Potato

MATURITY & QUALITY

Maturity Indices

Immature potatoes, generally harvested in spring or early summer, have a thin, poorly developed periderm, or skin. Irrigation and planting bed management, along with the option of vine-killing treatments, manage harvest “maturity”. Harvest preparedness generally begins once tubers have reached a desirable size for the variety or market. Immature potatoes are easily bruised and “skinning” leads to shriveling or decays. They are very perishable relative to late-crop potatoes and are only stored for short periods. Curing potatoes for 8 days at 15°C (59°F) and 95% RH will allow extended storage of up to 5 months at 4°C (39°F) and 95 to 98% RH, depending on variety. More typically, early-crop potatoes are harvested, cooled to 15°C (59°F), treated with a sprouting inhibitor, packed and shipped in a short period of time (i.e. 1 to 5 days).

Quality Indices

High quality traits, in commercial trade, include more than 70 to 80% of tubers well shaped, brightness of color (esp. reds, yellows, and whites), uniformity, firmness, freedom from adhering soil, freedom from bruising (black spot or shatter-bruising), scuffing or skinning, growth cracks, sprouting, insect damage, Rhizoctonia Black Scurf, decay, greening, or other defects. Commercial standards in use are typically higher than USDA grade standards. Differentiation of quality for potatoes is very complex.

U.S. Grades: Extra No. 1; No. 1; Commercial; No. 2 (Grade Standards established 1991) Potatoes may be sold as “Unclassified” to designate a lot, which has not been graded within the meaning of U.S. standards.

TEMPERATURE & CONTROLLED ATMOSPHERE (CA)

Optimum Storage Conditions

At optimum conditions, potatoes should have good quality after storage of 3 to 5 weeks. Storing immature potatoes below 10-13°C (50 – 55°F) for as few as 3 days may cause the accumulation of reducing sugars leading to excessive browning during frying/chipping. Storage for less than 3 weeks is recommended to maintain good visual and sensory quality of immature potatoes.

Rates of Respiration

*To calculate heat production, multiply ml CO₂/kg·hr by 440 to get BTU/ton/day or by 122 to get kcal/metric ton /day.

Note: Immature potato tubers, which are susceptible to bruising and skinning, can have high respiration rates. Cooler temperatures and/or increased air movement are effective methods to ameliorate.

Rates of Ethylene Production

Very low;<0.1 µl/kg·hr at 20°C (68°F)
Bruised, cut or otherwise damaged tubers have greatly increased ethylene production rates.

Responses to Ethylene

Potato tubers are not very sensitive to external ethylene. Low levels of external ethylene have been shown to elevate respiration, especially in immature potatoes, and result in weight loss and mild shriveling. After aging for 2-3 months at temperatures above 5 °C (41°F) and in the absence of sprouting inhibitor application, low levels of ethylene may retard sprouting. High concentrations of external ethylene may induce sprouting.

Responses to Controlled Atmosphere (CA)

Controlled or modified atmospheres offer little benefit to potato. Periderm develop Controlled ment and wound healing is delayed at atmospheres below 5% O₂. Injury from low O₂ Atmosphere (CA) (<1.5%) or elevated CO₂ (>10%) will induce off-odors, off-flavors, internal discoloration, and increased decay.