Effects of cooking on mollusk shell structure and chemistry: Implications for archeology and paleoenvironmental reconstruction

Mollusk shells excavated from archeological sites have been used to reconstruct paleoenvironment, human foraging, and migratory patterns. To retrieve information on past environment or human behavior, chemical signatures such as oxygen stable isotopes (δ18Oshell) are analyzed. Shell archeological remains usually represent food waste. Thermal treatments such as boiling and roasting may influence shell structure and biochemical composition. However, little is known about the relationship between changes at macro-, microstructural and chemical levels. This work is a calibration study on modern Phorcus (Osilinus) turbinatus shells. A simulation of two different cooking methods (boiling and roasting) was carried out at four temperatures (100 °C, 300 °C, 500 °C and 700 °C) for two durations (20 min and 60 min). The structure and biochemistry of shells boiled at 100 °C did not significantly change. However, treatments at higher temperatures strongly affected both the structure and the biochemistry of the shells. At 300 °C the external coloration, as well as nacre iridescence, were altered. Raman spectroscopy revealed that, at this temperature, the aragonite-calcite polymorphic transformation starts. Scanning electron microscope (SEM) analysis showed drastic changes in the microstructural organization also beginning at 300 °C. Furthermore, the isotopic δ18Oshell values were significantly affected. Increasing cooking temperatures resulted in an enhancement of the above-mentioned alterations. These results provide a set of temperature-related morphological, structural and biochemical characteristics for investigating the thermal behavior of biocarbonates and for estimating different cooking treatments in archeological record.