3.2 MOISTURE

The research into air moisture levels in sugar beet root storage is less extensive than that into air temperature. This is likely due to the situation in naturally ventilated post-harvest storage where air relative humidity levels have been found to be consistently very high and losses from dehydration very low (Huijbregts et al., 2013; Zavrazhnov et al., 2021). Dehydration is the movement of water from the sugar beet root into the surrounding air. It is driven by a differential in the water vapour pressure between the root and the air, and the resistance of the skin of the root to this movement (Carta, 2021b). As relative humidity tends to 100 %, the water vapour pressure differential and thus dehydration tends to zero. For any given relative humidity, the water vapour pressure differential will increase with temperature. The research that has looked at dehydration resulting from low relative humidity originates mainly from North America, where post-harvest storage is longer and active ventilation is used to control temperature in the large pile storage system. It has been shown that lower relative humidity does lead to higher rates of weight loss of roots, but that the effect of temperature when within the ideal range for sugar beet storage (2 – 8 °C) is of little consequence for dehydration (Andales et al., 1980). Over a 15 week storage period in climate chambers at 3.3 to 8.9 °C, mean weight loss at 80-85 %RH was 31 %, while it was only 15 % at 95-100 %RH. Higher rates of electrolyte leakage and respiration rates have also been found in roots stored at lower relative humidity levels (Lafta & Fugate, 2009). Raffinose concentration can increase under mild dehydration, while severe dehydration was found to result in decreased concentrations of many non-sucrose carbohydrates with the exclusion of invert sugars (no change) (Lafta et al., 2020). Pathogen growth does not seem to be increased on dehydrated beets (Bugbee & Cole, 1979). Despite the importance of this transfer of water from sugar beet roots under post-harvest storage, there are no values known to be reported in the literature for rates of transfer per unit root surface area and time (mass flux). Further, it is not well known what the total expected weight loss from dehydration during commercial post-harvest storage is.

Independent of the relative humidity of air, there are also a collection of work that focuses on levels of moisture on the surface of the stored root. It has been noticed that rainfall events have large impacts on the measured dirttare and thus payment received. In a NBR supervised student project, Mårtensson (2017) compared the dirt-tare of samples split at harvest and either analysed directly or stored in boxes for between 15 and 50 days. The dirt-tare of the stored samples was stable at 10.15 % ± 0.85%. The dirt-tare of the samples analysed at harvest ranged from 9.6% to 19.6%, clearly increasing with the closeness of the harvest date to rainfall events. The conclusion was that the dehydration of the soil attached to the root is an important quality driver. At the national level, a correlation between increasing soil water at harvest and increasing average dirt-tare is a regularly observed phenomena (Ekelöf, 2017b).

The movement of moisture can also play an important role in the thermodynamics of the storage system. Cannon (1950) notes the cooling potential of evaporation, suggesting that at temperatures below 21 °C it could remove all the heat of respiration. Zavrazhnov et al. (2021) models the heat exchange at the surface of a 6.5 m high unventilated pile during November in the Kursk region of Russia. They estimate that evaporation at the beet surface accounts for approximately 20 % of the total heat exchange.