A refrigerator (colloquially fridge) is a commercial and home appliance consisting of a thermally insulated compartment and a heat pump (mechanical, electronic or chemical) that transfers heat from its inside to its external environment so that its inside is cooled to a temperature below the room temperature. The refrigerator should be kept at or below 40 F (4 C) and the freezer should be regulated at 0 F (-18 C). Refrigeration is an essential food storage technique around the world. The lower temperature lowers the reproduction rate of bacteria, so the refrigerator reduces the rate of spoilage. A refrigerator maintains a temperature a few degrees above the freezing point of water. Optimum temperature range for perishable food storage is 3 to 5 C (37 to 41 F). A similar device that maintains a temperature below the freezing point of water is called a freezer. The refrigerator replaced the icebox, which had been a common household appliance for almost a century and a half. The first cooling systems for food involved ice. Artificial refrigeration began in the mid-1750s, and developed in the early 1800s.