The difference between Peruvian (“white”) potatoes and sweet potatoes
Peruvian potatoes (sometimes mistakenly called Irish potatoes) are stem tubers in the nightshade family; sweet potatoes are root tubers in the Morning Glory family. Stem tubers have buds, nodes and internodes, and scaly leaves, and the ability to develop chlorophyll when exposed to light. Root tubers do not have these attributes. This article is only about sweet potatoes.
The difference between curing and storage
Some vegetables need to cure before storage and the curing conditions are different from those needed for storage. Curing allows skins to harden and some of the starches to convert to sugars. These changes help the tubers to store for months.
Curing sweet potatoes
Within an hour or two after harvest, field drying, sorting and crating, take the boxes of sweet potatoes to a warm, damp indoor space to cure. Curing allows the skin to thicken, cuts to heal, and some of the starches to convert to sugars. Uncured sweet potatoes are not very sweet, will not bake well, and are best used in dishes with other foods.
In addition to promoting the healing of wounds acquired during harvesting and handling, the curing conditions are necessary for development of a protective cork layer over the whole root. And a waxy material (suberin) is produced by the root’s outer cells and covers the skin. This layer acts as a barrier to disease organisms, and prevents excess moisture loss.
Curing involves optimizing three conditions: temperature, relative humidity and ventilation. Ideal curing conditions are 85°F–90°F (29°C–32°C), and 80–95% humidity for 4–7 days. Curing takes longer (as much as 3 weeks) if conditions are less than perfect. Dry air does not lead to good curing. If the air is below 66% humidity, timely good healing will not take place, and the sweet potatoes will not store well unless more time is allowed. The loss from decay in sweet potatoes cured at 50% is twice that of those cured at 82%. (Storage of sweetpotatoes, Jacob Martin Lutz, USDA, 1958)
In the past we used our greenhouse to cure sweet potatoes, but it really is too hot and sunny, and dry. Nowadays we use a heated basement. We stack our 4” (10cm) deep boxes of roots on pallets, with wood spacer bars between boxes in each stack, to ensure air flow. We use box fans to improve the airflow, and the basement already has some natural ventilation. We reckon on 10–14 days.
We get quite good temperatures, but keeping humidity up is difficult for us. We cover the flats with newspaper to hold in some moisture. Some people use perforated plastic. We have also used domestic humidifiers and we’ve tried hanging strips of wet cloth from the ceiling. The best result seems to come from splashing water on the concrete floor several times each day.
To test if curing is complete, rub two sweet potatoes together. If the skins scratch, they need to cure longer. Curing longer than needed leads to sprouting.
Sweet potato storage
Sweet potatoes can be stored in the same room they are cured in, but it is important to cool the room evenly and fairly rapidly from the curing temperature of 85°F–90°F (29°C–32°C), to the storage temp of 55–60°F (13°C–16°C) in 10 days or fewer.
Above 60°F (16°C), shrinking, pithiness, and internal cork (a symptom of a viral disease) when the virus is present may occur, and below 55°F (13°C), a permanent chilling injury (Hard Core) can happen. The potatoes remain hard no matter how long you cook them, and are useless. Do not ever let the temperature drop below 50°F (10°C). Ideal storage conditions for sweet potatoes include 60–70% humidity, up to 85 %, with one air change each day. If the heat circulation is uneven, hot spots can develop in front of the heaters and cause severe losses. Never let hot air blow directly on the sweet potatoes. Do not store in airtight containers, sweet potatoes need one complete air change per day.
Ken Allan, in Sweet Potatoes for the Home Garden, informs us that at about 60°F (16°C), the metabolism of the sweet potato slows to near zero, meaning it won’t grow. Temperatures above 70°F (21°C) are conditions that allow growth: although slow at 70°F (21°C), the rate increases to fast at 100°F (38°C).
We use a rodent-proof “cage” in our basement. We stack the boxes directly on top of each other and this seems to keep enough moisture in. This way, assuming we had a good enough harvest, we can still have sweet potatoes into May and early June. Shrinkage occurs at 1–2 % per month if cured, 2–5 % if uncured. In some cultivars, pithiness also increases with length of storage.
Sweet potatoes do not need to be in the dark. Dormancy is generally broken by moisture and warmth, not daylight. Green sweet potato sprouts are edible, not toxic, as white potato sprouts are.
The effects of ethylene
Ethylene is a naturally occurring, odorless, colorless gas produced by many fruits and vegetables, but it can also be produced by faulty heating units and combustion engines. Propane heaters should not be used, as propane combustion produces ethylene. Incomplete combustion of organic fuels can result in the production of carbon monoxide, ethylene and other byproducts. Do not use any unvented hydrocarbon fuel heaters near stored produce.
Ethylene is associated with ripening, sprouting and rotting. Some crops produce ethylene in storage—apples, cantaloupes, ripening tomatoes all produce higher than average amounts. Chilling, wounding and pathogen attack can all induce ethylene formation in damaged crops.
Some crops, including most cut greens, are not sensitive to ethylene and can be stored in the same space as ethylene-producing crops. Other crops are very sensitive and will deteriorate in a high-ethylene environment. Potatoes will sprout, ripe fruits will go over the top, carrots lose their sweetness and become bitter. Sweet potatoes are sensitive to ethylene and should not be stored with any crops or any heating systems that produce ethylene. Symptoms are difficult to diagnose, but ethylene can cause internal darkening and pithy areas, as well as sprouting.
Accidental sprouting of sweet potatoes
If your curing or storage conditions were not right, you may get early sprouting. If this happens, snap off the sprouts and use the sweet potatoes as soon as possible. If the sweet potato also has soft and wrinkly flesh, it’s an indication that it has lost nutrients. Left longer, spouted sweet potatoes become mushy and turn brown or black.
Intentional sprouting of sweet potatoes
Sweet potatoes that are intended for sprouting are kept under normal storage conditions, then conditioned for 2 weeks (or even 4), before you start to grow slips. Start 10–12 weeks before your planting date, conditioning at 75°F–85°F (24°C–29°C), 95% humidity for 2–4 weeks, then set to sprout. Set up a place with light, humidity and ventilation at 75°F–85°F (24°C–29°C) and 12″ (30 cm) of headroom.
Summary: Sweet potatoes are more likely to sprout if they were cured for too long; curing conditions were too far from ideal; temperatures stayed too high—above 60°F (16°C)—once the sweet potatoes were cured (especially at high humidity); they were exposed to low temperatures followed by higher ones; they were physically damaged or stored near ethylene sources.