Sustainable Practices for Vegetable Production in the South
Dr. Mary Peet, NCSU
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Potato

HARVEST AND POST-HARVEST

Timing of harvest is determined by whether or not the potatoes will be stored. In the southern states, potatoes for processing are harvested at the size required for market and immediately made into chips, whether or not they are physiologically mature. In the primary U.S. production areas, however, much of the crop is stored. To improve storage life, tubers are left in the field until they reach physiological maturity. At this time, usually about 90 to 100 days from planting, the skin or periderm thickens and 'sets', improving storage life. Dry matter content peaks at about this time, improving both storage life and processing quality.

Physiologically mature tubers have high starch and protein levels and low respiration, water content and sugar levels.

Vines are sometimes killed before harvest in the fall-harvest of 'late' states. While vines will eventually die back naturally, vines are often killed by rotobeating, herbicide application, or or burning 2 to 3 weeks before harvest to increase skin set, control late blight, facilitate mechanical harvest and stop tuber growth. Applying herbicides to kill vines when the soil is dry and the weather is not darkens the vascular ring of the tubers. This discoloration is reduced by shredding the vines before applying the herbicide, reducing the herbicide application rate, using a slower acting material, and irrigating the field. In the spring-harvested southern crop because of the potential for heat damage, tubers are rarely left in the field with the vines cut off. Even when vines are left on, high soil temperatures can darken the inside of the tuber, especially if soil moisture is also high.

Harvesting when the soil is wet and cold (under 45 degrees F) usually results in increased incidence of cracks, bruises, and cuts. Soil temperatures below 45 degrees F for several weeks or below 40 degrees F for a few days also increase reducing sugars, which results in an undesirable darkening during processing.

The harvester should be run at the proper speed and its speed coordinated with the ground speed. Potatoes should drop a maximum of 6 inches without padding. Bruising will result in black spot formation in storage. Harvested potatoes should be kept out of the heat and sun.

Storage and Curing

Tubers are sometimes cured at 60 degrees F and 95 percent relative humidity for two weeks to suberize the outer layer of potato skin and thicken the periderm. This reduces infection and water loss. In processing potatoes, venting must be adequate to avoid a CO2 buildup which would increase reducing sugars.

The temperature should be slowly reduced from the curing temperature to the storage temperature. Storage temperatures depend on final use: table and seed stock should be stored at 38 to 40 degrees F; french fries at 45 degrees F; and chipping potatoes at 50 to 60 degrees F. Storage temperature is particularly critical in tubers used for processing. High temperatures lead to breakdown of tubers, sprouting, and water loss. Lower temperatures lead to reducing sugar (glucose and fructose) accumulation. When heated, these sugars combine with proteins to produce a 'browning reaction' which results in dark-colored fries or chips. Processors may reject loads with reducing sugar content above 2 percent. Lowest levels of reducing sugars occur in well-matured tubers, tubers slowly cooled to storage temperatures; high dry matter tubers and tubers from plants not stressed or exposed to high temperatures in the field.

A natural internal dormancy keeps tubers from sprouting for the first 2 to 3 months of storage. Beyond this period, the hormone CIPC (chlorpro-phom) can be added through the ventilation system to prevent sprouting. Potatoes should be stored in the dark because light will stimulate solanine formation. Before removing from storage, tubers stored below 50 degrees F should be warmed to 50 to 60 degrees F to prevent bruising during shipping and handling.