3.1 TEMPERATURE

Temperature is often cited as the most important factor in post-harvest storage (Wills & Golding, 2016). Huijbregts et al. (2013) give the optimum temperature range for the post-harvest storage of non-frozen sugar beet roots under commercial conditions as 2 to 8 °C. The more restricted range of 4 to 6 °C is given in both Bugbee (1993) and English (2020). This minimises the loss of quality per unit time. The temperature should preferably be stable (Wyse, 1978). Within this optimal temperature range, the processes that cause the loss of sucrose stored at the cellular levelled are slowed, as too the rate of loss of processable material. The loss of sucrose is reduced owing to lower respiration (Dilley et al., 1970). Respiration is the primary source of harvested sugar beet roots generally have an early peak at approximately day 3 or 30 °Cd, reach a minimum at approximately 200 °Cd, and then show gradual increase (Akeson et al., 1974; Dilley et al., 1970; Huijbregts, 2009). The early peak has been stated as 2 to 10 times the minimum rate and abates approximately as quickly as it increases (Akeson et al., 1974).

At lower temperatures there will be lower rates of pathogen growth (Sviridov & Kolomiets (2012) quoted in Korobova et al., 2022) and even the avoidance of the growth of some common pathogens (Legrand & Wauters, 2012). Rates of moulds and rots are of increased importance when post-harvest storage extends beyond approximately 250 to 300 °Cd. A large study in six European countries found rates of loss during post-harvest storage correlated most strongly with rates of moulds and rots when storage extended beyond 339 °Cd and out to 996 °Cd (van Swaaij & Huijbregts, 2010). By setting a floor of 4 °C, the accumulation of invert sugars and the trisaccharide raffinose will be less than if it was a lower temperature (Haagenson et al., 2008; Wyse & Dexter, 1971).

Outside of this optimum, temperatures below -2 °C should be avoided (Milford et al., 2002; Wyse, 1978). By avoiding sub-zero temperatures, the severe and acute loss that results when cells freeze and thaw is also avoided. A maximum temperature limit is not known as post-harvest storage is not commonly practiced in climates with ambient temperatures persistently above 15 °C, but it is clear that temperatures above 25 °C should be avoided (Orleans & Cotton, 1952). Given respiration in plants is temperature dependent (Klotz et al., 2008; Vallarino & Osorio, 2019) and seems to increase exponentially in sugar beet roots for the range of temperatures they are commonly stored in post-harvest (Vukov, 1977), there exists the potential for a vicious cycle of temperature and respiration rate to develop in bulks of sugar beet roots. This can rapidly lead to large rates of quality loss.

A generally applicable relationship between temperature and rates of loss of sucrose in stored sugar beet roots under commercial conditions is that loss is 0.02 %/°Cd (Jaggard et al., 1997). Percent is of the total available sucrose at the time of entering post-harvest storage, and the temperature is taken as the ambient temperature. The experimental data that gave these results went out to approximately 700 °Cd. An alternative form of this relationship comes from Legrand and Wauters (2012). They give a rate of loss of 0.013 %/°Cd out to 270 °Cd, and 0.042 %/°Cd between 270 °Cd and 450 °Cd. This gives an average of 0.024 %/°Cd out to 450 °Cd. The increased rate in the second loss of stored sucrose in the early stages of post-harvest storage (out to ca. 300 degree-days (°Cd) – measured in ambient air). Respiration rates in period is again attributed to the increased rates of moulds and rots. Given the aforementioned early peak in rates of respiration, these relationships are only accurate when applied over a long-term post-harvest storage period.

The interaction between temperature and these many other factors suggest it could itself forma principle: that sugar beet roots store better when held within an optimal temperature range. There are, however, two important footnotes that should be appended. The first footnote is that during the initial post-harvest period in which the roots need to heal from the harvest process, the optimal temperature is most likely well above the 4-6 °C range. Fugate et al. (2016) showed that wound healing is more complete in roots stored at 12 °C compared to 6 °C. Only the respiration and transpiration rate of the wounded root stored at 12 °C returned to levels similar to the wound free controls. The loss per degree-day out to the end of the comparable data at ca. 170 °Cd was similar at both temperatures, but it could be expected that the roots initially kept at the higher temperature will store better thereafter. The second footnote is that the absolute best temperature to store roots at to maintain quality over time is that at which the root freeze and remain frozen until processing. An average minimum daily temperature below -9 °C has been suggested as necessary (Bugbee, 1993). This is not a practical solution in the Swedish context, but is achievable in some parts of North America (Backer et al., 1979; Bichel, 1988) and Russia.

The details of the role of temperature in the long-term storage of sugar beet roots is discussed in detail in the literature study written as part of this research project: English (2020).