Summary of 'Old food, new methods’: recent developments in lipid analysis for ancient foodstuffs'

Introduction

• Understanding ancient people's relationship with plants, animals, and natural resources is crucial for understanding past environmental changes, technological developments, and human adaptations.
• Organic Residue Analysis (ORA): Characterizing preserved lipid molecular signatures in archaeological material culture, especially pottery, has become a routine archaeological tool.
• Lipids are absorbed into pottery during use and can survive for millennia, providing insights into ancient diets and culinary practices.

Methodological Advancements

• Focus on the last 5-7 years of developments in archaeological lipid analysis, complementing recent major reviews.
• Addressed challenges include throughput capacity, analytical resolution, range of identifiable commodities, and data analysis.
• Two key extraction methods:
  • Solvent Extraction (SE): Recovers all intact, complex lipids.
  • Acidified-Methanol Extraction (AE): Faster, higher lipid yields but sacrifices hydrolysable compounds.
  • Applying both methods is becoming routine.
• AE more effectively extracts neutral compounds like alkanes and phytosterols, important for identifying plant use.
• Targeted protocols, like extracting alkylresorcinols for lipid-poor cereals, are being developed.

Instrumentation

• Analytical approaches remain grounded in gas chromatographic techniques.
• Comprehensive workflow includes:
  • GC coupled with a flame ionization detector (GC-FID) for pre-screening and semi-quantitation.
  • Single-quadrupole GC-MS for peak identifications.
  • Selected ion monitoring (SIM) for low-abundant biomarkers.
  • GC combined with combustion isotope ratio mass spectrometry (GC-C-IRMS) for compound-specific δ^{13}C analysis of FAMEs C16:0 and C18:0.
• High-resolution GC quadrupole time of flight MS (GC-Q-ToF MS) allows comprehensive analysis and "-omics" data processing.
• GC-QQQ (triple quadrupole) MS achieves sub-ng/mL detection limits for lipids and plant biomarkers in stone tools.
• LC-MS is increasingly applied, especially for high molecular weight lipids like triacylglycerols (TGs).

Reference Work and Ground-Truthing

• Ground-truthing involves experimental archaeology and reference profiling to test archaeological conclusions and propose new biomarkers.
• Cooking experiments help understand lipid absorption behavior.
• Experiments validate biomarker survival through processing and degradation.
• Simulation experiments verify lipid absorption and endurance under cooking/burial conditions.
• Experiments identify new compounds formed from thermal reactions, such as ω-(o-alkylphenyl)alkanoic acids (APAAs).

Interpretive Frameworks

• Landscape-wide studies are expanding to regional and continental scales.
• AROLD database: A searchable compilation of published archaeological lipid data for spatio-temporal studies.
• Non-targeted GC-Q-ToF MS workflows allow data mining of statistically significant lipids.
• Statistical modeling assesses food mixing in vessels using stable carbon isotope values.

Expanding Themes

• Focus on food consumption in ritual/religious spheres.
• ORA elucidates religious practices, feasting behavior, and mortuary rituals.
• Studies analyze pottery from burials, finding these contexts fruitful.

Ethnoarchaeological Lipid Profiling (ELP)

• ELP links observed lipid profiles with documented cooking practices and use frequency.
• ELP uses real-world examples to tie lipid profiles to cooking practices; however, real-world examples often lack archaeological-level diagenesis, complicating direct comparisons.
• Blind analysis and comparison with recorded life history revealed that ORA does not always detect the food type or cooking method.

Conclusions

• Advancements in methodology, instrumentation, reference work, and interpretation strengthen archaeological lipid studies.
• Move towards more specific extraction protocols for plant-based foods.
• Use of highly sensitive instruments with higher resolution and automated compound analysis.
• 'Ground-truthing' efforts are gaining traction, and developments in interpretive frameworks are moving the field in new directions.
• A pervasive issue is that most large studies are still predominantly focused on Europe. The future needs more substantial studies focused on Africa, Asia, and the Americas.