As a researcher on starch and an author of the book Ancient Starch
Research, let me add my own perspective on the issue of preservation,
which is something that I am very interested in understanding. I
certainly don't take it for granted that every starch grain seen on the
surface of stone tool or in sediment is necessarily 'ancient'; a case must
be made by the analyst as to their reasons for interpreting the material
as an archaeological residue. To that end I have published a few papers
that specifically deal with the argument of starch preservation and more
broadly on our understanding of what an organic residue actually is - if
not contamination, as is often claimed. Most Recently I published a paper
looking at residues on museum artefacts and discuss the preservation
taphonomy of starch in that particular context.

Barton, H. 2007. Starch residues on museum artefacts: implications for
determining tool use. JOurnal of Archaeological Science 34: 1752-1762.
 
In that study, I found that starch granules were often found in very
different states of preservation on a tool surface. Some granules were
heavily degraded by microbial activity - showing all the charactersitc
forms of enzyme attack that the literature predicts and fits with
expectations, however, many granules, from the same residue extraction
were recovered in a perfectly good state of preservation. Apparently
unaffected by enzyme decay. why? I suggest that the formation of
an 'organic residue' - which must generally consist of some mixture of
mashed up plant material, lipids, etc. dries rapidly and entombs some
starches preventing access to microorganisms - some starches are not so
protected and are degraded by all the mechanisms known to degrade starch.
Sometimes these old starches have different visual properties to modern
starch, and have been altered in some way; most likely changes to the semi-
crystalline layers judging by the changes seen in the extinction cross
under cross polarised light (see Fig 7 and Fig 8). Decay does occur, and
starch are clearly removed from the environment - probably in very large
numbers, but organic residues themselves help preserve starch - which is
itself an extremely stable compound. Everything we could find at the time
that discussed likely mechanisms of preservation and degradation of starch
was published in the Taphonomy chapter in Ancient Starch Research (Barton
and Matthews 2006). Old starch also stains with IKI and Trypan Blue as
per our expectations for modern starch - which is also telling us
something about the chemical and structural properties of preserved starch.

Barton, H., Torrence, R., Fullagar, R. 1998. Clues to Stone Tool Function
Re-examined: Comparing Starch Grain Frequencies on Used and Unused
Obsidian Artefacts. Journal of Archaeological Science 25: 1231-1238

In this early study (largely in response to the criticisms of residue
analysis justifiably raised by Roger Grace) I undertook a study with
Richard Fullagar that involved a blind test comparing starch granule
counts with assessments of use based on usewear analysis. After I had
undertaken a whole tool extraction and counted the starch recovered, the
sonic cleaned artefacts were given to Fullagar without any knowledge of my
counts and the results compared. That study found a strong correlation
between high starch counts (an order of magnitude higher) than either
unused flakes or the soils fromt the bag that contained the artefacts.
That helped satisfy me that the starch residues were a) a real phenomena
related to tool use (unless starch granules know which tools were used in
the past and migrate towards them) and b) that cross-contamination from
sediments does occur but will reflect the general counts of starch
encountered in soils. This of course raises further issues of depostional
context and the definition of 'contamination' (which becomes a scale
question). For example starch found on tools and in soils may relate to
the history of site use, ie, that plants were processed here, contributing
starch and tools into the archaeological record.

For a more recent blind test on starch granule identification see:

Mercader, J., et al. (2007) 4,300 year old chimpanzee sites and the
origins of percussive stone technology. PNAS 104: 3043-3048.

The starch identification test was undertaken by two analysts (Huw Barton
and Robert Tyler) working independently, in fact the results were never
disucussed by either party, so it gives a pretty good sense of what can be
done and of the reliablity of single granule identifications to species or
genus level (at least with this sample of nuts and yams)

In press currently I am publishing the results of small two-year study of
starch preservation on tools (some left on the surface and some buried).
It is a study I set up many years ago before I thought I would get into
starch analysis in a major way, so many variables are not recorded such as
soil Ph, etc. however, I found that starch (sweet potato) was preserved
for two years on tools left on the surface and buried. this study will
surface in a special volume being prepared in honour of Tom Loy. I also
noted that many starch granules were completely surrounded by very small
particulate matter, which I believe (and see arguments in the Taphonomy
chapter in Ancient Starch Research, pp. 84-85 on particulate organic
matter) can effectively shield starch from microbial attack - if the
microbes cannot reach the granule, it cannot be degraded, at least not by
that mechanism. I also think that once starch gets into a soil - of which
there will be a great quantity - it will be under most threat of decay in
the upper humic horizon, what escapes (and again see papers on the
properties of Particulate Organic Matter in soils) is essentially locked
up in the sediment, and variables such as depth of burial and pore size of
sediments will also be important - compaction can be such that bacteria
can no longer move. The presence of water is also extremely important -
as this is the vehicle for moving bacteria in soil.

Further good studies looking at starch in sediments are:

Balme, J and Beck, W. 2002. Starch and charocoal: useful measures of
activity areas in archaeological rockshelters. Journal of Archaeological
Science 29: 157-66.

Atchison, J. and Fullagar, R. 1998. Starch residues on pounding implements
from Jimnium rock-shelter. In Fullagar, R. ed. A Closer Look: recent
Australian Studies of Stone tools, pp. 109-126. Sydney University
ARchaeological Methods Series 6.

Chapter 8 by Robin Torrence, 'Starch in Sediments' in the Ancient Starch
Analysis book is also worth a look in the context of this debate and
reviews some useful qualitative and quantitative experiements.

Recent work of my own has also looked at starch in sediments from a cave
site in Borneo e.g.

Barton, H. 2005. The Case for Rainforest Foragers: The Starch record at
Niah Cave, Sarawak, Asian Perspectives 44(1): 56-72. (discusses the aroid,
tuber starch and sago starch in cave sediments - people must have carried
plant material into the site, its entrance is well above the forest floor
and decaying underground tubers are unlikely to produce quantities of
airborne starch -though a flour mill might.

Barton, H. and Paz, V. Subterranean Diets in the Tropical Rain Forests of
Sarawak, Malaysia, in Denham et al. (eds). Rethinking Agriculture, pp. 50-
77. this paper further contextualises the results of the starch granule
analysis by comparison with charred plant macro-remains from the site.

There is every reason to expect starch in 'ancient' sediments is in
fact 'ancient' starch. While some people have raised the issue of
possible post-depositional transport of modern starch into old sediments -
it must occur to some degree - but such a mechanism must equally apply to
all particulate matter of similar size, ie., pollen, phytoliths, and micro
charcaol etc. It cannot all be contamination - or a lot of pollen analysts
are going to be very unhappy people.

As for dating, that would be great experiment to do, but I am not sure
whether AMS could handle the one or even ten starch granules recovered
from most sediment extractions - dating a tool residue with starch might
be more likely to give a good result, even so, the quantites of organic
matter are very small.

I can also help out with starch mimics, if anyone wants to send me pics.
Also I had thought of setting up a web site with a lot of that material,
but wasn't sure there would be much interest-though this thread suggests
otherwise.