Starch grains, tiny granules that operate within plants as the main mechanism for food storage, have become an important tool archaeologists use to identify the diets of past peoples. Botanical remains at the microscopic level called plant residues have been part of the archaeological toolkit for some time, in the form of pollen grains and opal phytoliths. But over the past decade or so, Dolores Piperno and colleagues have widened their research to include grains of starch.
What is a Starch Grain?
Plants store food sources in various parts of their structure as various types of starch. Two main types of plant starch include transitory starch, which is found in the leaves of plants and acts as an ongoing energy source; and storage starch, which forms in grains within the seeds, tubers, roots, corms, fruits and rhizomes of plants. Storage starch is a long-term storage strategy to allow the plant to survive during adverse climate situations, in extremes of drought or heat or flood or cold situations. Archaeologists are most interested in starch granules, since they are built to last.
Starch granules are made primarily of glucose molecules with a quasi-crystalline structure, and they range in size from 1-100 microns and with shapes from spherical to elipsoid. When exposed to water (as in when a plant is cooked), starch grains swell but will return to their normal shape assuming the temperature stays beneath a specific temperature--the temperature varies, but 60 degrees C is typical for many of the plants.
If not exposed to overly high heat, large quantities of starch grains from plants such as manioc, potatoes, rice, yams, barley, sweet potato, chili, achira, beans, arrowroot and maize are identifiable to species, based on the shapes, sizes, and surface decorations.
Starch Grains and Archaeology
Starch grains have the potential to identify dietary elements, the use of a particular tool on which starch grains appear, and hints about the climate and vegetation of the region of the archaeological site. Starch grain residues have been found on stone tools as old as the Acheulean period. Starches have also been discovered in dry caves as well as on archaeological objects.
Light microscopy is the main method of analysis of starches, but also chemical studies have identified the presence of cellulose residues.
Research History
Early researchers in the field included E.T. Reichert in 1913, who identified and described 300 starch grains, and archaeologists Don Ugent in the 1980s, and T.H. Loy in the 1990s, who identified starch grain species in archaeological contexts. as well as Dolores Piperno's laboratory at the Smithsonian Institution Tropical Research Institute.
Sources
A great source for information about starches in ethnobotanical research is the page by Don Ugent and Linda Scott Cummings called Ethnobotanical Research: Starch Research PageHaslam, Michael 2004 The decomposition of starch grains in soils: implications for archaeological residue analyses. Journal of Archaeological Science 31(12):1715-1734.
Piperno, D.R. and Irene Holst. 1998. The presence of starch grains on prehistoric stone tools from the humid neotropics: Indications of early tuber use and agriculture in Panama. Journal of Archaeological Science 25: 765-776.
This glossary entry is part of the Dictionary of Archaeology. Any mistakes are the responsibility of Kris Hirst.
