What is Starch?
Starch is the common form of a carbohydrate, and is made up of 2 organic polymers (large molecules composed of repeated sequences of units, attached by covalent chemical bonds). The structure (or building blocks) of starch is made up of 2 different chains, known as linear helical amylase and branched amylopectin. Amylase is soluble, while amylopectin is insoluble. Starch is located in most plant tissues, particularly in storage organs such as rhizomes, tubers, and grains.
Why are archaeologists so interested in starch grains?
To date, grains have been removed from stone tools, ceramic sherds, organic materials, dental calculus, and sediments. Starch often preserves when actual plant remains do not, and therefore provides an additional tool for understand past plant use.
Advantages of using starch grains: •Often identifiable to species as grains are distinct • Their common occurrence make starch grain analysis very useful • Preserves in conditions that other remains may not (e.g. pollen, fragile seeds).
Disadvantages of the technique: •Factors such as heat and water absorption may affect the structure of the grains, making identification more difficult •Even if the remains are well preserved by waterlogging or desiccation: fungi and dehydration may destroy the starch • Within the same species, there are differences in shape and size • Different
sized grains will survive differentially in the archaeological record (Haslam 2004).
For a general procedure see below. Of course, depending on the questions that are being asked and the artifacts that are being analyzed, the methodology will vary.
Advantages of using starch grains: •Often identifiable to species as grains are distinct • Their common occurrence make starch grain analysis very useful • Preserves in conditions that other remains may not (e.g. pollen, fragile seeds).
Disadvantages of the technique: •Factors such as heat and water absorption may affect the structure of the grains, making identification more difficult •Even if the remains are well preserved by waterlogging or desiccation: fungi and dehydration may destroy the starch • Within the same species, there are differences in shape and size • Different
sized grains will survive differentially in the archaeological record (Haslam 2004).
For a general procedure see below. Of course, depending on the questions that are being asked and the artifacts that are being analyzed, the methodology will vary.
Identification of Starch Grains
Various starch grains.
Photo credit: (http://archaeobotany.dept.shef.ac.uk)
Plants of different species manufacture different starch grains. These differences are visible on a number of levels: For example:
Granule types: •Simple •Compound •Semi-compound
Size and shape: •Wide range of shapes and sizes •Those high in amylose tend to be more elongated and irregular •Range from 1 to 100 microns (tend to be on the smaller end) •Site of storage and age of granules influences size
Features that allow identification of starch grains include: presence of hilum, layering, birefringence, and extinction cross, and also testing for presence of starch using iodine (staining black to blue in presence of starch). See below for more on the morphology.
Although plants of different species make specific starch grains, there is still a lot of variation within species, and therefore it is important to have a solid reference collection to which one can compare the samples to.
Morphology of starch grains
Potato starch. Credit: http://archaeobotany.dept.shef.ac.uk/wiki/index.php/Image:PotatoIKI.jpg
Each individual starch grain is composed of the following characteristics:
Hilum: Core of the grain
Fissures: Present in some grains
Lamellae: Growth layers
Striations/Vacuoles/open hila: There are only found in some grains
Extinction cross: dark cross pattern seen as rotating when using polarizing light (as seen on image on the right). Image in the center shows the same grains in unpolarized light, and therefore without the extinction cross visible. This is also known as the Maltese Cross.
Hilum: Core of the grain
Fissures: Present in some grains
Lamellae: Growth layers
Striations/Vacuoles/open hila: There are only found in some grains
Extinction cross: dark cross pattern seen as rotating when using polarizing light (as seen on image on the right). Image in the center shows the same grains in unpolarized light, and therefore without the extinction cross visible. This is also known as the Maltese Cross.
Extraction of Starch grains
Heavy liquid separation of starch, using sodium polytungstate. The starch (invisible to the naked eye) floats to the surface, while the heavier chemical sinks to the bottom.
As already mentioned, the methods used to extract starch will vary depending on the artifact and research question asked. Below I include the basic methods, and for those interested in more in-depth information, I suggest consulting Torrence and Barton (2006), as well as consulting the Foundation for Archaeobotanical Research in Microfossils website which provides a protocol, as other important resources for those interested in doing this type of analysis.
Basic procedures for stone tools include
1) Decide if other studies are being performed besides starch (starch removal will usually be the first as other procedures may destroy the starch-grains)
2) Examine stone tool under a microscope (stereo-microscope/low magnification) to see if any starch is visible
3) Note use-wear and residue spatial patterns
4) Dry brush the artifact to remove adhered sediment
5) Remove residue from various locations on tools with water (using pipette, needle probe). A sonicator can also be used to remove starch-grains adhering to the tool.
6) Residues will usually be separated from other sediments through centrifugation and heavy-metal flotation
7) Mount aqueous extract on slide, put cover slip on
8) View under microscope, with a minimum magnification of 400x.
Basic procedures for stone tools include
1) Decide if other studies are being performed besides starch (starch removal will usually be the first as other procedures may destroy the starch-grains)
2) Examine stone tool under a microscope (stereo-microscope/low magnification) to see if any starch is visible
3) Note use-wear and residue spatial patterns
4) Dry brush the artifact to remove adhered sediment
5) Remove residue from various locations on tools with water (using pipette, needle probe). A sonicator can also be used to remove starch-grains adhering to the tool.
6) Residues will usually be separated from other sediments through centrifugation and heavy-metal flotation
7) Mount aqueous extract on slide, put cover slip on
8) View under microscope, with a minimum magnification of 400x.
Reference collection
Maize starch, as seen under non-polarized light. Comparative collection slide.
A reference collection consists of microscope slides that contain starch from modern plant species. These allow analysts to check and compare archaeological samples with a modern collection. This reference collection should consist of various parts of the same plant.
1. Design a collection with specific vegetation areas/types in mind
2. Identify sources of material, for example seed banks or botanical gardens
3. Preparation and storage of slides, which can be temporary or permanent mounts.
1. Design a collection with specific vegetation areas/types in mind
2. Identify sources of material, for example seed banks or botanical gardens
3. Preparation and storage of slides, which can be temporary or permanent mounts.
Starch Grain Database Project: I am interested in compiling a database of images of starch grains from as many different plant species as possible. If you are interested in contributing images, please contact me.
Additional general references
Haslam, Michael, Gail Robertson, Alison Crowther, Sue Nugent and Luke Kirkwood, editors. 2011. Archaeological Science Under a Microscope (Terra Australis 30) Studies in Residue and Ancient DNA Analysis in Honour of Thomas H. Loy. ANU E Press.
Hather, J.G. (ed.) 1994. Tropical Archaeobotany: Applications and New Developments, pp. 86-114. Routledge, London.
Messner, Timothy C. 2011. Acorns and Bitter Roots: Starch Grain Research in the Prehistoric Eastern Woodlands. University of Alabama Press, Tuscaloosa, AL.
Torrence, Robin, and Huw Barton, editors. 2006. Ancient Starch Research. Left Coast Press, Walnut Creek, CA.
Additional references from studies around the world
Cagnato and Ponce 2017. Ancient Maya manioc (Manihot esculenta Crantz) consumption: Starch grain evidence from late to terminal classic (8th–9th century CE) occupation at La Corona, northwestern Petén, Guatemala. Journal of Archaeological Science: Reports.
Dickau et al. 2009. Starch grain evidence for the preceramic dispersals of maize and root crops into tropical dry and humid forests of Panama. PNAS 104(9): 3651-3656.
Duncan et al. 2009. Gourd and squash artifacts yield starch grains of feasting foods from preceramic Peru. PNAS 106 (13): 202–206.
Gong et al. 2011. Investigation of ancient noodles, cakes, and millet at the Subeixi Site, Xinjiang, China. Journal of Archaeological Science 38: 470-479.
Henry et al. 2010. Microfossils in calculus demonstrate consumption of plants and cooked foods in Neanderthal diets (Shanidar III, Iraq; Spy I and II, Belgium). PNAS 108(2): 486-491.
Horrocks et al. 2008. Introduced taro (Colocasia esculenta) and yams (Dioscorea spp.) in Podtanean (2700-1800 years BP) deposits from Me´ Aure´ Cave (WMD007), Moindou, New Caledonia. Journal of Archaeological Science 35: 169-180.
Zarillo and Kooyman 2006. Evidence for Berry and Maize Processing on the Canadian Plains from Starch Grain Analysis. American Antiquity 71(3): 473-499.
Dickau et al. 2009. Starch grain evidence for the preceramic dispersals of maize and root crops into tropical dry and humid forests of Panama. PNAS 104(9): 3651-3656.
Duncan et al. 2009. Gourd and squash artifacts yield starch grains of feasting foods from preceramic Peru. PNAS 106 (13): 202–206.
Gong et al. 2011. Investigation of ancient noodles, cakes, and millet at the Subeixi Site, Xinjiang, China. Journal of Archaeological Science 38: 470-479.
Henry et al. 2010. Microfossils in calculus demonstrate consumption of plants and cooked foods in Neanderthal diets (Shanidar III, Iraq; Spy I and II, Belgium). PNAS 108(2): 486-491.
Horrocks et al. 2008. Introduced taro (Colocasia esculenta) and yams (Dioscorea spp.) in Podtanean (2700-1800 years BP) deposits from Me´ Aure´ Cave (WMD007), Moindou, New Caledonia. Journal of Archaeological Science 35: 169-180.
Zarillo and Kooyman 2006. Evidence for Berry and Maize Processing on the Canadian Plains from Starch Grain Analysis. American Antiquity 71(3): 473-499.