We've noticed that adding an acidic ingredient such as tomatoes to a pot of beans can make them take longer to cook and even prevent them from ever fully softening. We wondered if adding an alkaline ingredient like baking soda would have the opposite effect and soften the beans more quickly. We set up a series of tests to find out.
We filled three pots, each with 5 cups of water. To one we added 1 percent baking soda by weight to turn it alkaline (about 8 on the pH scale) and to another we added enough citric acid to increase its acidity to 3. We left the third pot untreated so that it registered a neutral pH of 7. We stirred a cup of black beans into each pot, brought them all to a simmer, covered the pots, and put them in the same 350-degree oven to cook. We removed all three pots from the oven when the beans in the alkaline water had turned tender, or about 45 minutes. We repeated the test three times.
When the beans in the alkaline environment turned tender, the others were far behind. The beans in the plain water had only slightly softened after 45 minutes. We returned this pot to the oven and they required another 15 minutes of cooking time, an hour in total. Those in the acidic water were still rock-hard after 45 minutes. They required a total cooking time of one hour and 45 minutes to soften fully.
At the 45-minute mark, we removed some beans from each pot and placed them on the counter. We then applied a 5-pound weight on top. The beans cooked with baking soda were incredibly tender and squished down beautifully. The beans cooked in a neutral environment squished slightly but were still hard. The beans cooked with acid were rock-hard; the weight barely made a difference.
It turns out that an alkaline environment starts a chemical reaction that causes the cell structure of legumes to break down. When we add baking soda to a pot of cooking beans, it results in tender beans in less time.
On the flip side, adding acid causes the cell structure of legumes to remain firm. If there is too much acid in the pot, the beans may never soften enough to be ready to eat. This means that you should be careful when cooking beans with acidic ingredients, especially tomatoes, citrus juices, and vinegar. We find it is best to add citrus juices and vinegars at the end of the cooking process—when the beans are already softened. (This also preserves the flavor of these acidic ingredients.) Tomatoes generally need some cooking time, so we often add tomatoes (including all canned tomato products) partway through the cooking process, after the beans have softened considerably.
The lesson? Along with brining and soaking, baking soda can work wonders on beans, saving you up to an hour of cooking time. Just be sure not to add more than a pinch—too much and the beans can end up tasting soapy and unpleasant.