Changes in physico-mechanical properties of water caltrop fruit (Trapa natans L.) during the drying process

The cosmopolitan water caltrop plant (Trapa natans L.) produces nuts, which in the maturing process develop very hard pericarps. This hardness, together with structure and shape (external spikes) of pericarp and seed, and the water contained in the fruit are responsible for their mechanical properties. This study determined the force needed to break Trapa natans nuts at various drying stages, with tests having been carried out at weekly intervals until the fruit dried completely. The amount of force necessary for cracking nuts at each of the 6 drying stages was determined, as well as the work of crushing calculated until the greatest compressive force (crushing force) was reached. The force needed to rupture the hydrated fruit in the horizontal plane was higher than that necessary for the rupture of dried fruit. The experiment showed that the maximum force needed to crush the fruit was 828.7 N and occurred when crushing the fruit after 2 weeks of drying, while the largest calculated crushing work was 2185.5 mJ for the same fruit. Other strength parameters were introduced to characterize mechanical properties of water caltrop in a more extensive scope. These are hardness defined as a ratio of compressive force increment to strain increment, specific crushing energy defined as a ratio of crushing work to water caltrop's mass, and unit crashing force defined as a ratio of crushing force to caltrop's thickness. All these parameters reached their highest mean values for pericarps after 2 weeks of desiccation. Mass measurements were also applied in modelling the desiccation process by the exponential function. The very dense pericarp material, after reaching maturity, slightly changes during drying. It can be used industrially as an extremely durable and biodegradable biological material. Results also suggest that the great evolutionary success of the species may result from the ability of the pericarp to protect its seeds, leading to the spread of this species in aquatic environments.