Anywhere outside of Central America the tomatillo is little more than an afterthought—something we have come to know through globalism and restaurants brave enough to affront our rutted palates with the unfamiliar. Even its diminutive name, meaning ‘little tomato,’ suggests it came second, though most archeologists believe the tomatillo was cultivated by ancient Mesoamericans long before its world-famous cousin.
Roughly the size of a cherry tomato, cloaked in an attractive, lantern-shaped husk, the tomatillo looks almost like a tomato, though hold a tomatillo in one hand and a cherry tomato of the same size in the other, and you will begin to feel the difference. With drier flesh, the buoyant tomatillo seems less substantial. Take a bite of each. Unlike a tomato, whose juices ooze with concentrated flavor compounds and sugars, a raw tomatillo comes off as overly lean—bitter acidity and merely a hint of sweetness.
Cook that same tomatillo in a small amount of water or on the grill and its sharpness mellows, cell walls bursting open, releasing pectin that thickens the tartly sweet juices into syrup. The tomatillo, parading as simple and slight, creates its own luscious sauce with only the application of heat. Its vegetal, citrus-infused flavor cuts through lipids like a cool breeze on an August afternoon, explaining the tomatillo’s common association with fatty taco fillings or buttery guacamole.
That the raw tomatillo seems to the tongue like a lime in a poorly executed tomato costume is no coincidence. Lime flavor is dominated by acidic compounds, which occur mostly as citric acid, with ten-percent dashes of malic (from the Latin word for apple; associated with tart things like rhubarb and sour candies) and succinic acids, both of which add to the fruit’s complexity (and are nearly absent in lemons). Tomatillo acidity is primarily citric and malic, a combination that lends its flavor that lime-without-the-peel quality; lime flavor, in its fullness, is a cocktail of acidity, sugar, and aromatic compounds released from the skin.
In traditional Mayan and Aztec cuisines, tomatillos played the role of a pre-Columbian citrus. It’s hard to imagine a plate of Mexican food without that quintessential slice of lime, but citrus trees hail from Asia and did not reach Central America until the 16th Century. The acidic resonance between tomatillos, a long-time staple, and limes, introduced (along with other citrus varieties) by Spanish conquistadors, may explain why these cuisines took to limes much more so than lemons—it was familiar; they already knew what to do with it.
One pre-Columbian use of the tomatillo was to tenderize meat. Stewed with chiles and perhaps a handful of quelites (wild greens, such as purslane or amaranth leaves), tomatillo acidity softened lean cuts the way citrus juice will. Remnants of this technique are evident in dishes like Chile Verde (pork braised in green salsa). Tomatillos perform similar to (and likely predate the use of) lime juice in various salsas, invigorating avocado’s heaviness in a well-balanced guacamole, or kicking up the acidity of chile- and tomato-based salsas.
Although the temptation to toss tomatillo husks out the back door to tumble around the yard like balls of lace must have been irresistible even to the Mayans and Aztecs, their most enterprising cooks discovered arguably better uses. It turns out, for reasons I could not find a definitive explanation of, tomatillo husks contain a leavening agent. Used to improve breads and tamale dough, water boiled with ten or so tomatillo husks somehow imparts masa with a fluffy lightness the way baking soda does in modern recipes. Many sources attribute this to the husk’s acidity. Since the water-husk infusions often (but, tellingly, not always) cite the inclusion of a particular kind of alkaline mineral salt called tequesquite, the basic idea is that the salt and the husk react something like baking soda and vinegar.
In an interesting exchange I found between a Mexican food blogger and the scientist-author Harold McGee, McGee explains that the chemistry of this theory doesn’t pan out. Prepared as an infusion, the gas release that results from the contact between the alkali and the acid would occur during the boiling process, meaning that the fluff-producing magic would extinguish long before it is added to the masa. McGee postulates instead that the leavening could be the result of pectin and other thickening agents; released from rigid cell walls by boiling (with or without the alkaline salt), they might lend enough elasticity to the dense dough to allow air bubbles to expand during cooking.
With no more than a high school chemistry course under my belt, I have nothing to add, except another comment I came across a few times on unrelated sites. Though I hesitate to call it a “tradition” without further evidence, it appears that several Mexican grandmothers have been known to boil tomatillo husks with cactus paddles in order to reduce the paddles’ slime (something akin to okra’s). Sounds like magic, but maybe the slime isn’t disappearing, just thickening, in which case, Harold might be on to something.
So the next time you take home those ‘little tomatoes,’ don’t think tomato at all. Think lime and lightness and silky rich sauce, maybe even husk and all.