Why special treatments are needed!
Sections 3.3., 5.2., and 8.2. of this manual describe
various basic treatments applied to fresh produce in the field, at the packing
house, and prior to storage. In addition, some fresh produce commodities may
require special treatments in order to slow down their rate of deterioration
and minimize losses. The crops involved are for the most part those which are
seasonal and subjected to long-term storage, or which are highly perishable and
are transported over long distances to market. Some of the special treatments,
such as hot water dipping of mango and degreening of citrus, are often used Just
to make the produce acceptable for a given market rather than for any great
need to prevent their spoilage.
These special treatments may be applied before, during or
after packing of the produce and are supplements to the normal and basic
practices of grading, selecting, cleaning, and temperature and humidity
management and should never be considered as replacements for such practices.
Fruits, because of their generally higher value, seasonality and greater
perishability are the most common commodities to be given special post-harvest
treatments, and these are summarized in Table 9.. Root crops are often 'cured' to prolong their storage life
and minimise losses, while crops such as onions and potatoes may also be
treated with sprout suppressants prior to long-term storage. Fungicides are now
in widespread use for decay control in many fruits, vegetables and root crops.
These and other treatments are described in more detail below.
Ripening and degreening of fruits
Oranges and grapefruit grown in the Eastern Caribbean remain
green, partly green, or will reach full colouration depending on various
environmental factors but especially night-time temperatures. Regardless of
colour, the local markets recognize that the fruits are perfectly good to eat
and consumers worry more about fullness of flavour, juiciness and sweetness
than appearance of the fruits. The export markets demand full-coloured fruits
and so a degreening process is necessary.
Degreening is the process where the green chlorophyll pigments
in the peel are broken down and the yellow and orange xanthophyll and
carotenoid pigments are formed. In citrus, this natural process is stimulated
by exposing the fruits to 10 to 20 parts per million (ppm) of ethylene gas
under controlled conditions of temperature, humidity and ventilation, in
special degreening rooms operated by skilled management. Degreening is
generally considered uneconomic unless large quantities of fruit can be treated
at one time. The Citrus Growers Cooperative in Dominica uses large degreening
rooms in Roseau to initiate colouring of their export citrus prior to sea
shipment in refrigerated vessels.
Ripening of fruits is a perfectly natural and highly
desirable phenomonen leading to increased sweetness, flavour development and
softening of the edible tissue. However, ripening of certain fruits presents a
dilemma to the export industry because the very act of ripening clearly marks
the onset of senescence rapidly leading to decay and spoilage. The socalled
'climacteric' fruits such as banana, avocado and mango are harvested and
shipped for export in the green state while still hard and capable of surviving
the physical handling with minimum damage and spoilage during transit. This is
a very familiar operation in the Eastern Caribbean where large volumes of
bananas are shipped green every week on the Geest boats to England, where they
are held in store until just before they are needed by the consumer. The
transformation from green unripe to yellow ripe bananas is achieved by stacking
the boxed fruit in special airtight rooms where they are exposed to 1,000 ppm
of ethylene gas under controlled temperature and high humidity. The ripening
process is described in the text to Table
TABLE:RIPENING OF BANANAS
Number of Days to Ripen
|
Fruit Temperature (pulp) in °F
|
||||||
Day 1
|
Day 2
|
Day 3
|
Day 4
|
Day 5
|
Day 6
|
Day 7
|
|
4
|
64
|
64
|
62
|
60
|
-
|
-
|
-
|
5
|
62
|
62
|
62
|
62
|
60
|
-
|
-
|
6
|
62
|
62
|
60
|
60
|
60
|
58
|
-
|
7
|
60
|
60
|
60
|
60
|
60
|
58
|
58
|
PLEASE NOTE:
a. The temperatures indicated are for
pulp temperature NOT air temperature.
b. Air temperatures will usually be
lower than pulp temperatures.
c. Do not start ripening with ethylene
until pulp temperatures stable at desired temperature.
d. Seal ripening room and gas with 1000
ppm ethylene for 12 to 15 hours.
e. After initial gassing thoroughly
ventilate ripening roam with fresh air for 20 to 30 minutes every 12 hours.
f. Inspect fruit regularly and adjust
air temperatures to control pulp temperature in desired range.
g. Ripening time given is time from
hard green to Colour Stage 4 ( green-yellow with dark green tips)
h. Always use a proper pulp
thermometer.
In the tropics, banana ripening is traditionally achieved by
harvesting of fuller fruit and simply waiting for it to ripen at ambient. On
occasion, some traders may resort to triggering ripening in the same way as
with ethylene but using acetylene generated from small quantities of calcium
carbide and water in an enclosed room. This practice tends to give fruit which
are overly soft for their appearance and with a short market life. In addition,
it is very dangerous to use acetylene because of its explosive properties. In Barbados, quite large quantities of bananas from local
growers and from imported sources are now regularly ripened under reasonably
controlled conditions using ethylene gas in the same way as Geest, in order to
supply the increased consumer demand for "properly ripened" bananas,
but also to enable the high volume retailers, especially supermarkets, to better
supply their customers and control their post-harvest losses. Ripening of avocado and mango can also be achieved using
controlled ethylene exposure but its use in the tropical exporting countries
will depend upon the market to be supplied and individual importers/buyers
requirements.
Curing of root crops and onions
One of the most important methods of reducing post-harvest
losses in many root crops such as yam and sweet potato and also for onions, is
the use of surface drying and curing processes. Curing is a natural wound healing process which in sweet
potato and Irish potato replaces and stengthens damaged areas by forming a
corky layerwhich protects against water loss and infection by decay organisms. In contrast, the curing of onions is mainly a drying process
where excess moisture is removed from the outer skin and neck of the onion. At
the same time, by exposing the onion to higher temperatures the colour of the
skin darkens and natural fungicidal compounds accumulate in the skin. Both
processes together ensure the formation of protecting layer which greatly
reduces water loss and serves as a physical and chemical barrier to infection.
1. Root Crop Curing
The Table summarizes the specific details for the curing of
different root crops, but regardless of which root crop is to be cured, the
following conditions should be established for all:
- the roots and tubers must be kept at the right temperature to stimulate skin growth, and this is normally above ambient temperature;
- the air around the roots or tubers must be moist but without free moisture on the surface dry air will cause injured surfaces to dry out quickly but free moisture will allow spoilage organisms entry into the tuber before the protective layer forms;
- skin growth needs oxygen so ventilation is needed but not too much or the produce will dry out and temperatures are also likely to drop.
All root and tuber crops suffer some damage during harvest
and handling so curing should be carried out as soon as possible after harvest.
Sweet potato in the Eastern Caribbean is still largely traded both domestically
and regionally without a proper curing treatment. Often the uncured tubers are
bundled straight into crocus bags with damp soil still attached to the surface
and the poorly ventilated bags roughly handled and loaded into unventilated
ships holds. It should not be surprising that postharvest losses are often very
high. Considering that sweet potato is indigenous to the area it is a pity that
most farmers and exporters have very little understanding of how best to
harvest and handle the crop. The most simple curing practice for sweet potato in the
Eastern Caribbean involves firstly careful harvest (not when the soil is too
wet) and simple stacking of the produce in field crates or in small heaps, off
the ground in a shaded, sheltered and well ventilated spot under ambient
conditions. The curing process should be completed in 3 to 5 days after which
the tubers can be washed if necessary, graded, packed and distributed.
Onion Curing
Curing of onions is most coveniently performed in the field
by the 'windrowing' method. Windrowing of onions is performed by carefully
pulling or lifting the onions at harvest and simply laying them in their places
to dry with the leaves of one row covering the bulbs of the next row in order
to promote thorough drying of the tops while protecting the bulbs from undue
sunburn. the bulbs should be turned regularly to ensure even drying and curing
and should always be turned after a rain shower to make sure they are not
touching wet soil continuously. If rainfall persists, then the onions must be
dried and cured on special racks as shown in Figure 9.1. (Use the search box to find and See FIGURE: DRYING AND CURING RACKS FOR ONIONS)
which can be easily and cheaply constructed from locally available materials.
Polythene sheets should be fixed to the edge of the roof to let down quickly in
the event of heavy rain showers and removed afterwards.
Curing is considered complete when the outer scales and neck
are sufficiently dry to 'rustle' when handled and they form a tight cover over
the bulb. Development of skin colour is also completed at this stage, some 10
to 12 days after harvest.
Unfortunately, most onions grown in the Eastern Caribbean
are not sufficiently adapted to the local conditions and many of the 'Texas
Grano' types are seldom cured sufficiently, in the field or otherwise, before
marketing and post-harvest losses due to rots and sprouting tend to be high.
The introduction of more suitable red-skinned short day onion varieties with
better postharvest characteristics is strongly recommended.
Sprout inhibition
When crops such as onion and potato are placed in long-term
storage in temperate countries they tend to sprout and eventually rot.
Sprouting can be avoided under such conditions by using varieties with long
dormancy periods, proper curing (see above) and the use of chemical sprout
suppressants such as 'Tecnazene' (TNCB), 'Chlorpropham' (CIPC), or other
proprietary chemicals.
In the Eastern Caribbean storage is seldom for long enough
periods to worry about sprout control, but nevertheless some sprouting of
onions and potatoes does occur during distribution and home storage before
consumption. The best remedy is avoidance by making sure that these commodities
are not stored under high humidity, such as in unventilated plastic bags, and
not exposed to light for significant periods.
Fungicide application
Why Fungicides Are Necessary!
Most post-harvest losses eventually result from invasion and
breakdown of the produce by micro-organisms, although physical injury and
stress caused by poor handling may predispose the produce to such attacks. In
the spoilage of vegetables, bacteria can be the most important spoilage
organisms, but since bactericides are not suitable for application to fresh
produce, control must be largely by other methods. Chlorinated disinfectants
are beneficial in cooling and washing water but are difficult to use
effectively because the chlorine tends to combine readily with dirt and debris
and ceases to be effective.
Fungi are usually the primary agents in the spoilage of
fresh produce and control is possible by the application of fungicides at dose
rates which do not harm the produce nor the consumer of the produce. If the
produce is to be marketed and consumed rapidly after harvest then fungicides
may not be necessary or their use may be uneconomic if the value of the produce
is low or the effect of the fungicide is not sufficiently significant.
Fruits in particular suffer from fungal infection. Often
infections established in the field may remain dormant until after harvest and
decay only develops during post-harvest ripening. These so-called 'latent'
infections are of great importance to crops such as bananas, mangoes and
avocadoes, where anthracnose infections are not seen by the exporter but may
totally prevent their purchase in the export market if not controlled.
Application Methods for Fungicides
Fungicides are nearly always applied in the form of an
aqueous solution or suspension and thus only to produce which is normally
washed before packing.
Washed produce should be drained of excess water before the
fungicide is applied or dilution will occur and the fungicide may not work properly
- a waste of money and time.
Almost all fungicides used for post-harvest application are
in the form of wettable powders (WP) or emulsifiable concentrates (EC), and
when mixed with water form suspensions, not solutions. They are both liable to
settle out from the suspensions so it is ESSENTIAL THAT THE TANK MIX BE
CONTINUOUSLY AGITATED TO KEEP THE FUNGICIDE IN SUSPENSION. Failure to do this
will mean that the fungicide will collect at the bottom of the tank instead of
going on to the produce - no fungicide application means no effect and a waste
of time and money.
The most common application methods are:
i.
Spray
or mist
- when application is by a hand-held knapsack sprayer for small-scale
operations, or a mechanical spray set up on a moving belt or roller-conveyor
for large-scale commercial operations. Both systems normally include provision
for agitation of the spray mixture. Spraying should be done to the point of
run-oof to ensure a complete cover of fungicide.
ii.
Drenching - is a simple mechanical re-circulating
system in which the fungicide is pumped in a cascade over produce passing
beneath it on a belt or roller-conveyor. It has the advantage that there are no
spray nozzles to wear away or become blocked. The combination of reservoir
design and a high flow-rate pump keep the mixture agitated. A disadvantage of
the system is that the high flow-rate and agitation may cause the fungicide to
be dragged out of suspension and float on the foam generated by the agitation.
It may be necessary to add a non-toxic anti-foam agent to prevent this
happening.
iii.
Dipping - is normally used where small
quantities are to be treated. The fungicide mixture is made up in a small bath
and produce dipped by hand. Excess fungicide is allowed to drain back into the
bath. The fungicide mixture must of course be agitated frequently by hand. It
is advisable to wear rubber gloves because some people develop allergic skin
reactions to the fungicide mixture.
iv.
Smoke
or fumigant
- is rarely used as an application method for fungicides. Tecnazene, which is a
fungicide as well as a sprout suppressant, is applied in the form of volatile
granules in long-term Irish potato stores, and 'diphenyl' wraps or pads may be
used for citrus.
Caution When Using Post-Harvest Fungicides!
Pesticides used in the field are subject to removal from the
crop by rain, wind, solar degradation and physical removal by rubbing of leaves
etc. by handling of produce at harvest, and the produce may also be washed
after harvest. However, fungicides applied post-harvest in the packhouse, or
wherever, are not subject to these factors, and what is applied will largely
remain until the produce is purchased by the consumer. The fungicide will thus
remain on the produce and help to control fungal infection but it will also be
there as a possibly toxic chemical at the time of consumption.
For these reasons only a small number of chemicals is
available for post-harvest treatment of fresh produce and is limited to those
chemicals with a very low toxicity. The manufacturers specify strict
limitations on the concentrations to be used so that residues do not exceed the
permitted levels for human consumption. Extra-regional export markets regularly
check on fungicide and other pesticide residue levels on imported produce and if
the permitted levels are exceeded, the produce is rejected and a warning given
that all produce from that source must in future comply with the legislation or
a total ban will be implemented. It makes good sense from an economic as well
as an ethical standpoint to use fungicides carefully and accurately.
Hot water treatment
Although not used for treatment of vegetables and delicate
fruits, hot water immersion of mango and pawpaw has been shown to help control
latent 'Anthracuose' development. The hot water treatment is usually combined
with fungicide application to promote the most effective control and save
additional handling and equipment.
In the Eastern Caribbean, hot-water treatment is recommended
for mangoes exported to the extra-regional markets. The details of the process
have still to be optimized with regard to water temperature and duration of the
dip, and the requirements of different varieties from different growing areas,
and possibly also from different harvest times. Experiments are being conducted
by post-harvest technologists in Dominica at the Government's Produce Chemist's
Laboratory, and at the CENTENO Post-Harvest Research Unit of the Ministry of
Food Production in Trinidad, as well as individual experimentation by
independent exporters including CATCO.
Generally, there are two main variants of the hot water
treatment:
i.
Dip
in hot water at 55øC for 5 minutes. Heat injury can develop when the mangoes
are placed in cold storage after treatment - do not therefore dip in hot water
when fruits are to be stored.
ii.
Dip
in hot water at 52øC with Benomyl (Benlate) at 0.1% concentration, for 1 to 3
minutes.
Hot water treatment requires a lot more management and
supervision to ensure that the water temperature and dipping time are strictly
adhered to because there is little margin for error. Commercial scale equipment
for treating larger volumes of fruit have yet to be acquired in the region and
even in more developed countries the apparatus is still undergoing further
development.
Fumigation and vapour-heat
Fumigation of fruits was traditionally for control of
disease, as in the case of sulphur dioxide fumigation of grapes to control
Botrytis and other fungus diseases, or for control of insect pests for
quarantine purposes, principly against fruit flies.
Quarantine regulations in the USA and Japan require that
certain produce from areas where fruit fly is endemic must be disinfested on or
during importation, by approved and controlled methods. This was formerly
achieved by fumigation with ethylene dibromide (EDB) or methyl bromide (MB).
These chemicals have since been banned by many countries fo fear of toxic
residues on the treated fruits. For the present, there are no suitable
alternatives and areas with fruit fly in the Eastern Caribbean have no
opportunity to export fruits such as mango to the USA unless, like Grenada
recently, they are cleared as fruit fly free by USDA inspectors.
Vapour heat treatment may be used where other fumigants may
damage the produce. The treatment consists of circulating saturated water
vapour at high temperature around the produce in an enclosed store until the
produce reaches the required temperature, usually in about eight hours, and the
produce is then held at that temperature for a further six hours. Clearly most
fruits such as avocado, lemon and most vegetables would injured by this
treatment, but other kinds of citrus may be treated with vapour heat without
serious Injury. The method is not in widespread use on account of its cost and
impracticality. The method is not considered a viable alternative to EDB
fumigation for the Eastern Caribbean.