9. Some special post-harvest treatments
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9.1. 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 9.1., below.
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.
9.2. 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 9.2..
TABLE 9.2. 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|
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.
9.3. 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.
9.3.1. Root Crop Curing
Table 9.3. 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:
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.
9.3.2. 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. (See FIGURE 9.1. 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.
9.4. 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.
9.5. Fungicide application
9.5.1. 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.
9.5.2. 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:
9.5.3. 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.
9.6. 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:
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.
9.7. 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.
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