Commercial Postharvest Handling of Potatoes
(Solanum tuberosum)
James P. Yanta and Cindy Tong
Copyright ©
2009 Regents of the University of
Minnesota. All rights reserved.
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.
Harvesting
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.
Storage
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.
Packaging
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:Disorder | Symptoms | Control |
---|
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 |
Diseases
Postharvest diseases
include the following:Disease | Causal Agent | Symptoms |
---|
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 Minnesota
Extension Service publications. They are available from your county extension
office or by writing to the University of Minnesota Extension Store, 20 Coffey Hall, 1420 Eckles
Avenue, University of Minnesota, St. Paul, Minnesota 55108-6064.
FS-6236 Commercial Postharvest Handling of Fresh Market Asparagus (Asparagus officinalis)
FS-6237 Commercial Postharvest Handling of Strawberries (Fragaria spp.)
FS-6238 Commercial Postharvest Handling of Fresh Market Apples (Malus sp.)
BU-1880 Commercial Vegetable Pest Management Production Guide1996
BU-5886 Nutrient Management for Commercial Fruit and Vegetable Crops in Minnesota
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