Commercial Postharvest Handling of Potatoes (Solanum tuberosum)
The potato tuber is a shortened, enlarged, underground stem. Eighty percent of the potato tuber is water, with the remainder being carbohydrate, protein, ash, and fat. Potatoes are living organisms that respire, using oxygen and giving off carbon dioxide, moisture, and heat.
Three key conditions should be met in order to minimize harvest injury: 1) apply chemical vine killer two weeks before harvest, 2) harvest when soil conditions are dry, and 3) avoid harvesting in the late fall, when soil and air temperatures are below 45 degrees F. Killing vines before harvest allows them to dry thoroughly and allows time for pathogens to die, reducing the chances of transporting them into storage. It also allows for tuber skin maturation, reducing skinning and bruising. Harvesting when soils are dry decreases bruising due to soil clods and transport of soil into storage where it can block air circulation through the potato pile. Harvesting at temperatures below 45 degrees F will injure potatoes more than at higher temperatures. Avoid harvesting when temperatures are above 60 degrees F to minimize water loss and shrinkage.
One of the biggest problems to be controlled during harvest and handling is bruising. When bruising occurs in the early stages of postharvest handling, the potatoes get nicked and become breeding grounds for microorganisms, allowing them to spread rapidly and cause serious losses. To prevent bruising, inspect and adjust harvesting equipment frequently. Reduce drop heights, bouncing, and rolling by padding impact areas and adjust contact surfaces.
Tubers are usually conveyed from the harvesters into dump trucks or bins. When transporting tubers from the field to the packing shed, cover the load with a tarp to reduce exposure to high temperatures and sunlight. Exposure to light can cause greening.
If harvesting by hand, be careful not to nick potatoes with your digger. Carefully transfer tubers into bins, shade bins, then transport to your packing shed.
After harvest, potatoes are dormant for 6 to 12 weeks, depending on variety and storage temperatures. After the dormancy period, potatoes may begin to sprout after 2 to 3 months in storage. High storage temperatures will induce earlier sprouting. To avoid sprouting, you can apply a sprout inhibitor, such as chloroisopropyl-N phenylcarbamate (CIPC) or maleic hydrazide. They should not be used on seed potatoes. CIPC is applied as a gas after curing is completed. Maleic hydrazide is applied in the field during late full bloom to postbloom, and needs to be transported from the leaves to the tubers to be effective.
Before storage, potatoes should be culled and cured. Cull and discard any damaged, diseased or frozen tubers. Curing potatoes heals the skin, making it less susceptible to damage and disease. Cure potatoes by exposing them to temperatures between 50 and 60 degrees F and 95% relative humidity for 10 to 14 days.
Potatoes are either stored in refrigerated warehouses or nonrefrigerated bulk bins up to 20 feet deep. In the bulk bins, air should be forced from the floor through corrugated metal ducts up through the pile. This ensures good distribution of cool, humid air, which decreases shrinkage, sprouting, and decay. For table stock, ventilate at 0.6 to 0.7 cubic meters per minute per ton. For chipping stock, use 0.8 to 1 cubic meter per minute per ton. If airflow is too high, the relative humidity surrounding the potatoes may drop, causing weight loss. Air-cooled storage rooms may also be used, but you must ensure that night temperatures are low enough to keep your storage room cool and high enough to prevent freezing.
Hold table potatoes at 38 to 40 degrees F, decreasing field temperature 5 degrees per week to the desired storage temperature. Store processing potatoes at 50 to 55 degrees F, although Russet Burbank for processing can be stored at 45 degrees F. Cool processing potatoes to the final storage temperature at a rate of 3 to 4 degrees per week. Processing potatoes stored below 40 degrees F will build up sugars that will cause the flesh to turn brown or black when fried. Once the desired holding temperature is reached, keep the temperature differential about 2 degrees F between the top and bottom of the pile. Do not allow potatoes to remain at temperatures below 30 degrees F, or freezing injury will occur, leading to rot. For all types of potatoes, storage humidity should be 95%, but avoid moisture condensation on tubers and storage walls and ceilings. When diseases such as late blight and Pythium leak are severe, maintain lower humidity during storage and ensure good air circulation.
When ready for market, potatoes can be packed into perforated plastic bags that will help retain moisture but provide for air circulation and proper cooling during transport. Potatoes can also be packed into cardboard boxes with ventilation holes.
Mechanical and Physiological Disorders
Besides sprouting, potato disorders include:
|Greening||surface turns green with light treatment||minimize exposure to light|
|Black heart||sharply defined, purplish-grey to black area in center or cavities due to oxygen starvation||provide good air circulation to prevent heating and oxygen deprivation; avoid chilling injury|
|Chilling injury||gray to red-brown areas or black heart||store tubers above 37 degrees F|
|Freezing injury||vascular tissue turns black and tubers leak when thawed||store tubers above 37 degrees F|
|Blackspot||internal black spots due to bruising; can cause shatter in some potatoes||minimize bruising; warm to 60 degrees F before grading|
Postharvest diseases include the following:
|Dry rot||Fusarium spp.||brown, firm, sunken flesh; sunken and wrinkled surfaces with blue or white protuberances|
|Soft rot||Erwinia carotovora||soft, water cavities in flesh, foul smell; in non-russeted varieties, shallow, round lesions around lenticels|
|Leak||Pythium||oozing tubers; well defined areas between healthy and diseased flesh; pink then black flesh with granular, mushy rot|
|Late blight||Phytophthora infestans||small, shrunken, dark spots in flesh; foul smell|
|Ring rot||Cornybacterium sepedonicum||vascular ring yellow|
For More Information
For information covering related areas, consult the following University of Minnesota Extension publications. They are available from your county extension office.