2.06 MECHANICAL DAMAGE
Mechanical damage is the damage to the crop that occurs from physical actions. For the sugar beet crop, this will be damage related to the actions of machinery. Mechanical damage can occur during the growing season from mechanical weed control or the movement of machinery through the field. It seems reasonable to assume that this source of damage will not be of significance to post-harvest storage. No studies relating storability to mechanical damage occurring during the growing season are known. Mechanical damage at harvest and transport is, conversely, omnipresent and of large consequence. All harvest damage has been shown to reduce the storability of sugar beet roots.
Quality is lost from roots that have suffered mechanical damage through various mechanisms. Cells that have suffered physical damage will heal themselves through a sequence of steps that leads to the development of suberin and lignin-like substances (Ibrahim et al., 2001). This costs energy, which will be drawn from the vacuole stores of sucrose. In damaged cells, the contents including the sucrose stored in cell vacuoles can simply leak away. An open wound is also a relatively easy point of entry to the root for a pathogen. In their study on the infection of Penicillium and Botrytis in storage piles, Mumford and Wyse (1976) suggest that an open wound is “essential for fungus infection”. Finally, if the damage leads to separation of fragments of the root, these may be left in the field. If small fragments do make it to the factory and end up in the test sample, this will likely lead to an increased dirt-tare as they will be washed out of the sample.
Numerous works show higher rates of quality loss during storage as a result of higher rates of damage. Rates of damage are often quantified as rate of exposure to a damage inducing action, such as tumbling in a rotating drum (Kenter et al., 2006). Kenter et al. (2006) found a five to six times greater loss of sucrose during storage from roots exposed to a very high rate of a damage inducing action in comparison to roots harvested under standard conditions. In a commercial setting over 50 days, Ingelsson (2003) found an average loss of sucrose per day of 0.19 % for root that experienced hard harvester cleaning, compared to 0.14 % for a more gentle cleaning. They also reported much higher incidence of moulds post-storage.
Specific types of damage are sometimes quantified, although they are usually found to exist simultaneously. For example, Akeson and Stout (1978) found that with increasing rates of impact, damage types expanded from just bruising, to bruising and surface wounds, and finally to bruising, surface
wounds, plus cracking. Ultimately, the pathway to loss of quality from the categories of physical damage are those discussed above.
Akeson and Stout (1978) showed that even at low fall impacts where no surface wounds or cracks were visible, loss of sucrose and accumulation of invert sugars during post-harvest storage was elevated. This was attributed to damage as bruising. Brown et al. (2002) attribute approximately 12.5 % percent of total sucrose losses from damage to bruising.
Damage to the surface of the root, like bruising, is not as obvious as cracking and may be obscured by soil. A NBR supervised student project (Skyggeson, 2016) found high levels of surface damage increases risk of frost damage. Machine harvested but crack free roots, and hand harvested roots were stored at -3 °C for a short period, then 8 °C for 18 days. It was observed that all the machine harvested roots showed signs of frost damage, while none of the hand-harvested roots displayed frost damage. This was attributed to surface damage.
The most extreme versions of mechanical damage are cracks (from impacts) and slices (from machinery), with the extreme version of cracks and slices being when entire fragments of the roots are detached. Mechanical harvest will result in cracks and slices. The tap root will need to be broken for the root to be lifted and the removal of the top of the root by slicing is a requirement of processors to ensure standards of quality. Acknowledging this, the test standards for harvest assessment from the IIRB take a root tip
break of two centimetres or less, and a topping diameter of five centimetres or less, as the zero-loss reference levels (Schulze Lammers et al., 2015). Akeson et al. (1974) showed that topping induces high rates of respiration from wound healing, and high rates of moulds and rots later in the post-harvest storage period. This ultimately lead to higher rates of sucrose loss, with 12.6 % lower total sucrose after post-harvest storage in topped roots compared to non-topped roots.
Mechanical harvest and handling in general provides numerous opportunities for mechanical damage: defoliating, topping, lifting, cleaning, transport within the machinery, and transfer between intermediary steps. Olsson (2008) found that 80 – 90 % of mechanical damage occurs in the harvester. The use of force sensors through a harvester showed repeated force applied on the roots of up to 75 impacts over a 12 second period (Tordeur, 2018). The largest transfer of energy has been found to be when there is a large fall, be it into the hopper tank on the harvester, at transfer to a chaser bin, or unloading into a clamp (Steven Aldis, (BBRO, England) 2018-07-11, personal communications). Post-storage, it is commonly observed that loading into transport to the factory is a point at which much kinetic energy is transferred to noise energy when roots land in the trailer: that is, there are large impacts.