OUTLOOK

Healthier roots

The results of this project suggest that there are a number of avenues the work on plant health and successful post-harvest storage should pursued. The first is a focus on the “environment” source of variation in storability. This includes soil physical properties and soil microbiology. The methods of analysis exist or are rapidly developing, they just need to be applied. If gains are made in the understanding of the soil microbiome, this can likely be applied to quality related issue outside of long-term post-harvest storage. The COBRI network of which NBR is a member, is well placed to take up this work, given it is established, effective, and has the background scientific knowledge. It also has the interesting test case of the Netherlands. Some of the leading scientific knowledge lies outside of NBR and COBRI, so new partnerships may be necessary. The assessment of the established metrics such as mechanical properties and mechanical damage should not be excluded to the preference of the new. If gains are made reducing rates of mechanical damage in harvest and transport equipment, the scale of use of these machines means the gains will diffuse relatively quickly and internationally. The importance of mechanical properties in the down-stream process of root slicing is an important factor to remember.

Root morphology, including root size, is also an interesting avenue of exploration. The factors of root size pull in different directions. Larger roots have more energy during a fall impact, suggesting more damage. The opposite was observed in this research project, but what about in reality? Larger roots suggest larger pores in the clamp and thus higher air permeability. This was seen in this research project, but it did not seem to be of large consequence. The bottom line for post-harvest storage is how does damage and loss per unit weight of sucrose vary with root size. Even then, that is not sufficient as it does not take into account loss in the field through the whole season. It will be interesting to see what the doctoral research project out of Harper Adams College in the UK focusing on root morphology reveals.

A more controlled storage environment

Dramatic changes to the general form of the post-harvest storage system adopted in Sweden in the near term seem unlikely. There is not a strong demand from industry to investigate new systems of storage, and there does not seem to be plans to dramatically increase production and thus a need for greatly extended long-term storage campaigns. The system of forced ventilation developed through the NBR lead innovation project was successful in decreasing post-harvest storage losses, but further development has been paused until the economics demand it. This said, any incremental innovation that includes the right combination of changes in quality, risk, and cost including management effort, will always be adopted. An example of a low risk research project that extends the work of this current project is in the search for practical solutions to the issue of frozen roots at the edge of the clamp. The effects of different cover types can be relatively easily tested with the CFD model. A more exploratory extension could be in the search for systems that permit a controlled dehydration through greater passive ventilation. Computational Fluid Dynamics modelling could also be used in support of this work.

Modelling

The continued development of the CFD model to include buoyancy, soil thermal properties, radiation, and a baseline version of the transport of water are all achievable without further experimentation. The work would always benefit from the collection of the appropriate field data for model validation, with information on the airflow in the clamp being most critical. Improvements to the functionality of the model are also possible to pursue directly. There are a number of achievable extensions of the model, including connecting it with a clamp temperature reporting network. It would then be able to act as a live digital twin, which growers can use to make better informed decisions around the use of covers.

Cross validation of heat and mass transfer

It has been discussed above that the CFD model can be expanded to consider mass transfer through the inclusion of the Sherwood-Reynolds-Schmidt correlation to calculate convective mass transfer coefficients (kc). An alternative approach to the inclusion of this correlation alongside the Ranz-Marshall correlation in the model is to include one and then apply the appropriate analogy between energy and mass transfer: the Lewis analogy. The Lewis number is Le = hsf ÷ (kc × ρa × Cf), where ρa is the density of air and Cf is the heat capacity of air. By setting this equal to 1, the Lewis analogy states hsf= kc × ρa × Cf . Thus, with either of the Sherwood-Reynolds-Schmidt or Ranz-Marshall correlations, the other convective transfer coefficient can be derived. The value of testing such an approach is that it can cross-validate the accuracy of the results presented in Papers III and IV. The coefficients hsf and kc were calculated from two different experiments, conducted under different conditions, times and locations. The gains from such an approach in the form of increased computational efficiency are likely minimal.

Understanding the different storage systems

In North America, it is common that no on-farm post-harvest storage is used (N. Wulfekuhle (Minn-Dak, USA), personal communications, 2022-11-08; J. Anderson (Lantic, Canada), personal communications, 2022-11-09; M. Garner-Skiba, personal communications, 2022-11-10). The majority of harvesters have very small hopper tank capacity, with roots loaded directly into transport moving alongside the harvester. Post-harvest storage occurs not on farm but in large piles at the processing factory. This system is not employed in Sweden and an exact explanation for this is not known. It would be interesting to dig deeper into the reasons, benefits, and costs of the different storage systems employed for the sugar beet crop around the world. This thesis has benefited from looking at the different systems, but a more structured and collaborative investigation of why the different systems are employed and what each system can learn from the other could prove valuable.