Vegetable Research & Information Center, UC Cooperative Extension

Postharvest Handling Systems: Fruit vegetables

With the exceptions of peas and broad beans, fruit vegetables are warm-season crops, and with the exception of sweet corn and peas, all are subject to chilling injury. Fruit vegetables are not generally adaptable to long-term storage. Exceptions are the hard rind (winter) squashes and pumpkin. A useful classification for postharvest discussion of the fruit vegetables is based on the stage of maturity at harvest. This presents an overview of the general postharvest requirements and handling systems for this group of commodities.

Immature fruit vegetables

Legumes: snap, lima, and other beans, snow pea, sugar snap and garden peas

Cucurbits: cucumber, soft rind squashes, chayote, bitter melon, luffa

Solanaceous vegetables: eggplant, peppers, tomatillo

Others such as okra and sweet corn

Mature fruit vegetables

Cucurbits: cantaloupe, honeydew, and other muskmelons; watermelon, pumpkin, hard-rind squashes

Solanaceous vegetables: mature green and vine-ripe tomatoes, ripe peppers


Field Operations

Harvesting

The harvest index for most immature fruit vegetables is based principally on size and color. Immature soft-rind squashes, for example, may be harvested at several sizes or stages of development, depending upon market needs. Fruit that are too developed are of interior internal quality and show undesirable color change after harvest. This also applies to other immature fruit vegetables such as cucumber and bell peppers.

The harvest index for mature fruit vegetables depends on several characteristics, and proper harvest maturity is the key to adequate shelf life and good quality of the ripened fruit. For cantaloupe, the principal harvest indices are surface color and the development of the abscission zone.

Most fruit vegetables are harvested by hand. Some harvest aids may be used, including pickup machines and conveyors for melons. Cantaloupe is also harvested with "sack" crews who empty the melons into bulk trailers. Crenshaw and other specialty melons are easily damaged and require special care in handling and transport to the packing area. Mature green tomatoes are usually hand harvested into buckets and emptied into field bins or gondolas. Almost all fresh market tomatoes grown in California are bush type, and the plants are typically harvested only once or twice. At the time of harvest, 5 to 10 percent of the tomatoes have pink and yellow color and are separated out later on the packing line as vine-ripest

Immature fruit vegetables generally have very tender skins that are easily damaged in harvest and handling. Special care must be taken in all handling operations to prevent product damage and subsequent decay. Sweet corn, snap beans, and peas may be harvested mechanically or by hand.

Many of the mature fruit vegetables are hauled to packinghouses, storage, or loading facilities in bulk bins (hard rind squashes, peppers, pink tomatoes), gondolas (mature green tomatoes and peppers), or bulk field trailers or trucks (muskmelons, hard rind squashes).

Harvesting at night, when products are the coolest, is common for sweet corn and is gaining in use for cantaloupe. Products reach their lowest temperature near daybreak. Night harvest may reduce the time and costs of cooling products, may result in better and more uniform cooling, and helps maintain product quality. Fluorescent lights attached to mobile packing units have permitted successful night harvesting of cantaloupe in California.

Field packing

The trend is increasing toward field packing of fruit vegetables. Grading, sorting, sizing, packing, and palletizing are carried out in the field. The products are then transported to a central cooling facility. Mobile packing facilities are commonly towed through the fields for cantaloupe, honeydew melon, eggplant, cucumber, summer squashes, and peppers. Field-pack operations entail much less handling of products than in packinghouses. This reduces product damage and, therefore, increases packout yield of products. In melons, for example, field packing means less rolling, dumping, and dropping and thus helps reduce the "shaker" problem, in which the seed cavity loosens from the pericarp wall. It also reduces scuffing of the net which reduces subsequent water loss. Handling costs are also reduced in field pack operations. One difficulty with field packing, however, is the need for increased supervision to maintain consistent quality in the packed product. Field packing is not used for commodities that require classification for both color and size, such as tomato.

Packinghouse Operations

Receiving

Loaded field vehicles should be parked in shade to prevent product warming and sunburning. Products may be unloaded by hand (soft rind squashes, eggplant, some muskmelons, cucumber, watermelon), dry-dumped onto sloping, padded ramps (cantaloupe, honeydew melon, sweet peppers) or onto moving conveyor belts (tomatoes), or wet-dumped into tanks of moving water to reduce physical injury (honeydew melon, tomatoes, and peppers). Considerable mechanical damage occurs in dry-dumping operations; bruising, scratching, abrading and splitting are common examples. The water temperature in wet-dump tanks for tomatoes should be slightly warmer than the product temperature to prevent uptake of water and decay-causing organisms into the fruits. The dump tank water needs to be chlorinated. An operation may have two tanks separated by a clean water spray to improve overall handling sanitation.

Preliminary operations

Presizing. For many commodities, fruit below a certain size are eliminated manually or mechanically by a presizing belt or chain. Undersize fruit are diverted to a cull conveyor or used for processing.

Sorting or selection. The sorting process eliminates cull, overripe, misshapen, and otherwise defective fruit and separates products by color, maturity, and ripeness classes (e.g. tomato and muskmelons). Electronic color sorters are used in some tomato operations.

Grading. Fruit are sorted by quality into two or more grades according to U.S. standards, California grade standards, or a shipper's own Trade standards.

Waxing. Food grade waxes are commonly applied to cucumber, eggplant, sweet peppers, cantaloupe, and tomato, and occasionally to some summer squashes. The purpose is to replace some of the natural waxes removed in the washing and cleaning operations, to reduce water loss, and to improve appearance. Waxing may be done before or after sizing, and fungicides may be added to the wax. Application of wax and postharvest fungicides must be indicated on each shipping container. Waxing and fungicides are used only in packinghouse handling of fruit vegetables. European cucumbers are frequently shrink-wrapped rather than waxed.

Sizing. After sorting for defects and color differences, the fruit vegetables are segregated into several size categories. Sizing is done manually for many of the fruit vegetables, including the legumes, soft and hard rind squashes, cucumber, eggplant, chili peppers, okra, pumpkin, rnuskmelons, and watermelon. Cantaloupes may be sized by volumetric weights, or diverging roll sizers, sweet peppers are sized commonly by diverging bar sizers, and tomatoes are sized by diameter with belt sizers or by weight.

Packing. Mature green and pink tomatoes, sweet and chili peppers, okra, cucumber, and legumes are commonly weight- or volume-filled into shipping containers. All other fruit type vegetables and many of the above are place-packed into shipping containers by count, bulk bins (hard rind squashes. pumpkin, muskmelons, and watermelon) or bulk trucks (watermelon). Fruit type vegetables that are place-packed are often sized during the same operation.

Palletizing. Packed shipping containers of most fruit vegetables in large-volume operations are palletized for shipment. This is a common practice with cantaloupe, muskmelons, sweet peppers, and tomato. Except for sweet corn, the immature fruit vegetables are often handled in low volume operations, where palletizing is not common because of lack of forklifts. In these cases, the products are palletized at a centralized cooling facility or as they are loaded for transport. Palletizing is usually done after hydrocooling or package-ice cooling, but before forced-air cooling. In field-pack operations, palletizing is generally done in the field.

Cooling

Various methods are used for cooling fruit vegetables. The most common methods are discussed here.

Forced-air cooling is used for beans, cantaloupe, cucumbers, muskmelons, peas, peppers, soft rind squashes, and tomato. Forced-air evaporative cooling is used to a limited extent on chilling-sensitive commodities such as squashes, peppers, eggplant, and cherry tomato.

Hydrocooling is used before grading, sizing, and packing of beans, cantaloupe, sweet corn, and okra. Sorting of defective products is done both before and after cooling. Hydrocooling cycles are rarely long enough during hot weather. The need to maintain a continuous, adequate supply of cantaloupes to the packers often results in the melons being incompletely cooled. This can be remedied if, after packing and palletizing, enough time is allowed in the cold room to cool the product to recommended temperatures before loading for transport to markets.

Package icing and liquid-icing are used to a limited extent for cooling cantaloupe and routinely as a supplement to hydrocooling for sweet corn.

Temporary cold storage. In large-volume operations, most fruit vegetables are placed in cold storage rooms after cooling and before shipment. Cold rooms are less used in small farm operations; the products are often transported to central cooperatively owned or distributor-owned facilities for cooling and short-term storage.

Loading for transport. Some tomatoes, cantaloupe, and other muskmelons are shipped in refrigerated railcars, but most fruit vegetables are shipped in refrigerated trucks or container vans. Except for the major volume products such as cantaloupe and tomato, most are shipped in mixed loads, sometimes with ethylene-sensitive commodities. Among the immature fruit type vegetables, products such as cucumber, legumes, bitter melon, and eggplant are sensitive to ethylene exposure. Among the mature fruit types, watermelon is detrimentally affected by ethylene, resulting in softening of the whole fruit, flesh mealiness, and rind separation.

Special Treatments

Ripening

For uniform and controlled ripening, ethylene is often applied to mature green tomatoes and sometimes to honeydew, casaba, and Crenshaw melons. Ethylene treatments may be done at the shipping point or the destination, although final fruit quality is generally considered best if the treatment is applied at the shipping point soon after harvest. Satisfactory ripening occurs at 12.5° to 25°C (55° to 77°F), the higher the temperature, the faster the ripening (table 29.3). Above 30°C (86°F), red color development of tomato is inhibited. An ethylene concentration of about 100 ppm is commonly used. Honeydew melons (usually class 12 melons) are sometimes held in ethylene up to 24 hours; tomatoes are usually held at 20°C (68°F) and treated for up to 3 days.

Tomatoes may be ethylene-treated before or after packing, but most are treated after packing. An advantage of treating before packing is that the warmer conditions favor development of any decay-causing pathogens on the fruit, so infected fruit can be eliminated before final packout. Packing after ethylene treatment also permits a more uniform packout. Because most of the mature green tomatoes produced in California are packed and then treated with ethylene, "checkerboarding" may still occur and make a repack operation necessary.

Modified atmospheres

Modified atmospheres are seldom used commercially for these commodities, although shipments of melons and tomato under modified atmospheres are being tested for long-distance markets. Consumer packaging of vine-ripe tomatoes may also involve the use of modified atmospheres. For tomatoes held at recommended temperatures, oxygen levels of 3 to 5 percent slow ripening, with carbon dioxide levels held below 5 percent to avoid injury. Muskmelons have been less studied, but recommended atmospheres under normal storage conditions are 3 to 5 percent oxygen and 10 to 20 percent carbon dioxide.

Recommended storage/transit conditions

For mature fruit type vegetables temperature can effectively control the rate of ripening. Most mature-harvested fruit vegetables are sensitive to chilling injury when held below the recommended storage temperature. Chilling injury is cumulative, and its severity depends on the temperature and the duration of exposure. In the case of tomato, exposure to chilling temperatures below 10°C (50° F) results in lack of color development decreased flavor, and increased decay

The optimum temperatures for short-term storage and transport are:

Mature green tomatoes, pumpkin, and hard rind squashes: 12.5° to 15° C (55° to 60° F)

Partially to fully ripe tomatoes, muskmelons (except cantaloupe): 10° to 12.5° C (50° to 55° F).

Honeydew melons that are ripening naturally or have been induced with ethylene are best held at 5° to 7.5° C (41° to 45° F).

Watermelon: 7° to 10° C (45° to 50° F)

Cantaloupe: 2.5° to 5° C (36° to 41° F)

The optimum relative humidity range is 85 to 90 percent for tomato and muskmelons (except cantaloupe), 90 to 95 percent for cantaloupe, and 60 to 70 percent for pumpkin and hard rind squashes.

Immature fruit vegetables
All fruit vegetables harvested immature are sensitive to chilling injury. Exceptions are the peas and sweet corn, which are stored best at 0° C (32° F) and 95 percent RH.

The optimum product temperatures with RH at 90 to 95 percent for short-term storage and transport are as follows:

Eggplant, cucumber, soft rind squashes, okra: 10° to 12.5° C (50° to 55° F)

Peppers: 5° to 7° C (41° to 45° F)

Lima beans, snap beans: 5° to 8° C (41° to 46° F)

(Reprinted from Postharvest Technology, UC DANR Publication 3311)

 

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