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Starch analysis in archaeology

Starch is ubiquitous in the archaeological record. There are records of its existence dating back as far as two million years ago. Ancient starch analysis is not a new technique within archaeology, although it has became exponentially more popular in the last ten to twenty years. Starch analysis can contribute to a large number of archaeological research problems, ranging from diet to environmental reconstruction. For this reason, a very broad set of methods are required to fully analyze starch.

Biology of Starch

Starch is produced in several parts of the plant as a form of energy storage. The formation of starts begins with the process of photosynthesis, where the energy of su nlight is converted into a solid form of potential energy, in the plant's chloroplast. This energy starts a series of reactions that split water into hydrogen and oxygen, and then recombines the free hydrogen with absorbed carbon dioxide to form glucose. This glucose is the basis of all substances required by the plant, including starch. The formation of a starch granule is started at the hilum, with an additional layer added to the granule added each day under normal conditions. When the plant is in need of energy, the storage starch is converted back into glucose and distributed to the areas in need. When the photosynthesis rate is low during the day, small transient starch granules of indeterminate shape are formed in the chloroplast. During the night, these starches are reconverted back into sugar and used as energy or transformed into storage starch in the amyloplasts. Transient starch granules are less likely to be identifiable to plant taxa because their shapes are not species specific. Starch granules can be classified as simple, compound, or semi-compound depending on how they are formed in the amyloplast. Simple granules only have one component, while compound granules have several parts called subgranules or granula. Semi-compound granules have one exterior surface and two or more hila. Starch reacts to several stains in different ways, which is useful when for identification.

Microscopy

Starch granules are usually under 100 microns. Due to their small size, they are best observed under compound microscopes equipped with various lighting conditions and magnifications from x200 to x800.

Low magnification

Research focused on residues adhering to artifacts or macroremains should start at lower magnifications with a stereoscope. Most data obtained at this stage are mostly qualitative, but are very important to the next analysis stage. Magnifications between x10 and x50 are usually enough to locate potential residues. At low power, it is easier to detect and describe features and to see internal structures of macroremains or gather information about tool residues.

High magnification

Modern light microscopes used for high-power magnification have an internal light source often with both transmitted and reflected illumination. These microscopes can provide a maximum magnification of up to x1000. This is good enough to resolve clear images of starch granules as small as a few microns in diameter.

Reference Collection

A reference collection is the major tool for identifying starch granules to the correct taxa. Before building this collection, the size and scale should be defined by a range of factors including whether starch morphology varies with season or growth or environmental conditions. The most important factor is the potential range of plants materials appropriate to the research questions being posed. Collecting and preparing reference material will assist with observations of the variability of starch production and morphology between plant species.

Description, Classification, and Identification

There are two basic methods for identifying starch. The first one involves diagnostic tests of chemical and physical properties of starches, whereas the second one uses the optical properties of the granules. It is unclear whether or not these tests are necessarily useful to archaeologists because they often result in destruction of the morphological integrity of the starch granules.

There are four levels of classification. The first level determines whether or not evidence for the exploitation of plants is present. The second is the study of assemblage variation. The third targets the presence of one or just a few particular species. The fourth level assigns a significant percentage of starch granules within a sample to a particular taxon and presents quantitative data about their relative abundance in the sample. Identification of ancient starch can be done fairly easily for the first three levels of classification. The fourth level requires continued improvements in the description, classification, and identification of starch granules.

Starch in Sediments

Starch granules retrieved from sediments are often used to reconstruct habitats such as those associated with human land use. These granules can answer questions divided into two categories; landscapes, and specific contexts such as settlements or activity areas. The first category concentrates on the reconstruction of past plant communities at the widest scale of the environment or landscape. The second focuses on a smaller scale of individual archaeological sites or separate contexts within them. An example of this would be using starch to help identify specific human activities at a particular location. The stages involved in the analysis of starch from sediments are; sampling, extraction of starch, slide mounting and viewing, and interpretation.

Sampling
The first step in analysis is collecting the sediment samples from the archaeological record. Deciding how the samples will be taken depends on the nature of the archaeological problem and the questions addressed. Sampling a sediment core or sratigraphic profile to gather information about an environment requires a detailed understanding of the way the sediments were formed. Also, the number of samples required will be decided by the desired temporal resolution.

Extraction
After acquiring the samples, it's time to extract the starch granules from the sediment. Most extraction methods adhere to the following general procedure: sample preparation (sieving, drying, or soaking), disaggregation and deflocculation to break up the elements of the sample into single particles, removal of undesired particles (sands, silts, minerals, organics), slide mounting, and viewing. Through time, research is continually conducted to help improve protocols including determining which chemicals will preserve the starch granules the best without destroying them when they are submerged in them.

Slide Mounting and Viewing
After extracting, starch granules are mounted onto a slide and viewed as appropriate for identification and counting. It is important that the material is dried thoroughly before being mounted. This is to ensure that no further destruction is done to the starch by leaving it in liquid. The starch residue can be placed in many different mounting medias including but not limited to water, glycerol, and glycerine jelly.

Interpretation
After the starch granules have been examined, the findings are then recorded and interpreted based on the research questions that are being investigated.

Starch on Artifacts

Artifacts, stone tools in particular, collect starch granules and protect them from decay due to microorganisms, thus providing excellent conditions for long-term preservation. Two types of analysis can be performed given this situation. The first of which gives one the opportunity to study how these artificats were used in the past by combining analyses of starch granules with other approaches, such as the study of use-wear or additional residue types. The second type allows researchers to use the identification of starch granules on artifacts to aid them in the reconstruction of past plant use and diet.

Tool Function
Until recent years, the main focus of starch residues was to reconstruct how tools were used in the past. These studies can help researchers understand a more broader range of behaviorial questions, although just knowing whether or not an artifact has been used for a specific function can be a very useful finding. Starch residues also have the ability to assist researchers in identifying tool functions that may have been overlooked by the implementation of use-wear analysis.

Plant Use
The fact that starch is often concentrated within residues makes artifacts a great place to look for evidence of past plant use. Many studies have been conducted to illustrate this principle. Most of these studies have focused on stone artifacts, mainly those used in food preparation, so there is still an abundance of additional behaviors that could be explored including craft activities involving the preparation of adhesives, medicines, and other nonfood items.

Modified Starch

Modified starch is created when the morphological or physico-chemical structure of native starch is disrupted in some way, such as in food preparation. The most common way to modify starch is to apply heat. Cooking pits, hearths, and ovens that may have come into contact with heated starchy material are popular locations where modified starches can be found. Modified starch is only likely to be preserved in particular conditions such as arid because of its succeptibility to organic decay. Studies of ancient modified starch help researchers understand ancient food technology and variations in cuisine among different social groups, as well as provide an understanding of the function of food-processing equipment. Preserved forms of modified starch include discrete desiccated macroremains, attached desiccated residues, and charred residues. Discrete desiccated macroremains are coherent foods that are not attached to any other object and are among the most recognizable ancient starchy prepared foodstuffs. They can be either intended final prepared foods, like loaves of bread, or intermediate products of the food processing sequence like starch-rich, chaffy lumps. Attached desiccated residues are collections of starchy foodstuffs adhered to a container or vessel. The ability to identify these residues may be affected by the quantity of the existing residue, its appearance, and the awareness of the excavators. This collections with obvious plant tissues in them are most easily recognizable, while thin smears are not as easy to recognize. Charred residues are normally the result of accidental overcooking and can be preserved as discrete fragments or reamain stuck to the cooking vessel. Due to their charred nature, these residues are very difficult to identify. The retrieval methods chosen for the particular archaeological contexts depends greatly on the nature of the residue. All residues found on potsherds or other artifacts should not be washed and shoulbe be handled with care to prevent damage to them. Desiccated and charred discrete residues should be carefully placed inside plastic bags labeled with their provenience upon recognition. Fragments of charred starchy remains can be retrieved if water flotation has been used to recover plant remains. Observations should be made with the naked eye and as the analysis advances, higher levels of magnification can be implemented.

Looking Ahead

While ancient starch analysis is a relatively new concept examined by archaeologists, significant advances in this topic have been made since its earliest records of application in the the 1980s. The ubiquity of ancient starch in time and space, and the fact that its existence is not confined to any particular region or climate , makes it obvious that starch analysis will be used for many years to come and wil play an important role in answering research questions involving the role of plants in human societies. Including ancient starch analysis in the paleobotanists' or archaeologists' arsenal of investigative tools can potentially create many new areas of research that have yet to be examined and also reveal new findings that were previously looked over. The implementation of this fairly new field of study alongside the older, more conventional fields will definitely ensure that the field of plant research in archaeology is constantly advancing.