In archaeology, Organic Residue Analysis (ORA) refers to the study of micro-remains trapped in or adhered to artifacts from the past. These organic residues can include lipids, proteins, starches, and sugars. By analyzing these residues, ORA can reveal insights into ancient dietary behaviors, agricultural practices, housing organization, technological advancements, and trade interactions. Furthermore, it provides information on the use of cosmetics, arts, crafts, medicine, and burial preparations in ancient societies. ORA's broad applicability encompasses a variety of amorphous materials such as substances used in mummification, pastes, glues, binders, and colorants. These materials can be preserved in pottery, stone tools, the mineral matrix of bones, dental calculus, as well as in habitation floors or pits. The unique value of ORA lies in its ability to provide direct evidence of the materials and substances utilized by ancient peoples, often offering insights that other archaeological techniques cannot. For instance, analyzing organic residues in pottery can disclose specific dietary components, such as animal and plant fats, shedding light on ancient dietary habits and food sources. Similarly, the study of ancient adhesives and pigments can enhance our understanding of the production techniques and materials used in ancient art and craftsmanship. Moreover, ORA plays a crucial role in uncovering ancient medical knowledge, cosmetic usage, and the processes involved in creating artworks and handicrafts. Utilizing modern chemical analysis techniques, ORA offers archaeologists a useful tool to directly explore and understand the daily lives, cultural practices, and technological progress of ancient societies.
Organic residues The makeup of an organic residue depends on the organic components that got absorbed, trapped, or attached to unglazed and porous materials, like pottery, crusts, wood, and stones. The formation of organic residues can occur during a wide range of pre- and post-depositional events, including food preparation, cooking, storage, transportation, reparation and sealing. Given the complexities of organic residues, they can be defined at five nested scales: tissues, cells, macromolecules (e.g. lipids, proteins, metabolites, DNA and starches), molecules (e.g. fatty and amino acids) and atoms (e.g. carbon, nitrogen and hydrogen). We're especially interested in the residues at the level of biomolecules, particularly fats and metabolic products, because of how well they can be preserved and their importance to archaeology. Discussions about proteins are covered elsewhere under the topic of Paleoproteomics.
Lipids Lipids (lipos, gr. fat) are a class of biomolecules that mostly fall within the ester family and include fats and oils, waxes, terpenes, steroids, triglycerides (TAGs), free fatty acids, ketones, alcohols, dicarboxylic acids, bituminous substances and resins. By definition, lipids are hydrophobic biomolecules. Fatty acids consist of long carbon chains with various lengths and double bonds, ending with a carboxyl group. In biological systems, most fatty acids have an even number of carbon atoms, mainly between C14 and C24, maximising at C16 and C18. Waxes are simple esters of long-chain carboxylic acids and alcohols, fats and oils have a more complex structure. These are formed from triglycerides. Each triglyceride contains three long-chain carboxylic acids attached to a triester of 1,2,3-propantriol (glycerin). Typically, the fatty acids are unbranched and have an even number of carbon atoms. TAGs are important storage lipids and are the main constituent (~ 99%) of vegetable oils and food. Lipids are hydrophobic and therefore cannot be 'washed out' or accidentally 'washed into' the matrix of an analysed material. The ceramic matrix serves as a protective environment which favours significant lipid preservation.
Metabolites Metabolites are small organic molecules usually involved in metabolism either as a substrate or product. They can also refer to endogenous compounds, which includes organic acids, lipids, sugars, amino acids, and phenolic compounds, produced during metabolic processes. The field of metabolomics (Section 6) generally revolves around these molecules with a molecular weight between 50 – 1500 daltons (Da). Recently, the application of metabolomics in ancient residue analysis has been established where it was successfully used to determine distinct molecular residue markers in archaeological pottery.
Development of the method During the 1950s and 1960s, the emergence of chromatographic methods, especially those linking Gas Chromatography (GC) and Mass Spectrometry (MS), resulted in a methodology used to resolve and recognise molecules. One of the earliest papers using GC analysis applied to archaeological material was published by Thornton et al. (1970) and, investigated the composition of ancient bog butter.
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