Commercial postharvest handling of fresh market apples
Bridgette Matzinger and Cindy Tong
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Apples are a popular and nutritious horticultural product. Consumers demand a high-quality product that is free from bruises, cuts, punctures, physiological disorders, and pathogens. Apples are 84% water and are climacteric, meaning that they produce an increased amount of carbon dioxide as they ripen. During ripening, apples can change color, soften, and become sweeter and less astringent.
The generally accepted commercial practice is to pick fruit before the onset of the respiratory climacteric. Unless your market is near your orchard and can absorb all of your fresh product, you will want to harvest your apples before they begin to ripen. It is important to know the appropriate harvest dates for your apple varieties. Apples picked too early are susceptible to shrivel, scald, and bitter pit. They also may not ripen appropriately after harvest. Apples picked too late may begin the respiratory rise, which will decrease their shelf life and lead to disorders such as flesh browning and breakdown.
Commonly used harvest indexes are based on days from bloom, external and internal fruit color, flesh firmness, ease of separation from spurs, and starch, sugar, or acid content. No one index is a completely reliable measure of harvest readiness, but days from full bloom gives the most reliable guide.
Hand-pick fruit into bags, transfer gently into field bins, shade fruit in bins, then transport to packing sheds. At the shed, submerge the fruit in water dumps, wash, and sort into fresh-market, processing, and cull fruit. In general, small to medium sized apples keep the longest, while the most mature have the shortest shelf life and should be removed from storage first. Cool fruit as rapidly as possible following harvest, using forced air or hydrocooling.
Packaging keeps the product in convenient units for handling and protects it during marketing and storage. It should be easy to handle, protect the fruit from mechanical damage and temperature extremes, allow for rapid cooling, and allow for standardization. Apples for roadside stands will need minimal packaging. Apples that will be stored or shipped can be packed into plastic bags or corrugated cardboard boxes (either volume-filled or with individually wrapped fruit in trays).
Temperature and Humidity. It is important to cool your apples as quickly as possible after harvest and keep them cool regardless of the time in storage. Optimal temperatures for long term storage are 30 to 32 degrees F. Keep in mind that apples respire and soften twice as fast at 40 degrees F than at 32 degrees F. Storage rooms can be either air-cooled or mechanically refrigerated. In air-cooled storage, a well-insulated building is cooled by night air and kept closed during the day. If you use this system, you must ensure that the night temperatures are low enough to keep your storage room cool without drying out the room. Mechanical refrigeration will give you more control of the storage atmosphere. You must provide adequate air circulation (28 meters of air per minute per short ton of refrigeration capacity) to keep disease to a minimum while maintaining high humidity (95% relative humidity at 32 degrees F). You may need to use supplemental humidification, such as fog spray nozzles.
Atmosphere. Addition or removal of gases resulting in atmospheric composition different from normal air is being used in controlled atmosphere (CA) or hypobaric (low-pressure) storage to extend the shelf life of apples. It is an adjunct to low temperature storage, not a substitute. CA reduces metabolic activity of the fruit, and reduces the fruit's rate of ethylene production as well as sensitivity to ethylene, which can hasten fruit softening and color change. Ripening fruit can give off ethylene, which will hasten the ripening of surrounding fruit.
Typical atmospheric composition of a CA facility are 7 to 25% carbon dioxide and 2 to 4% oxygen. The optimum composition varies with temperature and apple variety, but oxygen does not normally go below 2%, or anaerobic respiration can occur, leading to off flavors and internal browning. In hypobaric storage, an atmospheric pressure of 102 mm mercury (0.16 atmosphere) is often used.
You should protect your fruit from mechanical damage and extreme temperatures during transport. Pack fruit carefully, use proper refrigeration (32 to 34 degrees F) and relative humidity (95%), and insulation. In mixed loads, apples can be shipped with berries, cherries, pears, plums, and quince.
Mechanical and physiological disorders
Bruising is the most common defect of apples. The symptoms include flattened areas with brown flesh underneath. To avoid bruising, carefully evaluate every step in your harvesting, packing, and handling operation. Pad areas of high impact and decrease drop heights. Use water dumps. Eliminate sharp corners. Immobilize the fruit during transport.
Bitter pit is a serious disorder in apples, and although it develops during fruit growth, it can be enhanced during storage by delayed cooling and high storage temperatures. Bitter pit looks like small brown spots in the flesh, usually near the surface and around the calyx end of the apple. Warm weather and moisture stress during fruit maturity, harvesting too early, heavy pruning, excessive nitrogen fertilizer application, and low fruit calcium can all contribute to the development of bitter pit. Well-timed irrigation, calcium nitrate foliar sprays (3 or 4 sprays at 1 or 2 week intervals before harvest), controlled atmosphere storage, and postharvest waxing can all help reduce the extent of bitter pit.
Superficial scald, a browning of the skin and flesh, is an apple defect that can develop when apples are held too long in cold storage. Skin browning begins at the calyx end of the fruit and is most severe on late harvested fruit.
Other disorders that are uncommon under proper storage conditions include:
|Sunburn scald||brown to black color on areas damaged by sunlight|
|Senescent breakdown||brown, mealy flesh, occurring in overmature, overstored fruit|
|Low temperature breakdown||cortex browning|
|Soft or deep scald||soft, sunken, brown to black, sharply defined areas on the surface and flesh|
|Jonathan spot||superficial spotting of lenticels, occurring at high temperatures|
|Senescent blotch||gray, superficial blotches on overstored fruit|
|Core flash||browning within coreline|
|Water core||brown, translucent areas in flesh|
|Brown heart||sharply defined brown areas in flesh, cavities|
Postharvest diseases due to fungi, bacteria, and viruses are often due to mechanical or insect damage, followed by the invasion of infecting organisms. The most common diseases of apples are blue mold rot, caused by Penicillium expansum, and gray mold rot, caused by Botrytis cinerea. Blue mold rot infects fruit after harvest, and is more common where apples are moved in water at the packing shed. To control this fungus, use benzimidazole fungicides (benomyl, thiabendazole, thiophanate methyl, methyl benzimidazole carbamate) that are labeled for postharvest use. The residue tolerance for these chemicals is 0 to 10 ppm. Gray mold rot actually infects fruit in the orchard at petal fall, but the fungus does not grow until the fruit begins to mature. The fungus can grow at temperatures as low as 36 degrees F, and can infect surrounding fruit during storage.
For more information
For information covering related areas, consult the following University of Minnesota Extension publications. They are available from your county extension office.
Commercial Postharvest Handling of Strawberries (Fragaria spp.)
Commercial Postharvest Handling of Potatoes (Solanum tuberosum)
Nutrient Management for Commercial Fruit and Vegetable Crops in Minnesota
Bridgette Matzinger and Cindy Tong