The potato is botanically a tuber, a swollen part of the underground stem of the plant, Solanum tuberosum. It functions as a food storage unit, packed with starches to fuel the next generation. Growth emerging from its surface indicates the potato is a living, biologically active organism attempting propagation. This new growth is triggered by environmental cues, meaning the potato has broken dormancy and is preparing to sprout a new plant.
Identifying Shoots and Roots
The most common growth emerging from a potato’s surface are shoots, often called sprouts, which appear from small indentations known as “eyes.” These shoots are typically pale white or yellowish when emerging in the dark, but turn green if exposed to light. Less frequently, thin, fibrous white threads growing from the eyes or the base of the tuber are adventitious roots. Both sprouts and roots indicate the potato is consuming its starch reserves to fuel this new growth.
The primary safety concern with sprouts is the concentration of natural toxins called glycoalkaloids, primarily solanine and chaconine. These toxins are highest in the shoots themselves. If the sprouts are small and the potato remains firm, safely remove the sprouts and the surrounding skin before cooking. However, if the sprouts are long, numerous, or the potato has become soft and shriveled, the toxin levels throughout the tuber have likely increased significantly, and the entire potato should be discarded.
Understanding Potato Greening and Solanine
Sometimes the potato develops a distinct green patch or tint beneath the skin, which is often mistaken for physical growth. This green color is chlorophyll, the pigment plants use for photosynthesis, caused directly by light exposure. While chlorophyll is harmless, its production signals that the potato has also increased its synthesis of the neurotoxin solanine in the same area. The potato produces this bitter-tasting alkaloid as a defense mechanism against insects and diseases.
High levels of solanine can cause gastrointestinal distress, including vomiting, abdominal pain, and diarrhea, and rarely, neurological problems. Cooking methods like boiling or baking do not fully eliminate solanine, as the compound is heat-stable. Therefore, cut away and discard all green areas and any deep eyes, since the highest concentration of solanine is found just beneath the skin. If the greening is widespread or the potato tastes noticeably bitter, throw the tuber away entirely to avoid consuming harmful levels of the chemical.
When Growth Is Actually Mold or Fungus
Growth on a potato is not always part of the plant’s life cycle; it can be contamination in the form of mold or fungus. This spoilage often appears as fuzzy, cotton-like patches, black spots, or a soft, mushy texture. Unlike firm shoots, mold growth is typically a sign of bacterial or fungal infection, thriving in environments with high humidity and poor air circulation. Common culprits include Fusarium or Penicillium species.
The danger with mold is not just the visible surface growth, but the thread-like structures, or hyphae, that penetrate deep into the tuber’s dense flesh. These fungal structures can produce mycotoxins, which are heat-stable and cannot be destroyed by cooking. Because these toxins can spread far beyond the visible patch, discard the entire potato immediately if you find any sign of mold, soft rot, or a sour smell.
Proper Storage to Stop Growth
Preventing unwanted growth, whether sprouting, greening, or mold, requires controlling the storage environment. Potatoes should be kept in a cool, dark, and dry location, with a consistent temperature ideally ranging between 45 and 50 degrees Fahrenheit. This range is low enough to slow the metabolic processes that trigger dormancy breaking and sprouting. However, temperatures that are too cold, such as in a refrigerator, cause starches to convert to sugars, resulting in an undesirable sweet flavor.
Keeping potatoes in the dark prevents the light exposure that initiates chlorophyll and solanine production. Ventilation is also important, so store them in a paper bag, basket, or open container rather than a sealed plastic bag, which traps moisture and encourages mold. Store potatoes away from ethylene-producing produce, specifically onions, apples, and bananas. Ethylene gas acts as a ripening hormone that accelerates the potato’s aging process, speeding up the onset of sprouting.