Does Field Shading Affect the Cold Storage Performance of Mandarins?

The use of shade nets in citrus orchards has become widespread in many production areas to protect trees against heat waves, wind and sunburn. This agronomic practice raises a key question for packhouses and exporters: can field shading change the ability of the fruit to withstand long periods of cold storage and subsequent shelf life at ambient temperature?

A recent study published in the scientific journal Horticulturae addresses this question using the mandarin cultivar "Nadorcott" as a case study, one of the most important soft citrus varieties in export programs. The results indicate that, under the conditions tested, the use of shade nets in the orchard did not modify the postharvest behaviour of the fruit during cold storage, providing relevant information for regions where shade nets and long cold chains are combined.

A trial under conditions close to commercial reality

The work was carried out in a commercial "Nadorcott" orchard in Citrusdal (Western Cape, South Africa), comparing two management systems: trees grown under a white 20% shade net and trees without netting. Fruit was harvested over two consecutive seasons, which allowed the researchers to check whether the behaviour was consistent between years with different climatic conditions.

After harvest, fruit was stored at 4 °C or −0.6 °C for 14, 27 and 34 days, followed by 7 days of simulated shelf life at 20 °C. Fruit was also analysed at harvest to establish a clear reference point before entering cold rooms.

Quality parameters monitored during storage and shelf life

To evaluate the possible effect of shading on cold storage performance, the researchers monitored internal and external parameters directly related to the commercial quality of mandarins:

• Peel and pulp colour

• Carotenoid content in the flavedo

• Soluble solids content (SSC), titratable acidity and SSC/acid ratio

• Weight loss during storage

• Incidence of physiological rind disorders, in particular rind staining, which can affect visual acceptance of the fruit

This monitoring was carried out both during cold storage and after the shelf life period, in order to reproduce the situation of fruit destined for export.

Shading did not alter postharvest behaviour

The results showed that no significant differences were detected between fruit from trees grown under shade nets and fruit from trees without netting for any of the postharvest parameters evaluated. In particular, preharvest shading in the orchard did not change:

• The evolution of peel and pulp colour during cold storage and shelf life

• Carotenoid levels in the peel

• The values and evolution of SSC, titratable acidity or SSC/acid ratio

• The percentage of weight loss

• The incidence and severity of rind staining on the fruit surface

In one of the seasons, rind disorders such as staining only appeared after 34 days of storage at both 4 °C and −0.6 °C, with no differences between shaded and non-shaded treatments.

The role of storage duration and seasonal conditions

The study did show a clear effect of storage duration on some quality parameters, such as increased weight loss and certain changes in carotenoid content as time in cold storage increased, regardless of the use of shade nets.

Differences between seasons were also observed in some results, which the authors relate to the specific conditions of each year and to the condition of the fruit at harvest.

For postharvest managers, these data reinforce the importance of factors such as harvest date, fruit condition and pre-storage handling, which can significantly influence the response of mandarins during cold storage, beyond the presence or absence of shade nets in the orchard.

Implications for producing regions combining shading and long cold chains

Based on the results of this trial, the authors conclude that the use of a white 20% shade net in "Nadorcott" orchards does not compromise the potential for cold storage at 4 °C or under more intense low-temperature regimes used for disinfestation (−0.6 °C), at least under the growing and handling conditions evaluated.

This message is of particular interest for producing regions where shading is used to protect fruit in the field and, at the same time, long cold chains are required for distant export markets. Managing storage duration, controlling weight loss and monitoring possible rind disorders remain critical points, but shading itself does not appear as a limiting factor for postharvest quality in this variety.

The authors also point out that it would be useful to extend this type of study to other citrus species and cultivars, since it cannot be assumed that all will respond in the same way to shade netting when later subjected to prolonged storage.

Source

Niemann, J., Hoffman, E. W., Zacarías, L., Kidd, M., & Cronjé, P. J. R. (2024). Postharvest Cold-Storage Behaviour of ‘Nadorcott’ Mandarin Fruit Remains Unaffected by Preharvest Shade Netting. Horticulturae, 10(8), 782. https://doi.org/10.3390/horticulturae10080782