Lipid analysis is very useful in the identification of vessel contents because lipids are contained in virtually all human food, both plant and animal. Moreover, lipids, in particular fatty acids, are very well preserved, as compared with carbohydrates and proteins. Standard chromatographic procedures are used to analyze ancient material, whose lipids are then identified by cross-referencing them to a reference library of known lipids. Depending upon the plant or animal, a single lipid or combination of lipids may be sufficient to determine the source of a sample (e.g., erucic acid is typical of the mustard family). In general, a sample of the interior fabric of a pottery vessel is best, since it has been subjected to relatively less degradation and contamination than the surfaces of the vessel. Special circumstances, such as a permanently dry or wet environment, help to preserve lipids, apparently because bacterial life and autoxidation are reduced.