Adding acid too early in slow cooking can prevent vegetables from softening properly, resulting in a dish where the meat is tender but the vegetables remain firm.
When you're slow cooking, the goal is to break down tough cuts of meat and allow flavors to meld over a long period. Vegetables also need time to soften and release their flavors. Acid, such as tomatoes, vinegar, or lemon juice, interferes with the breakdown of pectin, a substance found in plant cell walls. Pectin is what helps vegetables soften during cooking. If acid is present from the beginning, it stabilizes the pectin, preventing the vegetables from becoming tender. The meat, on the other hand, will still tenderize because the acid helps break down the proteins in the meat. Therefore, adding acidic ingredients too early can lead to a textural imbalance in your slow-cooked dish.
Add acidic ingredients like tomatoes, wine, or vinegar during the last hour or so of slow cooking to ensure your vegetables soften properly while still allowing the acid to contribute to the overall flavor of the dish.