Starch and Starch Grain Analysis in Archaeology

Why Are Archaeologists Interested in Ancient Starch Grains?

Starched Shirts
The cellulose in plant starches is used in the modern day to stiffen shirts and thicken sauces. David Singleton

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.

Identifying Starch Characteristics

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 and become gelatinized; they will return to their normal (identifiable) shape if the temperature stays beneath a specific range—the range varies, but 60-85 degrees Centigrade is typical for many of the plants. At that temperature, the plant has taken in enough water to make a water/starch ratio of ~1.5-2:1.

Over that temperature, and cooked starch will lose its structure permanently and be unidentifiable archaeologically. See Crowther 2012 for a detailed analysis of how the gelatinization process happens.

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 location for 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

Crowther A. 2012. The differential survival of native starch during cooking and implications for archaeological analyses: a review. Archaeological and Anthropological Sciences 4(3):221-235.

Dickau R, Ranere AJ, and Cooke RG. 2007. Starch grain evidence for the preceramic dispersal of maize and root crops into tropical dry and humid forests of Panama. Proceedings of the National Academy of Sciences 104(9):3651-3656.

Duncan NA, Pearsall DM, and Benfer J, Robert A. 2009. Gourd and squash artifacts yield starch grains of feasting foods from preceramic Peru. Proceedings of the National Academy of Sciences 106(32):13202-13206.

Haslam M. 2004. The decomposition of starch grains in soils: implications for archaeological residue analyses. Journal of Archaeological Science 31(12):1715-1734.

Liu L, Field J, Fullagar R, Bestel S, Chen X, and Ma X. 2010. What did grinding stones grind? New light on Early Neolithic subsistence economy in the Middle Yellow River Valley, China. Antiquity 84(325):816–833.

Perry L, Dickau R, Zarrillo S, Holst I, Pearsall DM, Piperno DR, Berman MJ, Cooke RG, Rademaker K, Ranere AJ et al. . 2007. Starch Fossils and the Domestication and Dispersal of Chili Peppers (Capsicum spp. L.) in the Americas. Science 315:986-988.

Piperno DR. 2009. Identifying crop plants with phytoliths (and starch grains) in Central and South America: A review and an update of the evidence. Quaternary International 193(1-2):146-159.

Sandweiss DH. 2007. Small is big: The microfossil perspective on human–plant interaction. Proceedings of the National Academy of Sciences 104(9):3021-3022.

Zarrillo S, Pearsall DM, Raymond JS, Tisdale MA, and Quon DJ. 2008. Directly dated starch residues document early formative maize (Zea mays L.) in tropical Ecuador. Proceedings of the National Academy of Sciences 105(13):5006-5011.