Tomatillos (Physalis philadelphica) are a distinctive fruit belonging to the nightshade family (Solanaceae), which includes tomatoes, potatoes, and eggplants. Characterized by a sticky, round fruit encased in a papery husk, the tomatillo is a staple in Mexican cuisine, used primarily for salsa verde. The fruit is typically harvested while still green, offering a bright, tart flavor. Because tomatillos are nightshades, concerns about the toxicity of the unripe fruit are common. Like its relatives, the tomatillo plant produces compounds that act as a natural defense mechanism, especially when the fruit is still developing.
The Presence of Tomatine in Unripe Tomatillos
The caution regarding unripe tomatillos stems from the concentration of naturally occurring protective compounds called glycoalkaloids. The unripe, deep-green fruit contains higher levels of tomatine, a steroidal alkaloid. The plant synthesizes tomatine to deter consumption before the seeds are mature.
Tomatine is chemically related to solanine, found in green potatoes, but is generally less potent in its toxic effects. The highest concentrations occur when the fruit is very immature, small, and hard. Ingesting a large quantity of these highly unripe fruits can cause a reaction as the body struggles to process the elevated alkaloid load.
Tomatine gives the very unripe fruit an intensely sour and bitter taste, which acts as a sensory deterrent. This bitterness indicates a glycoalkaloid concentration that could cause discomfort if consumed in excess. For culinary use, tomatillos are typically harvested when they are green but have grown large enough to fill the husk, a stage where tomatine levels are significantly lower.
The Reduction of Toxicity During Ripening
The transition from an unripe to a mature state involves a natural biochemical process that reduces toxicity. As the fruit matures, the plant’s metabolism actively breaks down glycoalkaloids like tomatine.
A tomatillo is considered fully mature when the fruit expands to completely fill the papery husk, often causing the husk to dry out and split open. At this stage, the concentration of tomatine has diminished to a negligible level, making the fruit safe to eat. While most common varieties remain green, some heirloom types change color to yellow or purple upon peak ripeness.
The key distinction for safety is the fruit’s stage of development, indicated by its size relative to the husk, not simply its green color. A fruit that has burst the husk has completed the necessary internal chemical changes. Selecting fruit that has filled the husk is recommended for the lowest level of naturally occurring toxicity and best flavor.
Practical Safety: Recognizing Inedible Parts and Symptoms
For safe culinary use, certain parts of the tomatillo plant are never edible, regardless of the fruit’s maturity. The leaves, stems, and the papery husk (calyx) contain high concentrations of glycoalkaloids and must be discarded. These parts maintain their toxic compounds throughout the growing season.
Even mature tomatillos require thorough washing after the husk is removed due to a sticky, waxy residue. This residue can contain trace elements from the husk and should be scrubbed off under cool running water. This step removes external contaminants or residual alkaloids from the non-edible parts.
If a person consumes a significant quantity of highly unripe fruit, symptoms of mild glycoalkaloid poisoning typically involve gastrointestinal distress. Effects may include nausea, vomiting, abdominal cramps, and diarrhea. Severe toxicity from consuming the fruit is extremely rare, but any severe symptoms, such as circulatory depression or extreme confusion, necessitate immediate medical attention.