|Characteristics and fitness for container transport
Fruit comprises the edible fruits and/or seeds of perennial woody or semiwoody plants (trees, shrubs) and herbaceous perennials.
The following classification based on the botanical features of the fruits has proved effective for transportation purposes and thus for container transport:
Tropical and subtropical fruit includes types such as bananas, pineapples, citrus fruit, kiwifruit, mangoes, avocados, together with exotic fruit such as cherimoya, granadillas, guava, Japanese persimmons, starfruit, kumquats, lychees, papaya, passion fruit, ground cherries and others (see Fig. 107).
Fruit exhibits high, 2nd order biotic activity (BA 2), i.e. respiration processes predominate due to separation from the parent plant and the consequent absence of any further supply of new nutrients.
The high water contents typical of fruit, which range from 80% (plums) to 85% (strawberries), mean that fruit is placed in water content class 3 (WCC 3) and is therefore highly perishable.
The sugar/acid content of fruit is the dominant feature. Examples of constituents are:
Most types of fruit are harvested at the preclimacteric stage (picking or industrial ripeness) because they are capable of post-ripening; they are also climacteric to a greater or lesser extent, i.e. they produce carbon dioxide and ethylene in the course of ripening.
Transport temperatures for fruit are often far above freezing point, as they may be at risk of chilling damage (see Table 11).
Table 11: Temperature and moisture/humidity conditions for fresh fruit storage during transport;
Fruit requires temperature, humidity/moisture and ventilation conditions (SC VII) because respiration processes must be specifically controlled and dormancy temperatures must be established. Ventilation ensures, on the one hand, that the necessary oxygen is supplied and, on the other, that harmful gases (carbon dioxide, ethylene) are removed. Refrigerated containers with a fresh air supply and CA containers are the most suitable for the temperature-controlled transport of fruit.
Transport instructions and damage
Before packing, the containers must be thoroughly cleaned, ventilated and, if required, disinfected.
The cargo is prepared for shipment in packing stations:
A written cooling order must be obtained from the shipper. The temperature specified in the order is a compromise between a temperature which prevents premature ripening ("dormancy temperature") and a temperature which is still high enough to prevent chilling damage.
It is clear from Table 11 that, especially for tropical and subtropical fruit, transport temperatures are far above freezing point as chilling damage is to be anticipated even at temperatures above freezing.
Fruit is also particularly sensitive to temperature variations.
Table 12 shows transport temperatures for the frozen storage of fruit of between -18 and -20°C, i.e. at which 80 to 90% of the cytosol is frozen, such that microorganisms and enzymes are made virtually inactive.
Table 12: Transport temperatures
for the frozen storage of fruit;
If the temperature falls below the refrigeration or dormancy temperature, chilling damage may occur, as for example in bananas whose skin turns pinkish-brown. Stripes appear in or under the skin. The flesh of the fruit remains hard and dry and has a mouth-puckering (astringent) flavor. The fruit also loses its ability to ripen. With pineapples, the skin loses its gloss, the flavor is insipid and the crown leaves become detached. In citrus fruit, spots form in the peel (peel discoloration) and the flavor becomes bitter. The commonest kind of chilling damage in fruit, especially in pomaceous fruit, is internal breakdown. In pineapple, chilling increases its susceptibility to black rot.
Interruption of the cold chain and exceeding the specified temperature may result, depending on the length of interruption, in premature ripening of the fruit. Once fruit ripening has begun, many kinds of fruit start to release the ripening gas ethylene. This gas causes other fruit to begin ripening (allelopathy) and, depending on its concentration, considerably accelerates ripening. In some kinds of fruit, e.g. the banana, ripening is a process which, once started, cannot be stopped. Exceeding the maximum upper temperature limit specified for a particular variety of bananas results in "cooked bananas". For this reason, strict compliance with the cold chain must be ensured.
Table 11 shows that relative humidity must generally be very high, around 80 - 90%, if flabbiness/shriveling and weight loss are to be avoided. Citrus fruit is often waxed to provide protection from drying-out. Pineapples and other kinds of fruit are at particular risk of drying-out and may suffer weight loss of up to 1% and, in extreme cases, even more.
Pomaceous and stone fruit, being climacteric fruit, produce large quantities of ethylene. Early and late varieties of apple, for example, must accordingly not be stowed together in a container as the metabolic activity of the early varieties will reduce the storage life of the late varieties. The carbon dioxide content of the container atmosphere should be maintained at 0.5 - 1.0 vol.% by supplying fresh air, so also keeping the content of other ripening gases such as ethylene within reasonable limits. Inadequate ventilation may, as a result of a build-up of carbon dioxide and an insufficient supply of atmospheric oxygen, cause the fruit to change over to anaerobic respiration, which rapidly results in fermentation and rot.
As has already been mentioned, most kinds of fruit are harvested and transported at picking ripeness (preclimacteric). It should, however, be noted that stone fruit (cherries, plums, apricots, peaches, mangoes, avocados) ought to be harvested as shortly as possible before eating ripeness (climacteric) as they do not post-ripen well and otherwise have a turnipy flavor and cannot easily be separated from their stone. However, harvesting at this stage means that only little time is available for transport. This shortcoming may, however, usually be overcome by using CA containers. One exception is grapes (dessert grapes), which are harvested at the climacteric, as they do not post-ripen.
The root cause of many types of damage, such as mold and rot, is mechanical damage, such as squashing, cracking, bruising. Pineapples, for example, are extremely pressure-sensitive and can develop "pressure sores" and are accordingly specially packaged in "Japanese garden" type (compartment) packaging. Squashed citrus fruit forms "goose pimples". Mangoes are highly perishable as even the smallest of cracks results in spoilage. All stem-ends must be removed from kiwifruit as they could injure adjoining fruits.
Due to their high content of highly volatile essential oils in the peel, citrus fruits in particular are a highly odor-contaminating cargo and must thus not be stowed in a container together with other kinds of fruit and vegetables. Care must also be taken to deodorize the containers before subsequent packing with other foodstuffs.
The Mediterranean fruit fly (Ceratitis capitata), for example, is a quarantine pest which in particular attacks thin-peeled citrus varieties. Mangoes infested by the mango fruit fly must be destroyed. Peaches infested by the San José scale cannot be exported (quarantine pest). Attack by the cherry fruit fly is manifested externally by a brown, sunken area near the stem-end.
Damage by molds and bacteria
Cut stems on bananas and pineapples are treated with a fungicidal paste to prevent mold. Citrus fruits are susceptible to green and blue mold and, after washing, optional finishing (dye bath) and waxing, are accordingly treated with fungicides, such as diphenyl, orthophenylphenol (OPP) and thiabendazole (TBZ). Since fungicides impair the flavor of the peel, indication of their use is mandatory. Grapes are highly susceptible to grey mold rot. Pomaceous fruit is affected by various rots, e.g. blue mold rot (apples, pears), brown rot (apples), grey mold rot (pears).
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