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Fermentation: Basic Guide

I’m always looking for new ways to use up tomatoes. I have also been getting into fermentation, fermenting everything from hot sauce to kimchi to sourdough. I thought, why not ferment some tomatoes?

Fermentation creates pleasantly salty, funky, tangy, and complex flavors. Fermented vegetables are truly unique, and they can give an extra punch of flavor to a wide variety of dishes. As a great source of probiotics, fermented foods can also improve gut health.

Fermented foods are easy to pick up at the grocery store and even easier to prepare yourself. Lacto-fermentation requires 3 ingredients: vegetables, sea salt, and bottled water. Salt is very important here. It discourages the growth of unhealthy organisms in your ferments while encouraging the growth of healthy bacteria. Make sure the salt is free of any anticaking agents and that it is NOT iodized, as this could ruin your ferment. I recommend plain sea salt, and I recommend NOT using table salt. Tap water is chlorinated, which could ruin the ferment, so use bottled.

You’ll also need 3 basic tools: a food scale (that measures in grams), a glass jar, and a fermentation weight. Avoid metal materials, as the acidic fermented products could react with metal. I recommend buying this fermentation kit. It has glass fermentation weights and an air-lock lid, which allows carbon dioxide to escape without needing to burp the jars.

For these fermented tomatoes, we’ll be using a brining fermentation method, where vegetables are submerged in a saltwater brine. For this method, I calculate a set percentage of salt in grams based on the weight of the vegetables and water. 2%-4% salt is usually recommended for fermentation. I usually stick with a 3% solution to achieve an end result that isn’t overly salty.

Fermented cherry tomatoes are my favorite, but you could ferment any tomato. It is important to prick tomatoes with a toothpick or cut bigger tomatoes in half. This allows the brine to get into the tomato.

How Much Salt?

Grab your calculators. Below are some sample vegetable + water and salt amounts, based on a 3% salt ratio. Simply multiply the weight of your vegetables and water in grams by 0.03, or 3%.

500g vegetables + water	15g salt
1,000g vegetables + water	30g salt
1,234g vegetables + water	37.02g salt ≈ 37g salt

Start with a clean (doesn’t have to be sanitized like in canning) glass jar. Make sure to zero out (tare) the weight of your glass jar before adding in your vegetables. To measure the water, simply pour enough water to cover the vegetables in the jar, leaving some room or “headspace” at the top of the jar to allow for breathing and prevent overflows.

Record the weight of the vegetables + water and strain the water into a separate bowl, leaving the vegetables in the jar. Add the calculated amount of salt to the water and stir to completely dissolve. Feel free to round any decimals in the salt calculation up or down to the nearest gram.

Pour the salt brine back into the jar. Weigh vegetables down with a fermentation weight or a small glass jar or bowl. Some people use a plastic bag filled with water, but I find that the creasing of the bag can lead to mold growth. Make sure that all vegetables are submerged below the brine. Even one floating tomato could spoil the whole ferment.

And now, we wait. If using a regular lid, quickly open (burp) jars each day to let carbon dioxide escape while not letting any oxygen in. Let ferment for 3-7 days, or until it reaches your desired taste, at room temperature. 70-72ºF (21-22ºC) is the sweet spot in my experience.

Use common sense. If you have any concerns about the appearance or smell of the end product, throw it away. It should have a pleasant vinegary and slightly funky smell. It is normal for the brine to become cloudy. If you see any fuzzy mold on top (white or any other color), throw it away. If it smells or looks bad, throw it out. Sometimes, a harmless yeast called Kahm yeast forms on top of the ferment. It’s powdery and smooth in texture, thin, and white. You can scoop the Kahm off (it tastes bad) and eat safely. If you are not 100% confident that a ferment is safe to eat, throw it away. When in doubt, throw it out.

When satisfied with the taste, store the ferment in the refrigerator. It will continue to ferment at a much slower rate.

Which Tomato Varieties Are Best?

You can use any tomato in a ferment. However, I think cherry, grape, or currant tomatoes taste the best as they retain the most texture. Some of my favorites to ferment are Spoon Tomatoes, Sungold Tomatoes, Purple Bumblebee Tomatoes, and Sweet Million Tomatoes. Beware: if you use Spoon Tomatoes or any other currant/wild tomato, it will take FOREVER to pierce with toothpicks. However, its fun and unique, caper-like flavor and appearance make everything worthwhile. Unripe green tomatoes at the end of the season are also good fermented.