Lipid Residue Analysis

An example of the type of foodcrust that adheres to pottery.

During cooking, molecules coming from the food will become trapped and preserved within the porosity of the vessel or in charred deposit. Lipids, a very stable molecular family, can be recovered by extraction and breakdown to the analysis of individual molecules. The lipid composition, the presence of specific molecules (biomarker) or the isotopic signature of targeted molecules will indicate the original content of the pot such as plants, ruminant meat, milk or fish.

This is done using a suite of organic-geochemical techniques: bulk isotope analysis, Gas Chromatography (GC), Gas Chromatography-Mass Spectrometry (GC-MS), Combustion Isotope Ratio Mass Spectrometry (GC-c-IRMS). In bulk isotope analysis the ratio of carbon and nitrogen isotopes indicates the source in a foodweb that these isotopes were acquired from. Examining the 12C/13C isotope ratio, it is possible to determine whether animals ate predominantly C3 or C4 plants. Potential C3 food sources include rice, tubers, fruits, nuts and many vegetables, while C4 food sources include millet and sugar cane.

Foodcrust residues are analysed by GC and GC-MS to generate spectra like the one here.A single one of the compounds separated can be measured isotopically and plotted against modern food references to show what the original class of food was.

As the trophic level increases from plant, to herbivore, to omnivore, to carnivore there is a fractionation in the isotopes that means different food sources can be distinguished to broad categories. Gas Chromatography is a technique for separating and analysing compounds, generating information about the class lipid that is present in a sample. In combination with Mass Spectrometry, the mass of molecules within those compounds can be targeted for analysis, refining the identification of the original food that they came from.

Some foods contain chemical 'biomarkers', a unique molecular signature that identifies the original compound. For example, beeswax, aquatic foods, and birch bark tar all contain biomarkers and can be identified archaeologically with a high degree of precision. GC-c-IRMS is a technique that measures the carbon isotope value of a single compound, separated by Gas Chromatography. Rather than being an isotope measure of the entire food contents of a vessel; protein, carbohydrate, and lipid, like with bulk isotope analysis, single compound isotope analysis measures only the isotope contribution from the lipid fraction. The results are a more precise isotopic measure of the major chemical contributor to the food residue.