The analysis of starch grains, tiny granules that operate within plants as the main mechanism for food storage, has 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 Starch?
Starch is the main energy source for plants, stored as a food source in various parts of plant structure: leaves, stems, seeds, roots, etc. Two main types of plant starch include transitory starch, which is found in the leaves of plants and acts as an ongoing energy source for the plant; and storage starch, which forms in granules within the seeds, tubers, roots, corms, fruits and rhizomes of plants. Storage starch is a long-term energy strategy that allows the plant to survive during adverse climate situations by accessing the protected granules during extremes of drought or heat or flood or cold. Archaeologists are most interested in storage starch granules: because they are constructed to survive in adverse climates, storage starch granules continue to exist long after the plant itself has died and rotted away.
Starch granules are made primarily of glucose molecules with a quasi-crystalline structure, and they range in size from 1-100 microns and in shapes from spherical to ellipsoid. When exposed to water (as in when a plant is cooked), starch grains swell; they will return to their normal (identifiable) shape if the temperature stays beneath a specific range—the range varies, but 60 degrees Centigrade is typical for many of the plants.
If they have not been exposed to warping levels of heat, starch grains from plants such as manioc, potatoes, rice, yams, barley, sweet potatoes, chili peppers, achira, beans, arrowroot and maize are all identifiable to species, because they have species-specific shapes, sizes, and surface decorations.
Starch and Archaeology
Starch grains have been used by archaeologists to identify elements of the diet of a particular human group; what a particular tool on which starch grains appear was used for; and even climate and vegetation of the region of an archaeological site in which the starch grains were discovered. Starch grains have been found on stone tools as old as the Middle Paleolithic Acheulean period and as recently as yesterday. Starches have been discovered in the sediments of dry caves, in middens and storage pit soils, and clinging to archaeological artifacts.
Archaeologists primarily use a light microscope to identify the shapes and patterns of starch grains; but chemical studies have also been successfully used to identify the presence of specific cellulose residues of ancient plant starches.
Starch and Research History
Early researchers in the field included E.T. Reichert in 1913, who identified and described 300 different species-specific starch grains, and archaeologists Don Ugent in the 1980s and T.H. Loy in the 1990s, who first identified starch grain species in archaeological contexts. Dolores Piperno's laboratory at the Smithsonian Institution Tropical Research Institute is an important locus of current investigation into the archaeological uses of starch.
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 Page
Haslam, 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.
