2.08 MECHANICAL PROPERTIES
Mechanical properties are a dimension of plant health similar to strength or balance in human health. It relates to the physical robustness and the ability of the plant to be functional in its physical environment. Mechanical properties are quantified in a multitude of ways and in reference to how the plant structure reacts to physical force. Many mechanical properties relate to the strength of a root, but properties on the way a root deforms under stress have also been noted as relevant (Vukov, 1977). A healthy sugar beet root is one with adequate mechanical properties. Too little of a mechanical property will mean a root will not tolerate mechanical harvest and handling. Given the need to slice roots at processing, it is also not desirable to have too much of certain properties. The current status of the commercially available sugar beet genetic material is such that there is little concern for there being too much of certain properties.
A recently completed doctoral research project has studied mechanical properties of sugar beet roots in detail (Kleuker, 2022). That project shares Paper I of this thesis. That thesis focused on the mechanical properties assessed and method developed in Kleuker and Hoffmann (2019). This includes the resistance to puncture forces at the outer five millimetres of a root, and to compression forces in the core of the root. Clear links between higher values for the mechanical properties and reduced loss of quality during post-harvest storage were found (Hoffmann et al., 2022; Kleuker & Hoffmann, 2020, 2021, 2022). The causal mechanism was consistently identified as reduced mechanical damage and thus reduced need for wound healing and reduced mould establishment. The strongest correlations with mechanical properties were with variety. Prior to Kleuker (2022) and its standardised method, similar results were found in Gorzelany and Puchalski (2000, 2003); Nedomová et al. (2017).
In reference to the post-storage process of slicing at the factory, Vukov (1977) notes that other mechanical properties should be considered. Resistance to cutting is a property that is ideally measured directly, although states that it is comparable to a measure that in its description seems similar to puncture resistance as defined in Kleuker and Hoffmann (2019). This is an example of a non-monotonic dimension of plant health, where roots are graded on their resistance to cutting on the scale Soft – Normal – Suberized – Woody – Extremely woody. Vukov (1977) also cites the modulus of elasticity as an important descriptor of how well roots will slice. It is noted that elastic behaviour will also have a bearing on bulk density and porosity of a bulk of sugar beet roots. It has further been suggested that more elastic fruit should suffer less damage than less elastic fruit from a given impact (Ruiz-Altisent, 1991).
While not a mechanical property per se, cell wall content is another property of sugar beet roots that has been linked to storability. Alcohol insoluble residue (AIR) content is a measure of the cell wall content of the root (van Soest et al., 1991) and shows strong correlation with variety and mechanical properties (Kleuker & Hoffmann, 2022). Similarly, marc content is an indication of the post processing pulp content of the root and had been linked to variety and storability (Hoffmann et al., 2018; Vukov, 1977).