Lacto-Fermented Salsa Made with Ingredients from a Mexico City Market

On Saturday morning, when you step into Mercado de la Merced, the spicy air rising from the piles of chiles hits your nose first. I breathe in that smell as I pick out the ingredients I'll ferment today. Every stall is heaped with tomatillos, habaneros, serranos, and chile varieties whose names I don't even know. This salsa uses no heat. Salt, time, and invisible lactic acid bacteria do all the work.

The first time I thought of applying lacto-fermentation to salsa, it was because I couldn't bear to throw away the leftover brine from making kimchi at home. I dropped a few tomatillos into that brine, and three days later they had become something entirely different. Uncooked yet deep, raw yet complex. From that point on I kept making this salsa with nothing but market ingredients, and now I ferment almost a jar a week.

Preparing the Ingredients

What you need to buy at the mercado is simple.

• 500 g tomatillos, husks not dried out, firm to the touch
• 3 serrano chiles and 1 habanero (adjust heat by removing the seeds)
• 4 cloves of garlic
• Half a white onion
• A handful of cilantro
• Salt, unrefined sea salt is best. Iodized salt can inhibit fermentation
• A clean 1 L glass jar

The total cost of ingredients comes to around 60 pesos. You can use whatever glass jar you have at home. The beauty of this recipe is that it requires no special equipment.

Process

Peel the papery husks from the tomatillos and wash them in lukewarm water. There's a sticky film on the surface, but you don't need to remove it completely. Natural lactic acid bacteria actually live on that film.

Quarter the tomatillos. Remove the stems from the serranos and habanero and cut them in half lengthwise. If you want less heat, scrape out the seeds and the white membrane inside. I tend to leave them in. Crush the garlic with the flat of a knife and roughly chop the onion.

Put all the ingredients in a bowl and weigh out salt equal to 2.5% of the total weight, then sprinkle it over everything. This ratio is key. Below 2%, unwanted bacteria can proliferate. Above 3%, fermentation slows down too much. I measure with a kitchen scale every time. Eyeballing it leads to inconsistent results.

Massage the salted ingredients by hand to draw out their moisture. After about five minutes of pressing and mixing, juice will pool at the bottom. This juice serves as the brine.

Pack the ingredients tightly into the glass jar. Press down firmly so the juice rises above the ingredients. If anything is exposed to air, mold can develop, so everything must stay submerged beneath the liquid. I cut the root end of an onion and place it on top like a lid, using it in place of a fermentation weight. It's not perfect, but it works well.

Don't screw the lid on all the way. Leave it slightly loose. Carbon dioxide builds up during fermentation. You can also open the lid once a day to release the gas.

Fermentation and Waiting

Place the jar somewhere out of direct sunlight. Indoor temperatures in Mexico City typically range from 20 to 25 degrees Celsius, and at that range 3 to 5 days is enough. On hotter days it takes about 3 days; on cooler days, closer to 5.

Starting on the second day, small bubbles will begin rising inside the jar. When you open the lid, you'll notice a slightly tart, fruity aroma. That means fermentation is going well. If you detect an unpleasant stench or see black mold on the surface, discard it. But if you followed the salt ratio, that almost never happens.

Once it reaches the level of tartness you want, add the cilantro and blend. The cilantro is not fermented. It's sensitive to heat and acid, so its flavor fades quickly. Adding it at the very end preserves its fresh, herbaceous aroma. Give it just a few rough pulses in the blender. Over-blending destroys the texture and makes it bland.

When you taste the finished salsa, you'll notice layers that set it apart from the salsa verde you commonly find at the market. The sourness isn't sharp like vinegar. It's round. Behind the heat of the chiles, there's an undercurrent of umami. It's a flavor you could never get by cooking.

Storage and Serving

Refrigeration slows fermentation to a near standstill. It easily keeps for about a month. My favorite way to eat it is on tacos de canasta. A spoonful of this salsa on top of a taco bought from a street vendor, and the fermented acidity cuts right through the greasiness. It's also great on a tlayuda with cheese, or simply scooped up with totopos.

The first time you make it, just get the salt ratio right. The lactic acid bacteria will take care of the rest. Once you've got it down, you can start experimenting. Swap in different chile varieties, or toss in mango chunks for a fruit-fermented salsa variation. Adding one new ingredient every time you go to the market, that's what keeps me making this salsa.