Freezing is a food-preservation technique that lowers a food's temperature until most of its water content crystallizes into ice, slowing the microbial growth and enzymatic activity responsible for spoilage. People have used naturally cold environments, such as caves and snow, to preserve food for millennia, but freezing as a controlled kitchen technique dates to the development of mechanical refrigeration in the late 19th and early 20th centuries. It is used on raw ingredients, such as meat, fish, fruit, and vegetables, and on fully cooked dishes like soups, stews, and casseroles, extending shelf life well beyond what refrigeration alone allows. Quality depends on freezing quickly, packaging to exclude air, and holding a steady freezer temperature; done poorly, it causes freezer burn and texture or flavor loss on thawing.