Potatoes (Solanum tuberosum) are one of the most widely consumed foods in the world. Though the French coined the term “apple of the earth” or pomme de terre, potatoes were brought to old-world countries by Spanish explorers.
The Incas discovered potatoes growing wild in the Andes Mountains and had been cultivating them for nearly two centuries for a variety of food and medicinal purposes.
Introduced to the Spanish in the 1500s, the explorers mistook potatoes for a kind of truffle, and took them back to Europe.
Acceptance, however, was slow; as part of the Solanacceae family (which includes some poisonous plants as well as tomatoes, peppers, and eggplant), a few well-publicized poisonings—from Europeans eating the harmful green parts of the potato plant (not the actual stem or potato)—resulted in widespread fear.
Today, potatoes are the U.S.’s top vegetable crop, with more than half being sold to processors for French fries and chips, dehydration, or used for other potato products. The remainder, often referred to as ‘table potatoes’ are sold fresh, used for animal feed, or set aside as seed tubers to grow new crops.
Types & Varieties
Potatoes come in a wide variety of shapes, colors, and sizes. Fresh market spuds include white, russet, red, gold, fingerling, purple or blue, mini, new, and yellow varieties.
Potatoes with white flesh can vary widely in skin color, and there are more exotic types too, sporting deeply colored skin and even blue or purple flesh.
White potatoes, often called Irish potatoes, are commonly used for chips, French fries, and baking. Red-skinned potatoes with white flesh are often used for mashed potatoes, while yellows, more common in Europe, are becoming more popular in the United States. Potato skins vary widely from rough and thick to thin and smooth. Shapes range from round to oblong to the narrower fingerling.
Potatoes require cooler weather and ideally a long, cool season to thrive. Generally planted in early spring, potatoes are not seeded, but are planted using portions of harvested potatoes that contain at least one or two buds or “eyes.”
Potatoes require loose, fertile soil high in organic matter and well drained. Soil temperature should be from 60 to 70°F. Growth stops at soil temperatures of 80°F or higher.
Planting is via hilling with ample space between rows. Seed potatoes are planted eye-side up approximately 2 to 3 inches deep. Two weeks before harvest, growers may apply a chemical to dry up vines, which also kills pathogens that might otherwise thrive in storage. Potato skins also mature during this step, so the crop is less likely to suffer bruising and injury.
Ideal harvest conditions are dry and cool, between 45 and 60°F. Drier soil decreases bruising from large soil clumps and minimizes the detritus accompanying the crop, which can impede air flow in storage and encourage disease. Temperature fluctuations can make potatoes more prone to injury, shrinkage, and water loss.
Some potato crops are harvested while immature, which increases susceptibility to bruising and can only be stored for short periods. However, even late-crop potatoes are susceptible to bruising, which can cause significant loss.
Bruises and nicks allow microorganisms to take hold and spread; harvest equipment should be well maintained to minimize drop height and rough handling.
Pests & Diseases
Pests of concern include potato leafhoppers, flea beetles, Colorado potato beetles, wireworms, and root knot nematodes.
Potatoes are especially vulnerable to bacterial diseases when injured. Diseases specifically caused by poor preharvest practices or postharvest handling include blackheart, black spot, chilling injury, greening, internal brown spot, and net necrosis.
Other diseases of concern during growing are blight, scab, fusarium wilt, bacterial soft rot, brown rot, and water rot. More serious diseases of immature potatoes are pink eye and grey mold.
Storage & Packaging
After harvest, while in bins or trucks, containers should be covered as exposure to sunlight can cause greening, and to keep potatoes from getting too hot. Crops are cured for 8 to 14 days at 50 to 60°F with 95% relative humidity. Depending on variety, potatoes can be stored for several months at 39°F with 95 to 98% relative humidity, but less than 3 weeks is considered optimal.
Immature potatoes are cooled to 59°F, treated with a sprouting inhibitor, and packed and shipped in 1 to 5 days. Storage also varies depending on intended use. Fresh market potatoes are ideally stored at 45°F and 98% relative humidity; frying potatoes between 50 to 59°F and 95% relative humidity; processing potatoes between 59 and 68°F and 95% relative humidity.
Stored potatoes should be cooled to the desired temperature gradually by decreasing temperatures about 5°F per week. Storing potatoes below 30°F results in chilling injury and rot.
Cool, humid ventilation is crucial for potatoes in bins versus air-cooled storage rooms. Poor ventilation results in shrinkage, sprouting, and decay. Too much airflow or too little humidity may cause weight loss. Potatoes are usually packed into perforated plastic bags to promote air circulation while retaining moisture; they can also be packed in ventilated cardboard boxes.
References: Purdue University Extension, UC Davis Postharvest Technology Center, University of Illinois Extension, University of Minnesota Extension, University of Wisconsin-Madison.
GRADES & GOOD ARRIVAL
Potatoes are divided into U.S. No. 1, U.S. Commercial, and U.S. No. 2 grades (though there are separate categories for processed and seed potatoes).
Generally speaking, the percentage of defects shown on a timely government inspection certificate should not exceed the percentage of allowable defects, provided: (1) transportation conditions were normal; (2) the USDA or CFIA inspection was timely; and (3) the entire lot was inspected.
|U.S. Grade Standards||Days Since Shipment||% of Defects Allowed||Optimum Transit Temp. (°F)|
|8-[5 int, 5 ext]-1||5
|14-[10 int, 10 ext]-3
13-[10 int, 10 ext]-3
12-[9 int, 9 ext]-3
11-[8 int, 8 ext]-2
10-[7 int, 10 ext]-2
Canadian good arrival guidelines (unless otherwise noted) are broken down into five parts as follows: maximum percentage of defects, maximum percentage of permanent defects, maximum percentage for any single permanent defect, maximum percentage for any single condition defect, and maximum for decay. Canadian destination guidelines are 15-15*-5-10-3 (the 15% permanent defects allows for no more than 3% hollow heart, 5% undersized, and 5% oversized potatoes).
References: DRC, PACA, USDA.
• At destination, not more than 5% of potatoes may have sprouts longer than a half-inch in length, or clusters materially affecting the appearance
• Silver scurf is scored as a defect, as damage, when more than 50% of the surface is affected
• Internal black spot, normally found affecting the shoulders and stem end, is scored as a defect when affecting more than 5% of the total weight of the potato
• Size ‘A’ potatoes must meet a minimum diameter of 1.875 inches and at least 40% must be 2.5 inches in diameter or 6 ounces in weight or larger.
Source: Tom Yawman, International Produce Training, www.ipt.us.com.