Curing is conducted immediately after harvesting. It strengthens the skin. The process is induced at relatively higher temperature and humidity involving suberization of outer tissues followed by the development of wound periderm which acts as an effective barrier against infection and water loss. It is favoured by high temperature and high humidity. Potato, sweet potato, colocasia, onion and garlic are cured prior to storage or marketing. In sweet potato, this condition is most rapid at 33 0C and relative humidity of 95%. Potato tubers are held at 18 0C for 2days and then at 7 0C – 10 0C for 10-12 days at 90 % relative humidity. Curing also reduced the moisture content especially in onion and garlic. Drying of superficial leaves of onion bulbs protects them from microbial infection in storage. Maximum safe temperature for onion curing at field is 37.8 0C for 3-5 days. Artificial curing of onions in crates at 40 0C for 16 hours reduces rot losses in storage.
Degrening is the process of decomposing green pigments in fruits usually by applying ethylene or other similar metabolic inducers to give a fruit its characteristic colour as preferred by consumers. It is applicable to banana, mango, citrus, and tomato. The time required to degree a fruit depends upon the degree of natural colour break and maturity. The higher the green colour and more mature a fruit is the less time is required to reduce the chlorophyll to a desired level.
Degreening is carried out in special treating rooms with controlled temperature and humidity in which low concentration of ethylene ( 20ppm) is applied. The ethylene should be supplied from a gas cylinders. These rooms are thoroughly ventilated to keep the Co2 level below 1% which does not allow higher colouring. If kerosene fumes are placed outside the degreening room , they enter the room through ducts by forced ventilation. Despite the fire hazard involved, kerosene fumes produce better coloured fruits than pure ethylene. It is due to good ventilation. The best Degreening temperature is 27 0C. Higher temperatures delay Degreening. The relative humidity should be 85-90%. Higher humidity levels cause condensation during Degreening and are associated with slow Degreening and increase in decay. Low humidity though checks decay, causes excessive shrinkage , shrivelling and peel break down.
In another method, frits in containers are sealed by 2 sheets of plastic film and water. the PVC ( 0.2mm thick) can also be used. The ethylene is introduced from a can of 4.2 litres capacity in the film which can cover 1.2 tonnes of fruits, the ethylene concentration becomes nearly 1000 ppm resulting in satisfactory colouring. After 15 hours the film is removed enforcing the fruit to air. Degreening takes 3 days. Ethylene accelerates decomposition of chlorophyll without significant affecting the synthesis of carotenoid pigments.
High temperatures are detrimental to keeping quality of fruits and vegetables , especially when harvesting is done during hot days. Pre-cooling is a means of removing the field heat. It slows down the respiration of the produce, ,minimizes susceptibility to attack of microorganism , reduces water loss and eases the load on cooling system of storage or transport. Peas and okra which deteriorate fast, need to prompt cooling. Sometimes stage of ripening and level of field heat of produce also determine the need for pre-cooling. For example, unless tomatoes are avove 26.7 0C and ripening and level of field heat of produce also determine the need for precolling. For example , unless tomatoes are above 26.7 0C and ripening is to be delayed there is no need for pre- cooling.
In air cooling, cool air can be obtained from cold storage. Temperature should not be less than -10C to avoid freezing. Where night temperatures are low, doors of the store rooms can be opened for cooling in the night.
In water cooling, field heat is removed quickly. It is used for leafy vegetables to retain their texture and freshness. Ice can be added to bring down the temperature. However, temperature should be controlled to avoid chilling injury in cold sensitive fruits and vegetables. Hydro cooling at 12 0C to 15 0C with 500 ppm Bavistin increases shelf-life of mango. In Alphonso mango, it also reduces the incidence of spongy tissue.
4) Washing and Drying:
Most of the fruits and vegetables are washed after harvesting to improve their appearances prevent wilting and remove primary inculum load of microorganisms. Hence a fungicide should be used in washing water. washing improves shelf-life of bananas by delaying their ripening. After washing, excess should be removed which would other encourage microbial spoilage. Root and tuber crops are often washed to remove the soil adhering to these.
5) Sorting and Grading:
Immature diseased and badly bruised fruits and vegetables are sorted out, most of the countries have their own set of standards of domestic trade and for international trade, standards have also been defined. Grades are based on size, weight, colour and shape. Grading is done manually or mechanically.
Papaya, mango, melon and other fruits are susceptible to fruit fly attacks. Disinfestations is done either by vapour heat treatment at 43 0C with air saturated with later vapour for 6-8 hours by ethylene dibromide fumigation ( 18-22 g of EBD/ cubic meter for 2-4 hours. Residues of inorganic bromide must not exceed to 10vg/g) or by cold treatment ( exposure of fruits to near freezing temperature for a specified period).
7) Post-Harvest Treatment:
Post –harvest application of Bavistin ( 0.1%) and Topsin ( 0.1%) controls storage disease in mango. In Nagpur mandarins , hot water treatment with Imazalil ( 0.1%) , Bavistin ( 0.1%) and Benlate ( 0.1%) is most effective. A complete inhibition of sprouting of cool chamber ( evaporatively cooled) stored potatoes for 4 months and 5 months is achieved by spraying them with an aqueous emulsion of CIPC @ 50 mg/kg of tubers respectively before completion of dormancy period.
Fruits and vegetables have a natural waxy layer on their outer surface which is partly removed by washing. An extra discontinuous layer of wax applied artificially with sufficient thickness and consistency to prevent anaerobic condition within the fruits provides necessary protection against decay organisms. Waxing is especially important if tiny injuries and scratches on their surface are present. These can be sealed by wax. Waxing also enhances the gloss of fruits or vegetables. Therefore, appearance is improved, making them more acceptable.
If refrigerated storage facilities are not available , protective skin coating with wax increases the storage life of fresh fruits and vegetables at ambient temperatures.
There are two types of wax emulsion: Wax ‘W’ and Wax ‘O’.
The composition ‘W’ does not impart any gloss to fruits and vegetables and hence where gloss is required for improving marketability of the produce , composition wax ‘O’ is recommended. Both these emulsion contain 12 % total solids.
The application of wax emulsion to freshly harvested healthy produce protects them against excessive moisture loss, higher rate of respiration, heat buildup or thermal decomposition. Texture and quality of the fresh produce is maintained as near the fresh condition as possible for a long time.
The wax emulsion without fungicide does not protect fruits and vegetables against microbial spoilage. Therefore, suitable fungicides are added to the wax emulsion.
9) Ripening of Fruits:
Ripening transforms a physically mature but inedible plant organ into a visually attractive taste and smell sensation. It marks the completion of development and commencement of senescence with life of a fruit and is normally an irreversible event. Ripening can be achieved by the application of ethylene.
Accurate quantity of ethylene should be used in the ripening room at regular interval. A concentration of CO2 about 1% delays ripening. Hence, through ventilation Is essential ) marketing is alkaline using caustic soda.
10) Pre-packaging in Plastic Film:
Pre-packaging increases the shelf life by create a modified atmosphere with an increases in concentration of Co2 in the package. The packaging material used should provide reasonable access to oxygen. For this, beginning films like polystyrene and cellulose acetate are used. But together LDPE films which have high O2 and Co2 transpiration rates are more durable , the pouches must have perforation to transmit oxygen and carbon dioxide rapidly enough for the respiration of fresh produce. The pouch used reduces bruising facilitates inspection , reduces moisture loss and prevents dehydration. It also creates modified atmosphere.
In pre-packaging leaves, stalk, stem , etc. are trimmed washed cleaned and weighted quantities are put in pouches. Ethylene absorbents may be added to the package wherever required to retard the ripening process. Hydrated lime inserts may also be beneficial in controlling CO2 concentration within the film package.