Impact of storage temperature, storage duration, and harvest date on sugarbeet raffinose metabolism

Raffinose negatively impacts sugarbeet (Beta vulgaris L.) processing by decreasing extractable sucrose yield and altering sucrose crystal morphology which reduces filtration rates and slows processing. Although increased raffinose concentrations have been observed during cold storage, the physiological and biochemical mechanisms associated with raffinose accumulation in sugarbeet are poorly understood. The objective of this study was to characterize the impact of storage temperature, storage duration, and harvest date on raffinose metabolism. Field-grown sugarbeets were harvested 7 September, 27 September, and 26 October 2004, and stored for 2, 10, or 18 weeks at 2 °C or 6 °C. Raffinose concentrations were approximately double at 2 weeks of storage, nearly threefold higher at 10 weeks, and decreased slightly at 18 weeks. Delaying harvest date increased raffinose concentration at harvest (0 weeks), but decreased concentrations at 18 weeks of storage. Storage temperature did not affect crown raffinose concentrations, but root tissues stored at 2 °C had 19% higher raffinose concentrations than at 6 °C. Biosynthetic or catabolic enzyme activities accounted for less than 15% of the variation in raffinose content in storage, although a small positive correlation (r = 0.28) between raffinose synthase activity and raffinose concentration in root tissues was observed. Galactinol synthase was highly expressed in tissues collected in late October and at 2 weeks of storage, and α-galactosidase activity increased 55% in roots stored for 18 weeks at 6 °C. Factors contributing to sugarbeet raffinose accumulation in storage are complex as raffinose concentrations were impacted by storage duration, harvest date, and storage temperature.