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Postharvest Handling of Baby Corn

Postharvest handling operations for baby corn (Zea mays L.) are relatively simple, but require precooling and cold-storage facilities to prevent quality losses after harvest. Harvesting at optimum maturity, minimizing mechanical damage during handling, and controlling temperature until the product reaches the market can extend the storage and shelf life to 7 to 10 days. Inadequate or slow cooling, poor temperature maintenance, or inappropriate packaging can rapidly lead to postharvest problems and quality loss in baby corn.

Quality Criteria

First-quality baby corn should be uniform in maturity, color, shape, size, and kernel coverage and be free from defects. Baby corn ears showing signs of disease or damage from insects, mechanical injury, or water loss should be discarded. Cobs that do not have sufficient kernel coverage should also be rejected. Different classes are normally based on the length and diameter of the corn.

Varieties

Although any variety of corn can be used, some varieties produce higher-quality baby corn than others, and some have been developed specifically for baby corn production. Principal varieties are Supersweet Corn (generic), Golden Baby (H-102), Sundance, Summer Sweet, and‹from Thailand‹Suwan 2, Suwan 3, and Rangsit 1. Early-maturity field varieties of baby corn are preferred because they reach full flowering sooner than full-season field varieties.

Harvest Maturity

Young ears for baby corn are harvested at 47 days, compared with 100 days for grain. The tassel of the male flower should be removed 45 days after planting or before pollen shedding, depending on the variety of corn. This stimulates fast baby corn development and improved weight.

Ears are ready for harvesting three to five days after detasseling, when the top ears have a maximum inner cob size of 9 centimeters in length and 1.0 to 1.5 centimeters in width, and 2.5 centimeters of long silk emerge from the tips. (This size is also associated with firming of the cob). The lower ears are ready for harvesting within three to five days of the top ears. Normally, two to three ears should be obtained per plant.

Baby ears that are destined for the fresh market are harvested as soon as the silks emerge, or up to a maximum of two days afterward; silks that have been exposed for four days or more are used for processing, not for fresh export. If the silks are pollinated, the quality and the price of the corn are normally lower.

The harvest index varies depending on ambient conditions, the nutritional status of the plant, planting distance, soil humidity, and other factors.

Harvesting

When possible, harvesting should be carried out in the morning, when the corn¹s moisture content is highest and product and ambient temperatures are low. (Morning harvesting also saves on energy costs and time needed for precooling.) Harvesting is conducted every other day and normally lasts 8 to 14 days, except in the case of Golden Baby harvesting, which lasts 8 to 10 days. Ears are harvested using a knife or secateurs to cut the stem. Ears should not be snapped or broken because either action puts pressure on the cob and can cause mechanical damage. Ten kilograms of harvested baby corn typically yield 1.5 to 1.8 kilograms of cob without husk, or about 15 to 20 percent of the harvested weight.

Precooling must be carried out within one to two hours of harvest. If ears are transported from the field in bulk, the temperature of the uncooled ears will increase as a result of the heat that is produced from the respiration of the corn.

If harvested corn is to be left in field crates or the transportation carrier for more than an hour, either it should be kept in the shade and prevented from heating up, or the cooling process should be started with the application of a constant flow of clean river or well water. Water must be distributed evenly over the load of corn at a rate of 2 liters per kilogram of corn per hour.

Packhouse Operations

The taste and quality of baby corn are judged mainly by the corn¹s sugar content, which decreases rapidly after harvest if the corn is maintained at ambient temperatures. The speed and efficiency of packhouse operations are therefore essential to good postharvest handling practices, which include proper precooling time and rate and maintenance of the cold chain until the product reaches the consumer.

Another consequence of improper temperature maintenance is moisture loss, which causes denting of the kernels. Water loss of only 2 percent can cause visible reduction in corn quality. To prevent transpiration and respiration, the corn must be cooled rapidly.

Baby corn should be trimmed uniformly to remove flag leaves and long shanks; if these are left on the corn, the ears will be difficult to pack and water loss will occur as moisture moves from the kernels to the cob and out the leaves. When the outside leaves of uncooled baby corn begin to wilt and turn pale green, overheating and water loss have already begun to damage the corn.

Precooling

The best method of precooling baby corn is hydrocooling, which can be done to ears with or without their husks. With corn that still has its husk, cooling takes slightly longer than with dehusked ears, because the leaves tend to act as insulation and slow down the cooling process. With dehusked ears, however, more care must be taken to prevent mechanical damage to the exposed kernels.

Field temperatures must be rapidly reduced to a range of 0 ° C to 1 ° C. As noted above, precooling must be performed within one to two hours of harvest. For every hour of delay in cooling after harvest, highly perishable produce such as corn loses 10 to 12 hours of shelf life.

Hydrocooling by drenching or immersion of the corn in near-freezing water is the most effective method of precooling; cooling in this manner normally takes 20 to 30 minutes. Top ice cooling may also be used. With this method, ice is used to remove field and respiratory heat, plus other heat emitted in the vehicle or store. Normally half a kilogram of ice is used for every 2.25 kilograms of baby corn. With these cooling methods, the baby corn is protected from water loss and in some cases may even absorb water. Conversely, forced-air cooling is not recommended for baby corn because of the slower cooling time it entails and the high possibility of rapid water loss; it is, however, better than no cooling at all.

Hydrocoolers incorporating a refrigeration system or crushed ice are suitable for precooling; crushed ice is normally used in small-volume operations. Several types of hydrocooling methods are suitable for baby corn, including the conveyor-shower system, the batch-shower system, and the conveyor-immersion system. The choice of which system to use depends partly on the type of packaging the corn has been placed in and its compatibility with hydrocooling. (In some cases, the product may be packed after cooling.) Carton ventilation and stacking affect the flow of water inside and over the product and, therefore, the rate of cooling. Baby corn packed in open plastic field crates cools rapidly because the arrangement allows water to move easily down through the product.

The conveyor-shower hydrocooling process passes the baby corn along a conveyor and under a shower of chilled water; the corn can be in field crates or in waxed cartons. The conveyor moves 1 foot per minute, with the water running through the crates and over the product. Uniform stacking of the crates and positioning of the corn are necessary to prevent water channeling and nonuniform cooling. The length of the conveyor and the volume and temperature of the water are determining factors in the degree of cooling. Large quantities of water‹800 liters per minute per square meter are recommended ‹are required.

Batch coolers cool by the pallet load with the product remaining station ary; cold water is poured over the top of the pallet load, collected at the bottom, recooled, and recycled. Volume capacity is normally lower than with the conveyor systems. Care must be taken to ensure uniform water movement and cooling.

Conveyor-immersion hydrocoolers are normally large shallow boxes that contain chilled moving water. Crates of corn are loaded onto the conveyor at its entrance, submerged, moved through the chilled water, and removed at the other end. Critical factors are water temperature, water movement versus product movement, conveyor speed, and container ventilation and design. This system is normally more efficient than the two hydro-cooling methods described above, because the cold water comes in contact with the entire product and is not susceptible to channeling or nonuniform cooling. With this method, the water must be agitated and moved against the flow of the product to obtain maximum heat transfer. If the water is cooled using ice rather than refrigeration, 135 kilograms of ice are needed initially to cool 400 liters of water. Ice is added as needed afterward to maintain water temperature.

When hydrocooling operations are used, postharvest diseases may develop, particularly in products that have sustained mechanical damage. With recirculating systems, it is easy to spread disease from a few ears of corn to all of the cooled product. Disease development can be reduced by using chlorine as a disinfectant in the water at rates of 100 parts per million to 150 parts per million. Chlorine can be added in the form of sodium hypochlorite or dry as calcium hypochlorite. The water¹s temperature, pH, and cleanliness all affect the efficiency of the chlorine treatment and should be monitored.

It should be remembered that chlorine treatments are surface treatments and kill disease-causing organisms in the vegetative form but not necessarily in the spore form. Spores may remain on the surface of the corn and develop later, if the corn is removed from the cold chain.

Grading and Packing

Baby corn is graded by size: large cobs should be 6 to 9 centimeters long and 1.5 centimeters wide; small cobs, 6 centimeters long and 1.0 centimeter wide. Size requirements vary according to the market, but the following can be used as a guide:

• 6 to 8 centimeters;
  
  
• 8 to 10 centimeters; and
  
  
• 10 to 12 centimeters.

Packing is normally carried out after cooling. Most baby corn is packed dehusked in perforated plastic clamshells or punnets for retail markets. Polystyrene or plastic trays wrapped with film can also be used. These trays may weigh 125 grams, 150 grams, 170 grams, 200 grams, or 250 grams. Clamshells, punnets, or trays are typically packed 12 to a carton. Some wholesale markets use 5- to 10-pound waxed cartons containing corn with husks intact.

Packaging

Packaging consists of either one- or two-piece self-locking telescopic cartons with a minimum breaking force of 175 pounds per square inch. Ventilation is required in all four side walls. Carton dimensions vary depending on net weight and market; the most common packs weigh 2 to 2.5 kilograms and have the following external dimensions, which are compatible with ISO pallets: 40 centimeters long by 30 centimeters wide by 9 centimeters high. If clamshells are used, compatible flats are required; these are normal°ly of standard size and fit ISO pallets.

Packages should bear all the labeling information required by the importing country; clamshells may also be labeled at their point of origin. Carton labeling may include the following:

• Country of origin;
  
  
• Product/variety;
  
  
• Net weight; and
  
  
• Exporter name/importer name.