Why won’t store-bought diced tomatoes cook down? Because they have a secret ingredient.

You followed all the instructions diligently and did everything that’s in the recipe. One of those steps is to add diced tomatoes and simmer until cooked down. You simmered the tomatoes, that’s for sure, but they never really cooked down.

What’s the reason for that?

Most store-bought diced tomatoes don’t cook down because their producers tend to add a firming agent called calcium chloride, which limits the tomatoes’ ability to break down when cooked.

It goes like this: ripe tomatoes are harvested and transported to the cannery, which is usually near the fields where the plants are grown.

Once in the cannery, the skin is removed from the tomatoes and they are diced into cubes. The cubes are dipped, sprayed, or immersed in a solution of calcium chloride and water. Then, they are sealed with tomato puree in cans—and the cans are sterilized.

Voilà, the diced tomatoes ready for you to buy and use.

The calcium chloride serves two purposes:

First, it limits the so-called enzymatic browning that takes place as soon as the tomato is diced. “Enzymatic browning” is the tendency of most fruits and vegetables to become brown shortly after they are bruised or sliced.

Basically, the walls of the cells that make up the fruit or veg are crushed, so the plant activates its natural defense mechanisms that harm microbes or insects. The same mechanisms cause collateral damage to the plant by browning it.

(In case you’re wondering, this is the same thing that happens to apples, avocados, bananas, and potatoes when they are crushed or cut.)

Second, the calcium chloride helps the tomato cubes to stay firm, even during the prolonged cooking necessary for canning. Thus, you get tomato cubes as they appear on the label, and not mush.

This makes the diced tomatoes more appetizing to you and me, the consumers, when we open the can. Alas, it also prevents them from cooking down in a sauce, soup, or stew the way we would (rightfully) expect them to.

Simply put, calcium chloride (and diced tomatoes that don’t cook down) is one of the compromises we have become accustomed to as part of 21st century city life. Despite lore to the contrary, turning up the heat won’t fix it, nor will simmering the tomatoes for longer.

What Can You Do About It?

Although the remedy for any cooking problem is limited only by the cook’s imagination, there are three things you can do to counteract the fact that most supermarket diced tomatoes don’t really cook down.

Throw the diced tomatoes in the blender or food processor:

By far the simplest solution for diced tomatoes that do not cook down is to puree them in the blender or grind them in a food processor before adding them to the cooking liquid. The result is a consistency very similar to tomato purée, which tends to get the job done.

Buy diced tomatoes that don’t contain any calcium chloride:

The secret to diced tomatoes that cook down well starts at the cannery and grocery store, and not in your kitchen. When buying diced tomatoes, take the time to read the labels carefully, and look for cans that don’t contain calcium chloride.

Nine times out of ten, such cans will contain diced tomatoes, sea salt, and citric acid. If you shop frugally, brace yourself: Should you follow our advice, you will have to pay up for the premium cans on the shelves at the store.

Still, the difference won’t be more than a dollar or two per can. Unless you eat tomatoes all day, every day, you probably won’t break the bank, and you will put a much tastier dinner on the table.

Dice fresh tomatoes. And add them to your dish with tomato purée:

If you want to make your dish meaty—we’ve written many times that diced tomatoes are an excellent thickener in recipes that welcome them—you should peel and dice fresh tomatoes and add them to the pot along with tomato purée.

Now, fresh tomatoes have a less cooked, more acidic flavor compared to their diced (and whole peeled, and crushed) counterparts. So do keep in mind that, with this substitution method, your dish will come out more zesty and zippy if you add fresh tomatoes.

Practical Tips for Cooking With Diced Tomatoes

To bring out the aromas and flavors of the tomatoes, add something sweet, such as a teaspoon of sugar, honey, or maple syrup, and something sour, such as lemon juice, vinegar, or wine.

Cook over high heat only if you want to reduce the cooking liquid and thicken your dish. Otherwise, simmer the dish over medium heat to keep the tomatoes from splattering all over the stovetop.

Cover the pan or pot with a lid only when you want to increase the pressure, which raises the boiling point of the water slightly above 212°F (100°C). In this way, the moisture is retained and the dish cooks faster. In contrast, cooking without a lid promotes the evaporation of moisture and the thickening of the sauce.

Tomatoes are an acidic ingredient. As such, they react with cast iron and carbon steel cooking vessels, causing them to leach dietary iron into your home-cooked meals and giving them a metallic aftertaste. (The longer the simmer, the greater the amount leached). Cook recipes that call for tomatoes in ceramic, non-stick, stainless steel, or enameled cast iron cookware instead.