The marinade's acid (be it derived from lemon juice, wine, or some other ingredient) is the potent substance that chemically softens the connective tissue. Acid also reduces the time required for heat to convert collagen to gelatin. To be effective, sufficient marinating time is necessary. With some dishes you can achieve adequate results in as few as several hours, though there are specialties that call for a weeklong marination. It all depends on the recipes.