One of the most durable pieces of culinary wisdom, passed down through generations of cooks, is that acidic ingredients such as tomatoes help to tenderize tough cuts of meat. That's why most marinades include acidic ingredients, such as wine, tomato juice or fruit juice, and why meat is often cooked in tomato-based sauces. It's true that tomatoes' acidity can help tenderize meats, but the effect is more complicated than most cooks realize.
Why Meats are Tough
Some cuts of meat are tender and some are tough, even in the youngest of veal or lamb. That's because regular use makes muscle fibers grow dense and resilient, with lots of tough connective tissue. Heavily used muscles, such as the heart, shoulder and shanks, require skilled and patient cooking to become edible. In contrast, tender cuts come mostly from muscles along the backbone and rib sections, which serve primarily to support the animal's skeleton. Those can be cooked with little or no preparation, but tough cuts require some help.
Meats and Acidity
The protein molecules that make up much of the meat's structure are bound up into a relatively tight form. When they come into contact with an acidic ingredient, the acidity causes the bonds holding the proteins together to loosen. This is why a few drops of lemon juice are often added to egg whites, to help them whip up more easily into a foam. Brief exposure to acidity, or longer exposure to mild acidity, can legitimately provide a small degree of tenderization to the exposed surfaces of the meat. However, the effect is limited to the meat's surface, and if it's exposed to acidity for too long, the protein strands will contract again and toughen the meat.
Tomatoes and Tenderness
If you're cooking a tough cut of meat and want tomatoes to provide you with some tenderization, start with small pieces of meat. Thinly slicing meats maximizes their surface area to take advantage of acidity's gentle tenderizing characteristics. Marinate sliced meat briefly in tomato juice before cooking. For a more reliable tenderizing effect, cook thinly sliced or diced meats in a tomato-based sauce. Slow-cooking naturally tenderizes tough cuts of meat, and the surface tenderizing effect of the tomatoes will speed that process. Tomato-based sauces improve the texture of ground meats in the same way, although the grinding leaves them tender enough without the tomatoes' acidity.
Of course, the modest tenderizing effect of tomatoes isn't the only reason for cooking them with your favorite meats. Tomatoes are also richly flavored in their own right, and provide a versatile base for many sauces. They're also filled with natural compounds that accentuate the flavors of other ingredients, making your meats and sauces taste more savory. The effect is especially strong when they're mixed with mushrooms, celery, onions and other savory ingredients that share the same characteristics.