All of the commodities covered in this handbook are alive and carry on processes characteristics of all living things. One of the most important of these is respiratory metabolism. The process of respiration involves combing O2 in the air with organic molecules in the tissue (usually a sugar) to form various intermediate compounds and eventually CO2 and water. The energy produced by the series of reactions comprising respiration can be captured as high energy bonds in compounds used by the cell in subsequent reactions, or lost as heat. The energy and organic molecules produced during respiration are used by other metabolic processes to maintain the health of the commodity. Heat produced during respiration is called vital heat and contributes to the refrigeration load that must be considered in designing storage rooms.