The design and use of an optical punch for maize kernel internal crack detection

The mechanical forces exerted by agricultural machinery on grain kernels is a critical factor in ensuring efficient harvesting. Traditional methods that assess kernel compressive strength solely based on fracture force, defined as the point of complete breakage, often overlook the formation of internal cracks. These internal cracks can significantly compromise germination rates and post-harvest handling quality, making their detection essential. To address this limitation, an “optical punch” device was developed to enable real-time observation of internal crack initiation and propagation during single-kernel compression tests. This method fills a gap in conventional quasi-static compression testing, which lacks the capability to monitor internal damage in real time. Using this system, force–displacement data were synchronised with video and audio recordings. Yellow dent maize kernels at 10.5 and 15.5 % moisture contents were tested using the optical punch. The results reveal the stages of crack development, with differences between the forces required to initiate internal cracks and those leading to kernel fracture. A linear correlation between crack initiation force and fracture force was observed, expressed as $Fc=m·F_f+b$, with parameters (m, b) = (0.99, 40.99 N) for 10.5 % moisture and (1.13, 52.20 N) for 15.5 %. Both crack initiation and fracture force followed log-normal distributions. The results also reveal that increasing moisture content (up to 15.5 %) raises the forces required for both crack initiation and complete fracture. The optical punch provides a useful method for linking crack formation to external mechanical loading, although it is limited by single-angle observation and the inability to detect sub-millimetre cracks.