When starch grains are heated in the presence of water, the intermolecular bonds within the starch begin to break down and the molecular order within the crystalline region of the grains is disrupted. This process, known as gelatinization, causes starch grains to lose their birefringent properties. Gelatini-zation may occur over a relatively narrow temperature range (a window of 5-10 °C) and is complete when all granules in a population exhibit irreversible changes in optical properties. Gelatinization temperature varies with size and species, with large granules apparently the most susceptible to gelatini-zation (from approximately 50 °C). Remarkably, ethnographic studies of the preparation of toxic starchy plants in tropical north Queensland, Australia, have shown that starch may retain its birefringent properties after baking of whole material (at temperatures in excess of 100 °C), followed by pounding and leaching in water.
The mechanisms determining the survival of starch in sediments and on stone tools are poorly understood. That it does survive for thousands of years in archaeological contexts is well documented. Some archaeological/fossil starch survives aggressive chemical treatment, including heating, during the extraction process. Starch in sediments has been recovered from arid zone contexts dating to over 100 000 years and from soils and artifacts from tropical sites dated to over 5000 years. Suggested explanations for the protection of starch from degradation include the formation of some type of silica coating making it inert, and association with/adsorption onto mineral surfaces, such as stone tools, soil minerals, ceramics, or bone mineral. Further research is needed to fully understand the processes involved in starch preservation and degradation in the burial environment.
World History
