4. SUGAR LOSS : NEGATIVE TEMPERATURES
Within the cumulative temperature framework, the degree days relationship only holds within the bounds of a limited range of positive temperatures on the Celsius scale. As such, a negative temperature does not contribute to a reduction in degree-days. Indeed, it is usual that exposure to negative temperatures ultimately results in increased total sugar loss compared to the positive yet low temperature range. Should a beet cell freeze, it is usual that processability is rapidly lost upon thaw. Any resulting rupturing of cell walls permits the leaching of electrolytes and the entry of pathogens. Bacteria that enter the cell after thawing result in the production of the non-processable invert sugars and polysaccharide gums levan and dextran (Oldfield et al (1971) cited in Campbell and Klotz, 2006; Milford et al., 2002). Taking from Wyse (1978, figure 5); using release of carbohydrates as a direct proxy for cell damage, the pre-freezing release level as the base rate in the experiment, and the -8°C release rate as 100%, approximately 8% of sugar beet cells are damaged at -1°C, 14% at -2°C, 39% at -3°C, and 78% at -4°C. Wyse (1978) themselves suggest all cells are frozen or destroyed at -5°C, and about half are thus at -3°C.
It is possible for a beet to withstand short durations of negative temperatures without freezing, owing in large to the lower freezing point of a solution with high sugar content. At 17% sucrose, and 77% water, the freezing point of the liquid component of the beet is approximately -1.2°C (own calculations 1). The rate at which the cold penetrates the beet and their ability to produce their own heat also contribute. Further into the negative degrees, the losses from a beet that is frozen but not thawed are less than those of the non-frozen beet. Indeed, in commercial processing systems in which it is likely that very low temperatures of ca. -9°C and less will be realised during many weeks and months, beets are usually permitted to freeze in storage. This has been shown to be an effective means of minimising sucrose loss in long-term storage, with temperatures so far below freezing needed to counter the heat generated within the pile of beets (Backer et al., 1979). Frozen beets are usually processed thus to avoid losses from thawing. 2
Campbell, L. and K. L. Klotz (2006). Storage. Sugar Beet. A. P. Draycott. Oxford, Blackwell Publishing Ltd: 387-408.
Milford, G., et al. (2002). FROST damage to sugar beet – estimating the risk. British Sugar Beet Review. Peterborough, Cambridgeshire, UK, British Sugar plc. 70: 41-45.
Wyse, R. E. (1978). “Effect of Low and Fluctuating Temperatures on the Storage Life of Sugarbeets.” Journal of the American Society of Sugar Beet Technologists 20(1): 33-42.
Backer, L. F., et al. (1979). Ventilation and freezing of sugarbeet storage piles.
1 ΔT1 = Kfm, Kf water = 1.86, gsucrose = 17, Molecular weight of sucrose = 342.32g.mole-1, molesucrose = 17/342.32, kgH2O = 0.077, m = molal = molesucrose.kg-1
2Direct extract from: English, W. (2020). Long Term Storage of Sugar Beets and the Role of Temperature. Introductory paper at the Faculty of Landscape Architecture, Horticulture and Crop Production Science. Alnarp, Sweden, Faculty of Landscape Architecture, Horticulture and Crop Production Science, Swedish University of Agricultural Science. 2020:14.