They soften and denature it. Tenderizing occurs in food when muscle tissue is separated, torn, or bruised. Tenderizing, for example, occurs when a cook pounds a chicken breast or a veal scallop with a kitchen mallet.
Marinades soften or denature tissue with their acid ingredients.
Marinades do not penetrate deeply into muscle tissue. The first is acid, such as wine, vinegar, citrus juice, or yogurt, acting as a softening agent. The second is oil, which adds flavor and moisture. The third is the aromatics that give the marinade its aroma and flavor. Whisk in olive oil a little at a time.