintenance of
quality in perishable plant products is now widely recog-
nised, and is discussed for the individual commodities in
the following chapters within this topic.
Ethylene gas (C
2
H
4
) is produced naturally by most plant
tissues, especially ripening fruit. It is a plant hormone that
controls many biological processes. Ethylene is a gas at
ambient temperatures, so that if one plant or plant organ
starts to produce ethylene, nearby plant tissues are also
affected. For plants many processes involving tissue death,
such as leaf drop in deciduous trees, petal drop in flowers
and over-ripening of fruit, are actively controlled as part of
the natural life cycle, and are controlled or stimulated by
ethylene. For this reason, during handling of fresh produce
exposure to ethylene can speed up deterioration.
As it controls so many processes associated with the
quality of fruit and vegetables, ethylene is an extremely
important chemical for the fresh produce handling indus-
try. On the one hand, it is used to trigger ripening in fruits.
Thus bananas are transported green to the United Kingdom
and are then stimulated to ripen by being fumigated with
ethylene within warm ripening rooms. On the other hand,
as set out above ethylene will stimulate deterioration and
senescence. Ethylene control strategies are therefore key
for maintenance of quality.
The concentrations at which ethylene can affect produce
are very low. There is evidence that many products are
1-Methylcyclopropene
An important development in the management of ethylene
during post-harvest handling of fresh produce was the
discovery of the chemical 1-methylcyclopropene (1-MCP).
1-MCP acts as an ethylene antagonist by binding ethylene
receptors within the cells of the plant tissues and thereby
blocking the ethylene response. Other ethylene antagonists
such as silver thiosulphate and ethylene synthesis inhibitors
such as aminoethyoxyvinylglycine (AVG) were already
known. However, 1-MCP has turned out to be an extremely
useful chemical both as a tool to investigate ethylene
physiology and as a commercial post-harvest treatment to
counteract ethylene effects and extend shelf life (for a
review, see Blankenship & Dole 2003). The use of 1-MCP
for the handling of individual commodities is discussed in
many chapters of this topic.
POST-HARVEST TECHNOLOGY, FOOD
SUPPLY AND INCOME GENERATION IN
DEVELOPING ECONOMIES
Many of the technological advances in the post-harvest
handling of perishable commodities rely on a level of infra-
structure that is not present in many parts of the developing
