Starch taphonomy

The interpretation of ancient starch residues should always take into consideration assemblage formation processes. Starch, like other microremains, can move around in archaeological deposits as a result of post-depositional processes such as bioturbation, water percolation, animal activity, as well as cultural processes such as sweeping, trampling, digging pits or holes, and other activities. Starch may be transfered to the surface of an artefact from the surrounding sediment matrix, or incidentally by handling.

Assessing whether starches recovered from sediments, artefacts, or other contexts relate directly to that context or are intrusive is therefore an important consideration. Examining distribution patterns of starches on artefacts (e.g. comparing starches from used and unused surfaces) and in surrounding sediments is the most common approach. Experimental studies have also provided some basic parameters for understanding the extent of starch movement in sediments from water percolation (Haslam 2009; Therin 1999). Some contexts such as charred cooking pot residues and dental calculus may be considered more 'secure' for starch recovery because the microfossil are trapped and sealed relatively soon after deposition. Starches may also be sealed in situ on the surface of house floors that have been covered over by subsequent floor layers, which may be assessed using micromorphology.

The effect of deposition, preservation and recovery biases on the composition of ancient starch assemblages should also be considered. These biases affect how the presence or absence of starches in a particular context are interpreted, and how assemblages from different contexts are compared. In terms of deposition, is clear that certain plants will contribute more starch to the archaeological record than others, which may need to be taken into account when interpreting quantitative data. Preservation is likely to differ not only between depositional environments but also between starch types depending on their physicochemical properties. It has been suggested, for example, that the surfaces of stone tools (where starches can be trapped in pits and crevices or embedded in plant residues that adsorb to the tool surface) provide a more stable microenvironment for starch preservation than sediments, in which starch is loosely bound in the matrix and less protected from mechanical abrasion and microbial attack (Perry 2002, Perry 2007). It is not yet clear how this factor affects comparisons between starch assemblages on stone tools and in the surrounding sediments (see discussion in Haslam 2004).

Cultural activities can also affect starch assemblages, as many of the processing techniques commonly used to prepare starchy foods will damage, degrade and even destroy starch granules. It is possible that certain starches will degrade preferentially during food processing, owing to different physical and structural properties. Laboratory recovery techniques may also bias assemblages towards certain starch types. For example, the protocol recommended by Lentfer and Therin (2006) for separating starches using heavy liquid flotation is designed to recover granules >5 µm diameter - although smaller granules are routinely recovered using this method, some may be lost during decanting after centrifugation. Likewise, small starch granules as well as damaged starches are often much harder to locate during microscopy and can be overlooked.