STARCH : INTRODUCTION
Starch occurs in plants as morphologically distinctive microscopic granules. These granules are produced in large quantities in storage organs, including many of the seeds, grains, nuts, roots, tubers and rhizomes that are staples of human diet or were a component of past subsistence strategies. Starch granule morphology varies depending on botanical origin and may be used to differentiate between families, genera and even species. In some cases, plants that do not produce other diagnostic microfossil types (such as phytoliths or pollen) produce starch. Starch granules are preserved in a wide variety of archaeological contexts, including situations where the survival of plant macroremains is poor.
Starch has been found as residues of plant-processing activities on artefact surfaces and in site sediments, as evidence of diet and consumption in dental calculus, coprolites, stomach contents and cess pits, and in charred and desiccated food remains. Starch granules are therefore an important archaeobotanical tool for documenting the presence of plants through time and across space, as well as for providing a more direct link between the archaeobotanical record and past human-plant interactions at various stages of procurement, storage, processing and consumption (Torrence and Barton 2006; see also the bibliography of case studies in ancient starch research). The study of ancient starch generally requires chemical extraction and slide preparation for light microscopic examination under magnifications ranging from 400-1000x.
Starch production in higher plants
Most starch granules of archaeological interest occur in plant storage organs such as seeds, roots and tubers, where they are synthesised by the plant as a long-term energy reserve. These granules range in size from <1 µm to about 200 µm in size (note: 1000 µm = 1 mm) and have a variety of shapes, including round, ovate, lenticular, reniform, bell and polygonal. Storage starch granules form in specialised plastids called amyloplasts, where they can occur either as single ‘simple’ granules or compound aggregates. Starch granules are also produced in chloroplasts within stems, leaves and other photosynthetic tissues. These starches have a very transitory existence. They are synthesised on a daily basis and are thus found to be generally small and morphometrically non-diagnostic (Gott et al. 2006; Haslam 2004; Therin et al. 1997). Transitory starch has been little studied, however, and more research is required to assess its significance in archaeological analyses.
Starch granule composition, structure and properties
Starch granules grow by apposition (the laying down of concentric layers) around a central growth point known as the hilum. The layers are composed mainly of two polysaccharides, amylose and amylopectin, which are arranged in alternating amorphous and semi-crystalline lamellae. Amylose has an essentially linear structure and is mainly found in the amorphous regions, while amylopectin, which is highly branched, occurs chiefly in the crystalline lamellae (Buléon et al. 1998). Starch granules also contain minor quantities of lipids, proteins, moisture and phosphorus. The highly ordered molecular structure of the crystallites gives starch granules semi-crystalline properties.
The relative abundance of amylose (usually ~20-40%) and amylopectin (~60-80%), as well as the structural arrangement of the crystallites vary between taxa and determine a granule’s chemical and physical properties. These include degree of resistance to chemical and enzymatic hydrolysis, intensity of staining reaction with iodine (which forms a blue-coloured complex with amylose), and the temperature at which granules gelatinise (become amorphous in the presence of heat and moisture) (Banks and Greenwood 1975; Gott et al. 2006; Loy 1994).
Deposition of starch in the archaeological record
Starch can be deposited in the archaeological record through general vegetative decay or as the result of human activities involving plants as food, craft, medicine and animal fodder. Plant foods containing starch grains often need to be processed prior to consumption, to make them more palatable, less toxic, more easily digestible (by altering the starch polysaccharides to a form more accessible to gut enzymes), or to transform them into a form suitable for the preparation of different types of food, e.g. grinding seeds to prepare flour. These activities often leave residues containing starch granules on the surfaces of food-processing implements such as grinding stones, pounders, mortars and pestles, pottery vessels (used for cooking, storing, mixing, serving), flaked stone tools (used to peel, cut or slice tubers or starchy-fruits, or used in pith extraction). Starch granules can also accumulate in the sediments immediately surrounding these activities. Food products themselves, which may be preserved through charring or desiccation, can also contain identifiable starch granules. When starchy foods are consumed, they can be incorporated into dental calculus (mineralised plaque deposits on teeth), or trapped in cavities. A proportion of starch granules in consumed foods can escape breakdown by starch-degrading enzymes in the gut and be excreted in faecal material, from which they can be recovered archaeologically from coprolites or cess deposits. Starch granules have also been found in the stomach contents of Egyptian mummies.
Starch degradation and preservation
The mechanisms of starch survival and degradation are still being understood, but it is clear from the growing number of archaeological studies that they can preserve in a variety of contexts and depositional environments, despite their susceptibility to degradation by a number of physical, biological, chemical and thermal processes. These processes can affect starch survival prior to deposition in the archaeological record such as during plant-processing (grinding, cooking, charring, fermentation, etc.) or digestion in the gut, as well as post-deposition owing to soil-borne starch-degrading enzymes, mechanical attrition by sediment particles, stress fracture from repeated cycles of wetting and drying, or soil chemical properties (e.g. extreme acidity or alkalinity, presence of chaotropic salts) (see Barton and Matthews 2006; Haslam 2004).
Given their very small size, it is easy for starch grains to get trapped or embedded in places where they are protected from degradation, such as cracks or pores on the surface of an artefact, soil aggregates, or charred or calcified matrices (e.g. pottery residues or dental calculus). Desiccating or anaerobic environments and the presence of certain microbe-inhibiting components such as metals (e.g. copper, manganese) and tannic acids are also likely to promote starch survival (Barton and Matthews 2006; Haslam 2004). Overall, a stable microenvironment in which temperature and hydration are regulated and movement is restricted will be favourable for starch longevity. Considering also that starch is produced in very large quantities in plant storage organs (for example, a single wheat grain contains 2-3 million starch granules) (Chojecki et al. 1986), assemblages large enough for archaeobotanical identification and inference can still be recovered, even if survival rates are extremely low.
References
Banks, W.T. and C.T. Greenwood (1975) Starch and its Components. Edinburgh: Edinburgh University Press.
Barton, H. and P. Matthews (2006) Taphonomy. In R. Torrence and H. Barton (eds) Ancient Starch Research, pp. 75-94. Walnut Creek, California: Left Coast Press.
Buléon, A., P. Colonna, V. Planchot and S. Ball (1998) Starch granules: structure and biosynthesis. International Journal of Biological Macromolecules23:85-112.
Chojecki, A.J.S., M.D. Gale and M.W. Bayliss (1986) The number and sizes of starch granules in the wheat endosperm, and their association with grain weight. Annals of Botany 58:819-831.
Gott, B., H. Barton, D. Samuel and R. Torrence (2006) Biology of starch. In R. Torrence and H. Barton (eds) Ancient Starch Research, pp. 35-45. Walnut Creek: Left Coast Press.
Haslam, M. (2004) The decomposition of starch grains in soils: implications for archaeological residue analyses. Journal of Archaeological Science31:1715-1734.
Loy, T.H. (1994) Methods in the analysis of starch residues on prehistoric stone tools. In J.G. Hather (ed.) Tropical Archaeobotany: Applications and New Developments, pp. 86-114. London: Routledge.
Therin, M., R. Torrence and R.L.K. Fullagar (1997) Australian Museum starch reference collection. Australian Archaeology 44:52-53.
Torrence, R. and H. Barton (2006) Ancient Starch Research. Walnut Creek: Left Coast Press.