Gastric Plant Matter and Stomach Content Analysis

Gastric content collection is the pathologist's responsibility, but the forensic botanist should communicate sample requirements to the autopsy team before the examination begins. The stomach should be clamped at both the cardia and pylorus before it is opened, to prevent loss of liquid contents. The full stomach volume is weighed before and after emptying, and the emptied contents are described macroscopically: colour, consistency, odour, presence of recognisable food particles, and estimated degree of digestion.

For botanical analysis, a portion of the gastric contents, typically 10-20 grams of solid material or an equivalent volume of liquid gastric juice, is retained in a labelled container with formalin fixation or refrigeration at 4 degrees Celsius. Formalin preserves cellular structures but prevents molecular analysis; if plant DNA identification is a possibility (rare but applicable in some cases), an unfixed aliquot should be frozen at minus 20 degrees Celsius separately.

Gastric content samples are processed using a combination of wet-sieving, flotation, and centrifugation to separate the resistant plant fractions from the organic soup of digested material. The standard workflow begins with washing the sample through a sieve stack (typically 500 micrometres and 125 micrometres) to remove large undigested particles from a finer residue. The coarser fraction is examined macroscopically and photographed before any microscopic work.

1. Macroscopic examination
   Sieved material is spread on a white tray and examined with the naked eye and a hand lens. Large food fragments, seed coats, fruit skins, vegetable fibre, and gritty particles are sorted and described. Identifiable items are photographed and retained.
2. Starch grain preparation
   A drop of gastric liquid or smeared residue is placed on a microscope slide, covered with a coverslip, and examined under polarised light. Starch grains polarise strongly to a 'Maltese cross' pattern and are readily visible. Shape, size, and hilum character are documented and compared to a reference starch collection.
3. Cuticle preparation
   Residue is treated with a mild bleach solution to lighten pigmentation and then mounted. Cuticle fragments are identified by their cellular patterning under transmitted light microscopy and compared to cleared reference leaf material.
4. Pollen and phytolith recovery
   The fine residue fraction (passing the 125-micrometre sieve) is subjected to acetolysis (for pollen) or heavy-liquid flotation (for phytoliths) using the same protocols as environmental sample processing. Counts and identifications are made against reference slides.

Starch grains are among the most informative plant micro-fossils in gastric analysis: they are abundant in most human diets and morphologically distinctive across the major food plant groups. Under polarised light, intact starch grains show a characteristic extinction cross ('Maltese cross'), and the shape, size, and hilum detail allow genus-level identification for most staple foods.

Starch digestion by salivary and pancreatic amylase begins before food reaches the stomach and continues in the small intestine. This means that starch grains in gastric contents are often partially digested, showing eroded surfaces, broken hilum structure, or incomplete extinction crosses. Reference collections of partially degraded starch, produced by controlled acid or amylase treatment of known samples, are needed to interpret partially digested material correctly and avoid misidentification of degraded forms.

Leaf and fruit cuticles consumed with vegetables and salads pass through the stomach largely intact because the waxy cutin polymer is not targeted by any human digestive enzyme. Under transmitted light microscopy after mild bleaching, they reveal the same epidermal cell patterns used for botanical identification from crime scene evidence: stomatal complex type, anticlinal wall undulation, and trichome remnants. For case purposes, identifying cuticle fragments from food plants such as tomato skin (with its characteristic glandular trichome bases), capsicum, or leafy vegetables provides confirmation of the meal's plant component even when the soft flesh has digested away completely.

Seed coat fragments from commonly eaten foods are equally useful. Raspberry, strawberry, sesame, poppy, tomato, and fig seeds all have distinctive testa surface textures that are identifiable to species in gastric residue. These fragments are abundant: a single portion of raspberry jam contains thousands of seeds, and their testa fragments persist well into the small intestine. In several forensic cases, the identification of specific seed types in gastric contents has allowed the investigating team to narrow down the victim's likely last meal to a specific type of food even when no witness could provide direct information.

Airborne pollen is inhaled with every breath and swallowed in saliva continuously, so low levels of wind-pollinated pollen in stomach contents are a normal background finding and not evidence of deliberate consumption or a specific location. The forensically useful pollen signal in gastric contents comes from higher concentrations of insect-pollinated species, which are not normally present in airborne pollen rain at significant levels.

Routes of deliberate pollen ingestion include honey (which concentrates entomophilous pollen from the plant species the bees visited), herbal teas (which carry pollen from the plant material used), pollen supplements, and strongly flavoured fresh herbs eaten raw. Each of these routes produces a characteristic assemblage: honey pollen is dominated by a few heavily visited species; herbal teas produce high concentrations of the target plant's pollen mixed with occasional contaminants from other herbs stored nearby.

In a homicide investigation, the presence of an unusual pollen type in the stomach, not typical of the local background and not explained by the victim's known diet, can indicate either ingestion of a plant product from a specific region or travel to a location where that pollen is locally concentrated. Combined with other pollen evidence from clothing or from the deposition site, it can contribute to reconstructing the victim's movements in the final hours.

Stomach content evidence is not a standalone PMI method. Its value comes from integration with other lines of evidence. The pathologist provides an estimate of gastric emptying stage based on the volume and consistency of remaining contents; the forensic botanist tells the pathologist which plant species are present and at what stage of digestion. Both experts must understand gastric emptying physiology to interpret the findings correctly, and neither should reach a time-since-meal conclusion without the other's input.

Several factors confound simple gastric emptying timetables. A large, fat-rich meal empties more slowly than a small carbohydrate meal. Alcohol slows gastric motility. Fear and physical stress can cause gastric stasis, where the stomach stops emptying even though a meal has been consumed. In violent deaths, gastric emptying may have been interrupted by the physiological response to trauma, meaning that the stomach content volume at autopsy may overestimate the time between the last meal and death. A careful report acknowledges these confounders explicitly rather than presenting a single time estimate as definitive.

Gastric plant analysis answers some questions precisely (which plant species were eaten) and others only approximately (when they were eaten). The time-since-meal question is the most frequently asked in court, and it is also the most susceptible to overstatement. Gastric emptying ranges in published studies span several hours for any given meal type, and individual variation within those ranges is large. An expert who claims to identify the last meal time to within an hour is likely overstating the method's precision.

Post-mortem redistribution of gastric contents is another consideration in decomposed bodies. Gastric acid continues to act on the stomach wall after death, and gas pressure from putrefaction can force gastric contents back into the oesophagus and even the airway. Finding plant material in unexpected anatomical locations therefore requires caution in interpretation, and any unusual distribution should be noted in the report rather than ignored.