Internal structure of carbonized wheat (Triticum spp.) grains: relationships to kernel texture and ploidy

Abstract

The identification of wheat grains to the species level is problematic in many archaeobotanical samples, yet this is key to better understanding wheat phylogeny and agricultural trajectories. This study was conducted to see if the pronounced differences in kernel texture (grain hardness) which exist among einkorn (very soft), emmer/durum (very hard), and bread wheat/spelt (soft) would manifest themselves after charring (carbonization). Grains of these three species were either broken and then charred, or charred and then broken (charring at 250, 270 and 400 °C). All specimens were then examined using field emission scanning electron microscopy. At 250 °C, grains broken before charring showed distension across the broken exposed endosperm surface. In the two soft wheats (Triticum monococcum, T. spelta; einkorn and spelt), starch granules were still evident, whereas in T. dicoccum, emmer, the fracture surface was nearly smooth. When charring occurred before breaking, a flat fracture face resulted. At 270 °C, distension increased for grains broken before charring whereas in those broken after charring, the fracture plane was again perpendicular to the longitudinal axis and was present as an open sponge-like topography. At 400 °C, distension was pronounced and represented “puffing” in grains that were broken before charring. Those that were charred and then broken produced a markedly open porous structure, but again with aspects of an angular fracture plane. In conclusion, several morphological features related to grain hardness, and therefore ploidy and genus were present at the lower charring temperatures. These differences largely disappeared at 400 °C. Characteristics of the carbonized grains provide some diagnostic features as to possible food processing or damage during recovery.