Sweet potatoes (Ipomoea batatas) are a tropical crop, making them extremely susceptible to cold temperatures at every stage of their life cycle. Understanding cold tolerance involves two concerns: how the living plant tolerates low temperatures in the field, and how the harvested storage root reacts to cooling during transport and storage.
Cold Tolerance in the Growing Stage
The sweet potato plant is highly sensitive to cold during the growing season. The vines and foliage are quickly damaged by even a light frost (around 32°F). When the leaves are frosted, decay can move down the stems into the underground storage roots, causing severe quality loss even if the roots have not yet frozen.
Soil temperature is critical for root development. Growth is severely stunted when soil temperatures fall below 60°F, and the plant will stop producing storage roots altogether. Prolonged exposure to soil temperatures below 50°F causes chilling injury to developing roots, leading to poor quality after harvest.
For a successful harvest, it is important to dig the sweet potatoes before the first hard frost kills the vines and significantly cools the soil. If a light frost does occur, the roots must be harvested immediately to prevent decay from spreading from the dead foliage.
Chilling Injury Thresholds for Stored Roots
The harvested sweet potato root is highly vulnerable to chilling injury, which occurs at temperatures well above freezing. Irreversible damage begins when the roots are exposed to temperatures below 55°F (13°C). This threshold is the most important temperature to monitor after harvest, as the injury is cumulative and can be caused by exposure to cold for even a short period.
Visible symptoms of chilling injury severely impact marketability and eating quality. External signs include pitting and sunken lesions on the root surface, along with accelerated weight loss and shriveling. Internally, the flesh may show darkening or browning of the tissue, often resulting from biochemical changes triggered by cold stress.
One significant result of chilling injury is a physiological disorder known as “hardcore,” where sections of the cooked root remain hard and inedible after preparation. Hardcore can develop after only three days of storage at 50°F, or in as little as a single day if the roots are exposed to 35°F. Chilling also significantly increases the roots’ susceptibility to post-harvest decay and fungal infections, which further shortens their storage life.
Curing and Optimal Long-Term Storage Conditions
The primary defense against cold damage is the post-harvest process known as curing. Curing involves holding the newly harvested roots in a high-heat, high-humidity environment for a short duration. The recommended parameters are a temperature of 80–85°F (27–29°C) and a relative humidity of 85–95% for four to seven days.
This controlled environment promotes the rapid healing of wounds and abrasions that occurred during harvest. The high heat stimulates the formation of a protective layer of cells, known as wound cork, which effectively thickens the skin. This process prevents moisture loss and creates a physical barrier against decay-causing organisms and subsequent cold stress.
After curing is complete, the roots must be moved to long-term storage conditions that balance quality preservation and chilling injury risk. The optimal temperature range for cured sweet potatoes is 55–60°F (13–16°C), maintained with a relative humidity of 85–90%. Storing the roots in this narrow range prevents chilling injury while also slowing the metabolic processes that lead to sprouting.
Standard household refrigeration, which maintains temperatures around 40°F or lower, is detrimental to sweet potatoes. Exposure to these cold temperatures quickly induces chilling injury, leading to the development of off-flavors and the breakdown of the root tissue.