2. SUGAR LOSS : DEGREE DAYS
A meaningful method for defining the relationship between sugar loss and temperature is to measure temperature as accumulated thermal time, given as degree-days (°Cd). It has been shown that accumulated thermal time is able to provide more generalisable relationships with loss, with the relationship between °Cd and loss holding up relatively well against daily, locational, and seasonal variations. In post-harvest storage, degree-days are counted in the positive Celsius scale. For example, storage for 24 hours at 10°C is equivalent to 10°Cd, storage for 10 days at 1°C is equivalent to 10°Cd, or storage for 54 days at 5°C is equivalent to 270°Cd.
In a trial conducted by Jaggard et al. (1997) under commercial operating conditions across 18 farms in the UK for storage periods of up to 85 days, accumulated thermal time was a strong predictor of sugar loss. They estimated 0.0188% of the initially available sugar was lost per degree-day on average. For this to be equivalent to 0.1% per day, the clamp must be at 5.3°C. At 10°C, the loss per day is 0.18%. Other studies, such as those of Legrand and Wauters (2012) have found comparable values, although with more nuanced conditions. Legrand and Wauters showed that after 270°Cd, the rates of sugar loss increase greatly. For the period up to 270°Cd, an average loss of approximately 0.013% of initial sugar per degree-days was calculated. Thereafter, for the period from 270 to 450°Cd, an average loss of 0.042% of initial sugar per degree day was found. The average for the total period of 450°Cd was approximately 0.024% per °Cd. Legrand and Wauters’ figures are equivalent to Jaggard’s at 337.5°Cd – a value that would be considered to be entering the high risk phase of storage for commercially harvested beets. Table 1 summarises these findings.
Further to adding nuance to the relationship between accumulated temperature and sugar loss, the study of Legrand and Wauters (2012) showed that the relationship between degree-days and sugar loss in sugar beets was comparable for temperature ranges of 5 to 20 °C (see Table 1 for a selection of these results). It is important to note that in Legrand and Wauters (2012), the 270°Cd is taken as the temperature of the ambient air outside of the clamp. This, they note, corresponds to temperatures inside the clamp of 300 to 350°Cd, or ca. 11% to 30% higher.1
Jaggard, K. W., et al. (1997). “Changes in the weight and quality of sugarbeet (Beta vulgaris) roots in storage clamps on farm.” The Journal of Agricultural Science 129(3): 287-301.
Legrand, G. and A. Wauters (2012). New experiments on long term storage of sugar beets: effect of different storage temperatures according to the thermal time and effect of the harvesting time according to different varieties. 73rd IIRB congress, Brussels,.
1Direct 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.